Arabidopsis ARGONAUTE1 is an RNA Slicer that selectively recruits microRNAs and short interfering RNAs

ARGONAUTE (AGO) RNA-binding proteins are involved in RNA silencing. They bind to short interfering RNAs (siRNAs) and microRNAs (miRNAs) through a conserved PAZ domain, and, in animals, they assemble into a multisubunit RNA-induced silencing complex (RISC). The mammalian AGO2, termed Slicer, directs siRNA- and miRNA-mediated cleavage of a target RNA. In Arabidopsis, there are 10 members of the AGO family, and the AGO1 protein is potentially the Slicer component in different RNA-silencing pathways. Here, we show that AGO1 selectively recruits certain classes of short silencing-related RNA. AGO1 is physically associated with miRNAs, transacting siRNAs, and transgene-derived siRNAs but excludes virus-derived siRNAs and 24-nt siRNAs involved in chromatin silencing. We also show that AGO1 has Slicer activity. It mediates the in vitro cleavage of a mir165 target RNA in a manner that depends on the sequence identity of amino acid residues in the PIWI domain that are predicted by homology with animal Slicer-competent AGO proteins to constitute the RNase catalytic center. However, unlike animals, we find no evidence that AGO1 Slicer is in a high molecular weight RNA-induced silencing complex. The Slicer activity fractionates as a complex of approximately 150 kDa that likely constitutes the AGO1 protein and associated RNA without any other proteins. Based on sequence similarity, we predict that other Arabidopsis AGOs might have a similar catalytic activity but recruit different subsets of siRNAs or miRNAs.     

Stabilization of hepatitis C virus RNA by an Ago2-miR-122 complex

MicroRNAs (miRNAs) are small noncoding RNAs that regulate eukaryotic gene expression by binding to regions of imperfect complementarity in mRNAs, typically in the 3' UTR, recruiting an Argonaute (Ago) protein complex that usually results in translational repression or destabilization of the target RNA. The translation and decay of mRNAs are closely linked, competing processes, and whether the miRNA-induced silencing complex (RISC) acts primarily to reduce translation or stability of the mRNA remains controversial. miR-122 is an abundant, liver-specific miRNA that is an unusual host factor for hepatitis C virus (HCV), an important cause of liver disease in humans. Prior studies show that it binds the 5' UTR of the messenger-sense HCV RNA genome, stimulating translation and promoting genome replication by an unknown mechanism. Here we show that miR-122 binds HCV RNA in association with Ago2 and that this slows decay of the viral genome in infected cells. The stabilizing action of miR-122 does not require the viral RNA to be translationally active nor engaged in replication, and can be functionally substituted by a nonmethylated 5' cap. Our data demonstrate that a RISC-like complex mediates the stability of HCV RNA and suggest that Ago2 and miR-122 act coordinately to protect the viral genome from 5' exonuclease activity of the host mRNA decay machinery. miR-122 thus acts in an unconventional fashion to stabilize HCV RNA and slow its decay, expanding the repertoire of mechanisms by which miRNAs modulate gene expression.     

Type 2 diabetes mellitus‐related genetic polymorphisms in microRNAs and microRNA target sites (MicroRNAs中与2型糖尿病相关的基因多态性及microRNA靶位)

MicroRNAs (miRNAs) are important endogenous regulators in eukaryotic gene expression and a broad range of biological processes. MiRNA‐related genetic variations have been proved to be associated with human diseases, such as type 2 diabetes mellitus (T2DM). Polymorphisms in miRNA genes (primary miRNAs, precursor miRNAs, mature miRNAs, and miRNA regulatory regions) may be involved in the development of T2DM by changing the expression and structure of miRNAs and target gene expression. Genetic polymorphisms of the 3′‐untranslated region (UTR) in miRNA target genes may destroy putative miRNA binding sites or create new miRNA binding sites, which affects the binding of UTRs with miRNAs, finally resulting in susceptibility to and development of T2DM. Therefore, focusing on studies into genetic polymorphisms in miRNAs or miRNA binding sites will help our understanding of the pathophysiology of T2DM development and lead to better health management. Herein, we review the association of genetic polymorphisms in miRNA and miRNA targets genes with T2DM development.     

MicroRNA-based therapeutic approaches in the cardiovascular system

MicroRNAs (miRNAs) are endogenous small ribonucleotides that participate in the orchestration of the genome by regulating target messenger RNA translation. MiRNAs control physiological processes and misexpression of miRNAs is pathogenically involved in many diseases including cardiovascular diseases. Normalization of miRNA expression and thus downstream target networks may have enormous therapeutic chances but also risks. We here highlight and discuss recent advances in the development and use of miRNA therapeutics to target miRNAs in vivo that may translate into novel therapeutic strategies for cardiovascular diseases in the future.     

Human polymorphism at microRNAs and microRNA target sites

MicroRNAs (miRNAs) function as endogenous translational repressors of protein-coding genes in animals by binding to target sites in the 3' UTRs of mRNAs. Because a single nucleotide change in the sequence of a target site can affect miRNA regulation, naturally occurring SNPs in target sites are candidates for functional variation that may be of interest for biomedical applications and evolutionary studies. However, little is known to date about variation among humans at miRNAs and their target sites. In this study, we analyzed publicly available SNP data in context with miRNAs and their target sites throughout the human genome, and we found a relatively low level of variation in functional regions of miRNAs, but an appreciable level of variation at target sites. Approximately 400 SNPs were found at experimentally verified target sites or predicted target sites that are otherwise evolutionarily conserved across mammals. Moreover, approximately 250 SNPs potentially create novel target sites for miRNAs in humans. If some variants have functional effects, they might confer phenotypic differences among humans. Although the majority of these SNPs appear to be evolving under neutrality, interestingly, some of these SNPs are found at relatively high population frequencies even in experimentally verified targets, and a few variants are associated with atypically long-range haplotypes that may have been subject to recent positive selection.     

Extensive 3' modification of plant small RNAs is modulated by helper component-proteinase expression

RNA silencing is an evolutionarily conserved process in eukaryotes that represses gene expression by using 21- to 24-nt guide RNAs to mediate mRNA cleavage or translational inhibition. Plants have two distinct groups of silencing-associated small RNAs (smRNAs): the micro RNAs (miRNAs) and the small interfering RNAs (siRNAs). A recent report by Yu et al. [Yu, B., Yang, Z., Li, J., Minakhina, S., Yang, M., Padgett, R. W., Steward, R. & Chen, X. (2005) Science 307, 932-935] has shown that plant miRNAs are modified at their 3' termini with a methyl group. Here, we show that a large fraction of all silencing-associated smRNAs in tobacco are modified; this modification occurs on the 2' hydroxyl of the terminal ribose and significantly reduces the cloning efficiency of these modified smRNAs. Expression of the strong silencing suppressor P1/helper-component proteinase results in a marked decrease in the 3'-terminal modification of viral siRNAs but does not significantly affect the modification of endogenous miRNAs and 24-nt siRNAs. The differential modification mediated by helper-component proteinase expression implies that exogenous and endogenous smRNAs are processed through independent pathways that are isolated by subcellular compartmentalization and/or the association with distinct Dicer complexes. The degree of terminal modification may play an important role in regulating the extent to which primary smRNA signals can be amplified by RNA-dependent RNA polymerases.     

The mevalonate pathway regulates microRNA activity in Caenorhabditis elegans

The mevalonate pathway is highly conserved and mediates the production of isoprenoids, which feed into biosynthetic pathways for sterols, dolichol, ubiquinone, heme, isopentenyl adenine, and prenylated proteins. We found that in Caenorhabditis elegans, the nonsterol biosynthetic outputs of the mevalonate pathway are required for the activity of microRNAs (miRNAs) in silencing their target mRNAs. Inactivation of genes that mediate multiple steps of the mevalonate pathway causes derepression of several miRNA target genes, with no disruption of the miRNA levels, suggesting a role in miRNA-induced silencing complex activity. Dolichol phosphate, synthesized from the mevalonate pathway, functions as a lipid carrier of the oligosaccharide moiety destined for protein N-linked glycosylation. Inhibition of the dolichol pathway of protein N-glycosylation also causes derepression of miRNA target mRNAs. The proteins that mediate miRNA repression are therefore likely to be regulated by N-glycosylation. Conversely, drugs such as statins, which inhibit the mevalonate pathway, may compromise miRNA repression as well as the more commonly considered cholesterol biosynthesis.     

Platelet microRNA-mRNA coexpression profiles correlate with platelet reactivity

MicroRNAs (miRNAs) regulate cell physiology by altering protein expression, but the biology of platelet miRNAs is largely unexplored. We tested whether platelet miRNA levels were associated with platelet reactivity by genome-wide profiling using platelet RNA from 19 healthy subjects. We found that human platelets express 284 miRNAs. Unsupervised hierarchical clustering of miRNA profiles resulted in 2 groups of subjects that appeared to cluster by platelet aggregation phenotypes. Seventy-four miRNAs were differentially expressed (DE) between subjects grouped according to platelet aggregation to epinephrine, a subset of which predicted the platelet reactivity response. Using whole genome mRNA expression data on these same subjects, we computationally generated a high-priority list of miRNA-mRNA pairs in which the DE platelet miRNAs had binding sites in 3'-untranslated regions of DE mRNAs, and the levels were negatively correlated. Three miRNA-mRNA pairs (miR-200b:PRKAR2B, miR-495:KLHL5, and miR-107:CLOCK) were selected from this list, and all 3 miRNAs knocked down protein expression from the target mRNA. Reduced activation from platelets lacking PRKAR2B supported these findings. In summary, (1) platelet miRNAs are able to repress expression of platelet proteins, (2) miRNA profiles are associated with and may predict platelet reactivity, and (3) bioinformatic approaches can successfully identify functional miRNAs in platelets.     

Inhibition of hepatitis C virus replication using adeno-associated virus vector delivery of an exogenous anti-hepatitis C virus microRNA cluster

RNA interference (RNAi) is being evaluated as an alternative therapeutic strategy for hepatitis C virus (HCV) infection. The use of viral vectors encoding short hairpin RNAs (shRNAs) has been the most common strategy employed to provide sustained expression of RNAi effectors. However, overexpression and incomplete processing of shRNAs has led to saturation of the endogenous miRNA pathway, resulting in toxicity. The use of endogenous microRNAs (miRNAs) as scaffolds for short interfering (siRNAs) may avoid these problems, and miRNA clusters can be engineered to express multiple RNAi effectors, a feature that may prevent RNAi-resistant HCV mutant generation. We exploited the endogenous miRNA-17-92 cluster to generate a polycistronic primary miRNA that is processed into five mature miRNAs that target different regions of the HCV genome. All five anti-HCV miRNAs were active, achieving up to 97% inhibition of Renilla luciferase (RLuc) HCV reporter plasmids. Self-complementary recombinant adeno-associated virus (scAAV) vectors were chosen for therapeutic delivery of the miRNA cluster. Expression of the miRNAs from scAAV inhibited the replication of cell culture-propagated HCV (HCVcc) by 98%, and resulted in up to 93% gene silencing of RLuc-HCV reporter plasmids in mouse liver. No hepatocellular toxicity was observed at scAAV doses as high as 5 × 10(11) vector genomes per mouse, a dose that is approximately five-fold higher than doses of scAAV-shRNA vectors that others have shown previously to be toxic in mouse liver. Conclusion: We have demonstrated that exogenous anti-HCV miRNAs induce gene silencing, and when expressed from scAAV vectors inhibit the replication of HCVcc without inducing toxicity. The combination of an AAV vector delivery system and exploitation of the endogenous RNAi pathway is a potentially viable alternative to current HCV treatment regimens.     

Genome-wide impact of a recently expanded microRNA cluster in mouse

Variations in microRNA (miRNA) gene and/or target repertoire are likely to be key drivers of phenotypic differences between species. To better understand these changes, we developed a computational method that identifies signatures of species-specific target site gain and loss associated with miRNA acquisition. Interestingly, several of the miRNAs implicated in mouse 3' UTR evolution derive from a single rapidly expanded rodent-specific miRNA cluster. Located in the intron of Sfmbt2, a maternally imprinted polycomb gene, these miRNAs (referred to as the Sfmbt2 cluster) are expressed in both embryonic stem cells and the placenta. One abundant miRNA from the cluster, miR-467a, functionally overlaps with the mir-290-295 cluster in promoting growth and survival of mouse embryonic stem cells. Predicted novel targets of the remaining cluster members are enriched in pathways regulating cell survival. Two relevant species-specific target candidates, Lats2 and Dedd2, were validated in cultured cells. We suggest that the rapid evolution of the Sfmbt2 cluster may be a result of intersex conflict for growth regulation in early mammalian development and could provide a general model for the genomic response to acquisition of miRNAs and similar regulatory factors.     

Plant secondary siRNA production determined by microRNA-duplex structure

Processing of microRNA (miRNA) precursors results in the release of a double-stranded miRNA/miRNA* duplex. The miRNA or guide strand, is loaded onto the Argonaute (AGO) effector, and the miRNA* or passenger strand is typically degraded. The loaded AGO-containing RNA-induced silencing complex specifically recognizes a target mRNA, leading to its degradation or translational inhibition. In plants, miRNA-mediated cleavage of a target triggers in some cases the production of secondary small interfering RNAs (siRNAs), which in turn can silence other genes in trans. This alternative pathway depends on the length of the miRNA and the specific AGO in the effector complex. However, 22-nt miRNAs are sufficient, but not essential for this pathway. Using a combination of computational and experimental approaches, we show that transitivity can be triggered when the small RNA that is not retained in AGO is 22-nt long. Moreover, we demonstrate that asymmetrically positioned bulged bases in the miRNA:miRNA* duplex, regardless of miRNA or miRNA* length, are sufficient for the initiation of transitivity. We propose that the RNA-induced silencing complex reprogramming occurs during the early steps of miRNA loading, before the miRNA duplex is disassembled and the guide strand is selected.     

MicroRNA binding to the HIV-1 Gag protein inhibits Gag assembly and virus production

MicroRNAs (miRNAs) are small, 18–22 nt long, noncoding RNAs that act as potent negative gene regulators in a variety of physiological and pathological processes. To repress gene expression, miRNAs are packaged into RNA-induced silencing complexes (RISCs) that target mRNAs for degradation and/or translational repression in a sequence-specific manner. Recently, miRNAs have been shown to also interact with proteins outside RISCs, impacting cellular processes through mechanisms not involving gene silencing. Here, we define a previously unappreciated activity of miRNAs in inhibiting RNA–protein interactions that in the context of HIV-1 biology blocks HIV virus budding and reduces virus infectivity. This occurs by miRNA binding to the nucleocapsid domain of the Gag protein, the main structural component of HIV-1 virions. The resulting miRNA–Gag complexes interfere with viral–RNA-mediated Gag assembly and viral budding at the plasma membrane, with imperfectly assembled Gag complexes endocytosed and delivered to lysosomes. The blockade of virus production by miRNA is reversed by adding the miRNA’s target mRNA and stimulated by depleting Argonaute-2, suggesting that when miRNAs are not mediating gene silencing, they can block HIV-1 production through disruption of Gag assembly on membranes. Overall, our findings have significant implications for understanding how cells modulate HIV-1 infection by miRNA expression and raise the possibility that miRNAs can function to disrupt RNA-mediated protein assembly processes in other cellular contexts.To trigger viral budding, Gag proteins must extensively multimerize at the plasma membrane (PM), forming a tightly packed lattice that remodels into a virus particle containing a few thousand Gag molecules (1). Whereas recruitment of the viral genome by Gag is driven by specific and highly efficient interactions between Gag and the genome’s Psi-element (2, 3), increasing evidence suggests Gag multimerization can also be mediated by nonspecific Gag–RNA interactions. For example, long-stranded RNAs not possessing a Psi-element can drive Gag assembly, serving as a scaffold for concentrating Gag molecules (4–7). In in vitro systems, Gag can multimerize by binding nonspecifically to RNA through its nucleocapsid (NC) domain, with the degree of multimerization proportional to the length of the input RNA (4). Finally, in living cells, Gag binding to either HIV-1 viral RNA or cellular mRNA promotes viral particle assembly (5, 6). The proposed role of nonspecific Gag–RNA interactions in facilitating Gag multimerization and viral budding prompted us to investigate whether miRNAs could prevent viral budding by competing with viral RNA for Gag binding via an unconventional mechanism not involving gene silencing like others have shown (8–11).     

微小RNA与缺血性卒中

微小RNA（microRNA,miRNA）是内源性非编码单链小分子RNA,可通过与mRNA的3＆#39;-非编码区特异性结合引发转录后基因沉默效应来调节mRNA表达.最近的研究显示,miRNA广泛参与了动脉粥样硬化、脑缺血缺氧耐受、脑水肿、神经细胞死亡等多种病理生理学过程,提示miRNA可能作为缺血性卒中早期诊断的生物学标记物以及早期治疗的生物学靶标.文章对miRNA的生物学特性及其与缺血性卒中的关系和作用机制进行了综述,以期为miRNA在缺血性卒中的诊断和治疗方面提供新的依据和思路.