The C-terminal half of human Ago2 binds to multiple GW-rich regions of GW182 and requires GW182 to mediate silencing

Hypermethylation and suppression of microRNA219a-2 activates the ALDH1L2/GSH/PAI-1 pathway for fibronectin degradation in renal fibrosis

Epigenetic regulations, such as DNA methylation and microRNAs, play an important role in renal fibrosis. Here, we report the regulation of microRNA219a-2 by DNA methylation in fibrotic kidneys, unveiling the crosstalk between these epigenetic mechanisms. Through genome-wide DNA methylation analysis and pyrosequencing, we detected the hypermethylation of microRNA219a-2 in renal fibrosis induced by unilateral ureteral obstruction (UUO) or renal ischemia/reperfusion, which was accompanied by a significant decrease in microRNA-219a-5p expression. Functionally, overexpression of microRNA219a-2 enhanced fibronectin induction during hypoxia or TGF-β1 treatment of cultured renal cells. In mice, inhibition of microRNA-219a-5p suppressed fibronectin accumulation in UUO and ischemic/reperfused kidneys. Aldehyde dehydrogenase 1 family member L2 (ALDH1L2) was identified to be the direct target gene of microRNA-219a-5p in renal fibrotic models. MicroRNA-219a-5p suppressed ALDH1L2 expression in cultured renal cells, while inhibition of microRNA-219a-5p prevented the decrease of ALDH1L2 in injured kidneys. Knockdown of ALDH1L2 enhanced plasminogen activator inhibitor-1 (PAI-1) induction during TGF-β1 treatment of renal cells, which was associated with fibronectin expression. In conclusion, the hypermethylation of microRNA219a-2 in response to fibrotic stress may attenuate microRNA-219a-5p expression and induce the upregulation of its target gene ALDH1L2, which reduces fibronectin deposition by suppressing PAI-1.

Inorganic arsenic modulates cell apoptosis by regulating Argonaute 2 expression via the p53 pathway

This study explores the role of Argonaute 2 (AGO2) in the induction of apoptosis by arsenic in 16HBE cells and investigates the association between AGO2 expression and arsenic exposure in a human population. By silencing AGO2 with siRNA, we examined its impact on cell viability and apoptosis using CCK-8, HO-PI, and JC-1 assays, complemented by qRT-PCR and Western blot analyses for gene and protein expressions. Our findings revealed a significant correlation between AGO2 expression and levels of exposure to inorganic arsenic (iAs), which was more pronounced than with other arsenic forms such as monomethylarsonic (MMA) and dimethylarsinic acids (DMA). The results showed that silencing AGO2 not only reduced cell viability but also intensified apoptosis, highlighting its role in activating the p53 pathway. This was further supported by increased phosphorylation of p53 at Ser392 and Thr55, reinforcing AGO2's involvement in apoptotic processes. The study underscores the potential of AGO2 as a therapeutic target in arsenic-related pathologies and highlights the critical need for managing occupational exposure to arsenic.

Plant bioactive compounds driven microRNAs (miRNAs): A potential source and novel strategy targeting gene and cancer therapeutics

Irrespective of medical technology improvements, cancer ranks among the leading causes of mortality worldwide. Although numerous cures and treatments exist, creating alternative cancer therapies with fewer adverse side effects is vital. Since ancient times, plant bioactive compounds have already been used as a remedy to heal cancer. These plant bioactive compounds and their anticancer activity can also deregulate the microRNAs (miRNAs) in the cancerous cells. Therefore, the deregulation of miRNAs in cancer cells by plant bioactive compounds and the usage of the related miRNA could be a promising approach for cancer cure, mainly to prevent cancer and overcome chemotherapeutic side effect problems. Hence, this review highlights the function of plant bioactive compounds as an anticancer agent through the underlying mechanism that alters the miRNA expression in cancer cells, ultimately leading to apoptosis. Moreover, this review provides insight into using plant bioactive compounds -driven miRNAs as an anticancer agent to develop miRNA-based cancer gene therapy. They can be the potential resource for gene therapy and novel strategies targeting cancer therapeutics.

Sponging of five tumour suppressor miRNAs by lncRNA-KCNQ1OT1 activates BMPR1A/BMPR1B-ACVR2A/ACVR2B signalling and promotes chemoresistance in hepatocellular carcinoma

Diverse mechanisms have been established to understand the chemoresistance of hepatocellular carcinoma (HCC), but the contribution of non-coding RNAs is not surveyed well. Here, we aimed to explore the lncRNA-miRNA axis in Hepatitis C and B virus (HCV and HBV) infected HCC to investigate the molecular mechanism of chemoresistance and to identify a potential therapeutic target for HCC. The small RNA transcriptome analysis followed by qRT-PCR validation with the liver tissues of both HCV and HBV infected HCC patients revealed that miR-424-5p, miR-136-3p, miR-139-5p, miR-223-3p, and miR-375-3p were the most downregulated miRNAs in HCC compared to normal (log2 fold change ≤-1.5, Padj ≤ 0.05). In silico pathway analysis with the validated targets of each miRNA revealed that the signalling pathway regulating pluripotency of stem cells is commonly targeted by these five miRNAs. Subsequent validation by 3'UTR-luciferase assay and western blot analysis unveiled that these five miRNAs impeded either same or diverse genes, but all linked to BMP signalling pathway such as BMPR1A/BMPR1B by miR-139-5p, miR-136-3p, and miR-375-3p, and ACVR2A/ACVR2B by miR-424-5p and miR-223-3p. Furthermore, restoration of each miRNA in Huh7/SNU449 cells inhibited phosphorylation of downstream SMAD1/5 and ERK1/2, and attenuated Epithelial-mesenchymal transition, stemness, spheroid formation, chemoresistance, invasion and migration of cells. To investigate the mechanism of suppression of these miRNAs, "DIANA" tool was employed and lncRNA-KCNQ1OT1 was retrieved as interacting partner of all the five miRNAs. In vitro RNA pull-down assay revealed that lncRNA-KCNQ1OT1 physically interacted and sequestered these five miRNAs in the cytoplasm. Hence, KCNQ1OT1 was suppressed in Huh7/SNU449 cells using CRISPR technology and observed regression of oncogenic properties with enhanced chemosensitivity and reduced metastasis in cancer cells. Shrinkage of tumour size and volume in NOD-SCID mice injected with KCNQ1OT1-sgRNA cells further strengthened our observations. Thus, lncRNA-KCNQ1OT1 is the main regulator, which reduces the level of beneficiary miRNAs in the tumour milieu and modulates BMP signalling pathway to promote chemoresistance in HCC, suggesting lncRNA-KCNQ1OT1 might have robust potential to be a therapeutic target in HCC.

Argonaute3-SF3B3 complex controls pre-mRNA splicing to restrain type 2 immunity

Argonaute (AGO) proteins execute microRNA (miRNA)-mediated gene silencing. However, it is unclear whether all 4 mammalian AGO proteins (AGO1, AGO2, AGO3, and AGO4) are required for miRNA activity. We generate Ago1, Ago3, and Ago4-deficient mice (Ago134Δ) and find AGO1/3/4 to be redundant for miRNA biogenesis, homeostasis, or function, a role that is carried out by AGO2. Instead, AGO1/3/4 regulate the expansion of type 2 immunity via precursor mRNA splicing in CD4+ T helper (Th) lymphocytes. Gain- and loss-of-function experiments demonstrate that nuclear AGO3 interacts directly with SF3B3, a component of the U2 spliceosome complex, to aid global mRNA splicing, and in particular the isoforms of the gene Nisch, resulting in a dysregulated Nisch isoform ratio. This work uncouples AGO1, AGO3, and AGO4 from miRNA-mediated RNA interference, identifies an AGO3:SF3B3 complex in the nucleus, and reveals a mechanism by which AGO proteins regulate inflammatory diseases.

miR-7 is recruited to the high molecular weight RNA-induced silencing complex in CD8+ T cells upon activation and suppresses IL-2 signaling

Increasing evidence suggests mammalian Argonaute (Ago) proteins partition into distinct complexes within cells, but there is still little biochemical or functional understanding of the miRNAs differentially associated with these complexes. In naïve T cells, Ago2 is found almost exclusively in low molecular weight (LMW) complexes which are associated with miRNAs but not their target mRNAs. Upon T-cell activation, a proportion of these Ago2 complexes move into a newly formed high molecular weight (HMW) RNA-induced silencing complex (RISC), which is characterized by the presence of the GW182 protein that mediates translational repression. Here, we demonstrate distinct partitioning of miRNAs and isomiRs in LMW versus HMW RISCs upon antigen-mediated activation of CD8+ T cells. We identify miR-7 as highly enriched in HMW RISC and demonstrate that miR-7 inhibition leads to increased production of IL-2 and up-regulation of the IL-2 receptor, the transferrin receptor, CD71 and the amino acid transporter, CD98. Our data support a model where recruitment of miR-7 to HMW RISC restrains IL-2 signaling and the metabolic processes regulated by IL-2.

The CHARGE syndrome-associated protein FAM172A controls AGO2 nuclear import

CHARGE syndrome is a neural crest-related disorder mainly caused by mutation of the chromatin remodeler-coding gene CHD7 Alternative causes include mutation of other chromatin and/or splicing factors. One of these additional players is the poorly characterized FAM172A, which we previously found in a complex with CHD7 and the small RNA-binding protein AGO2 at the chromatin-spliceosome interface. Focusing on the FAM172A-AGO2 interplay, we now report that FAM172A is a direct binding partner of AGO2 and, as such, one of the long sought-after regulators of AGO2 nuclear import. We show that this FAM172A function mainly relies on its classical bipartite nuclear localization signal and associated canonical importin-α/β pathway, being enhanced by CK2-induced phosphorylation and abrogated by a CHARGE syndrome-associated missense mutation. Overall, this study thus strengthens the notion that noncanonical nuclear functions of AGO2 and associated regulatory mechanisms might be clinically relevant.

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.

Hypermethylation suppresses microRNA-219a-2 to activate the ALDH1L2/GSH/PAI-1 pathway for fibronectin degradation in renal fibrosis

Epigenetic regulations, such as DNA methylation and microRNAs, play an important role in renal fibrosis. Here, we report the regulation of microRNA-219a-2 (mir-219a-2) by DNA methylation in fibrotic kidneys, unveiling the crosstalk between these epigenetic mechanisms. Through genome-wide DNA methylation analysis and pyro-sequencing, we detected the hypermethylation of mir-219a-2 in renal fibrosis induced by unilateral ureter obstruction (UUO) or renal ischemia/reperfusion, which was accompanied by a significant decrease in mir-219a-5p expression. Functionally, overexpression of mir-219a-2 enhanced fibronectin induction during hypoxia or TGF-β1 treatment of cultured renal cells. In mice, inhibition of mir-219a-5p suppressed fibronectin accumulation in UUO kidneys. ALDH1L2 was identified to be the direct target gene of mir-219a-5p in renal fibrosis. Mir-219a-5p suppressed ALDH1L2 expression in cultured renal cells, while inhibition of mir-219a-5p prevented the decrease of ALDH1L2 in UUO kidneys. Knockdown of ALDH1L2 enhanced PAI-1 induction during TGF-β1 treatment of renal cells, which was associated with fibronectin expression. In conclusion, the hypermethylation of mir-219a-2 in response to fibrotic stress attenuates mir-219a-5p expression and induces the up-regulation of its target gene ALDH1L2, which may reduce fibronectin deposition by suppressing PAI-1.

KRAS Hijacks the miRNA Regulatory Pathway in Cancer

Extensive studies have focused on the misregulation of individual miRNAs in cancer. More recently, mutations in the miRNA biogenesis and processing machinery have been implicated in several malignancies. Such mutations can lead to global miRNA misregulation, which may promote many of the well-known hallmarks of cancer. Interestingly, recent evidence also suggests that oncogenic Kristen rat sarcoma viral oncogene homolog (KRAS) mutations act in part by modulating the activity of members of the miRNA regulatory pathway. Here, we highlight the vital role mutations in the miRNA core machinery play in promoting malignant transformation. Furthermore, we discuss how mutant KRAS can simultaneously impact multiple steps of miRNA processing and function to promote tumorigenesis. Although the ability of KRAS to hijack the miRNA regulatory pathway adds a layer of complexity to its oncogenic nature, it also provides a potential therapeutic avenue that has yet to be exploited in the clinic. Moreover, concurrent targeting of mutant KRAS and members of the miRNA core machinery represents a potential strategy for treating cancer.

tRNA derived fragments:A novel player in gene regulation and applications in cancer

The heterogeneous species of tRNA-derived fragments (tRFs) with specific biological functions was recently identified. Distinct roles of tRFs in tumor development and viral infection, mediated through transcriptional and post-transcriptional regulation, has been demonstrated. In this review, we briefly summarize the current literatures on the classification of tRFs and the effects of tRNA modification on tRF biogenesis. Moreover, we highlight the tRF repertoire of biological roles such as gene silencing, and regulation of translation, cell apoptosis, and epigenetics. We also summarize the biological roles of various tRFs in cancer development and viral infection, their potential value as diagnostic and prognostic biomarkers for different types of cancers, and their potential use in cancer therapy.

PSMC3 promotes RNAi by maintaining AGO2 stability through USP14

BACKGROUND

Argonaute 2 (AGO2), the only protein with catalytic activity in the human Argonaute family, is considered as a key component of RNA interference (RNAi) pathway. Here we performed a yeast two-hybrid screen using the human Argonaute 2 PIWI domain as bait to screen for new AGO2-interacting proteins and explored the specific mechanism through a series of molecular biology and biochemistry experiments.

METHODS

The yeast two-hybrid system was used to screen for AGO2-interacting proteins. Co-immunoprecipitation and immunofluorescence assays were used to further determine interactions and co-localization. Truncated plasmids were constructed to clarify the interaction domain. EGFP fluorescence assay was performed to determine the effect of PSMC3 on RNAi. Regulation of AGO2 protein expression and ubiquitination by PSMC3 and USP14 was examined by western blotting. RT-qPCR assays were applied to assess the level of AGO2 mRNA. Rescue assays were also performed.

RESULTS

We identified PSMC3 (proteasome 26S subunit, ATPase, 3) as a novel AGO2 binding partner. Biochemical and bioinformatic analysis demonstrates that this interaction is performed in an RNA-independent manner and the N-terminal coiled-coil motif of PSMC3 is required. Depletion of PSMC3 impairs the activity of the targeted cleavage mediated by small RNAs. Further studies showed that depletion of PSMC3 decreased AGO2 protein amount, whereas PSMC3 overexpression increased the expression of AGO2 at a post-translational level. Cycloheximide treatment indicated that PSMC3 depletion resulted in a decrease in cytoplasmic AGO2 amount due to an increase in AGO2 protein turnover. The absence of PSMC3 promoted ubiquitination of AGO2, resulting in its degradation by the 26S proteasome. Mechanistically, PSMC3 assists in the interaction of AGO2 with the deubiquitylase USP14(ubiquitin specific peptidase 14) and facilitates USP14-mediated deubiquitination of AGO2. As a result, AGO2 is stabilized, which then promotes RNAi.

CONCLUSION

Our findings demonstrate that PSMC3 plays an essential role in regulating the stability of AGO2 and thus in maintaining effective RNAi.

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.

N-terminal Ago-binding domain of GW182 contains a tryptophan-rich region that confer binding to the CCR4-NOT complex

GW182 family proteins are a key component of microRNA-protein complex eliciting translational repression and/or degradation of microRNA-targets. MicroRNAs in complex with Argonaute proteins bind to target mRNAs, and GW182 proteins are recruited by association with Argonaute proteins. The GW182 protein acts as a scaffold that links the Argonaute protein to silencing machineries including the CCR4-NOT complex which accelerates deadenylation and inhibits translation. The carboxyl-terminal effector domain of GW182 protein, also called the silencing domain, has been shown to bind to the subunits of the CCR4-NOT complex, the CNOT1 and the CNOT9. Here we show that a small region within the amino-terminal Argonaute-binding domain of human GW182/TNRC6A can associate with the CCR4-NOT complex. This region resides between the two Argonaute-binding sites and contains reiterated GW/WG-motifs. Alanine mutation experiments showed that multiple tryptophan residues are required for the association with the CCR4-NOT complex. Furthermore, co-expression and immunoprecipitation assays suggested that the CNOT9 subunit of the CCR4-NOT complex is a possible binding partner of this region. Our work, taken together with previous studies, indicates that the human GW182 protein contains multiple binding interfaces to the CCR4-NOT complex. This article is protected by copyright. All rights reserved.

Identification of Silencing Suppressor Protein Encoded by Strawberry Mottle Virus

Strawberry mottle virus (SMoV) is associated with strawberry decline disease, causing losses to fruit yield and quality. In this study, using a screening system that enables detection of both local and systemic plant host (RNA silencing) defense responses, we found that Pro2Glu and P28, encoded by SMoV RNA2 genome, functioned to suppress local and systemic RNA silencing triggered by single- but not double-stranded GFP RNA. Subcellular localization assay revealed that both Pro2Glu and P28 were localized to nucleus and cytoplasm. The deletion of 11 amino acid residues at the C-terminus destabilized Pro2Glu protein, and the disruption of two conserved GW motifs deprived Pro2Glu of ability to suppress RNA silencing. Additionally, SMoV Pro2Glu and P28 enhanced the accumulation of potato virus X (PVX) in Nicotiana benthamiana 22 days post-infiltration, and P28 exacerbated significantly the symptoms of PVX. Collectively, these data indicate that the genome of SMoV RNA2 encodes two suppressors of RNA silencing. This is the first identification of a stramovirus suppressor of RNA silencing.

Protein S-Nitrosation: Biochemistry, Identification, Molecular Mechanisms, and Therapeutic Applications

Protein S-nitrosation (SNO), a posttranslational modification (PTM) of cysteine (Cys) residues elicited by nitric oxide (NO), regulates a wide range of protein functions. As a crucial form of redox-based signaling by NO, SNO contributes significantly to the modulation of physiological functions, and SNO imbalance is closely linked to pathophysiological processes. Site-specific identification of the SNO protein is critical for understanding the underlying molecular mechanisms of protein function regulation. Although careful verification is needed, SNO modification data containing numerous functional proteins are a potential research direction for druggable target identification and drug discovery. Undoubtedly, SNO-related research is meaningful not only for the development of NO donor drugs but also for classic target-based drug design. Herein, we provide a comprehensive summary of SNO, including its origin and transport, identification, function, and potential contribution to drug discovery. Importantly, we propose new views to develop novel therapies based on potential protein SNO-sourced targets.

siRNA Seed Region Is Divided into Two Functionally Different Domains in RNA Interference in Response to 2'-OMe Modifications

In RNA interference (RNAi), small interfering RNA (siRNA) functions to suppress the expression of its target mRNA with perfect sequence complementarity. In a mechanism different from above, siRNA also suppresses unintended mRNAs with partial sequence complementarities, mainly to the siRNA seed region (nucleotides 2-8). This mechanism is largely utilized by microRNAs (miRNAs) and results in siRNA-mediated off-target effects. Thus, the siRNA seed region is considered to be involved in both RNAi and off-target effects. In this study, we revealed that the impact of 2'-O-methyl (2'-OMe) modification is different according to the nucleotide positions. The 2'-OMe modifications of nucleotides 2-5 inhibited off-target effects without affecting on-target RNAi activities. In contrast, 2'-OMe modifications of nucleotides 6-8 increased both RNAi and off-target activities. The computational simulation revealed that the structural change induced by 2'-OMe modifications interrupts base pairing between siRNA and target/off-target mRNAs at nucleotides 2-5 but enhances at nucleotides 6-8. Thus, our results suggest that siRNA seed region consists of two functionally different domains in response to 2'-OMe modifications: nucleotides 2-5 are essential for avoiding off-target effects, and nucleotides 6-8 are involved in the enhancement of both RNAi and off-target activities.

Inducible and reversible inhibition of miRNA-mediated gene repression in vivo

Although virtually all gene networks are predicted to be controlled by miRNAs, the contribution of this important layer of gene regulation to tissue homeostasis in adult animals remains unclear. Gain and loss-of-function experiments have provided key insights into the specific function of individual miRNAs, but effective genetic tools to study the functional consequences of global inhibition of miRNA activity in vivo are lacking. Here we report the generation and characterization of a genetically engineered mouse strain in which miRNA-mediated gene repression can be reversibly inhibited without affecting miRNA biogenesis or abundance. We demonstrate the usefulness of this strategy by investigating the consequences of acute inhibition of miRNA function in adult animals. We find that different tissues and organs respond differently to global loss of miRNA function. While miRNA-mediated gene repression is essential for the homeostasis of the heart and the skeletal muscle, it is largely dispensable in the majority of other organs. Even in tissues where it is not required for homeostasis, such as the intestine and hematopoietic system, miRNA activity can become essential during regeneration following acute injury. These data support a model where many metazoan tissues primarily rely on miRNA function to respond to potentially pathogenic events.

Role of hsa‑miR‑105 during the pathogenesis of paclitaxel resistance and its clinical implication in ovarian cancer

More than 70% of patients with epithelial ovarian cancer (EOC), one of the leading cause of gynecological cancer-related deaths worldwide, are diagnosed at an advanced stage of the disease. Currently, the mainstay for treatment of advanced EOC is tumor debulking surgery followed by combined platinum- and paclitaxel (PTX)-based chemotherapy. However, most patients eventually develop chemoresistance, which remains a major obstacle to successful treatment. Herein, by using clinical specimens and experimentally induced cell models, we found that the expression levels of hsa-miR-105 were significantly decreased in PTX-resistant EOC tissues and cell lines. Follow-up functional experiments demonstrated that repression of hsa-miR-105 conferred resistance to paclitaxel in EOC cells, whereas restoration of hsa-miR-105 expression in situ via intratumoral injection of hsa-miR-105 micrON™ agomir potentiated in vivo sensitivity to PTX and thereafter significantly inhibited tumor growth in a PTX-challenged xenograft model. Mechanistically, hsa-miR-105 exerted its tumor suppressor function by directly inhibiting the zinc and ring finger 2 (ZNRF2) signaling pathway. Importantly, aberrant expression of hsa-miR-105 in both tumor and circulating samples predicted a poor post-chemotherapy prognosis in EOC patients. These findings collectively suggest that hsa-miR-105 may act as a potent tumor suppressor miRNA during the progression of EOC, likely affecting cell proliferation, invasiveness and chemosensitivity to PTX, and functioning at least in part via inhibition of ZNRF2 signaling. The stability and availability and ease in measurement of circulating hsa-miR-105 make it a valuable diagnostic/prognostic biomarker candidate for chemotherapy of EOC.

Identification of Phosphorylated Amino Acids in Human TNRC6A C-Terminal Region and Their Effects on the Interaction with the CCR4-NOT Complex

Human GW182 family proteins have Argonaute (AGO)-binding domains in their N-terminal regions and silencing domains, which interact with RNA silencing-related proteins, in their C-terminal regions. Thus, they function as scaffold proteins between the AGO protein and RNA silencing-related proteins, such as carbon catabolite repressor4-negative on TATA (CCR4-NOT) or poly(A)-binding protein (PABP). Our mass spectrometry analysis and the phosphorylation data registered in PhosphoSitePlus, a post-translational modification database, suggested that the C-terminal region of a human GW182 family protein, TNRC6A, has at least four possible phosphorylation sites, which are located near the region interacting with the CCR4-NOT complex. Among them, two serine residues at amino acid positions 1332 and 1346 (S1332 and S1346) were certainly phosphorylated in human HeLa cells, but other two serine residues (S1616 and S1691) were not phosphorylated. Furthermore, it was revealed that the phosphorylation patterns of TNRC6A affect the interaction with the CCR4-NOT complex. When S1332 and S1346 were dephosphorylated, the interactions of TNRC6A with the CCR4-NOT complex were enhanced, and when S1616 and S1691 were phosphorylated, such interaction was suppressed. Thus, phosphorylation of TNRC6A was considered to regulate the interaction with RNA silencing-related factors that may affect RNA silencing activity.

Extracellular MicroRNAs as Intercellular Mediators and Noninvasive Biomarkers of Cancer

Simple Summary

There are an extensive number of publications regarding the role of endogenous miRNAs as regulators of gene expression in cancer. However, extracellular miRNAs have emerged as a novel mechanism of cell-to-cell communication in normal conditions and disease and have drawn a large amount of interest as regulators of gene expression and as potential non-invasive biomarkers in cancer. Despite this high interest and the abundance of research on the biology and role of extracellular miRNAs in cancer, they are not yet completely understood. The aim of this review is to highlight the relevant biological characteristics of extracellular miRNAs that enable them to function as intercellular mediators of gene expression regulation and provide the recently published evidence of the specific role of extracellular miRNAs in tumor development and progression.

Abstract

MicroRNAs (miRNAs) are released by different types of cells through highly regulated mechanisms under normal and pathological conditions. These extracellular miRNAs can be delivered into recipient cells for functional purposes, acting as cell-to-cell signaling mediators. It has been discovered that cancer cells release miRNAs into their surroundings, targeting normal cells or other cancer cells, presumably to promote tumor development and progression. These extracellular miRNAs are associated with oncogenic mechanisms and, because they can be quantified in blood and other bodily fluids, may be suitable noninvasive biomarkers for cancer detection. This review summarizes recent evidence of the role of extracellular miRNAs as intercellular mediators, with an emphasis on their role in the mechanisms of tumor development and progression and their potential value as biomarkers in solid tumors. It also highlights the biological characteristics of extracellular miRNAs that enable them to function as regulators of gene expression, such as biogenesis, gene silencing mechanisms, subcellular compartmentalization, and the functions and mechanisms of release.

The tumor suppressor microRNA let-7 inhibits human LINE-1 retrotransposition

Nearly half of the human genome is made of transposable elements (TEs) whose activity continues to impact its structure and function. Among them, Long INterspersed Element class 1 (LINE-1 or L1) elements are the only autonomously active TEs in humans. L1s are expressed and mobilized in different cancers, generating mutagenic insertions that could affect tumor malignancy. Tumor suppressor microRNAs are ∼22nt RNAs that post-transcriptionally regulate oncogene expression and are frequently downregulated in cancer. Here we explore whether they also influence L1 mobilization. We show that downregulation of let-7 correlates with accumulation of L1 insertions in human lung cancer. Furthermore, we demonstrate that let-7 binds to the L1 mRNA and impairs the translation of the second L1-encoded protein, ORF2p, reducing its mobilization. Overall, our data reveals that let-7, one of the most relevant microRNAs, maintains somatic genome integrity by restricting L1 retrotransposition.

Human Long INterspersed Element class 1 (LINE-1) elements are expressed and mobilized in many types of cancer, contributing to malignancy. Here the authors show that the tumor suppressor microRNA let-7 targets the LINE-1 mRNA and reduces LINE-1 mobilization.

microRNA strand selection: Unwinding the rules

microRNAs (miRNAs) play a central role in the regulation of gene expression by targeting specific mRNAs for degradation or translational repression. Each miRNA is post‐transcriptionally processed into a duplex comprising two strands. One of the two miRNA strands is selectively loaded into an Argonaute protein to form the miRNA‐Induced Silencing Complex (miRISC) in a process referred to as miRNA strand selection. The other strand is ejected from the complex and is subject to degradation. The target gene specificity of miRISC is determined by sequence complementarity between the Argonaute‐loaded miRNA strand and target mRNA. Each strand of the miRNA duplex has the capacity to be loaded into miRISC and possesses a unique seed sequence. Therefore, miRNA strand selection plays a defining role in dictating the specificity of miRISC toward its targets and provides a mechanism to alter gene expression in a switch‐like fashion. Aberrant strand selection can lead to altered gene regulation by miRISC and is observed in several human diseases including cancer. Previous and emerging data shape the rules governing miRNA strand selection and shed light on how these rules can be circumvented in various physiological and pathological contexts.

This article is categorized under:

RNA Processing > Processing of Small RNAs

Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs

Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs

MicroRNAs as Biomarkers and Therapeutic Targets in Inflammation- and Ischemia-Reperfusion-Related Acute Renal Injury

Acute kidney injury (AKI), caused mainly by ischemia-reperfusion, sepsis, or nephrotoxins (such as contrast medium), is identified by an abrupt decline in kidney function and is associated with high morbidity and mortality. Despite decades of efforts, the pathogenesis of AKI remains poorly understood, and effective therapies are lacking. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at the posttranscriptional level to control cell differentiation, development, and homeostasis. Additionally, extracellular miRNAs might mediate cell-cell communication during various physiological and pathological processes. Recently, mounting evidence indicates that miRNAs play a role in the pathogenesis of AKI. Moreover, emerging research suggests that because of their remarkable stability in body fluids, microRNAs can potentially serve as novel diagnostic biomarkers of AKI. Of note, our previous finding that miR-494 is rapidly elevated in urine but not in serum provides insight into the ultimate role of urine miRNAs in AKI. Additionally, exosomal miRNAs derived from stem cells, known as the stem cell secretome, might be a potential innovative therapeutic strategy for AKI. This review aims to provide new data obtained in this field of research. It is hoped that new studies on this topic will not only generate new insights into the pathophysiology of urine miRNAs in AKI but also might lead to the precise management of this fatal disease.

tRNA-derived fragments (tRFs) regulate post-transcriptional gene expression via AGO-dependent mechanism in IL-1β stimulated chondrocytes

OBJECTIVES

Recent studies have shown that tRNA-derived RNA fragments (tRFs) are novel regulators of post-transcriptional gene expression. However, the expression profiles and their role in post-transcriptional gene regulation in chondrocytes is unknown. Here, we determined tRFs expression profile and explored tRF-3003a role in post-transcriptional gene regulation in IL-1β stimulated chondrocytes.

METHODS

We used qPCR arrays to determine tRNAs and tRFs expression in age- and sex-matched primary human OA chondrocytes and TC28/I2 cells stimulated with IL-1β. Chondrocytes were transfected with tRNA-Cys^GCA overexpression plasmid or tRF-3003a mimic and 3'UTR luciferase reporter plasmids of mRNAs harboring predicted tRF target "seed sequence". The AGO-RNA-induced silencing complex (AGO-RISC)-dependent repressive activity of tRF-3003a was determined by siRNA-mediated knockdown of AGO2.

RESULTS

IL-1β increased the expression levels of specific tRNAs and of tRF-3003a, a type 3 tRF produced by the cleavage of tRNA-Cys^GCA. tRF-3003a "seed sequence" was identified in the 3'UTR of JAK3 mRNA and tRNA-Cys^GCA overexpression or transfection of a tRF-3003a mimic in chondrocytes downregulated JAK3 expression and significantly reduced the activity of the 3'UTR reporter. RIP assay showed enrichment of tRF-3003a into AGO2/RISC in IL-1β treated chondrocytes. The suppressive effect of tRF-3003a on JAK3 3'UTR reporter was abrogated with siRNA-mediated depletion of AGO2.

CONCLUSIONS

We demonstrate that under pathological conditions chondrocytes display perturbations in the expression profile of specific tRNAs and tRFs. Furthermore, a specific tRF namely tRF-3003a can post-transcriptionally regulate JAK3 expression via AGO/RISC formation in chondrocytes. Identification of this novel mechanism may be of value in the design of precision therapies for OA.

A novel microRNA boosts hyper-β-oxidation of fatty acids in liver by impeding CEP350-mediated sequestration of PPARα and thus restricts chronic hepatitis C

Imbalance in lipid metabolism induces steatosis in liver during Chronic hepatitis C (CHC). Contribution of microRNAs in regulating lipid homoeostasis and liver disease progression is well established using small RNA-transcriptome data. Owing to the complexity in the development of liver diseases, the existence and functional importance of yet undiscovered regulatory miRNAs in disease pathogenesis was explored in this study using the unmapped sequences of the transcriptome data of HCV-HCC liver tissues following miRDeep2.pl pipeline. MicroRNA-c12 derived from the first intron of LGR5 of chromosome 12 was identified as one of the miRNA like sequences retrieved in this analysis that showed human specific origin. Northern blot hybridization has proved its existence in the hepatic cell line. Enrichment of premiR-c12 in dicer-deficient cells and miR-c12 in Ago2-RISC complex clearly suggested that it followed canonical miRNA biogenesis pathway and accomplished its regulatory function. Expression of this miRNA was quite low in CHC tissues than normal liver implying HCV-proteins might be regulating its biogenesis. Promoter scanning and ChIP analysis further revealed that under expression of p53 and hyper-methylation of STAT3 binding site upon HCV infection restricted its expression in CHC tissues. Centrosomal protein 350 (CEP350), which sequestered PPARα, was identified as one of the targets of miR-c12 using Miranda and validated by luciferase assay/western blot analysis. Furthermore, reduced triglyceride accumulation and enhanced PPARα mediated transcription of β-oxidation genes upon restoration of miR-c12 in liver cells suggested its role in lipid catabolism. Thus this study is reporting miR-c12 for the first time and showed its' protective role during chronic HCV infection.

CSDE1 controls gene expression through the miRNA-mediated decay machinery

In animals, miRNAs are the most prevalent small non-coding RNA molecules controlling posttranscriptional gene regulation. The Argonaute proteins (AGO) mediate miRNA-guided gene silencing by recruiting multiple factors involved in translational repression, deadenylation, and decapping. Here, we report that CSDE1, an RNA-binding protein linked to stem cell maintenance and metastasis in cancer, interacts with AGO2 within miRNA-induced silencing complex and mediates gene silencing through its N-terminal domains. We show that CSDE1 interacts with LSM14A, a constituent of P-body assembly and further associates to the DCP1-DCP2 decapping complex, suggesting that CSDE1 could promote the decay of miRNA-induced silencing complex-targeted mRNAs. Together, our findings uncover a hitherto unknown mechanism used by CSDE1 in the control of gene expression mediated by the miRNA pathway.

ncRNAs in Type-2 Immunity

Immunological diseases, including asthma, autoimmunity and immunodeficiencies, affect a growing percentage of the population with significant unmet medical needs. As we slowly untangle and better appreciate these complex genetic and environment-influenced diseases, new therapeutically targetable pathways are emerging. Non-coding RNA species, which regulate epigenetic, transcriptional and translational responses are critical regulators of immune cell development, differentiation and effector function, and may represent one such new class of therapeutic targets. In this review we focus on type-2 immune responses, orchestrated by T_H2 cell-derived cytokines, IL-4, IL-5 and IL-13, which stimulate a variety of immune and tissue responses- commonly referred to as type-2 immunity. Evolved to protect us from parasitic helminths, type-2 immune responses are observed in individuals with allergic diseases, including Asthma, atopic dermatitis and food allergy. A growing number of studies have identified the involvement of various RNA species, including microRNAs (miRNA) and long non-coding (lncRNA), in type-2 immune responses and in both clinical and pre-clinical disease settings. We highlight these recent findings, identify gaps in our understanding and provide a perspective on how our current understanding can be harnessed for novel treat opportunities to treat type-2 immune-mediated diseases.

AGO unchained: Canonical and non-canonical roles of Argonaute proteins in mammals

Argonaute (AGO) proteins play key roles in animal physiology by binding to small RNAs and regulating the expression of their targets. In mammals, they do so through two distinct pathways: the miRNA pathway represses genes through a multiprotein complex that promotes both decay and translational repression; the siRNA pathway represses transcripts through direct Ago2-mediated cleavage. Here, we review our current knowledge of mechanistic details and physiological requirements of both these pathways and briefly discuss their implications to human disease.

Expression of TNRC6 (GW182) Proteins Is Not Necessary for Gene Silencing by Fully Complementary RNA Duplexes

The trinucleotide repeat containing 6 (TNRC6) family of proteins are core components of RNA interference (RNAi) and consist of three paralogs (TNRC6A, TNRC6B, and TNRC6C). The TNRC6 paralogs associate with argonaute (AGO) protein, the core RNAi factor, and bridge its interactions with other proteins. We obtained TNRC6A and TNRC6B single and double knockout cell lines to investigate how the TNRC6 paralogs contribute to RNAi. We found that TNRC6 proteins are not required for gene silencing when duplex RNAs are fully complementary. TNRC6 expression was necessary for regulation by a microRNA. TNRC6A, but not TNRC6B, expression was necessary for transcriptional activation by a duplex RNA targeting a gene promoter. By contrast, AGO2 is required for all three gene expression pathways. TNRC6A can affect the Dicer localization in cytoplasm versus the nucleus, but none of the three TNRC6 paralogs was necessary for nuclear localization of AGO2. Our data suggest that the roles of the TNRC6 paralogs differ in some details and that TNRC6 is not required for clinical therapeutic silencing mechanisms that involve fully complementary duplex RNAs.

KSHV RNA-binding protein ORF57 inhibits P-body formation to promote viral multiplication by interaction with Ago2 and GW182

Cellular non-membranous RNA-granules, P-bodies (RNA processing bodies, PB) and stress granules (SG), are important components of the innate immune response to virus invasion. Mechanisms governing how a virus modulates PB formation remain elusive. Here, we report the important roles of GW182 and DDX6, but not Dicer, Ago2 and DCP1A, in PB formation, and that Kaposi's sarcoma-associated herpesvirus (KSHV) lytic infection reduces PB formation through several specific interactions with viral RNA-binding protein ORF57. The wild-type ORF57, but not its N-terminal dysfunctional mutant, inhibits PB formation by interacting with the N-terminal GW-domain of GW182 and the N-terminal domain of Ago2, two major components of PB. KSHV ORF57 also induces nuclear Ago2 speckles. Homologous HSV-1 ICP27, but not EBV EB2, shares this conserved inhibitory function with KSHV ORF57. By using time-lapse confocal microscopy of HeLa cells co-expressing GFP-tagged GW182, we demonstrated that viral ORF57 inhibits primarily the scaffolding of GW182 at the initial stage of PB formation. Consistently, KSHV-infected iSLK/Bac16 cells with reduced GW182 expression produced far fewer PB and SG, but 100-fold higher titer of infectious KSHV virions when compared to cells with normal GW182 expression. Altogether, our data provide the first evidence that a DNA virus evades host innate immunity by encoding an RNA-binding protein that promotes its replication by blocking PB formation.

Biphasic regulation of RNA interference during rotavirus infection by modulation of Argonaute2

RNA interference (RNAi) is an evolutionary ancient innate immune response in plants, nematodes, and arthropods providing natural protection against viral infection. Viruses have also gained counter‐defensive measures by producing virulence determinants called viral‐suppressors‐of‐RNAi (VSRs). Interestingly, in spite of dominance of interferon‐based immunity over RNAi in somatic cells of higher vertebrates, recent reports are accumulating in favour of retention of the antiviral nature of RNAi in mammalian cells. The present study focuses on the modulation of intracellular RNAi during infection with rotavirus (RV), an enteric virus with double‐stranded RNA genome. Intriguingly, a time point‐dependent bimodal regulation of RNAi was observed in RV‐infected cells, where short interfering RNA (siRNA)‐based RNAi was rendered non‐functional during early hours of infection only to be reinstated fully beyond that early infection stage. Subsequent investigations revealed RV nonstructural protein 1 to serve as a putative VSR by associating with and triggering degradation of Argonaute2 (AGO2), the prime effector of siRNA‐mediated RNAi, via ubiquitin–proteasome pathway. The proviral significance of AGO2 degradation was further confirmed when ectopic overexpression of AGO2 significantly reduced RV infection. Cumulatively, the current study presents a unique modulation of host RNAi during RV infection, highlighting the importance of antiviral RNAi in mammalian cells.

CPEB3 inhibits translation of mRNA targets by localizing them to P bodies

Protein synthesis is crucial for the maintenance of long-term memory-related synaptic plasticity. The cytoplasmic polyadenylation element-binding protein 3 (CPEB3) regulates the translation of several mRNAs important for long-term synaptic plasticity in the hippocampus. In previous studies, we found that the oligomerization and activity of CPEB3 are controlled by small ubiquitin-like modifier (SUMO)ylation. In the basal state, CPEB3 is SUMOylated; it is soluble and acts as a repressor of translation. Following neuronal stimulation, CPEB3 is de-SUMOylated; it now forms oligomers that are converted into an active form that promotes the translation of target mRNAs. To better understand how CPEB3 regulates the translation of its mRNA targets, we have examined CPEB3 subcellular localization. We found that basal, repressive CPEB3 is localized to membraneless cytoplasmic processing bodies (P bodies), subcellular compartments that are enriched in translationally repressed mRNA. This basal state is affected by the SUMOylation state of CPEB3. After stimulation, CPEB3 is recruited into polysomes, thus promoting the translation of its target mRNAs. Interestingly, when we examined CPEB3 recombinant protein in vitro, we found that CPEB3 phase separates when SUMOylated and binds to a specific mRNA target. These findings suggest a model whereby SUMO regulates the distribution, oligomerization, and activity of oligomeric CPEB3, a critical player in the persistence of memory.

Tagger-A Swiss army knife for multiomics to dissect cell type-specific mechanisms of gene expression in mice

A deep understanding of how regulation of the multiple levels of gene expression in mammalian tissues give rise to complex phenotypes has been impeded by cellular diversity. A handful of techniques were developed to tag-select nucleic acids of interest in specific cell types, thereby enabling their capture. We expanded this strategy by developing the Tagger knock-in mouse line bearing a quad-cistronic transgene combining enrichment tools for nuclei, nascent RNA, translating mRNA, and mature microRNA (miRNA). We demonstrate that Tagger can capture the desired nucleic acids, enabling multiple omics approaches to be applied to specific cell types in vivo using a single transgenic mouse line.

This Methods and Resources paper describes Tagger, a knock-in mouse line bearing a quad-cistronic transgene that enables the capture of translating mRNAs, mature miRNAs, pulse-labeled total RNA, and the nucleus, all from specific cells of complex tissues.

Adenosine deaminase acting on RNA-1 (ADAR1) inhibits hepatitis B virus (HBV) replication by enhancing microRNA-122 processing

Adenosine deaminases acting on RNA-1 (ADAR1) involves adenosine to inosine RNA editing and microRNA processing. ADAR1 is known to be involved in the replication of various viruses, including hepatitis C and D. However, the role of ADAR1 in hepatitis B virus (HBV) infection has not yet been elucidated. Here, for the first time, we demonstrated ADAR1 antiviral activity against HBV. ADAR1 has two splicing isoforms in human hepatocytes: constitutive p110 protein and interferon-α (IFN-α)-responsive p150 protein. We found that overexpression of ADAR1 decreased HBV RNA in an HBV culture model. A catalytic-site mutant ADAR1 also decreased HBV RNA levels, whereas another adenosine deaminases that act on the RNA (ADAR) family protein, ADAR2, did not. Moreover, the induction of ADAR1 by stimulation with IFN-α also reduced HBV RNA levels. Decreases in endogenous ADAR1 expression by knock-down or knock-out increased HBV RNA levels. A major hepatocyte-specific microRNA, miRNA-122, was found to be positively correlated with ADAR1 expression, and exogenous miRNA-122 decreased both HBV RNA and DNA, whereas, conversely, transfection with a miRNA-122 inhibitor increased them. The reduction of HBV RNA by ADAR1 expression was abrogated by p53 knock-down, suggesting the involvement of p53 in the ADAR1-mediated reduction of HBV RNA. This study demonstrated, for the first time, that ADAR1 plays an antiviral role against HBV infection by increasing the level of miRNA-122 in hepatocytes.

MicroRNA‑20 induces methylation of hepatitis B virus covalently closed circular DNA in human hepatoma cells

Methylation was suggested to suppress the transcriptional activity of hepatitis B virus (HBV) covalently closed circular DNA (cccDNA) in hepatocytes. This may be associated with its low replicative activity during the inactive stage of chronic HBV infection; however, the exact mechanisms of methylation in HBV infection remain unknown. We have previously shown that short hairpin RNAs induced the methylation of the HBV genome in hepatoma cell lines. We also reported that the microRNA (miR) 17–92 cluster negatively regulates HBV replication in human hepatoma cells. In addition, miR-20a, a member of the miR 17–92 cluster, has sequence homology with the short hairpin RNA that induces HBV methylation. In the present study, we investigated whether miR-20a can function as an endogenous effector of HBV DNA methylation. The results indicated that overexpression of miR-20a could suppress the replicative activity of HBV and increased the degree of methylation of HBV cccDNA in the HepAD38 hepatoma cell line. Argonaute (AGO)1 and AGO2, effectors of the RNA-induced silencing complex, were detected in the nucleus of HepAD38 cells; however, only AGO2 was bound to HBV cccDNA. In addition, intranuclear AGO2 was determined to be bound with miR-20a. In conclusion, miR-20a may be loaded onto AGO2, prior to its translocation into the nucleus, inducing the methylation of HBV DNA in human hepatoma cells, leading to the suppression of HBV replication.

P7 and P8 proteins of High Plains wheat mosaic virus, a negative-strand RNA virus, employ distinct mechanisms of RNA silencing suppression

High Plains wheat mosaic virus (genus Emaravirus), an octapartite negative-sense RNA virus, encodes two RNA silencing suppressors, P7 and P8. In this study, we found that P7 and P8 efficiently delayed the onset of dsRNA-induced transitive pathway of RNA silencing. Electrophoretic mobility shift assays (EMSA) revealed that only P7 protected long dsRNAs from dicing in vitro and bound weakly to 21- and 24-nt PTGS-like ds-siRNAs. In contrast, P8 bound strongly and relatively weakly to 21- and 24-nt ds-siRNAs, respectively, suggesting size-specific binding. In EMSA, neither protein bound to 180-nt and 21-nt ssRNAs at detectable levels. Sequence analysis revealed that P7 contains a conserved GW motif. Mutational disruption of this motif resulted in loss of suppression of RNA silencing and pathogenicity enhancement, and failure to complement the silencing suppression-deficient wheat streak mosaic virus. Collectively, these data suggest that P7 and P8 proteins utilize distinct mechanisms to overcome host RNA silencing for successful establishment of systemic infection in planta.

Mechanistic Insights into MicroRNA-Mediated Gene Silencing

MicroRNAs (miRNAs) posttranscriptionally regulate gene expression by repressing protein synthesis and exert a broad influence over development, physiology, adaptation, and disease. Over the past two decades, great strides have been made toward elucidating how miRNAs go about shutting down messenger RNA (mRNA) translation and promoting mRNA decay.

Regulation of MicroRNA Machinery and Development by Interspecies S-Nitrosylation

Bioactive molecules can pass between microbiota and host to influence host cellular functions. However, general principles of interspecies communication have not been discovered. We show here in C. elegans that nitric oxide derived from resident bacteria promotes widespread S-nitrosylation of the host proteome. We further show that microbiota-dependent S-nitrosylation of C. elegans Argonaute protein (ALG-1)-at a site conserved and S-nitrosylated in mammalian Argonaute 2 (AGO2)-alters its function in controlling gene expression via microRNAs. By selectively eliminating nitric oxide generation by the microbiota or S-nitrosylation in ALG-1, we reveal unforeseen effects on host development. Thus, the microbiota can shape the post-translational landscape of the host proteome to regulate microRNA activity, gene expression, and host development. Our findings suggest a general mechanism by which the microbiota may control host cellular functions, as well as a new role for gasotransmitters.

Role of GW182 protein in the cell

GW182 proteins interact directly with the argonaute proteins and constitute key components of miRNA repressor complexes (miRISC) in metazoans. As argonautes are insufficient for silencing they recruit the GW182 s that act as scaffold proteins inducing downstream translational repression, target mRNA deadenylation and exonucleolytic mRNA degradation. Besides their role as part of repressor complexes inside the cell, they function in wide variety of cellular processes as highlighted in this review. The present review summarises and discusses in detail our current knowledge of the GW182 s and their role inside the cell.

MicroRNP-mediated translational activation of nonadenylated mRNAs in a mammalian cell-free system

MicroRNAs are small noncoding RNAs that regulate translation and mRNA stability by binding target mRNAs in complex with Argonaute (AGO) proteins. AGO interacts with a member of the TNRC6 family proteins to form a microRNP complex, which recruits the CCR4-NOT complex to accelerate deadenylation and inhibits translation. MicroRNAs primarily repress translation of target mRNAs but have been shown to enhance translation of a specific type of target reporter mRNAs in various experimental systems: G0 quiescent mammalian cells, Xenopus laevis oocytes, Drosophila embryo extracts, and HeLa cells. In all of the cases mentioned, a common feature of the activated target mRNAs is the lack of a poly(A) tail. Here, we show let-7-microRNP-mediated translational activation of nonadenylated target mRNAs in a mammalian cell-free system, which contains over-expressed AGO2, TNRC6B, and PAPD7 (TUTase5, TRF4-1). Importantly, translation of nonadenylated mRNAs was activated also by tethered TNRC6B silencing domain (SD), in the presence of PAPD7. Deletion of the poly(A)-binding protein (PABP) interacting motif (PAM2) from the TNRC6B-SD abolished the translational activation, suggesting the involvement of PABP in the process. Similar results were also obtained in cultured HEK293T cells. This work may provide novel insights into microRNP-mediated mRNA regulation.

Argonaute Utilization for miRNA Silencing Is Determined by Phosphorylation-Dependent Recruitment of LIM-Domain-Containing Proteins

As core components of the microRNA-induced silencing complex (miRISC), Argonaute (AGO) proteins interact with TNRC6 proteins, recruiting other effectors of translational repression/mRNA destabilization. Here, we show that LIMD1 coordinates the assembly of an AGO-TNRC6 containing miRISC complex by binding both proteins simultaneously at distinct interfaces. Phosphorylation of AGO2 at Ser 387 by Akt3 induces LIMD1 binding, which in turn enables AGO2 to interact with TNRC6A and downstream effector DDX6. Conservation of this serine in AGO1 and 4 indicates this mechanism may be a fundamental requirement for AGO function and miRISC assembly. Upon CRISPR-Cas9-mediated knockout of LIMD1, AGO2 miRNA-silencing function is lost and miRNA silencing becomes dependent on a complex formed by AGO3 and the LIMD1 family member WTIP. The switch to AGO3 utilization occurs due to the presence of a glutamic acid residue (E390) on the interaction interface, which allows AGO3 to bind to LIMD1, AJUBA, and WTIP irrespective of Akt signaling.

Phase Transitions in the Assembly and Function of Human miRISC

miRISC is a multi-protein assembly that uses microRNAs (miRNAs) to identify mRNAs targeted for repression. Dozens of miRISC-associated proteins have been identified, and interactions between many factors have been examined in detail. However, the physical nature of the complex remains unknown. Here, we show that two core protein components of human miRISC, Argonaute2 (Ago2) and TNRC6B, condense into phase-separated droplets in vitro and in live cells. Phase separation is promoted by multivalent interactions between the glycine/tryptophan (GW)-rich domain of TNRC6B and three evenly spaced tryptophan-binding pockets in the Ago2 PIWI domain. miRISC droplets formed in vitro recruit deadenylation factors and sequester target RNAs from the bulk solution. The condensation of miRISC is accompanied by accelerated deadenylation of target RNAs bound to Ago2. The combined results may explain how miRISC silences mRNAs of varying size and structure and provide experimental evidence that protein-mediated phase separation can facilitate an RNA processing reaction.

Identification and Evolutionary Characterization of ARGONAUTE-Binding Platforms

ARGONAUTE (AGO) proteins are eukaryotic RNA silencing effectors that interact with their binding partners via short peptide motifs known as AGO hooks. AGO hooks tend to cluster in one region of the protein to create an AGO-binding platform. In addition to the presence of AGO hooks, AGO-binding platforms are intrinsically disordered, contain tandem repeat arrays, and have weak sequence conservation even between close relatives. These characteristics make it difficult to identify and perform evolutionary analysis of these regions. Because of their weak sequence conservation, only a few AGO-binding platforms are characterized, and the evolution of these regions is only poorly understood. In this chapter we describe modules developed for computational identification and evolutionary analysis of AGO-binding platforms, with particular emphasis on understanding evolution of the tandem repeat arrays.

Long Non-coding Antisense RNA TNRC6C-AS1 Is Activated in Papillary Thyroid Cancer and Promotes Cancer Progression by Suppressing TNRC6C Expression

Context: Evidences have shown the important role of long non-coding antisense RNAs in regulating its cognate sense gene in cancer biology. Objective: Investigate the regulatory role of a long non-coding antisense RNA TNRC6C-AS1 on its sense partner TNRC6C, and their effects on the aggressiveness and iodine-uptake ability of papillary thyroid cancer (PTC). Design: TNRC6C-AS1 was identified as the target long non-coding RNA in PTC by using microarray analysis and computational analysis. In vitro gain/loss-of-function experiments were performed to investigate the effects of TNRC6C-AS1 and TNRC6C on proliferation, apoptosis, migration, invasion and iodine-uptake ability of TPC1 cells. Expression levels of TNRC6C-AS1 and TNRC6C of 30 cases of PTC tissues and its adjacent normal thyroid tissues were determined. Results: Downregulation of TNRC6C-AS1 or overexpression of TNRC6C inhibited proliferation, migration and invasion of TPC1 cells, while apoptosis and iodine uptake was promoted in TPC1 cells. Suppression of TNRC6C-AS1 significantly increased the expression of TNRC6C in TPC1 cells. The inhibitory effect of TNRC6C-AS1 knockdown on cell proliferation, migration and invasion was attenuated when the expression of TNRC6C was suppressed simultaneously, indicating TNRC6C is a functional target of TNRC6C-AS1. The expression of TNRC6C-AS1 was significantly higher, while the TNRC6C mRNA and protein were significantly lower in PTC tissues than normal adjacent tissues. There was a significant inverse correlation between TNRC6C-AS1 and TNRC6C mRNA in PTC tissue samples. Conclusions: TNRC6C-AS1 promotes the progression of PTC and inhibits its ability of iodine accumulation by suppressing the expression of TNRC6C. Targeting TNRC6C-AS1 - TNRC6C axis may be a new promising treatment for PTC.

An miRNA-mediated therapy for SCA6 blocks IRES-driven translation of the CACNA1A second cistron

Spinocerebellar ataxia type 6 (SCA6) is a dominantly inherited neurodegenerative disease characterized by slowly progressive ataxia and Purkinje cell degeneration. SCA6 is caused by a polyglutamine repeat expansion within a second CACNA1A gene product, α1ACT. α1ACT expression is under the control of an internal ribosomal entry site (IRES) present within the CACNA1A coding region. Whereas SCA6 allele knock-in mice show indistinguishable phenotypes from wild-type littermates, expression of SCA6-associated α1ACT (α1ACTSCA6) driven by a Purkinje cell-specific promoter in mice produces slowly progressive ataxia and cerebellar atrophy. We developed an early-onset SCA6 mouse model using an adeno-associated virus (AAV)-based gene delivery system to ectopically express CACNA1A IRES-driven α1ACTSCA6 to test the potential of CACNA1A IRES-targeting therapies. Mice expressing AAV9-mediated CACNA1A IRES-driven α1ACTSCA6 exhibited early-onset ataxia, motor deficits, and Purkinje cell degeneration. We identified miR-3191-5p as a microRNA (miRNA) that targeted CACNA1A IRES and preferentially inhibited the CACNA1A IRES-driven translation of α1ACT in an Argonaute 4 (Ago4)-dependent manner. We found that eukaryotic initiation factors (eIFs), eIF4AII and eIF4GII, interacted with the CACNA1A IRES to enhance α1ACT translation. Ago4-bound miR-3191-5p blocked the interaction of eIF4AII and eIF4GII with the CACNA1A IRES, attenuating IRES-driven α1ACT translation. Furthermore, AAV9-mediated delivery of miR-3191-5p protected mice from the ataxia, motor deficits, and Purkinje cell degeneration caused by CACNA1A IRES-driven α1ACTSCA6 We have established proof of principle that viral delivery of an miRNA can rescue a disease phenotype through modulation of cellular IRES activity in a mouse model.

microRNAs stimulate translation initiation mediated by HCV-like IRESes

MicroRNAs (miRNAs) are small non-coding RNAs that control gene expression by recognizing and hybridizing to a specific sequence generally located in the 3΄ untranslated region (UTR) of targeted mRNAs. miRNA-induced inhibition of translation occurs during the initiation step, most probably at the level of ribosome scanning. In this process, the RNA-induced silencing complex interacts both with PABP and the 43S pre-initiation complex to disrupt scanning of the 40S ribosome. However, in some specific cases, miRNAs can stimulate translation. Although the mechanism of miRNA-mediated upregulation is unknown, it appears that the poly(A) tail and the lack of availability of the TNRC6 proteins are amongst major determinants. The genomic RNA of the Hepatitis C Virus is uncapped, non-polyadenylated and harbors a peculiar internal ribosome entry site (IRES) that binds the ribosome directly to the AUG codon. Thus, we have exploited the unique properties of the HCV IRES and other related IRESes (HCV-like) to study how translation initiation can be modulated by miRNAs on these elements. Here, we report that miRNA binding to the 3΄ UTR can stimulate translation of a reporter gene given that its expression is driven by an HCV-like IRES and that it lacks a poly(A) tail at its 3΄ extremity.

Structural characterisation of TNRC6A nuclear localisation signal in complex with importin-alpha

The GW182/TNRC6 family of proteins are central scaffolds that link microRNA-associated Argonaute proteins to the cytoplasmic decay machinery for targeted mRNA degradation processes. Although nuclear roles for the GW182/TNRC6 proteins are unknown, recent reports have demonstrated nucleocytoplasmic shuttling activity that utilises the importin-α and importin-β transport receptors for nuclear translocation. Here we describe the structure of mouse importin-α in complex with the TNRC6A nuclear localisation signal peptide. We further show that the interactions observed between TNRC6A and importin-α are conserved between mouse and human complexes. Our results highlight the ability of monopartite cNLS sequences to maximise contacts at the importin-α major binding site, as well as regions outside the main binding cavities.

Multivalent Recruitment of Human Argonaute by GW182

In miRNA-mediated gene silencing, the physical interaction between human Argonaute (hAgo) and GW182 (hGW182) is essential for facilitating the downstream silencing of the targeted mRNA. GW182 can interact with hAgo via three of the GW/WG repeats in its Argonaute-binding domain: motif-1, motif-2, and the hook motif. The structure of hAgo1 in complex with the hook motif of hGW182 reveals a "gate"-like interaction that is critical for GW182 docking into one of hAgo1's tryptophan-binding pockets. We show that hAgo1 and hAgo2 have a single GW182-binding site and that miRNA binding increases hAgo's affinity to GW182. With target binding occurring rapidly, this ensures that only mature RISC would be recruited for silencing. Finally, we show that hGW182 can recruit up to three copies of hAgo via its three GW motifs. This may explain the observed cooperativity in miRNA-mediated gene silencing.

Trinucleotide repeat containing 6c (TNRC6c) is essential for microvascular maturation during distal airspace sacculation in the developing lung

GW182 (also known asTNRC6) family members are critically involved in the final effector phase of miRNA-mediated mRNA repression. The three mammalian paralogs, TNRC6a, b and c, are thought to be redundant based on Argonaute (Ago) binding, tethering assays, and RNAi silencing of individual members in cell lines. To test this idea, we generated TNRC6a, b and c knockout mice. TNRC6a mutants die at mid-gestation, while b- and c- deleted mice are born at a Mendelian ratio. However, the majority of TNRC6b and all TNRC6c mutants die within 24h after birth, the latter with respiratory failure. Necropsy of TNRC6c mutants revealed normal-appearing airways that give rise to abnormally thick-walled distal gas exchange sacs. Immunohistological analysis of mutant lungs demonstrated a normal distribution of bronchiolar and alveolar cells, indicating that loss of TNRC6c did not abrogate epithelial cell differentiation. The cellular kinetics and relative proportions of endothelial, epithelial, and mesenchymal cells were also not altered. However, the underlying capillary network was simplified and endothelial cells had failed to become tightly apposed to the surface epithelium in TNRC6c mutants, presumably causing the observed respiratory failure. TGFβ family mutant mice exhibit a similar lung phenotype of thick-walled air sacs and neonatal lethality, and qRT-PCR confirmed dynamic downregulation of TGFβ1 and TGFβR2 in TNRC6c mutant lungs during sacculation. VEGFR, but not VEGF-A ligand, was also lower, likely reflecting the overall reduced capillary density in TNRC6c mutants. Together, these results demonstrate that GW182 paralogs are not functionally redundant in vivo. Surprisingly, despite regulating a general cellular process, TNRC6c is selectively required only in the distal lung and not until late in gestation for proper expression of the TGFβ family genes that drive sacculation. These results imply a complex and indirect mode of regulation of sacculation by TNRC6c, mediated in part by dynamic transcriptional repression of an inhibitor of TGFβ family gene expression.