Multilayer checkpoints for microRNA authenticity during RISC assembly

MicroRNAs (miRNAs) function through the RNA-induced silencing complex (RISC), which contains an Argonaute (Ago) protein at the core. RISC assembly follows a two-step pathway: miRNA/miRNA* duplex loading into Ago, and separation of the two strands within Ago. Here we show that the 5' phosphate of the miRNA strand is essential for duplex loading into Ago, whereas the preferred 5' nucleotide of the miRNA strand and the base-pairing status in the seed region and the middle of the 3' region function as additive anchors to Ago. Consequently, the miRNA authenticity is inspected at multiple steps during RISC assembly.     

Arabidopsis ARGONAUTE7 selects miR390 through multiple checkpoints during RISC assembly

Plant ARGONAUTE7 (AGO7) assembles RNA‐induced silencing complex (RISC) specifically with miR390 and regulates the auxin‐signalling pathway via production of TAS3 trans‐acting siRNAs (tasiRNAs). However, how AGO7 discerns miR390 among other miRNAs remains unclear. Here, we show that the 5′ adenosine of miR390 and the central region of miR390/miR390* duplex are critical for the specific interaction with AGO7. Furthermore, despite the existence of mismatches in the seed and central regions of the duplex, cleavage of the miR390* strand is required for maturation of AGO7–RISC. These findings suggest that AGO7 uses multiple checkpoints to select miR390, thereby circumventing promiscuous tasiRNA production.     

The activity of siRNA in mammalian cells is related to the kinetics of siRNA-target recognition in vitro: mechanistic implications

The specificity of siRNA-mediated suppression of gene expression involves base-base recognition between siRNA and its single-stranded RNA target. We investigated the kinetics of this process in vitro by using full-length ICAM-1 target RNA and biologically active and inactive ICAM-1-directed siRNA, respectively. To mimic the situation in living cells we used the well-characterised facilitator of RNA-RNA annealing and strand exchange cetyltrimethylammonium bromide, which increases strongly the kinetics of RNA-RNA interactions at conditions that do not affect RNA structure nor the presumed structure-function relationship. For the biologically active siRNA si2B, we find faster binding, i.e. recognition of the target and a slower backward reaction when compared with the biologically inactive siRNA si1. This is reflected by an approximately 400-fold more favorable equilibrium constant of si2B. Kinetic evidence favors an associative mechanism of recognition of the target strand by the double-stranded siRNA. The minimal model for siRNA-target recognition described here is consistent with the high biological activity of si2B only if one assumes a step subsequent to target recognition, which might be degradation of the target RNA when complexed with the antisense strand of siRNA or when considering rapid destruction of the released sense strand of siRNA.     

RNA interference induced by siRNAs modified with 4'-thioribonucleosides in cultured mammalian cells

Short interfering RNAs (siRNAs) variously modified with 4'-thioribonucleosides against the Photinus luciferase gene were tested for their induction of the RNA interference (RNAi) activity in cultured NIH/3T3 cells. Results indicated that modifications at the sense-strand were well tolerated for RNAi activity except for full modification with 4'-thioribonucleosides. However, the activity of siRNAs modified at the antisense-strand was dependent on the position and the number of modifications with 4'-thioribonucleosides. Since modifications of siRNAs with 4'-thioribonucleosides were well tolerated in RNAi activity compared with that of 2'-O-methyl nucleosides, 4'-thioribonucleosides might be potentially useful in the development of novel and effective chemically modified siRNAs.     

Polerovirus protein P0 prevents the assembly of small RNA‐containing RISC complexes and leads to degradation of ARGONAUTE1

RNA silencing plays an important role in plants in defence against viruses. To overcome this defence, plant viruses encode suppressors of RNA silencing. The most common mode of silencing suppression is sequestration of double‐stranded RNAs involved in the antiviral silencing pathways. Viral suppressors can also overcome silencing responses through protein–protein interaction. The poleroviral P0 silencing suppressor protein targets ARGONAUTE (AGO) proteins for degradation. AGO proteins are the core component of the RNA‐induced silencing complex (RISC). We found that P0 does not interfere with the slicer activity of pre‐programmed siRNA/miRNA containing AGO1, but prevents de novo formation of siRNA/miRNA containing AGO1. We show that the AGO1 protein is part of a high‐molecular‐weight complex, suggesting the existence of a multi‐protein RISC in plants. We propose that P0 prevents RISC assembly by interacting with one of its protein components, thus inhibiting formation of siRNA/miRNA–RISC, and ultimately leading to AGO1 degradation. Our findings also suggest that siRNAs enhance the stability of co‐expressed AGO1 in both the presence and absence of P0.     

A retroviral vector for siRNA expression in mammalian cells

Utilization of RNA interference (RNAi) for knockdown of gene expression has become a standard tool for the study of gene function. Short hairpin RNAs (shRNAs) expressed from RNA polymerase III promoters are widely used to achieve stable knockdown of gene expression by RNAi. We have constructed a retroviral-based shRNA expression vector, pSiRPG, as a tool for shRNA-based functional genomic studies. This vector is based on a widely used shRNA expression system and was modified to harbor an enhanced green fluorescent protein (EGFP) and a puromycin selection marker. The functionality of the elements in the pSiRPG vector was validated. The H1(TetO₂) promoter in the vector facilitates doxycycline-inducible shRNA expression, which was demonstrated in cells expressing the Tet repressor (TetR). However, we also demonstrated limited efficiency of the inhibition of shRNA expression in an uninduced TetR-expressing cell line. This observation strongly indicates that the H1(TetO₂) promoter, which is used in a wide range of vectors, is not optimal for tightly regulated shRNA expression. Stable repression of the NDRG1 protein level was observed when introducing pSiRPG constructs expressing shRNAs targeting NDRG1 into two mammary epithelial cell lines by retroviral delivery. This vector should therefore facilitate functional studies in breast cell lines that are hard to transfect with conventional plasmid-based methods.     

Effects of ATP and cyclic AMP on the in vitro assembly and stability of mammalian brain microtubules

The relevance of protein phosphorylation, transphosphorylation and binding phenomena in the kinetics of the ATP-induced assembly of cycle-purified microtubule protein from mammalian brain were studied. ATP was able to induce the polymerization of microtubules of normal appearance. However, the assembled structures, were unstable and microtubules depolymerized after achievement of a transitory maximum. Cyclic AMP reduced the amplitude of the polymerization maximum in a concentration-dependent manner, correlating with the stimulation of the endogenous phosphorylation reaction. When microtubule assembly was induced by GTP, in the presence of various concentrations of ATP, the slope of the depolymerization phase was found to depend on the concentration of ATP. Fluoride ion inhibited the endogenous phosphorylation reaction and reduced the disassembly rate, in a concentration-dependent manner. Evidence is also presented indicating that ATP did not bind to phosphocellulose-purified tubulin. These results further contribute to indicate that ATP and cyclic AMP, acting coordinately to control the phosphorylation extent of microtubule proteins are important factors to determine microtubule stability within the cell. Some implications of this mechanism for the regulation by cAMP of the initiation of DNA synthesis and mitosis are considered.Abbreviations MAPs microtubule-associated proteins - MAP2 microtubule-associated protein 2, Mes-4-morpholinoethanesulfonic acid - EGTA ethylene-glycol bis (-aminoethyl ether)N,N,N,N-tetraacetic acid - PMSF phenylmethylsulfonyl fluoride - PEI polyethyleneimine - PC phosphocellulose - DEAE Diethylaminoethyl - SDS-PAGE polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate     

Dicer is dispensable for asymmetric RISC loading in mammals

In flies, asymmetric loading of small RNA duplexes into Argonaute2-containing RNA-induced silencing complex (Ago2-RISC) requires Dicer-2/R2D2 heterodimer, which acts as a protein sensor for the thermodynamic stabilities of the ends of small RNA duplexes. However, the mechanism of small RNA asymmetry sensing in mammalian RISC assembly remains obscure. Here, we quantitatively examined RISC assembly and target silencing activity in the presence or absence of Dicer in mammals. Our data show that, unlike the well-characterized fly Ago2-RISC assembly pathway, mammalian Dicer is dispensable for asymmetric RISC loading in vivo and in vitro.     

Cleavage of the siRNA passenger strand during RISC assembly in human cells

A crucial step in the RNA interference (RNAi) pathway involves the assembly of RISC, the RNA-induced silencing complex. RISC initially recognizes a double-stranded short interfering RNA (siRNA), but only one strand is finally retained in the functional ribonucleoprotein complex. The non-incorporated strand, or 'passenger' strand, is removed during the assembly process and most probably degraded thereafter. In this report, we show that the passenger strand is cleaved during the course of RISC assembly following the same rules established for the siRNA-guided cleavage of a target RNA. Chemical modifications impairing the cleavage of the passenger strand also impair the cleavage of a target RNA in vitro as well as the silencing of a reporter gene in vivo, suggesting that passenger strand removal is facilitated by its cleavage during RISC assembly. Interestingly, target RNA cleavage can be rescued if an otherwise non-cleavable passenger strand shows a nick at the scissile phosphodiester bond, which further indicates that the cleavage event per se is not essential.     

RNA干涉在提高植物营养价值上的应用

RNA干涉(RNAi)是由具同源性的外源dsRNA引起的序列特异性转录后基因沉默现象,广泛存在于各种动、植物中。人类大多数疾病,像心脏病和癌症,都能够通过食用添加具有特殊营养成分养的饮食来预防。应用RNAi技术可为人和动物提高植物的营养价值。     

Wig1 prevents cellular senescence by regulating p21 mRNA decay through control of RISC recruitment

Premature senescence, a key strategy used to suppress carcinogenesis, can be driven by p53/p21 proteins in response to various stresses. Here, we demonstrate that Wig1 plays a critical role in this process through regulation of p21 mRNA stability. Wig1 controls the association of Argonaute2 (Ago2), a central component of the RNA‐induced silencing complex (RISC), with target p21 mRNA via binding of the stem‐loop structure near the microRNA (miRNA) target site. Depletion of Wig1 prohibited miRNA‐mediated p21 mRNA decay and resulted in premature senescence. Wig1 plays an essential role in cell proliferation, as demonstrated in tumour xenografts in mice, and Wig1 and p21 mRNA levels are inversely correlated in human normal and cancer tissues. Together, our data indicate a novel role of Wig1 in RISC target accessibility, which is a key step in RNA‐mediated gene silencing. In addition, these findings indicate that fine‐tuning of p21 levels by Wig1 is essential for the prevention of cellular senescence.     

Dissecting beta-ring assembly pathway of the mammalian 20S proteasome

The 20S proteasome is the catalytic core of the 26S proteasome. It comprises four stacked rings of seven subunits each, alpha(1-7)beta(1-7)beta(1-7)alpha(1-7). Recent studies indicated that proteasome-specific chaperones and beta-subunit appendages assist in the formation of alpha-rings and dimerization of half-proteasomes, but the process involved in the assembly of beta-rings is poorly understood. Here, we clarify the mechanism of beta-ring formation on alpha-rings by characterizing assembly intermediates accumulated in cells depleted of each beta-subunit. Starting from beta2, incorporation of beta-subunits occurs in an orderly manner dependent on the propeptides of beta2 and beta5, and the C-terminal tail of beta2. Unexpectedly, hUmp1, a chaperone functioning at the final assembly step, is incorporated as early as beta2 and is required for the structural integrity of early assembly intermediates. We propose a model in which beta-ring formation is assisted by both intramolecular and extrinsic chaperones, whose roles are partially different between yeast and mammals.     

Rotenone inhibition of spindle microtubule assembly in mammalian cells

Rotenone, a potent inhibitor of mitochondrial respiration is also an effective antimitotic agent. The addition of either rotenone or Colcemid to exponentially growing Chinese hamster ovary cells resulted in a dramatic increase in mitotic index after 90 min. When the cultures were washed free of the drugs, mitosis was completed and the cells progressed into G 1 at approximately the same rate. Further similarity of rotenone-arrested cells to Colcemid-induced mitotic inhibition was apparent at the ultrastructural level. Mitotic cells treated by either drug contained monopolar spindles with chromosomes grouped around centriole pairs near the cell center. Occasional microtubules were seen near the kinetochore and centrioles. These observations, along with the fact that rotenone inhibited the binding of ³H-colchicine to isolated bovine brain tubulin, suggested that rotenone inhibited mitosis by binding directly to tubulin to prevent microtubule assembly.     

Conditional gene silencing in mammalian cells mediated by a stress-inducible promoter

MicroRNAs (miRNAs) are endogenous non-coding small RNAs, which negatively regulate gene expression in a sequence-specific manner through the RNA interference (RNAi) pathway. Here we describe a new miRNA-based conditional RNAi expression system that relies on cellular stress-response mechanisms in mammalian cells. In our constructs, expression of miRNA mimics is tightly controlled by a heat shock-inducible promoter. This system is highly effective in silencing permanently or conditionally expressed luciferase. The stress inducible vectors also effectively deplete co-expressed pro-apoptotic protein CHOP with heat shock. Furthermore, we demonstrate cloning of a protein-coding sequence between the stress-inducible promoter and the miRNA expression cassette allows simultaneous silencing of a target gene and activation of synthesis of a protein of choice in response to stress stimulation. This new conditional gene silencing approach could be an invaluable tool for various areas of basic and applied research and for therapeutic intervention.     

A Tetrahymena Hsp90 co‐chaperone promotes siRNA loading by ATP‐dependent and ATP‐independent mechanisms

The loading of small interfering RNAs (siRNAs) and microRNAs into Argonaute proteins is enhanced by Hsp90 and ATP in diverse eukaryotes. However, whether this loading also occurs independently of Hsp90 and ATP remains unclear. We show that the Tetrahymena Hsp90 co‐chaperone Coi12p promotes siRNA loading into the Argonaute protein Twi1p in both ATP‐dependent and ATP‐independent manners in vitro. The ATP‐dependent activity requires Hsp90 and the tetratricopeptide repeat (TPR) domain of Coi12p, whereas these factors are dispensable for the ATP‐independent activity. Both activities facilitate siRNA loading by counteracting the Twi1p‐binding protein Giw1p, which is important to specifically sort the 26‐ to 32‐nt siRNAs to Twi1p. Although Coi12p lacking its TPR domain does not bind to Hsp90, it can partially restore the siRNA loading and DNA elimination defects of COI12 knockout cells, suggesting that Hsp90‐ and ATP‐independent loading of siRNA occurs in vivo and plays a physiological role in Tetrahymena.     

Quantitative evaluation of siRNA delivery in vivo

Effective small interfering RNA (siRNA)-mediated therapeutics require the siRNA to be delivered into the cellular RNA-induced silencing complex (RISC). Quantitative information of this essential delivery step is currently inferred from the efficacy of gene silencing and siRNA uptake in the tissue. Here we report an approach to directly quantify siRNA in the RISC in rodents and monkey. This is achieved by specific immunoprecipitation of the RISC from tissue lysates and quantification of small RNAs in the immunoprecipitates by stem-loop PCR. The method, expected to be independent of delivery vehicle and target, is label-free, and the throughput is acceptable for preclinical animal studies. We characterized a lipid-formulated siRNA by integrating these approaches and obtained a quantitative perspective on siRNA tissue accumulation, RISC loading, and gene silencing. The described methodologies have utility for the study of silencing mechanism, the development of siRNA therapeutics, and clinical trial design.     

Network spandrels reflect ecological assembly

Ecological networks that exhibit stable dynamics should theoretically persist longer than those that fluctuate wildly. Thus, network structures which are over‐represented in natural systems are often hypothesised to be either a cause or consequence of ecological stability. Rarely considered, however, is that these network structures can also be by‐products of the processes that determine how new species attempt to join the community. Using a simulation approach in tandem with key results from random matrix theory, we illustrate how historical assembly mechanisms alter the structure of ecological networks. We demonstrate that different community assembly scenarios can lead to the emergence of structures that are often interpreted as evidence of ‘selection for stability’. However, by controlling for the underlying selection pressures, we show that these assembly artefacts—or spandrels—are completely unrelated to stability or selection, and are instead by‐products of how new species are introduced into the system. We propose that these network‐assembly spandrels are critically overlooked aspects of network theory and stability analysis, and we illustrate how a failure to adequately account for historical assembly can lead to incorrect inference about the causes and consequences of ecological stability.     

The errors in assembly of MuLV in interferon treated cells

Interferon treatment of JLSV-6 cells chronically infected with Rauscher MuLV leads to the formation of noninfectious particles (interferon virions) containing the structural proteins of env and gag genes as well as additional viral polypeptides. In the control virions the major glycoprotein detected is gp71, interferon virions contain in addition to gp71 and 85k dalton (gp85) glucosamine-containing, fucose-deficient glycoprotein which is recognized by antiserum to MuLV but not by the gp71 antiserum. The surface iodination of the intact virions indicates that both gp71 and gp85 are the major components of the external virions envelope. However, unlike the control virions in which gp71 associates with p15E (gp90), the gp71-p15E complex was not detected in interferon virions. The analysis of the iodinated proteins of the disrupted interferon virions revealed the presence of 85k and 65k dalton polypeptides preciptable with antiserum against MuLV, which are not present in the control virions. The difference in the polypeptide pattern of virions produced in the presence of interferon does not seem to be a consequence of the slowdown in the synthesis of viral proteins or their processing in the interferon-treated cells. Both the structural proteins of env and gag genes seem to be synthesized and processed at a comparable rate in the interferon-treated and -untreated cells. These results indicate an alteration of virus assembly in the presence of interferon.