The let-7 microRNA directs vulval development through a single target

CRISPR-based dissection of miRNA binding sites using isogenic cell lines is hampered by pervasive noise

Non-coding regulatory sequences play essential roles in adjusting gene output to cellular needs and are thus critical to animal development and health. Numerous such sequences have been identified in mammalian genomes ranging from transcription factors binding motifs to recognition sites for RNA-binding proteins and non-coding RNAs. The advent of CRISPR has raised the possibility of assigning functionality to individual endogenous regulatory sites by facilitating the generation of isogenic cell lines that differ by a defined set of genetic modifications. Here we investigate the usefulness of this approach to assign function to individual miRNA binding sites. We find that the process of generating isogenic pairs of mammalian cell lines with CRISPR-mediated mutations introduces extensive molecular and phenotypic variability between biological replicates confounding attempts at assigning function to the binding site. Our work highlights an important consideration when employing CRISPR editing to characterize non-coding regulatory sequences in cell lines and calls for the development and adoption of alternative strategies to address this question in the future.

In silico identification and characterization of microRNAs from rotifers, cladocerans, and copepods

MicroRNAs (miRNAs) are short non-coding RNA molecules that regulate post-transcription and influence various biological processes across species. Despite various studies of miRNAs in vertebrates, plants, and other organisms, miRNA data in aquatic invertebrates are insufficient. In this study, we identified miRNAs from four aquatic invertebrate species that are widely used in aquatic toxicology: the rotifer Brachionus koreanus, the water flea Daphnia magna, the cyclopoid copepod Paracyclopina nana, and the harpacticoid copepod Tigriopus japonicus, using next-generation sequencing and in silico analysis. We identified total 188, 41, 47, and 100 miRNAs from each species, and target genes were predicted based on 3'-untranslated region information. Target prediction and functional annotation results provided the biological processes of these miRNAs in various development-related mechanisms, signaling transduction, and metabolism-related pathways. Moreover, the network between the miRNAs and their targets concerning defense-related and antioxidant genes suggests the suitability of miRNAs as biomarkers in ecotoxicological studies.

A new perspective on microRNA-guided gene regulation specificity, and its potential generalization to transcription factors and RNA-binding proteins

Our conception of gene regulation specificity has undergone profound changes over the last 20 years. Previously, regulators were considered to control few genes, recognized with exquisite specificity by a 'lock and key' mechanism. However, recently genome-wide exploration of regulator binding site occupancy (whether on DNA or RNA targets) revealed extensive lists of molecular targets for every studied regulator. Such poor biochemical specificity suggested that each regulator controls many genes, collectively contributing to biological phenotypes. Here, I propose a third model, whereby regulators' biological specificity is only partially due to 'lock and key' biochemistry. Rather, regulators affect many genes at the microscopic scale, but biological consequences for most interactions are attenuated at the mesoscopic scale: only a few regulatory events propagate from microscopic to macroscopic scale; others are made inconsequential by homeostatic mechanisms. This model is well supported by the microRNA literature, and data suggest that it extends to other regulators. It reconciles contradicting observations from biochemistry and comparative genomics on one hand and in vivo genetics on the other hand, but this conceptual unification is obscured by common misconceptions and counter-intuitive modes of graphical display. Profound understanding of gene regulation requires conceptual clarification, and better suited statistical analyses and graphical representation.

CRISPR-based dissection of miRNA binding sites using isogenic cell lines is hampered by pervasive noise

Non-coding regulatory sequences play essential roles in adjusting gene output to cellular needs and are thus critical to animal development and health. Numerous such sequences have been identified in mammalian genomes ranging from transcription factors binding motifs to recognition sites for RNA-binding proteins and non-coding RNAs. The advent of CRISPR has raised the possibility of assigning functionality to individual endogenous regulatory sites by facilitating the generation of isogenic cell lines that differ by a defined set of genetic modifications. Here we investigate the usefulness of this approach to assign function to individual miRNA binding sites. We find that the process of generating isogenic pairs of mammalian cell lines with CRISPR-mediated mutations introduces extensive molecular and phenotypic variability between biological replicates making any attempt of assigning function to the binding site essentially impossible. Our work highlights an important consideration when employing CRISPR editing to characterize non-coding regulatory sequences in cell lines and calls for the development and adoption of alternative strategies to address this question in the future.

A comprehensive analysis of 3'UTRs in Caenorhabditis elegans

3'Untranslated regions (3'UTRs) are essential portions of genes containing elements necessary for pre-mRNA 3'end processing and are involved in post-transcriptional gene regulation. Despite their importance, they remain poorly characterized in eukaryotes. Here, we have used a multi-pronged approach to extract and curate 3'UTR data from 11533 publicly available datasets, corresponding to the entire collection of Caenorhabditis elegans transcriptomes stored in the NCBI repository from 2009 to 2023. We have also performed high throughput cloning pipelines to identify and validate rare 3'UTR isoforms and incorporated and manually curated 3'UTR isoforms from previously published datasets. This updated C. elegans 3'UTRome (v3) is the most comprehensive resource in any metazoan to date, covering 97.4% of the 20362 experimentally validated protein-coding genes with refined and updated 3'UTR boundaries for 23489 3'UTR isoforms. We also used this novel dataset to identify and characterize sequence elements involved in pre-mRNA 3'end processing and update miRNA target predictions. This resource provides important insights into the 3'UTR formation, function, and regulation in eukaryotes.

Interaction between a J-domain co-chaperone and a specific Argonaute protein contributes to microRNA function in animals

MicroRNAs (miRNAs) are essential regulators of several biological processes. They are loaded onto Argonaute (AGO) proteins to achieve their repressive function, forming the microRNA-Induced Silencing Complex known as miRISC. While several AGO proteins are expressed in plants and animals, it is still unclear why specific AGOs are strictly binding miRNAs. Here, we identified the co-chaperone DNJ-12 as a new interactor of ALG-1, one of the two major miRNA-specific AGOs in Caenorhabditis elegans. DNJ-12 does not interact with ALG-2, the other major miRNA-specific AGO, and PRG-1 and RDE-1, two AGOs involved in other small RNA pathways, making it a specific actor in ALG-1-dependent miRNA-mediated gene silencing. The loss of DNJ-12 causes developmental defects associated with defective miRNA function. Using the Auxin Inducible Degron system, a powerful tool to acutely degrade proteins in specific tissues, we show that DNJ-12 depletion hampers ALG-1 interaction with HSP70, a chaperone required for miRISC loading in vitro. Moreover, DNJ-12 depletion leads to the decrease of several miRNAs and prevents their loading onto ALG-1. This study uncovers the importance of a co-chaperone for the miRNA function in vivo and provides insights to explain how different small RNAs associate with specific AGO in animals.

Defining the contribution of microRNA-specific Argonautes with slicer capability in animals

microRNAs regulate gene expression through interaction with an Argonaute protein. While some members of this protein family retain an enzymatic activity capable of cleaving RNA molecules complementary to Argonaute-bound small RNAs, the role of the slicer residues in the canonical microRNA pathway is still unclear in animals. To address this, we created Caenorhabditis elegans strains with mutated slicer residues in the endogenous ALG-1 and ALG-2, the only two slicing Argonautes essential for the miRNA pathway in this animal model. We observe that the mutation in ALG-1 and ALG-2 catalytic residues affects overall animal fitness and causes phenotypes reminiscent of miRNA defects only when grown and maintained at restrictive temperature. Furthermore, the analysis of global miRNA expression shows that the slicer residues of ALG-1 and ALG-2 contribute differentially to regulate the level of specific subsets of miRNAs in young adults. We also demonstrate that altering the catalytic tetrad of those miRNA-specific Argonautes does not result in any defect in the production of canonical miRNAs. Together, these data support that the slicer residues of miRNA-specific Argonautes contribute to maintaining levels of a set of miRNAs for optimal viability and fitness in animals particularly exposed to specific growing conditions.

The rapidly evolving X-linked MIR-506 family fine-tunes spermatogenesis to enhance sperm competition

Despite rapid evolution across eutherian mammals, the X-linked MIR-506 family miRNAs are located in a region flanked by two highly conserved protein-coding genes (SLITRK2 and FMR1) on the X chromosome. Intriguingly, these miRNAs are predominantly expressed in the testis, suggesting a potential role in spermatogenesis and male fertility. Here, we report that the X-linked MIR-506 family miRNAs were derived from the MER91C DNA transposons. Selective inactivation of individual miRNAs or clusters caused no discernible defects, but simultaneous ablation of five clusters containing 19 members of the MIR-506 family led to reduced male fertility in mice. Despite normal sperm counts, motility, and morphology, the KO sperm were less competitive than wild-type sperm when subjected to a polyandrous mating scheme. Transcriptomic and bioinformatic analyses revealed that these X-linked MIR-506 family miRNAs, in addition to targeting a set of conserved genes, have more targets that are critical for spermatogenesis and embryonic development during evolution. Our data suggest that the MIR-506 family miRNAs function to enhance sperm competitiveness and reproductive fitness of the male by finetuning gene expression during spermatogenesis.

miR-483-5p orchestrates the initiation of protein synthesis by facilitating the decrease in phosphorylated Ser209eIF4E and 4E-BP1 levels

Eukaryotic initiation factor 4E (eIF4E) is a pivotal protein involved in the regulatory mechanism for global protein synthesis in both physiological and pathological conditions. MicroRNAs (miRNAs) play a significant role in regulating gene expression by targeting mRNA. However, the ability of miRNAs to regulate eIF4E and its phosphorylation remains relatively unknown. In this study, we predicted and experimentally verified targets for miR-483-5p, including eukaryotic translation initiation factor eIF4E and its binding proteins, 4E-BPs, that regulate protein synthesis. Using the Web of Science database, we identified 28 experimentally verified miR-483-5p targets, and by the TargetScan database, we found 1818 predicted mRNA targets, including EIF4E, EIF4EBP1, and EIF4EBP2. We verified that miR-483-5p significantly reduced ERK1 and MKNK1 mRNA levels in HEK293 cells. Furthermore, we discovered that miR-483-5p suppressed EIF4EBP1 and EIF4EBP2, but not EIF4E. Finally, we found that miR-483-5p reduced the level of phosphorylated eIF4E (pSer209eIF4E) but not total eIF4E. In conclusion, our study suggests that miR-483-5p's multi-targeting effect on the ERK1/ MKNK1 axis modulates the phosphorylation state of eIF4E. Unlike siRNA, miRNA can have multiple targets in the pathway, and thereby exploring the role of miR-483-5p in various cancer models may uncover therapeutic options.

Recent advances in understanding microRNA function and regulation in C. elegans

MicroRNAs (miRNAs) were first discovered in C. elegans as essential post-transcriptional regulators of gene expression. Since their initial discovery, miRNAs have been implicated in numerous areas of physiology and disease in all animals examined. In recent years, the C. elegans model continues to contribute important advances to all areas of miRNA research. Technological advances in tissue-specific miRNA profiling and genome editing have driven breakthroughs in understanding biological functions of miRNAs, mechanism of miRNA action, and regulation of miRNAs. In this review, we highlight these new C. elegans findings from the past five to seven years.

Evaluation of microRNA variant maturation prior to genome edition

Assessment of the functionality of individual microRNA/target sites is a crucial issue. Genome editing techniques should theoretically permit a fine functional exploration of such interactions, allowing the mutation of microRNAs or individual binding sites in a complete in vivo setting, therefore abrogating or restoring individual interactions on demand. A major limitation to this experimental strategy is the influence of microRNA sequence on its accumulation level, which introduces a confounding effect when assessing phenotypic rescue by compensatorily mutated microRNA and target site. Here we describe a simple assay to identify microRNA variants most likely to accumulate at wild-type levels even though their sequence has been mutated. In this assay, quantification of a reporter construct in cultured cells predicts the efficiency of an early biogenesis step, the Drosha-dependent cleavage of microRNA precursors, which appears to be a major determinant of microRNA accumulation in our variant collection. This system allowed the generation of a mutant Drosophila strain expressing a bantam microRNA variant at wild-type levels.

Looking for a needle in a haystack: de novo phenotypic target identification reveals Hippo pathway-mediated miR-202 regulation of egg production

Understanding microRNA (miRNA) functions has been hampered by major difficulties in identifying their biological target(s). Currently, the main limitation is the lack of a suitable strategy to identify biologically relevant targets among a high number of putative targets. Here we provide a proof of concept of successful de novo (i.e. without prior knowledge of its identity) miRNA phenotypic target (i.e. target whose de-repression contributes to the phenotypic outcomes) identification from RNA-seq data. Using the medaka mir-202 knock-out (KO) model in which inactivation leads to a major organism-level reproductive phenotype, including reduced egg production, we introduced novel criteria including limited fold-change in KO and low interindividual variability in gene expression to reduce the list of 2853 putative targets to a short list of 5. We selected tead3b, a member of the evolutionarily-conserved Hippo pathway, known to regulate ovarian functions, due to its remarkably strong and evolutionarily conserved binding affinity for miR-202-5p. Deleting the miR-202-5p binding site in the 3' UTR of tead3b, but not of other Hippo pathway members sav1 and vgll4b, triggered a reduced egg production phenotype. This is one of the few successful examples of de novo functional assignment of a miRNA phenotypic target in vivo in vertebrates.

The Rapidly Evolving X-linked miR-506 Family Finetunes Spermatogenesis to Enhance Sperm Competition

Despite rapid evolution across eutherian mammals, the X-linked miR-506 family miRNAs are located in a region flanked by two highly conserved protein-coding genes (Slitrk2 and Fmr1) on the X chromosome. Intriguingly, these miRNAs are predominantly expressed in the testis, suggesting a potential role in spermatogenesis and male fertility. Here, we report that the X-linked miR-506 family miRNAs were derived from the MER91C DNA transposons. Selective inactivation of individual miRNAs or clusters caused no discernable defects, but simultaneous ablation of five clusters containing nineteen members of the miR-506 family led to reduced male fertility in mice. Despite normal sperm counts, motility and morphology, the KO sperm were less competitive than wild-type sperm when subjected to a polyandrous mating scheme. Transcriptomic and bioinformatic analyses revealed that these X-linked miR-506 family miRNAs, in addition to targeting a set of conserved genes, have more targets that are critical for spermatogenesis and embryonic development during evolution. Our data suggest that the miR-506 family miRNAs function to enhance sperm competitiveness and reproductive fitness of the male by finetuning gene expression during spermatogenesis.

Defining the contribution of microRNA-specific slicing Argonautes in animals

microRNAs regulate gene expression through interaction with an Argonaute protein family member. While some members of this protein family retain an enzymatic activity capable of cleaving RNA molecules complementary to Argonaute-bound small RNAs, the role of the slicing activity in the canonical microRNA pathway is still unclear in animals. To address the importance of slicing Argonautes in animals, we created Caenorhabditis elegans strains, carrying catalytically dead endogenous ALG-1 and ALG-2, the only two slicing Argonautes essential for the miRNA pathway in this animal model. We observe that the loss of ALG-1 and ALG-2 slicing activity affects overall animal fitness and causes phenotypes, reminiscent of miRNA defects, only when grown and maintained at restrictive temperature. Furthermore, the analysis of global miRNA expression shows that the catalytic activity of ALG-1 and ALG-2 differentially regulate the level of specific subsets of miRNAs in young adults. We also demonstrate that altering the slicing activity of those miRNA-specific Argonautes does not result in any defect in the production of canonical miRNAs. Together, these data support that the slicing activity of miRNA-specific Argonautes function to maintain the levels of a set of miRNAs for optimal viability and fitness in animals particularly exposed to specific growing conditions.

microRNAs in action: biogenesis, function and regulation

Ever since microRNAs (miRNAs) were first recognized as an extensive gene family >20 years ago, a broad community of researchers was drawn to investigate the universe of small regulatory RNAs. Although core features of miRNA biogenesis and function were revealed early on, recent years continue to uncover fundamental information on the structural and molecular dynamics of core miRNA machinery, how miRNA substrates and targets are selected from the transcriptome, new avenues for multilevel regulation of miRNA biogenesis and mechanisms for miRNA turnover. Many of these latest insights were enabled by recent technological advances, including massively parallel assays, cryogenic electron microscopy, single-molecule imaging and CRISPR-Cas9 screening. Here, we summarize the current understanding of miRNA biogenesis, function and regulation, and outline challenges to address in the future.

Casein kinase 1 and 2 phosphorylate Argonaute proteins to regulate miRNA-mediated gene silencing

MicroRNAs (miRNAs) together with Argonaute (AGO) proteins form the core of the RNA-induced silencing complex (RISC) to regulate gene expression of their target RNAs post-transcriptionally. Argonaute proteins are subjected to intensive regulation via various post-translational modifications that can affect their stability, silencing efficacy and specificity for targeted gene regulation. We report here that in Caenorhabditis elegans, two conserved serine/threonine kinases - casein kinase 1 alpha 1 (CK1A1) and casein kinase 2 (CK2) - regulate a highly conserved phosphorylation cluster of 4 Serine residues (S988:S998) on the miRNA-specific AGO protein ALG-1. We show that CK1A1 phosphorylates ALG-1 at sites S992 and S995, while CK2 phosphorylates ALG-1 at sites S988 and S998. Furthermore, we demonstrate that phospho-mimicking mutants of the entire S988:S998 cluster rescue the various developmental defects observed upon depleting CK1A1 and CK2. In humans, we show that CK1A1 also acts as a priming kinase of this cluster on AGO2. Altogether, our data suggest that phosphorylation of AGO within the cluster by CK1A1 and CK2 is required for efficient miRISC-target RNA binding and silencing.

Post-transcriptional repression of CFP-1 expands the regulatory repertoire of LIN-41/TRIM71

The Caenorhabditis elegans LIN-41/TRIM71 is a well-studied example of a versatile regulator of mRNA fate, which plays different biological functions involving distinct post-transcriptional mechanisms. In the soma, LIN-41 determines the timing of developmental transitions between larval stages. The somatic LIN-41 recognizes specific mRNAs via LREs (LIN-41 Recognition Elements) and elicits either mRNA decay or translational repression. In the germline, LIN-41 controls the oocyte-to-embryo transition (OET), although the relevant targets and regulatory mechanisms are poorly understood. The germline LIN-41 was suggested to regulate mRNAs indirectly by associating with another RNA-binding protein. We show here that LIN-41 can also regulate germline mRNAs via the LREs. Through a computational-experimental analysis, we identified the germline mRNAs potentially controlled via LREs and validated one target, the cfp-1 mRNA, encoding a conserved chromatin modifier. Our analysis suggests that cfp-1 may be a long-sought target whose LIN-41-mediated regulation during OET facilitates the transcriptional reprogramming underlying the switch from germ- to somatic cell identity.

Convergence of multiple RNA-silencing pathways on GW182/TNRC6

The RNA-binding protein TRIM71/LIN-41 is a phylogenetically conserved developmental regulator that functions in mammalian stem cell reprogramming, brain development, and cancer. TRIM71 recognizes target mRNAs through hairpin motifs and silences them through molecular mechanisms that await identification. Here, we uncover that TRIM71 represses its targets through RNA-supported interaction with TNRC6/GW182, a core component of the miRNA-induced silencing complex (miRISC). We demonstrate that AGO2, TRIM71, and UPF1 each recruit TNRC6 to specific sets of transcripts to silence them. As cellular TNRC6 levels are limiting, competition occurs among the silencing pathways, such that the loss of AGO proteins or of AGO binding to TNRC6 enhances the activities of the other pathways. We conclude that a miRNA-like silencing activity is shared among different mRNA silencing pathways and that the use of TNRC6 as a central hub provides a means to integrate their activities.

A Compilation of the Diverse miRNA Functions in Caenorhabditis elegans and Drosophila melanogaster Development

MicroRNAs are critical regulators of post-transcriptional gene expression in a wide range of taxa, including invertebrates, mammals, and plants. Since their discovery in the nematode, Caenorhabditis elegans, miRNA research has exploded, and they are being identified in almost every facet of development. Invertebrate model organisms, particularly C. elegans, and Drosophila melanogaster, are ideal systems for studying miRNA function, and the roles of many miRNAs are known in these animals. In this review, we compiled the functions of many of the miRNAs that are involved in the development of these invertebrate model species. We examine how gene regulation by miRNAs shapes both embryonic and larval development and show that, although many different aspects of development are regulated, several trends are apparent in the nature of their regulation.

A specific type of Argonaute phosphorylation regulates binding to microRNAs during C. elegans development

Argonaute proteins are at the core of the microRNA-mediated gene silencing pathway essential for animals. In C. elegans, the microRNA-specific Argonautes ALG-1 and ALG-2 regulate multiple processes required for proper animal developmental timing and viability. Here we identified a phosphorylation site on ALG-1 that modulates microRNA association. Mutating ALG-1 serine 642 into a phospho-mimicking residue impairs microRNA binding and causes embryonic lethality and post-embryonic phenotypes that are consistent with alteration of microRNA functions. Monitoring microRNA levels in alg-1 phosphorylation mutant animals shows that microRNA passenger strands increase in abundance but are not preferentially loaded into ALG-1, indicating that the miRNA binding defects could lead to microRNA duplex accumulation. Our genetic and biochemical experiments support protein kinase A (PKA) KIN-1 as the putative kinase that phosphorylates ALG-1 serine 642. Our data indicate that PKA triggers ALG-1 phosphorylation to regulate its microRNA association during C. elegans development.

Feedback between a retinoid-related nuclear receptor and the let-7 microRNAs controls the pace and number of molting cycles in C. elegans

Animal development requires coordination among cyclic processes, sequential cell fate specifications, and once-a-lifetime morphogenic events, but the underlying timing mechanisms are not well understood. Caenorhabditis elegans undergoes four molts at regular 8 to 10 hour intervals. The pace of the cycle is governed by PERIOD/lin-42 and other as-yet unknown factors. Cessation of the cycle in young adults is controlled by the let-7 family of microRNAs and downstream transcription factors in the heterochronic pathway. Here, we characterize a negative feedback loop between NHR-23, the worm homolog of mammalian retinoid-related orphan receptors (RORs), and the let-7 family of microRNAs that regulates both the frequency and finite number of molts. The molting cycle is decelerated in nhr-23 knockdowns and accelerated in let-7(-) mutants, but timed similarly in let-7(-) nhr-23(-) double mutants and wild-type animals. NHR-23 binds response elements (ROREs) in the let-7 promoter and activates transcription. In turn, let-7 dampens nhr-23 expression across development via a complementary let-7-binding site (LCS) in the nhr-23 3' UTR. The molecular interactions between NHR-23 and let-7 hold true for other let-7 family microRNAs. Either derepression of nhr-23 transcripts by LCS deletion or high gene dosage of nhr-23 leads to protracted behavioral quiescence and extra molts in adults. NHR-23 and let-7 also coregulate scores of genes required for execution of the molts, including lin-42. In addition, ROREs and LCSs isolated from mammalian ROR and let-7 genes function in C. elegans, suggesting conservation of this feedback mechanism. We propose that this feedback loop unites the molting timer and the heterochronic gene regulatory network, possibly by functioning as a cycle counter.

Disrupting interaction between miR-132 and Mmp9 3'UTR improves synaptic plasticity and memory in mice

As microRNAs have emerged to be important regulators of molecular events occurring at the synapses, the new questions about their regulatory effect on the behavior have araised. In the present study, we show for the first time that the dysregulated specific targeting of miR132 to Mmp9 mRNA in the mouse brain results in the increased level of Mmp9 protein, which affects synaptic plasticity and has an effect on memory formation. Our data points at the importance of complex and precise regulation of the Mmp9 level by miR132 in the brain.

Identifying the Caenorhabditis elegans vulval transcriptome

Development of the Caenorhabditis elegans vulva is a classic model of organogenesis. This system, which starts with 6 equipotent cells, encompasses diverse types of developmental event, including developmental competence, multiple signaling events to control precise and faithful patterning of three cell fates, execution and proliferation of specific cell lineages, and a series of sophisticated morphogenetic events. Early events have been subjected to extensive mutational and genetic investigations and later events to cell biological analyses. We infer the existence of dramatically changing profiles of gene expression that accompanies the observed changes in development. Yet, except from serendipitous discovery of several transcription factors expressed in dynamic patterns in vulval lineages, our knowledge of the transcriptomic landscape during vulval development is minimal. This study describes the composition of a vulva-specific transcriptome. We used tissue-specific harvesting of mRNAs via immunoprecipitation of epitope-tagged poly(A) binding protein, PAB-1, heterologously expressed by a promoter known to express GFP in vulval cells throughout their development. The identified transcriptome was small but tightly interconnected. From this data set, we identified several genes with identified functions in development of the vulva and validated more with promoter-GFP reporters of expression. For one target, lag-1, promoter-GFP expression was limited but a fluorescent tag of the endogenous protein revealed extensive expression. Thus, we have identified a transcriptome of C. elegans vulval lineages as a launching pad for exploration of functions of these genes in organogenesis.

Ubiquitin ligases and a processive proteasome facilitate protein clearance during the oocyte-to-embryo transition in Caenorhabditis elegans

The ubiquitin-mediated degradation of oocyte translational regulatory proteins is a conserved feature of the oocyte-to-embryo transition. In the nematode Caenorhabditis elegans, multiple translational regulatory proteins, including the TRIM-NHL RNA-binding protein LIN-41/Trim71 and the Pumilio-family RNA-binding proteins PUF-3 and PUF-11, are degraded during the oocyte-to-embryo transition. Degradation of each protein requires activation of the M-phase cyclin-dependent kinase CDK-1, is largely complete by the end of the first meiotic division and does not require the anaphase-promoting complex. However, only LIN-41 degradation requires the F-box protein SEL-10/FBW7/Cdc4p, the substrate recognition subunit of an SCF-type E3 ubiquitin ligase. This finding suggests that PUF-3 and PUF-11, which localize to LIN-41-containing ribonucleoprotein particles, are independently degraded through the action of other factors and that the oocyte ribonucleoprotein particles are disassembled in a concerted fashion during the oocyte-to-embryo transition. We develop and test the hypothesis that PUF-3 and PUF-11 are targeted for degradation by the proteasome-associated HECT-type ubiquitin ligase ETC-1/UBE3C/Hul5, which is broadly expressed in C. elegans. We find that several GFP-tagged fusion proteins that are degraded during the oocyte-to-embryo transition, including fusions with PUF-3, PUF-11, LIN-41, IFY-1/Securin, and CYB-1/Cyclin B, are incompletely degraded when ETC-1 function is compromised. However, it is the fused GFP moiety that appears to be the critical determinant of this proteolysis defect. These findings are consistent with a conserved role for ETC-1 in promoting proteasome processivity and suggest that proteasomal processivity is an important element of the oocyte-to-embryo transition during which many key oocyte regulatory proteins are rapidly targeted for degradation.

Functional identification of microRNA-centered complexes in C. elegans

microRNAs (miRNAs) are crucial for normal development and physiology. To identify factors that might coordinate with miRNAs to regulate gene expression, we used 2'O-methylated oligonucleotides to precipitate Caenorhabditis elegans let-7, miR-58, and miR-2 miRNAs and the associated proteins. A total of 211 proteins were identified through mass-spectrometry analysis of miRNA co-precipitates, which included previously identified interactors of key miRNA pathway components. Gene ontology analysis of the identified interactors revealed an enrichment for RNA binding proteins, suggesting that we captured proteins that may be involved in mRNA lifecycle. To determine which miRNA interactors are important for miRNA activity, we used RNAi to deplete putative miRNA co-factors in animals with compromised miRNA activity and looked for alterations of the miRNA mutant phenotypes. Depletion of 25 of 39 tested genes modified the miRNA mutant phenotypes in three sensitized backgrounds. Modulators of miRNA phenotypes ranged from RNA binding proteins RBD-1 and CEY-1 to metabolic factors such as DLST-1 and ECH-5, among others. The observed functional interactions suggest widespread coordination of these proteins with miRNAs to ultimately regulate gene expression. This study provides a foundation for future investigations aimed at deciphering the molecular mechanisms of miRNA-mediated gene regulation.

Critical contribution of 3' non-seed base pairing to the in vivo function of the evolutionarily conserved let-7a microRNA

Base pairing of the seed region (g2–g8) is essential for microRNA targeting; however, the in vivo function of the 3′ non-seed region (g9–g22) is less well understood. Here, we report a systematic investigation of the in vivo roles of 3′ non-seed nucleotides in microRNA let-7a, whose entire g9–g22 region is conserved among bilaterians. We find that the 3′ non-seed sequence functionally distinguishes let-7a from its family paralogs. The complete pairing of g11–g16 is essential for let-7a to fully repress multiple key targets, including evolutionarily conserved lin-41, daf-12, and hbl-1. Nucleotides at g17–g22 are less critical but may compensate for mismatches in the g11–g16 region. Interestingly, a certain minimal complementarity to let-7a 3′ non-seed sequence can be required even for sites with perfect seed pairing. These results provide evidence that the specific configurations of both seed and 3′ non-seed base pairing can critically influence microRNA-mediated gene regulation in vivo.

Duan et al. find that microRNA-target pairing at g11–g16 is critical for the function of evolutionarily conserved microRNA let-7a; 3′ pairing is required for both perfect and imperfect seed in regulating multiple targets. These findings provide evidence that base pairing of specific microRNA non-seed nucleotides can critically contribute to target regulation.

New Roles for MicroRNAs in Old Worms

The use of Caenorhabditis elegans as a model organism in aging research has been integral to our understanding of genes and pathways involved in this process. Several well-conserved signaling pathways that respond to insulin signaling, diet, and assaults to proteostasis have defined roles in controlling lifespan. New evidence shows that microRNAs (miRNAs) play prominent roles in regulating these pathways. In some cases, key aging-related genes have been established as direct targets of specific miRNAs. However, the precise functions of other miRNAs and their protein cofactors in promoting or antagonizing longevity still need to be determined. Here, we highlight recently uncovered roles of miRNAs in common aging pathways, as well as new techniques for the ongoing discovery of miRNA functions in aging C. elegans.

MicroRNA 3'-compensatory pairing occurs through two binding modes, with affinity shaped by nucleotide identity and position

MicroRNAs (miRNAs), in association with Argonaute (AGO) proteins, direct repression by pairing to sites within mRNAs. Compared to pairing preferences of the miRNA seed region (nucleotides 2-8), preferences of the miRNA 3' region are poorly understood, due to the sparsity of measured affinities for the many pairing possibilities. We used RNA bind-n-seq with purified AGO2-miRNA complexes to measure relative affinities of >1000 3'-pairing architectures for each miRNA. In some cases, optimal 3' pairing increased affinity by >500 fold. Some miRNAs had two high-affinity 3'-pairing modes-one of which included additional nucleotides bridging seed and 3' pairing to enable high-affinity pairing to miRNA nucleotide 11. The affinity of binding and the position of optimal pairing both tracked with the occurrence of G or oligo(G/C) nucleotides within the miRNA. These and other results advance understanding of miRNA targeting, providing insight into how optimal 3' pairing is determined for each miRNA.

daf-16/FOXO blocks adult cell fate in Caenorhabditis elegans dauer larvae via lin-41/TRIM71

Many tissue-specific stem cells maintain the ability to produce multiple cell types during long periods of non-division, or quiescence. FOXO transcription factors promote quiescence and stem cell maintenance, but the mechanisms by which FOXO proteins promote multipotency during quiescence are still emerging. The single FOXO ortholog in C. elegans, daf-16, promotes entry into a quiescent and stress-resistant larval stage called dauer in response to adverse environmental cues. During dauer, stem and progenitor cells maintain or re-establish multipotency to allow normal development to resume after dauer. We find that during dauer, daf-16/FOXO prevents epidermal stem cells (seam cells) from prematurely adopting differentiated, adult characteristics. In particular, dauer larvae that lack daf-16 misexpress collagens that are normally adult-enriched. Using col-19p::gfp as an adult cell fate marker, we find that all major daf-16 isoforms contribute to opposing col-19p::gfp expression during dauer. By contrast, daf-16(0) larvae that undergo non-dauer development do not misexpress col-19p::gfp. Adult cell fate and the timing of col-19p::gfp expression are regulated by the heterochronic gene network, including lin-41 and lin-29. lin-41 encodes an RNA-binding protein orthologous to LIN41/TRIM71 in mammals, and lin-29 encodes a conserved zinc finger transcription factor. In non-dauer development, lin-41 opposes adult cell fate by inhibiting the translation of lin-29, which directly activates col-19 transcription and promotes adult cell fate. We find that during dauer, lin-41 blocks col-19p::gfp expression, but surprisingly, lin-29 is not required in this context. Additionally, daf-16 promotes the expression of lin-41 in dauer larvae. The col-19p::gfp misexpression phenotype observed in dauer larvae with reduced daf-16 requires the downregulation of lin-41, but does not require lin-29. Taken together, this work demonstrates a novel role for daf-16/FOXO as a heterochronic gene that promotes expression of lin-41/TRIM71 to contribute to multipotent cell fate in a quiescent stem cell model.

In adults and juveniles, tissue-specific stem cells divide as needed to replace cells that are lost due to injury or normal wear and tear. Many stem cells spend long periods of time in cellular quiescence, or non-division. During quiescence, stem cells remain multipotent, where they retain the ability to produce all cell types within their tissue. In this study, we define a new role for the FOXO protein DAF-16 in promoting multipotency during the quiescent C. elegans dauer larva stage. C. elegans larvae enter dauer midway through development in response to adverse environmental conditions. Epidermal stem cells are multipotent in C. elegans larvae but differentiate at adulthood, a process controlled by the “heterochronic” genes. We found that daf-16 blocks the expression of adult cell fate specifically in dauer larvae by promoting the expression of the heterochronic gene lin-41. lin-41 normally blocks adult fate by repressing the expression of another heterochronic gene, lin-29, but surprisingly, lin-29 is not needed for the expression of adult cell fate in this context. These findings may be relevant to mammals where the orthologs of daf-16 and lin-41 are important in stem cell maintenance and opposing differentiation.

phiC31 integrase for recombination mediated single copy insertion and genome manipulation in C. elegans

C. elegans benefits from a large set of tools for genome manipulation. Yet, the precise single-copy insertion of very large DNA constructs (>10 kb) and the generation of inversions are still challenging. Here, we adapted the phiC31 integrase system for C. elegans. We generated an integrated phiC31 integrase expressing strain flanked by attP sites that serves as a landing pad for integration of transgenes by recombination mediated cassette exchange (RCME). This strain is unc-119(-) so RMCE integrants can be produced simply by injection of a plasmid carrying attB sites flanking unc-119(+) and the gene(s) of interest. Additionally, phiC31 integrase is removed concomitantly with integration, eliminating the need to outcross away the integrase. Integrations were obtained for insert sizes up to ∼33.4 kb. Taking advantage of this integration method we establish a dual color fluorescent operon reporter system able to study post-transcriptional regulation of mRNA. Last, we show that large chromosomal segments can be inverted using phiC31 integrase. Thus, the phiC31 integrase system should be a useful addition to the C. elegans toolkit.

The promise of microRNA-based therapies in Alzheimer's disease: challenges and perspectives

Multi-pathway approaches for the treatment of complex polygenic disorders are emerging as alternatives to classical monotarget therapies and microRNAs are of particular interest in that regard. MicroRNA research has come a long way from their initial discovery to the cumulative appreciation of their regulatory potential in healthy and diseased brain. However, systematic interrogation of putative therapeutic or toxic effects of microRNAs in (models of) Alzheimer's disease is currently missing and fundamental research findings are yet to be translated into clinical applications. Here, we review the literature to summarize the knowledge on microRNA regulation in Alzheimer's pathophysiology and to critically discuss whether and to what extent these increasing insights can be exploited for the development of microRNA-based therapeutics in the clinic.

CRISPR/Cas9 small promoter deletion in H19 lncRNA is associated with altered cell morphology and proliferation

The imprinted H19 long non-coding RNA, a knowing oncofetal gene, presents a controversial role during the carcinogenesis process since its tumor suppressor or oncogenic activity is not completely elucidated. Since H19 lncRNA is involved in many biological pathways related to tumorigenesis, we sought to develop a non-cancer lineage with CRISPR-Cas9-mediated H19 knockdown (H19-) and observe the changes in a cellular context. To edit the promoter region of H19, two RNA guides were designed, and the murine C2C12 myoblast cells were transfected. H19 deletion was determined by DNA sequencing and gene expression by qPCR. We observed a small deletion (~ 60 bp) in the promoter region that presented four predicted transcription binding sites. The deletion reduced H19 expression (30%) and resulted in increased proliferative activity, altered morphological patterns including cell size and intracellular granularity, without changes in viability. The increased proliferation rate in the H19- cell seems to facilitate chromosomal abnormalities. The H19- myoblast presented characteristics similar to cancer cells, therefore the H19 lncRNA may be an important gene during the initiation of the tumorigenic process. Due to CRISPR/Cas9 permanent edition, the C2C12 H19- knockdown cells allows functional studies of H19 roles in tumorigenesis, prognosis, metastases, as well as drug resistance and targeted therapy.

microRNA-mediated translation repression through GYF-1 and IFE-4 in C. elegans development

microRNA (miRNA)-mediated gene silencing is enacted through the recruitment of effector proteins that direct translational repression or degradation of mRNA targets, but the relative importance of their activities for animal development remains unknown. Our concerted proteomic surveys identified the uncharacterized GYF-domain encoding protein GYF-1 and its direct interaction with IFE-4, the ortholog of the mammalian translation repressor 4EHP, as key miRNA effector proteins in Caenorhabditis elegans. Recruitment of GYF-1 protein to mRNA reporters in vitro or in vivo leads to potent translation repression without affecting the poly(A) tail or impinging on mRNA stability. Loss of gyf-1 is synthetic lethal with hypomorphic alleles of embryonic miR-35-42 and larval (L4) let-7 miRNAs, which is phenocopied through engineered mutations in gyf-1 that abolish interaction with IFE-4. GYF-1/4EHP function is cascade-specific, as loss of gyf-1 had no noticeable impact on the functions of other miRNAs, including lin-4 and lsy-6. Overall, our findings reveal the first direct effector of miRNA-mediated translational repression in C. elegans and its physiological importance for the function of several, but likely not all miRNAs.

A cohort of Caenorhabditis species lacking the highly conserved let-7 microRNA

The let-7 gene encodes a highly conserved microRNA with critical functions integral to cell fate specification and developmental progression in diverse animals. In Caenorhabditis elegans, let-7 is a component of the heterochronic (developmental timing) gene regulatory network, and loss-of-function mutations of let-7 result in lethality during the larval to adult transition due to misregulation of the conserved let-7 target, lin-41. To date, no bilaterian animal lacking let-7 has been characterized. In this study, we identify a cohort of nematode species within the genus Caenorhabditis, closely related to C. elegans, that lack the let-7 microRNA, owing to absence of the let-7 gene. Using Caenorhabditis sulstoni as a representative let-7-lacking species to characterize normal larval development in the absence of let-7, we demonstrate that, except for the lack of let-7, the heterochronic gene network is otherwise functionally conserved. We also report that species lacking let-7 contain a group of divergent let-7 paralogs-also known as the let-7-family of microRNAs-that have apparently assumed the role of targeting the LIN-41 mRNA.

Parallel genetics of regulatory sequences using scalable genome editing in vivo

How regulatory sequences control gene expression is fundamental for explaining phenotypes in health and disease. Regulatory elements must ultimately be understood within their genomic environment and development- or tissue-specific contexts. Because this is technically challenging, few regulatory elements have been characterized in vivo. Here, we use inducible Cas9 and multiplexed guide RNAs to create hundreds of mutations in enhancers/promoters and 3' UTRs of 16 genes in C. elegans. Our software crispr-DART analyzes indel mutations in targeted DNA sequencing. We quantify the impact of mutations on expression and fitness by targeted RNA sequencing and DNA sampling. When applying our approach to the lin-41 3' UTR, generating hundreds of mutants, we find that the two adjacent binding sites for the miRNA let-7 can regulate lin-41 expression independently of each other. Finally, we map regulatory genotypes to phenotypic traits for several genes. Our approach enables parallel analysis of regulatory sequences directly in animals.

Repressing Ago2 mRNA translation by Trim71 maintains pluripotency through inhibiting let-7 microRNAs

The regulation of stem cell fate is poorly understood. Genetic studies in Caenorhabditis elegans lead to the hypothesis that a conserved cytoplasmic double-negative feedback loop consisting of the RNA-binding protein Trim71 and the let-7 microRNA controls the pluripotency and differentiation of stem cells. Although let-7-microRNA-mediated inhibition of Trim71 promotes differentiation, whether and how Trim71 regulates pluripotency and inhibits the let-7 microRNA are still unknown. Here, we show that Trim71 represses Ago2 mRNA translation in mouse embryonic stem cells. Blocking this repression leads to a specific post-transcriptional increase of mature let-7 microRNAs, resulting in let-7-dependent stemness defects and accelerated differentiation in the stem cells. These results not only support the Trim71-let-7-microRNA bi-stable switch model in controlling stem cell fate, but also reveal that repressing the conserved pro-differentiation let-7 microRNAs at the mature microRNA level by Ago2 availability is critical to maintaining pluripotency.

KH domain containing RNA-binding proteins coordinate with microRNAs to regulate Caenorhabditis elegans development

MicroRNAs (miRNAs) and RNA-binding proteins (RBPs) regulate gene expression at the post-transcriptional level, but the extent to which these key regulators of gene expression coordinate their activities and the precise mechanisms of this coordination are not well understood. RBPs often have recognizable RNA binding domains that correlate with specific protein function. Recently, several RBPs containing K homology (KH) RNA binding domains were shown to work with miRNAs to regulate gene expression, raising the possibility that KH domains may be important for coordinating with miRNA pathways in gene expression regulation. To ascertain whether additional KH domain proteins functionally interact with miRNAs during Caenorhabditis elegans development, we knocked down twenty-four genes encoding KH-domain proteins in several miRNA sensitized genetic backgrounds. Here, we report that a majority of the KH domain-containing genes genetically interact with multiple miRNAs and Argonaute alg-1. Interestingly, two KH domain genes, predicted splicing factors sfa-1 and asd-2, genetically interacted with all of the miRNA mutants tested, whereas other KH domain genes showed genetic interactions only with specific miRNAs. Our domain architecture and phylogenetic relationship analyses of the C. elegans KH domain-containing proteins revealed potential groups that may share both structure and function. Collectively, we show that many C. elegans KH domain RBPs functionally interact with miRNAs, suggesting direct or indirect coordination between these two classes of post-transcriptional gene expression regulators.

Animal Behavior: Drosophila melanogaster Goes Social

A new study relates the properties of Drosophila melanogaster social networks to group composition and demonstrates how they may be altered using behavioral priming and genetic manipulations.

Dynamic global analysis of transcription reveals the role of miRNAs in synergistic stabilization of gene expression

Buffering exogenous perturbation is crucial to maintain transcriptional homeostasis during development. While miRNAs have been speculated to play a role in stability maintenance, previous studies seeking to check this conjecture focused on measurements of transcript levels at steady state or involved individual miRNA targets. We measured whole-genome expression dynamics by introducing a transient perturbation and establishing a perturbation and recovery system in Drosophila larvae. We inhibited all transcription and assayed transcriptomes at several time points during recovery from inhibition. We performed these experiments in the wild type and miRNA-deficient genetic backgrounds. Consistent with theories about miRNAs' function in stabilizing the transcriptome, we find that attenuating miRNA expression leads to weak impairment in degradation of targets but strong destabilization of target genes when transcription is re-activated. We further fitted a model that captures the essential aspects of transcription dynamics in our experiments and found that the miRNA target transcripts uniformly overshoot the original steady state as they recover from a general inhibition of transcription if global miRNA levels are reduced. Collectively, our results provide experimental evidence for the idea that miRNAs act cumulatively to stabilize the transcriptional regulatory network. We therefore found a promising approach to assess the effect of these molecules on transcription dynamics.

The multifaceted roles of microRNAs in differentiation

MicroRNAs (miRNAs) are major drivers of cell fate specification and differentiation. The post-transcriptional regulation of key molecular factors by microRNAs contributes to the progression of embryonic and postembryonic development in several organisms. Following the discovery of lin-4 and let-7 in Caenorhabditis elegans and bantam microRNAs in Drosophila melanogaster, microRNAs have emerged as orchestrators of cellular differentiation and developmental timing. Spatiotemporal control of microRNAs and associated protein machinery can modulate microRNA activity. Additionally, adaptive modulation of microRNA expression and function in response to changing environmental conditions ensures that robust cell fate specification during development is maintained. Herein, we review the role of microRNAs in the regulation of differentiation during development.

Differential expression, function and prognostic value of miR-17-92 cluster in ER-positive and triple-negative breast cancer

Recent evidence has shown that the miR-17-92 cluster can function either as oncogene or tumor suppressor in human cancers. The function of miR-17-92 in subtypes of breast cancer remains largely unknown. The expression of miR-17-92 is elevated in triple negative breast cancer (TNBC) but reduced in estrogen receptor (ER)-positive breast cancer (ERPBC). We show that increased expression of miRNAs belonging to the miR-17-92 cluster is associated with poor outcome in TNBC, whereas the expression of miR-17-92 miRNAs is with good outcome in ERPBC. We show that ectopic expression of miR-17-92 inhibited cell growth and invasion of ER-positive and HER2-enriched cells. On the contrary, miR-17-92 expression enhanced cell growth and invasion of TNBC cells. Further, we found that miR-17-92 expression sensitized MCF7 cells to chemotherapeutic compounds, whereas it rendered SKBR3 cells resistant to them. We found that expression of ADORA1 was reduced by miR-17-92-expressing breast cancer cells, specifically in ERPBC. We observed an inverse correlation between the expression of ADORA1 and miR-17-92 in human breast cancer. Treatment with DPCPX, a selective ADORA1 antagonist, abolished the difference in the growth of control and miR-17-92 overexpressing MCF7 cells and identified ADORA1 as a key functional target of miR-17-92 in ERPBC. Furthermore, increased expression of ADORA1 in ERPBC is associated with a poor outcome. Our observations underscore the context-dependent role of miR-17-92 in breast cancer subtypes and suggest that miR-17-92 could serve as novel prognostic markers in breast cancer.

Concepts and functions of small RNA pathways in C. elegans

A diversity of gene regulatory mechanisms drives the changes in gene expression required for animal development. Here, we discuss the developmental roles of a class of gene regulatory factors composed of a core protein subunit of the Argonaute family and a 21-26-nucleotide RNA cofactor. These represent ancient regulatory complexes, originally evolved to repress genomic parasites such as transposons, viruses and retroviruses. However, over the course of evolution, small RNA-guided pathways have expanded and diversified, and they play multiple roles across all eukaryotes. Pertinent to this review, Argonaute and small RNA-mediated regulation has acquired numerous functions that affect all aspects of animal life. The regulatory function is provided by the Argonaute protein and its interactors, while the small RNA provides target specificity, guiding the Argonaute to a complementary RNA. C. elegans has 19 different, functional Argonautes, defining distinct yet interconnected pathways. Each Argonaute binds a relatively well-defined class of small RNA with distinct molecular properties. A broad classification of animal small RNA pathways distinguishes between two groups: (i) the microRNA pathway is involved in repressing relatively specific endogenous genes and (ii) the other small RNA pathways, which effectively act as a genomic immune system to primarily repress expression of foreign or "non-self" RNA while maintaining correct endogenous gene expression. microRNAs play prominent direct roles in all developmental stages, adult physiology and lifespan. The other small RNA pathways act primarily in the germline, but their impact extends far beyond, into embryogenesis and adult physiology, and even to subsequent generations. Here, we review the mechanisms and developmental functions of the diverse small RNA pathways of C. elegans.

Robust partitioning of microRNA targets from downstream regulatory changes

The biological impact of microRNAs (miRNAs) is determined by their targets, and robustly identifying direct miRNA targets remains challenging. Existing methods suffer from high false-positive rates and are unable to effectively differentiate direct miRNA targets from downstream regulatory changes. Here, we present an experimental and computational framework to deconvolute post-transcriptional and transcriptional changes using a combination of RNA-seq and PRO-seq. This novel approach allows us to systematically profile the regulatory impact of a miRNA. We refer to this approach as CARP: Combined Analysis of RNA-seq and PRO-seq. We apply CARP to multiple miRNAs and show that it robustly distinguishes direct targets from downstream changes, while greatly reducing false positives. We validate our approach using Argonaute eCLIP-seq and ribosome profiling, demonstrating that CARP defines a comprehensive repertoire of targets. Using this approach, we identify miRNA-specific activity of target sites within the open reading frame. Additionally, we show that CARP facilitates the dissection of complex changes in gene regulatory networks triggered by miRNAs and identification of transcription factors that mediate downstream regulatory changes. Given the robustness of the approach, CARP would be particularly suitable for dissecting miRNA regulatory networks in vivo.

Cellular Expression and Functional Roles of All 26 Neurotransmitter GPCRs in the C. elegans Egg-Laying Circuit

Maps of the synapses made and neurotransmitters released by all neurons in model systems, such as Caenorhabditis elegans have left still unresolved how neural circuits integrate and respond to neurotransmitter signals. Using the egg-laying circuit of C. elegans as a model, we mapped which cells express each of the 26 neurotransmitter GPCRs of this organism and also genetically analyzed the functions of all 26 GPCRs. We found that individual neurons express many distinct receptors, epithelial cells often express neurotransmitter receptors, and receptors are often positioned to receive extrasynaptic signals. Receptor knockouts reveal few egg-laying defects under standard laboratory conditions, suggesting that the receptors function redundantly or regulate egg-laying only in specific conditions; however, increasing receptor signaling through overexpression more efficiently reveals receptor functions. This map of neurotransmitter GPCR expression and function in the egg-laying circuit provides a model for understanding GPCR signaling in other neural circuits.SIGNIFICANCE STATEMENT Neurotransmitters signal through GPCRs to modulate activity of neurons, and changes in such signaling can underlie conditions such as depression and Parkinson's disease. To determine how neurotransmitter GPCRs together help regulate function of a neural circuit, we analyzed the simple egg-laying circuit in the model organism C. elegans We identified all the cells that express every neurotransmitter GPCR and genetically analyzed how each GPCR affects the behavior the circuit produces. We found that many neurotransmitter GPCRs are expressed in each neuron, that neurons also appear to use these receptors to communicate with other cell types, and that GPCRs appear to often act redundantly or only under specific conditions to regulate circuit function.

In vivo CRISPR screening for phenotypic targets of the mir-35-42 family in C. elegans

In this study, Yang et al. devised a novel strategy to test the phenotypic impact of individual microRNA–target interactions by disrupting each predicted miRNA-binding site by CRISPR–Cas9 genome editing in C. elegans. They developed a multiplexed negative selection screening approach, in which edited loci are deep sequenced, and candidate sites are prioritized based on apparent selection pressure against mutations that disrupt miRNA binding.

Identifying miRNA target genes is difficult, and delineating which targets are the most biologically important is even more difficult. We devised a novel strategy to test the phenotypic impact of individual microRNA–target interactions by disrupting each predicted miRNA-binding site by CRISPR–Cas9 genome editing in C. elegans. We developed a multiplexed negative selection screening approach in which edited loci are deep sequenced, and candidate sites are prioritized based on apparent selection pressure against mutations that disrupt miRNA binding. Importantly, our screen was conducted in vivo on mutant animals, allowing us to interrogate organism-level phenotypes. We used this approach to screen for phenotypic targets of the essential mir-35-42 family. By generating 1130 novel 3′UTR alleles across all predicted targets, we identified egl-1 as a phenotypic target whose derepression partially phenocopies the mir-35-42 mutant phenotype by inducing embryonic lethality and low fecundity. These phenotypes can be rescued by compensatory CRISPR mutations that retarget mir-35 to the mutant egl-1 3′UTR. This study demonstrates that the application of in vivo whole organismal CRISPR screening has great potential to accelerate the discovery of phenotypic negative regulatory elements in the noncoding genome.

Molecular and biological functions of TRIM-NHL RNA-binding proteins

The TRIM-NHL family of proteins shares a conserved domain architecture and play crucial roles in stem cell biology, fertility, and development. This review synthesizes new insights that have revolutionized our understanding of the molecular and biological functions of TRIM-NHL proteins. Multiple TRIM-NHLs have been shown to bind specific RNA sequences and structures. X-ray crystal structures of TRIM-NHL proteins in complex with RNA ligands reveal versatile modes of RNA recognition by the NHL domain. Functional and genetic analyses show that TRIM-NHL RNA-binding proteins negatively regulate the protein expression from the target mRNAs that they bind. This repressive activity plays a crucial role in controlling stem cell fate in the developing brain and differentiating germline. To highlight these paradigms, we focus on several of the most-extensively studied TRIM-NHL proteins, specifically Drosophila and vertebrate TRIM71, among others. Brat is essential for development and regulates key target mRNAs to control differentiation of germline and neural stem cells. TRIM71 is also required for development and promotes stem cell proliferation while antagonizing differentiation. Moreover, TRIM71 can be utilized to help reprogram fibroblasts into induced pluripotent stem cells. Recently discovered mutations in TRIM71 cause the neurodevelopmental disease congenital hydrocephalus and emphasize the importance of its RNA-binding function in brain development. Further relevance of TRIM71 to disease pathogenesis comes from evidence linking it to several types of cancer, including liver and testicular cancer. Collectively, these advances demonstrate a primary role for TRIM-NHL proteins in the post-transcriptional regulation of gene expression in crucial biological processes. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Translation Regulation RNA Turnover and Surveillance > Regulation of RNA Stability.

miRNAs and Neural Alternative Polyadenylation Specify the Virgin Behavioral State

How are diverse regulatory strategies integrated to impose appropriately patterned gene expression that underlie in vivo phenotypes? Here, we reveal how coordinated miRNA regulation and neural-specific alternative polyadenylation (APA) of a single locus controls complex behaviors. Our entry was the unexpected observation that deletion of Bithorax complex (BX-C) miRNAs converts virgin female flies into a subjective post-mated behavioral state, normally induced by seminal proteins following copulation. Strikingly, this behavioral switch is directly attributable to misregulation of homothorax (hth). We localize specific CNS abdominal neurons where de-repressed Hth compromises virgin behavior in BX-C miRNA mutants. Moreover, we use genome engineering to demonstrate that precise mutation of hth 3' UTR sites for BX-C miRNAs or deletion of its neural 3' UTR extension containing most of these sites both induce post-mated behaviors in virgins. Thus, facilitation of miRNA-mediated repression by neural APA is required for virgin females to execute behaviors appropriate to their internal state.

A branched heterochronic pathway directs juvenile-to-adult transition through two LIN-29 isoforms

Robust organismal development relies on temporal coordination of disparate physiological processes. In Caenorhabditis elegans, the heterochronic pathway controls a timely juvenile-to-adult (J/A) transition. This regulatory cascade of conserved proteins and small RNAs culminates in accumulation of the transcription factor LIN-29, which triggers coordinated execution of transition events. We report that two LIN-29 isoforms fulfill distinct functions. Functional specialization is a consequence of distinct isoform expression patterns, not protein sequence, and we propose that distinct LIN-29 dose sensitivities of the individual J/A transition events help to ensure their temporal ordering. We demonstrate that unique isoform expression patterns are generated by the activities of LIN-41 for lin-29a, and of HBL-1 for lin-29b, whereas the RNA-binding protein LIN-28 coordinates LIN-29 isoform activity, in part by regulating both hbl-1 and lin-41. Our findings reveal that coordinated transition from juvenile to adult involves branching of a linear pathway to achieve timely control of multiple events.