Endogenous transcripts direct microRNA degradation in Drosophila, and this targeted degradation is required for proper embryonic development

Decoding microRNA arm switching: a key to evolutionary innovation and gene regulation

miRNA arm switching is a pivotal regulatory mechanism that allows organisms to fine-tune gene expression by selectively utilizing either the 5p or 3p strand of a miRNA duplex. This process, conserved across species, facilitates adaptive responses to developmental cues, environmental changes, and disease states. By dynamically altering strand selection, arm switching reshapes gene regulatory networks, contributing to phenotypic diversity and evolutionary innovation. Despite its growing recognition, the mechanisms driving arm switching-such as thermodynamic properties and enzyme-mediated processing-remain incompletely understood. This review synthesizes current findings, highlighting arm switching as a highly conserved mechanism with profound implications for the evolution of regulatory networks. We explore how this phenomenon expands miRNA functionality, drives phenotypic plasticity, and co-evolves with miRNA gene duplications to fuel the diversification of biological functions across taxa.

Insights into the target-directed miRNA degradation mechanism in Drosophila ovarian cell culture

Target-directed miRNA degradation (TDMD) is a process of post-transcriptional regulation of miRNA stability in animals induced by an extended pairing of Ago-bound miRNAs with specialized complementary RNA targets. As suggested by studies on human cell culture, Ago engaged with the extended duplex is recognized by the ZSWIM8 receptor of the Cullin-RING-ligase complex (CRL3), which also contains Cul3, EloB, and EloC proteins. The CRL activity is accelerated by the neddylation of Cul3 with the involvement of the E2 conjugating protein UbcE2M. The CRL ubiquitinates Ago, resulting in proteolysis of Ago and degradation of the released miRNAs. To date, the molecular mechanism of TDMD has not been studied in other species. To further characterize TDMD in animals, we investigated the protein Dora, the Drosophila ortholog of ZSWIM8, in the culture of Drosophila ovarian somatic cells (OSC). We showed that Dora in OSCs localizes in protein granules unrelated to P- and GW-bodies. The dora knockout resulted in the accumulation of multiple miRNAs, including miR-7-5p, and transcriptome-wide affected the mRNA targets of differentially expressed miRNAs. We also showed that Dora associates with proteins of the CRL3 complex, and the depletion of CRL3 components or inhibition of Cul3 neddylation upregulates miR-7-5p. We concluded that the molecular mechanism of TDMD is conserved in humans and Drosophila. Finally, we found that cells without Dora have an impaired Notch signaling pathway, indicating that TDMD in OSCs may contribute to the modulation of the Notch pathway.

The biogenesis and regulation of animal microRNAs

MicroRNAs (miRNAs) are small, yet profoundly influential, non-coding RNAs that base-pair with mRNAs to induce RNA silencing. Although the basic principles of miRNA biogenesis and function have been established, recent breakthroughs have yielded important new insights into the molecular mechanisms of miRNA biogenesis. In this Review, we discuss the metazoan miRNA biogenesis pathway step-by-step, focusing on the key biogenesis machinery, including the Drosha-DGCR8 complex (Microprocessor), exportin-5, Dicer and Argonaute. We also highlight newly identified cis-acting elements and their impact on miRNA maturation, informed by advanced high-throughput and structural studies, and discuss recently discovered mechanisms of clustered miRNA processing, target recognition and target-directed miRNA decay (TDMD). Lastly, we explore multiple regulatory layers of miRNA biogenesis, mediated by RNA-protein interactions, miRNA tailing (uridylation or adenylation) and RNA modifications.

EBAX-1/ZSWIM8 destabilizes miRNAs, resulting in transgenerational inheritance of a predatory trait

Environmental influences on traits and associated transgenerational epigenetic inheritance have widespread implications but remain controversial and underlying mechanisms poorly understood. We introduce long-term environmental induction experiments on alternative diets in Pristionchus pacificus, a nematode exhibiting mouth-form plasticity including predation, by propagating 110 isogenic lines for 101 generations with associated food-reversal experiments. We found dietary induction and subsequent transgenerational inheritance of the predatory morph and identified a role of ubiquitin ligase EBAX-1/ZSWIM8 in this process. Ppa-ebax-1 mutants are transgenerational inheritance defective, and Ppa-EBAX-1 destabilizes the clustered microRNA family miR-2235a/miR-35. Deletions of a cluster of 44 identical miR-2235a copies resulted in precocious and extended transgenerational inheritance of the predatory morph. These findings indicate that EBAX-1/ZSWIM8 destabilizes miRNAs, resulting in transgenerational inheritance, suggesting a role for target-directed miRNA degradation.

Circular RNA circASH1L(4,5) protects microRNA-129-5p from target-directed microRNA degradation in human skin wound healing

BACKGROUND

Skin wound healing involves a complex gene expression programme that remains largely undiscovered in humans. Circular RNAs (circRNAs) and microRNAs (miRNAs) are key players in this process.

OBJECTIVES

To understand the functions and potential interactions of circRNAs and miRNAs in human skin wound healing.

METHODS

CircRNA, linear RNA and miRNA expression in human acute and chronic wounds were analysed with RNA sequencing and quantitative reverse transcription polymerase chain reaction. The roles of circASH1L(4,5) and miR-129-5p were studied in human primary keratinocytes (proliferation and migration assays, microarray analysis) and ex vivo wound models (histological analysis). The interaction between circASH1L(4,5) and miR-129-5p was examined using luciferase reporter and RNA pulldown assays.

RESULTS

We identified circASH1L(4,5) and its interaction with miR-129-5p, both of which increased during human skin wound healing. Unlike typical miRNA sponging, circASH1L enhanced miR-129 stability and silencing activity by protecting it from target-directed degradation triggered by NR6A1 mRNA. Transforming growth factor-β signalling - crucial in wound healing - promoted circASH1L expression while suppressing NR6A1, thereby increasing the abundance of miR-129 at the post-transcriptional level. CircASH1L and miR-129 enhanced keratinocyte migration and proliferation, crucial processes for the re-epithelialization of human wounds.

CONCLUSIONS

Our study uncovered a novel role for circRNAs as protectors of miRNAs and highlights the importance of regulated miRNA degradation in skin wound healing.

The PIWI-interacting protein Gtsf1 controls the selective degradation of small RNAs in Paramecium

Ciliates undergo developmentally programmed genome elimination, in which small RNAs direct the removal of transposable elements (TEs) during the development of the somatic nucleus. Twenty-five nucleotide scanRNAs (scnRNAs) are produced from the entire germline genome and transported to the maternal somatic nucleus, where selection of scnRNAs corresponding to germline-specific sequences is thought to take place. Selected scnRNAs then guide the elimination of TEs in the developing somatic nucleus. How germline-specific scnRNAs are selected remains to be determined. Here, we provide important mechanistic insights into the scnRNA selection pathway by identifying a Paramecium homolog of Gtsf1 as essential for the selective degradation of scnRNAs corresponding to retained somatic sequences. Consistently, we also show that Gtsf1 is localized in the maternal somatic nucleus where it associates with the scnRNA-binding protein Ptiwi09. Furthermore, we demonstrate that the scnRNA selection process is critical for genome elimination. We propose that Gtsf1 is required for the coordinated degradation of Ptiwi09-scnRNA complexes that pair with target RNA via the ubiquitin pathway, similarly to the mechanism suggested for microRNA target-directed degradation in metazoans.

Widespread destabilization of Caenorhabditis elegans microRNAs by the E3 ubiquitin ligase EBAX-1

MicroRNAs (miRNAs) associate with Argonaute (AGO) proteins to form complexes that direct mRNA repression. miRNAs are also the subject of regulation. For example, some miRNAs are destabilized through a pathway in which pairing to specialized transcripts recruits the ZSWIM8 E3 ubiquitin ligase, which polyubiquitinates AGO, leading to its degradation and exposure of the miRNA to cellular nucleases. Here, we found that 22 miRNAs in Caenorhabditis elegans are sensitive to loss of EBAX-1, the ZSWIM8 ortholog in nematodes, implying that these 22 miRNAs might be subject to this pathway of target-directed miRNA degradation (TDMD). The impact of EBAX-1 depended on the developmental stage, with the greatest effect on the miRNA pool (14.5%) observed in L1 larvae, and the greatest number of different miRNAs affected (17) observed in germline-depleted adults. The affected miRNAs included the miR-35-42 family, as well as other miRNAs among the least stable in the worm, suggesting that TDMD is a major miRNA-destabilization pathway in the worm. The excess miR-35-42 molecules that accumulated in ebax-1 mutants caused increased repression of their predicted target mRNAs and underwent 3' trimming over time. In general, however, miRNAs sensitive to EBAX-1 loss had no consistent pattern of either trimming or tailing. Replacement of the 3' region of miR-43 substantially reduced EBAX-1 sensitivity, a result that differed from that observed previously for miR-35. Together, these findings broaden the implied biological scope of TDMD-like regulation of miRNA stability in animals, and indicate that a role for miRNA 3' sequences is variable in the worm.

An endogenous cluster of target-directed microRNA degradation sites induces decay of distinct microRNA families

While much is known about miRNA biogenesis and canonical miRNA targeting, relatively less is understood about miRNA decay. The major miRNA decay pathway in metazoans is mediated through target-directed miRNA degradation (TDMD), in which certain RNAs can "trigger" miRNA decay. All known triggers for TDMD base pair with the miRNA seed, and extensively base pair on the miRNA 3' end, a pattern that supposedly induces a TDMD-competent conformational change of Argonaute (Ago), allowing for miRNA turnover. Here, we utilized Ago1-CLASH to find that the Drosophila transcript Kah contains at least two triggers, a "trigger cluster", against miR-9b and the miR-279 family. One of these triggers contains minimal/non-canonical 3' end base pairing but is still sufficient to induce TDMD of the entire miR-279 family. We found that these clustered triggers likely lack cooperativity, the minimal 3' pairing is required for miR-279 family turnover, and probed the in-cell RNA structure of the Kah trigger cluster. Overall, this study expands the list of endogenous triggers and the unexpectedly complex regulatory network governing miRNA degradation.

Retargeting target-directed microRNA-decay sites to highly expressed viral or cellular miRNAs

MicroRNAs (miRNAs) are pervasive regulators of gene expression, necessitating the development of tools to inhibit individual miRNAs for functional studies or therapeutic targeting. Specialized base-pairing configurations between a miRNA and an RNA target site can trigger the degradation of the targeting miRNA through target-directed miRNA decay (TDMD). Previous work has identified several natural sites that induce TDMD of specific miRNAs. We explored retargeting known TDMD sites for the inhibition of heterologous miRNAs, including several encoded by Kaposi's Sarcoma-associated herpesvirus (KSHV). We focused particularly on miR-K11, a viral mimic of the oncogenic miRNA miR-155. miRNA pairing architectures based on the TDMD site in the long non-coding RNA Cyrano outperformed other retargeted sites. Cyrano-like inhibitors were specific for viral miR-K11 over cellular miR-155 and vice versa. Lentiviral delivery of a Cyrano-like miR-K11 inhibitor into KSHV-transformed primary effusion lymphoma (PEL) cells impaired their viability, showing that miR-K11 promotes KSHV-dependent PEL cell survival. Surprisingly, inactivation of ZSWIM8, a key mediator of TDMD, did not substantially affect miRNA inhibition by retargeted Cyrano-based inhibitors in 293T or PEL cells. Together, our results demonstrate the feasibility of retargeting natural TDMD sites to highly expressed viral or cellular miRNAs and further define features of effective encoded miRNA inhibitors.

Pseudogene GSTM3P1 derived long non-coding RNA promotes ischemic acute kidney injury by target directed microRNA degradation of kidney-protective mir-668

Long non-coding RNAs (lncRNAs) are a group of epigenetic regulators that have been implicated in kidney diseases including acute kidney injury (AKI). However, very little is known about the specific lncRNAs involved in AKI and the mechanisms underlying their pathologic roles. Here, we report a new lncRNA derived from the pseudogene GSTM3P1, which mediates ischemic AKI by interacting with and promoting the degradation of mir-668, a kidney-protective microRNA. GSTM3P1 and its mouse orthologue Gstm2-ps1 were induced by hypoxia in cultured kidney proximal tubular cells. In mouse kidneys, Gstm2-ps1 was significantly upregulated in proximal tubules at an early stage of ischemic AKI. This transient induction of Gstm2-ps1 depends on G3BP1, a key component in stress granules. GSTM3P1 overexpression increased kidney proximal tubular apoptosis after ATP depletion, which was rescued by mir-668. Notably, kidney proximal tubule-specific knockout of Gstm2-ps1 protected mice from ischemic AKI, as evidenced by improved kidney function, diminished tubular damage and apoptosis, and reduced kidney injury biomarker (NGAL) induction. To test the therapeutic potential, Gstm2-ps1 siRNAs were introduced into cultured mouse proximal tubular cells or administered to mice. In cultured cells, Gstm2-ps1 knockdown suppressed ATP depletion-associated apoptosis. In mice, Gstm2-ps1 knockdown ameliorated ischemic AKI. Mechanistically, both GSTM3P1 and Gstm2-ps1 possessed mir-668 binding sites and downregulated the mature form of mir-668. Specifically, GSTM3P1 directly bound to mature mir-668 to induce its decay via target-directed microRNA degradation. Thus, our results identify GSTM3P1 as a novel lncRNA that promotes kidney tubular cell death in AKI by binding mir-668 to inducing its degradation.

What goes up must come down: off switches for regulatory RNAs

Small RNAs base pair with and regulate mRNA translation and stability. For both bacterial small regulatory RNAs and eukaryotic microRNAs, association with partner proteins is critical for the stability and function of the regulatory RNAs. We review the mechanisms for degradation of these RNAs: displacement of the regulatory RNA from its protein partner (in bacteria) or destruction of the protein and its associated microRNAs (in eukaryotes). These mechanisms can allow specific destruction of a regulatory RNA via pairing with a decay trigger RNA or function as global off switches by disrupting the stability or function of the protein partner.

Dynamics of miRNA accumulation during C. elegans larval development

Temporally and spatially controlled accumulation underlies the functions of microRNAs (miRNAs) in various developmental processes. In Caenorhabditis elegans, this is exemplified by the temporal patterning miRNAs lin-4 and let-7, but for most miRNAs, developmental expression patterns remain poorly resolved. Indeed, experimentally observed long half-lives may constrain possible dynamics. Here, we profile miRNA expression throughout C. elegans postembryonic development at high temporal resolution, which identifies dynamically expressed miRNAs. We use mathematical models to explore the underlying mechanisms. For let-7, we can explain, and experimentally confirm, a striking stepwise accumulation pattern through a combination of rhythmic transcription and stage-specific regulation of precursor processing by the RNA-binding protein LIN-28. By contrast, the dynamics of several other miRNAs cannot be explained by regulation of production rates alone. Specifically, we show that a combination of oscillatory transcription and rhythmic decay drive rhythmic accumulation of miR-235, orthologous to miR-92 in other animals. We demonstrate that decay of miR-235 and additional miRNAs depends on EBAX-1, previously implicated in target-directed miRNA degradation (TDMD). Taken together, our results provide insight into dynamic miRNA decay and establish a resource to studying both the developmental functions of, and the regulatory mechanisms acting on, miRNAs.

Catalytic residues of microRNA Argonautes play a modest role in microRNA star strand destabilization in C. elegans

Many microRNA (miRNA)-guided Argonaute proteins can cleave RNA ('slicing'), even though miRNA-mediated target repression is generally cleavage-independent. Here we use Caenorhabditis elegans to examine the role of catalytic residues of miRNA Argonautes in organismal development. In contrast to previous work, mutations in presumed catalytic residues did not interfere with development when introduced by CRISPR. We find that unwinding and decay of miRNA star strands is weakly defective in the catalytic residue mutants, with the largest effect observed in embryos. Argonaute-Like Gene 2 (ALG-2) is more dependent on catalytic residues for unwinding than ALG-1. The miRNAs that displayed the greatest (albeit minor) dependence on catalytic residues for unwinding tend to form stable duplexes with their star strand, and in some cases, lowering duplex stability alleviates dependence on catalytic residues. While a few miRNA guide strands are reduced in the mutant background, the basis of this is unclear since changes were not dependent on EBAX-1, an effector of Target-Directed miRNA Degradation (TDMD). Overall, this work defines a role for the catalytic residues of miRNA Argonautes in star strand decay; future work should examine whether this role contributes to the selection pressure to conserve catalytic activity of miRNA Argonautes across the metazoan phylogeny.

Influence of RNA circularity on Target RNA-Directed MicroRNA Degradation

A subset of circular RNAs (circRNAs) and linear RNAs have been proposed to 'sponge' or block microRNA activity. Additionally, certain RNAs induce microRNA destruction through the process of Target RNA-Directed MicroRNA Degradation (TDMD), but whether both linear and circular transcripts are equivalent in driving TDMD is unknown. Here, we studied whether circular/linear topology of endogenous and artificial RNA targets affects TDMD. Consistent with previous knowledge that Cdr1as (ciRS-7) circular RNA protects miR-7 from Cyrano-mediated TDMD, we demonstrate that depletion of Cdr1as reduces miR-7 abundance. In contrast, overexpression of an artificial linear version of Cdr1as drives miR-7 degradation. Using plasmids that express a circRNA with minimal co-expressed cognate linear RNA, we show differential effects on TDMD that cannot be attributed to the nucleotide sequence, as the TDMD properties of a sequence often differ when in a circular versus linear form. By analysing RNA sequencing data of a neuron differentiation system, we further detect potential effects of circRNAs on microRNA stability. Our results support the view that RNA circularity influences TDMD, either enhancing or inhibiting it on specific microRNAs.

Convergent and divergent evolution of microRNA-mediated regulation in metazoans

The evolution of microRNAs (miRNAs) has been studied extensively to understand their roles in gene regulation and evolutionary processes. This review focuses on how miRNA-mediated regulation has evolved in bilaterian animals, highlighting both convergent and divergent evolution. Since animals and plants display significant differences in miRNA biogenesis and target recognition, the 'independent origin' hypothesis proposes that miRNA pathways in these groups independently evolved from the RNA interference (RNAi) pathway, leading to modern miRNA repertoires through convergent evolution. However, recent evidence raises the alternative possibility that the miRNA pathway might have already existed in the last common ancestor of eukaryotes, and that the differences in miRNA pathway and miRNA repertoires among animal and plant lineages arise from lineage-specific innovations and losses of miRNA pathways, miRNA acquisition, and loss of miRNAs after eukaryotic divergence. The repertoire of miRNAs has considerably expanded during bilaterian evolution, primarily through de novo creation and duplication processes, generating new miRNAs. Although ancient functionally established miRNAs are rarely lost, many newly emerged miRNAs are transient and lineage specific, following a birth-death evolutionary pattern aligning with the 'out-of-the-testis' and 'transcriptional control' hypotheses. Our focus then shifts to the convergent molecular evolution of miRNAs. We summarize how miRNA clustering and seed mimicry contribute to this phenomenon, and we review how miRNAs from different sources converge to degrade maternal messenger RNAs (mRNAs) during animal development. Additionally, we describe how miRNAs evolve across species due to changes in sequence, seed shifting, arm switching, and spatiotemporal expression patterns, which can result in variations in target sites among orthologous miRNAs across distant strains or species. We also provide a summary of the current understanding regarding how the target sites of orthologous miRNAs can vary across strains or distantly related species. Although many paralogous miRNAs retain their seed or mature sequences after duplication, alterations can occur in the seed or mature sequences or expression patterns of paralogous miRNAs, leading to functional diversification. We discuss our current understanding of the functional divergence between duplicated miRNAs, and illustrate how the functional diversification of duplicated miRNAs impacts target site evolution. By investigating these topics, we aim to enhance our current understanding of the functions and evolutionary dynamics of miRNAs. Additionally, we shed light on the existing challenges in miRNA evolutionary studies, particularly the complexity of deciphering the role of miRNA-mediated regulatory network evolution in shaping gene expression divergence and phenotypic differences among species.

Target-directed microRNA degradation: Mechanisms, significance, and functional implications

MicroRNAs (miRNAs) are small non-coding RNAs that play a fundamental role in enabling miRNA-mediated target repression, a post-transcriptional gene regulatory mechanism preserved across metazoans. Loss of certain animal miRNA genes can lead to developmental abnormalities, disease, and various degrees of embryonic lethality. These short RNAs normally guide Argonaute (AGO) proteins to target RNAs, which are in turn translationally repressed and destabilized, silencing the target to fine-tune gene expression and maintain cellular homeostasis. Delineating miRNA-mediated target decay has been thoroughly examined in thousands of studies, yet despite these exhaustive studies, comparatively less is known about how and why miRNAs are directed for decay. Several key observations over the years have noted instances of rapid miRNA turnover, suggesting endogenous means for animals to induce miRNA degradation. Recently, it was revealed that certain targets, so-called target-directed miRNA degradation (TDMD) triggers, can "trigger" miRNA decay through inducing proteolysis of AGO and thereby the bound miRNA. This process is mediated in animals via the ZSWIM8 ubiquitin ligase complex, which is recruited to AGO during engagement with triggers. Since its discovery, several studies have identified that ZSWIM8 and TDMD are indispensable for proper animal development. Given the rapid expansion of this field of study, here, we summarize the key findings that have led to and followed the discovery of ZSWIM8-dependent TDMD. This article is categorized under: Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA in Disease and Development > RNA in Development.

To kill a microRNA: emerging concepts in target-directed microRNA degradation

MicroRNAs (miRNAs) guide Argonaute (AGO) proteins to bind mRNA targets. Although most targets are destabilized by miRNA-AGO binding, some targets induce degradation of the miRNA instead. These special targets are also referred to as trigger RNAs. All triggers identified thus far have binding sites with greater complementarity to the miRNA than typical target sites. Target-directed miRNA degradation (TDMD) occurs when trigger RNAs bind the miRNA-AGO complex and recruit the ZSWIM8 E3 ubiquitin ligase, leading to AGO ubiquitination and proteolysis and subsequent miRNA destruction. More than 100 different miRNAs are regulated by ZSWIM8 in bilaterian animals, and hundreds of trigger RNAs have been predicted computationally. Disruption of individual trigger RNAs or ZSWIM8 has uncovered important developmental and physiologic roles for TDMD across a variety of model organisms and cell types. In this review, we highlight recent progress in understanding the mechanistic basis and functions of TDMD, describe common features of trigger RNAs, outline best practices for validating trigger RNAs, and discuss outstanding questions in the field.

A conserved nutrient responsive axis mediates autophagic degradation of miRNA-mRNA hybrids in blood cell progenitors

In animals, microRNAs are amongst the primary non-coding RNAs involved in regulating the gene expression of a cell. Most mRNAs in a cell are targeted by one or many miRNAs. Although several mechanisms can be attributed to the degradation of miRNA and mRNA within a cell, but the involvement of autophagy in the clearance of miRNA and its target mRNA is not known. We discover a leucine-responsive axis in blood cell progenitors that can mediate an autophagy-directed degradation of miRNA-bound mRNA in Drosophila melanogaster and Homo sapiens. This previously unknown miRNA clearance axis is activated upon amino acid deprivation that can traffic miRNA-mRNA-loaded Argonaute for autophagic degradation in a p62-dependent manner. Thus, our research not only reports a novel axis that can address the turnover of a catalytically active miRISC but also elucidates a slicer-independent mechanism through which autophagy can selectively initiate the clearance of target mRNA.

The catalytic activity of microRNA Argonautes plays a modest role in microRNA star strand destabilization in C. elegans

Many Argonaute proteins can cleave RNA ("slicing") as part of the microRNA-induced silencing complex (miRISC), even though miRNA-mediated target repression is generally independent of target cleavage. Here we use genome editing in C. elegans to examine the role of miRNA-guided slicing in organismal development. In contrast to previous work, slicing-inactivating mutations did not interfere with normal development when introduced by CRISPR. We find that unwinding and decay of miRNA star strands is weakly defective in the absence of slicing, with the largest effect observed in embryos. Argonaute-Like Gene 2 (ALG-2) is more dependent on slicing for unwinding than ALG-1. The miRNAs that displayed the greatest (albeit minor) dependence on slicing for unwinding tend to form stable duplexes with their star strand, and in some cases, lowering duplex stability alleviates dependence on slicing. Gene expression changes were consistent with negligible to moderate loss of function for miRNA guides whose star strand was upregulated, suggesting a reduced proportion of mature miRISC in slicing mutants. While a few miRNA guide strands are reduced in the mutant background, the basis of this is unclear since changes were not dependent on EBAX-1, a factor in the Target-Directed miRNA Degradation (TDMD) pathway. Overall, this work defines a role for miRNA Argonaute slicing in star strand decay; future work should examine whether this role could have contributed to the selection pressure to conserve catalytic activity of miRNA Argonautes across the metazoan phylogeny.

Representation of non-coding RNA-mediated regulation of gene expression using the Gene Ontology

Regulatory non-coding RNAs (ncRNAs) are increasingly recognized as integral to the control of biological processes. This is often through the targeted regulation of mRNA expression, but this is by no means the only mechanism through which regulatory ncRNAs act. The Gene Ontology (GO) has long been used for the systematic annotation of protein-coding and ncRNA gene function, but rapid progress in the understanding of ncRNAs meant that the ontology needed to be revised to accurately reflect current knowledge. Here, a targeted effort to revise GO terms used for the annotation of regulatory ncRNAs is described, focusing on microRNAs (miRNAs), long non-coding RNAs (lncRNAs), small interfering RNAs (siRNAs) and PIWI-interacting RNAs (piRNAs). This paper provides guidance to biocurators annotating ncRNA-mediated processes using the GO and serves as background for researchers wishing to make use of the GO in their studies of ncRNAs and the biological processes they regulate.

Recent progress in miRNA biogenesis and decay

MicroRNAs are a class of small regulatory RNAs that mediate regulation of protein synthesis by recognizing sequence elements in mRNAs. MicroRNAs are processed through a series of steps starting from transcription and primary processing in the nucleus to precursor processing and mature function in the cytoplasm. It is also in the cytoplasm where levels of mature microRNAs can be modulated through decay mechanisms. Here, we review the recent progress in the lifetime of a microRNA at all steps required for maintaining their homoeostasis. The increasing knowledge about microRNA regulation upholds great promise as therapeutic targets.

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.

Post-transcriptional gene silencing in a dynamic RNP world

MicroRNA (miRNA)-guided gene silencing is a key regulatory process in various organisms and linked to many human diseases. MiRNAs are processed from precursor molecules and associate with Argonaute proteins to repress the expression of complementary target mRNAs. Excellent work by numerous labs has contributed to a detailed understanding of the mechanisms of miRNA function. However, miRNA effects have mostly been analyzed and viewed as isolated events and their natural environment as part of complex RNA-protein particles (RNPs) is often neglected. RNA binding proteins (RBPs) regulate key enzymes of the miRNA processing machinery and furthermore RBPs or readers of RNA modifications may modulate miRNA activity on mRNAs. Such proteins may function similarly to miRNAs and add their own contributions to the overall expression level of a particular gene. Therefore, post-transcriptional gene regulation might be more the sum of individual regulatory events and should be viewed as part of a dynamic and complex RNP world.

ZSWIM8 destabilizes many murine microRNAs and is required for proper embryonic growth and development

MicroRNAs (miRNAs) pair to sites in mRNAs to direct the degradation of these RNA transcripts. Conversely, certain RNA transcripts can direct the degradation of particular miRNAs. This target-directed miRNA degradation (TDMD) requires the ZSWIM8 E3 ubiquitin ligase. Here, we report the function of ZSWIM8 in the mouse embryo. Zswim8 -/- embryos were smaller than their littermates and died near the time of birth. This highly penetrant perinatal lethality was apparently caused by a lung sacculation defect attributed to failed maturation of alveolar epithelial cells. Some mutant individuals also had heart ventricular septal defects. These developmental abnormalities were accompanied by aberrant accumulation of more than 50 miRNAs observed across 12 tissues, which often led to enhanced repression of their mRNA targets. These ZSWIM8-sensitive miRNAs were preferentially produced from genomic miRNA clusters, and in some cases, ZSWIM8 caused a switch in the dominant strand or isoform that accumulated from a miRNA hairpin-observations suggesting that TDMD provides a mechanism to uncouple coproduced miRNAs from each other. Overall, our findings indicate that the regulatory influence of ZSWIM8, and presumably TDMD, in mammalian biology is widespread and consequential, and posit the existence of many yet-unidentified transcripts that trigger miRNA degradation.

Target-directed microRNA degradation regulates developmental microRNA expression and embryonic growth in mammals

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play critical roles in development and disease. Target-directed miRNA degradation (TDMD), a pathway in which miRNAs that bind to specialized targets with extensive complementarity are rapidly decayed, has emerged as a potent mechanism of controlling miRNA levels. Nevertheless, the biological role and scope of miRNA regulation by TDMD in mammals remains poorly understood. To address these questions, we generated mice with constitutive or conditional deletion of Zswim8, which encodes an essential TDMD factor. Loss of Zswim8 resulted in developmental defects in the heart and lungs, growth restriction, and perinatal lethality. Small RNA sequencing of embryonic tissues revealed widespread miRNA regulation by TDMD and greatly expanded the known catalog of miRNAs regulated by this pathway. These experiments also uncovered novel features of TDMD-regulated miRNAs, including their enrichment in cotranscribed clusters and examples in which TDMD underlies "arm switching," a phenomenon wherein the dominant strand of a miRNA precursor changes in different tissues or conditions. Importantly, deletion of two miRNAs, miR-322 and miR-503, rescued growth of Zswim8-null embryos, directly implicating the TDMD pathway as a regulator of mammalian body size. These data illuminate the broad landscape and developmental role of TDMD in mammals.

Target-directed microRNA degradation regulates developmental microRNA expression and embryonic growth in mammals

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play critical roles in development and disease. Target-directed miRNA degradation (TDMD), a pathway in which miRNAs that bind to specialized targets with extensive complementarity are rapidly decayed, has emerged as a potent mechanism of controlling miRNA levels. Nevertheless, the biological role and scope of miRNA regulation by TDMD in mammals remains poorly understood. To address these questions, we generated mice with constitutive or conditional deletion of Zswim8 , which encodes an essential TDMD factor. Loss of Zswim8 resulted in developmental defects in heart and lung, growth restriction, and perinatal lethality. Small RNA sequencing of embryonic tissues revealed widespread miRNA regulation by TDMD and greatly expanded the known catalog of miRNAs regulated by this pathway. These experiments also uncovered novel features of TDMD-regulated miRNAs, including their enrichment in co-transcribed clusters and examples in which TDMD underlies 'arm switching', a phenomenon wherein the dominant strand of a miRNA precursor changes in different tissues or conditions. Importantly, deletion of two miRNAs, miR-322 and miR-503, rescued growth of Zswim8 null embryos, directly implicating the TDMD pathway as a regulator of mammalian body size. These data illuminate the broad landscape and developmental role of TDMD in mammals.

Screening of Drosophila microRNA-degradation sequences reveals Argonaute1 mRNA's role in regulating miR-999

MicroRNAs (miRNA) load onto AGO proteins to target mRNAs for translational repression or degradation. However, miRNA degradation can be triggered when extensively base-paired with target RNAs, which induces confirmational change of AGO and recruitment of ZSWIM8 ubiquitin ligase to mark AGO for proteasomal degradation. This target RNA-directed miRNA degradation (TDMD) mechanism appears to be evolutionarily conserved, but recent studies have focused on mammalian systems. Here, we performed AGO1-CLASH in Drosophila S2 cells, with Dora (ortholog of vertebrate ZSWIM8) knockout mediated by CRISPR-Cas9 to identify five TDMD triggers (sequences that can induce miRNA degradation). Interestingly, one trigger in the 3' UTR of AGO1 mRNA induces miR-999 degradation. CRISPR-Cas9 knockout of the AGO1 trigger in S2 cells and in Drosophila specifically elevates miR-999, with concurrent repression of the miR-999 targets. AGO1 trigger knockout flies respond poorly to hydrogen peroxide-induced stress, demonstrating the physiological importance of this TDMD event.

Profiling microRNA from peripheral blood mononuclear cells in early-onset familial Alzheimer's disease

MicroRNAs (miRNAs) refer to short in-length, noncoding RNAs that regulate numerous cellular functions by targeting mRNA, and numerous types of research have shown that miRNA is vitalin Alzheimer's disease. For identifying differentially expressed miRNAs in the peripheral blood mononuclear cell (PBMC) of early-onset familial Alzheimer's disease (EOFAD), we conducted this study which might give a reference for potential therapeutic targets or biomarkers for this disease. On the basis of high-throughput sequencing, we screened the miRNAs expression profiles in PBMC regarding both EOFAD patients and healthy controls, and the biological information was analyzed. Compared with the PBMC of healthy controls, 142 miRNAs were differentially expressed in EOFAD patients ( P  < 0.05), including 48 significantly differentially expressed miRNAs, 37 of which were significantly upregulated, including miR-3614-5p, miR-193A-5p, miR-2115-5p, miR-143-3p, etc. and 11 were significantly downregulated, including miR-484, miR-708-5p, miR-205-5p, miR-31-5p, etc. According to biological information analysis, 768 miRNA target genes were differentially expressed, which may be involved in multiple gene functions and cell cycle, cell senescence, and several signaling pathways, including FoxO, MAPK, Ras, mTOR, neurotrophin, etc. There are differential expressions of miRNAs in PBMC of EOFAD patients and controls, revealing their importance in Alzheimer's disease as indicated by co-expression network analysis; this may support basic information for new biomarkers or treatment exploring.

Noncoding RNA Regulation of Hormonal and Metabolic Systems in the Fruit Fly Drosophila

The importance of RNAs is commonly recognised thanks to protein-coding RNAs, whereas non-coding RNAs (ncRNAs) were conventionally regarded as 'junk'. In the last decade, ncRNAs' significance and roles are becoming noticeable in various biological activities, including those in hormonal and metabolic regulation. Among the ncRNAs: microRNA (miRNA) is a small RNA transcript with ~20 nucleotides in length; long non-coding RNA (lncRNA) is an RNA transcript with >200 nucleotides; and circular RNA (circRNA) is derived from back-splicing of pre-mRNA. These ncRNAs can regulate gene expression levels at epigenetic, transcriptional, and post-transcriptional levels through various mechanisms in insects. A better understanding of these crucial regulators is essential to both basic and applied entomology. In this review, we intend to summarise and discuss the current understanding and knowledge of miRNA, lncRNA, and circRNA in the best-studied insect model, the fruit fly Drosophila.

Network Regulation of microRNA Biogenesis and Target Interaction

MicroRNAs (miRNAs) are versatile, post-transcriptional regulators of gene expression. Canonical miRNAs are generated through the two-step DROSHA- and DICER-mediated processing of primary miRNA (pri-miRNA) transcripts with optimal or suboptimal features for DROSHA and DICER cleavage and loading into Argonaute (AGO) proteins, whereas multiple hairpin-structured RNAs are encoded in the genome and could be a source of non-canonical miRNAs. Recent advances in miRNA biogenesis research have revealed details of the structural basis of miRNA processing and cluster assistance mechanisms that facilitate the processing of suboptimal hairpins encoded together with optimal hairpins in polycistronic pri-miRNAs. In addition, a deeper investigation of miRNA-target interaction has provided insights into the complexity of target recognition with distinct outcomes, including target-mediated miRNA degradation (TDMD) and cooperation in target regulation by multiple miRNAs. Therefore, the coordinated or network regulation of both miRNA biogenesis and miRNA-target interaction is prevalent in miRNA biology. Alongside recent advances in the mechanistic investigation of miRNA functions, this review summarizes recent findings regarding the ordered regulation of miRNA biogenesis and miRNA-target interaction.