The Host Nonsense-Mediated mRNA Decay Pathway Restricts Mammalian RNA Virus Replication

In addition to classically defined immune mechanisms, cell-intrinsic processes can restrict virus infection and have shaped virus evolution. The details of this virus-host interaction are still emerging. Following a genome-wide siRNA screen for host factors affecting replication of Semliki Forest virus (SFV), a positive-strand RNA (+RNA) virus, we found that depletion of nonsense-mediated mRNA decay (NMD) pathway components Upf1, Smg5, and Smg7 led to increased levels of viral proteins and RNA and higher titers of released virus. The inhibitory effect of NMD was stronger when virus replication efficiency was impaired by mutations or deletions in the replicase proteins. Consequently, depletion of NMD components resulted in a more than 20-fold increase in production of these attenuated viruses. These findings indicate that a cellular mRNA quality control mechanism serves as an intrinsic barrier to the translation of early viral proteins and the amplification of +RNA viruses in animal cells.

The synthesis and application of a diazirine-modified uridine analogue for investigating RNA-protein interactions

The roles played by many ncRNAs remain largely unknown. Similarly, relatively little is known about the RNA binding proteins involved in processing ncRNA. Identification of new RNA/RNA binding protein (RBP) interactions may pave the way to gain a better understanding of the complex events occurring within cells during gene expression and ncRNA biogenesis. The development of chemical tools for the isolation of RBPs is of paramount importance. In this context, we report on the synthesis of the uridine phosphoramidite U Dz that bears a diazirine moiety on the nucleobase. RNA probes containing U Dz units were irradiated in the presence of single-stranded DNA binding protein (SSB), which is also known to bind ssRNAs, and shown to efficiently (15% yield) and selectively cross-link to the protein. The corresponding diazirine-modified uridine triphosphate U DzTP was synthesized and its capacity to act as a substrate for the T7 RNA polymerase was tested in transcription assays. U DzTP was accepted with a maximum yield of 38% for a 26mer RNA containing a single incorporation and 28% yield for triple consecutive incorporations. Thus, this uridine analogue represents a convenient biochemical tool for the identification of RNA binding proteins and unraveling the role and function played by ncRNAs.

Nonsense-mediated mRNA decay (NMD) restricts replication of mammalian RNA viruses

A genome-wide siRNA screen against host factors that affect the infection of Semliki Forest virus (SFV), a positive-strand (+)RNA virus, revealed that components of the nonsense-mediated mRNA decay (NMD) pathway restrict early, post-entry steps of the infection cycle. In HeLa cells and primary human fibroblasts, knockdown of UPF1, SMG5 and SMG7 leads to increased levels of viral proteins and RNA and to higher titers of released virus. The inhibitory effect of NMD was stronger when the efficiency of virus replication was impaired by mutations or deletions in the replicase proteins. Accordingly, impairing NMD resulted in a more than 20-fold increased production of these attenuated viruses. Our data suggest that intrinsic features of genomic and sub-genomic viral mRNAs, most likely the extended 3'-UTR length, make them susceptible to NMD. The fact that SFV replication is entirely cytoplasmic strongly suggests that degradation of the viral RNA occurs through the exon junction complex (EJC)-independent mode of NMD. Collectively, our findings uncover a new biological function for NMD as an intrinsic barrier to the translation of early viral proteins and the amplification of (+)RNA viruses in animal cells. Thus, in addition to its role in mRNA surveillance and post-transcriptional gene regulation, NMD also contributes to protect cells from RNA viruses.

Expression of the vault RNA protects cells from undergoing apoptosis

Non-protein-coding RNAs are a functionally versatile class of transcripts found in all domains of life exerting their biological role at the RNA level. Recently, we demonstrated that the vault-associated RNAs (vtRNAs) were significantly up-regulated in human B cells upon Epstein-Barr virus (EBV) infection [1,2]. vtRNAs are an integral part of the vault complex, a huge and evolutionarily conserved cytoplasmic ribonucleoprotein complex. The major vault protein (MVP) is the main structural component of the complex while vtRNA accounts for only 5% of its mass. Very little is known about the function(s) of the vtRNAs or the vault complex. In particular the role and significance of the previously observed vtRNA up-regulation upon EBV infection remained unclear. We individually expressed EBV-encoded genes in B cells and found the latent membrane protein 1 (LMP1) as trigger for vtRNA up-regulation. To unravel a putative functional interconnection between vtRNA expression and EBV infection, we ectopically expressed vtRNA1-1 in human B cells and observed an improved viral establishment. Furthermore, expression of vtRNA1-1 but not of the other vtRNA paralogs protected cells from undergoing apoptosis. Knock-down of MVP had no effect on these phenotypes thus revealing the vtRNA and not the vault complex to contribute to the enhanced EBV establishment and apoptosis resistance. Mutational analysis highlighted the central domain of the vtRNA to be involved in the anti-apoptotic effect. Ongoing research aims at characterizing the target of vtRNA1-1 in the apoptotic pathway. In summary, our data reveal a crucial cellular function for the so far elusive RNA biology of the vtRNAs.

Identification of gunshots to the head by detection of RNA in backspatter primarily expressed in brain tissue

Traces of backspatter recovered from the inside of the barrel of a gun that was used to deliver suicidal or homicidal contact shots may be a source of valuable forensic evidence and first systematic investigations of the persistence of victim DNA from inside firearms have been presented. The aim of the present study was to include victim RNA in such analyses to determine the origin of tissues in addition and parallel to standard DNA profiling for forensic identification purposes. In a first step, suitable mRNA (C1orf61) and micro-RNAs (miR-124a and miR-124*) that are primarily expressed in brain tissue were selected from potential candidates and confirmed using quantitative PCR (qPCR). Secondly, a co-extraction procedure for RNA and DNA was established and brain differentiability of the selected RNAs was demonstrated via qPCR using samples from experimental shots at ballistic models. In a third step, this procedure was successfully applied to analyse samples from real casework comprising eight cases of suicidal contact shots. In this pilot study, we are first to report the possibility of co-extracting mRNA, miRNA and DNA from ballistic trace samples collected from the inside of firearms and we demonstrate that RNA and DNA based analyses can be performed in parallel to produce informative and highly complementary evidence.

RNA interference screening identifies a novel role for PCTK1/CDK16 in medulloblastoma with c-Myc amplification.

Medulloblastoma (MB) is the most common malignant brain tumor in children and is associated with a poor outcome. cMYC amplification characterizes a subgroup of MB with very poor prognosis. However, there exist so far no targeted therapies for the subgroup of MB with cMYC amplification. Here we used kinome-wide RNA interference screening to identify novel kinases that may be targeted to inhibit the proliferation of c-Myc-overexpressing MB. The RNAi screen identified a set of 5 genes that could be targeted to selectively impair the proliferation of c-Myc-overexpressing MB cell lines: AKAP12 (A-kinase anchor protein), CSNK1α1 (casein kinase 1, alpha 1), EPHA7 (EPH receptor A7) and PCTK1 (PCTAIRE protein kinase 1). When using RNAi and a pharmacological inhibitor selective for PCTK1, we could show that this kinase plays a crucial role in the proliferation of MB cell lines and the activation of the mammalian target of rapamycin (mTOR) pathway. In addition, pharmacological PCTK1 inhibition reduced the expression levels of c-Myc. Finally, targeting PCTK1 selectively impaired the tumor growth of c-Myc-overexpressing MB cells in vivo. Together our data uncover a novel and crucial role for PCTK1 in the proliferation and survival of MB characterized by cMYC amplification.

Characterization of the role of the RNA-binding protein FUS in the DNA damage response

FUS/TLS (fused in sarcoma/translocated in liposarcoma), a ubiquitously expressed RNA-binding protein, has been linked to a variety of cellular processes, including RNA metabolism, microRNA biogenesis and DNA repair. However, the precise cellular function of FUS remains unclear. Recently, mutations in the FUS gene have been found in ∼5% of familial Amyotrophic Lateral Sclerosis, a neurodegenerative disorder characterized by the dysfunction and death of motor neurons. Since MEFs and B-lymphocytes derived from FUS knockdown mice display major sensitivity to ionizing radiation and chromosomal aberrations [1,2], we are investigating the effects of DNA damage both in the presence or in the absence of FUS. To this purpose, we have generated a SH-SY5Y human neuroblastoma cell line expressing a doxycycline-induced shRNA targeting FUS, which specifically depletes the protein. We have found that FUS depletion induces an activation of the DNA damage response (DDR). However, treatment with genotoxic agents did not induce any strong changes in ATM (Ataxia Telangiectasia Mutated)-mediated DDR signaling. Interestingly, genotoxic treatment results in changes in the subcellular localization of FUS in normal cells. We are currently exploring on one hand the mechanism by which FUS depletion leads to DNA damage, and on the other the functional significance of FUS relocalization after genotoxic stress.

Methylation of ribosomal RNA by NSUN5 is a conserved mechanism modulating organismal lifespan

Several pathways modulating longevity and stress resistance converge on translation by targeting ribosomal proteins or initiation factors, but whether this involves modifications of ribosomal RNA is unclear. Here, we show that reduced levels of the conserved RNA methyltransferase NSUN5 increase the lifespan and stress resistance in yeast, worms and flies. Rcm1, the yeast homologue of NSUN5, methylates C2278 within a conserved region of 25S rRNA. Loss of Rcm1 alters the structural conformation of the ribosome in close proximity to C2278, as well as translational fidelity, and favours recruitment of a distinct subset of oxidative stress-responsive mRNAs into polysomes. Thus, rather than merely being a static molecular machine executing translation, the ribosome exhibits functional diversity by modification of just a single rRNA nucleotide, resulting in an alteration of organismal physiological behaviour, and linking rRNA-mediated translational regulation to modulation of lifespan, and differential stress response.

To sense or not to sense viral RNA--essentials of coronavirus innate immune evasion.

An essential function of innate immunity is to distinguish self from non-self and receptors have evolved to specifically recognize viral components and initiate the expression of antiviral proteins to restrict viral replication. Coronaviruses are RNA viruses that replicate in the host cytoplasm and evade innate immune sensing in most cell types, either passively by hiding their viral signatures and limiting exposure to sensors or actively, by encoding viral antagonists to counteract the effects of interferons. Since many cytoplasmic viruses exploit similar mechanisms of innate immune evasion, mechanistic insight into the direct interplay between viral RNA, viral RNA-processing enzymes, cellular sensors and antiviral proteins will be highly relevant to develop novel antiviral targets and to restrict important animal and human infections.

Targeting membrane-bound viral RNA synthesis reveals potent inhibition of diverse coronaviruses including the middle East respiratory syndrome virus.

Coronaviruses raise serious concerns as emerging zoonotic viruses without specific antiviral drugs available. Here we screened a collection of 16671 diverse compounds for anti-human coronavirus 229E activity and identified an inhibitor, designated K22, that specifically targets membrane-bound coronaviral RNA synthesis. K22 exerts most potent antiviral activity after virus entry during an early step of the viral life cycle. Specifically, the formation of double membrane vesicles (DMVs), a hallmark of coronavirus replication, was greatly impaired upon K22 treatment accompanied by near-complete inhibition of viral RNA synthesis. K22-resistant viruses contained substitutions in non-structural protein 6 (nsp6), a membrane-spanning integral component of the viral replication complex implicated in DMV formation, corroborating that K22 targets membrane bound viral RNA synthesis. Besides K22 resistance, the nsp6 mutants induced a reduced number of DMVs, displayed decreased specific infectivity, while RNA synthesis was not affected. Importantly, K22 inhibits a broad range of coronaviruses, including Middle East respiratory syndrome coronavirus (MERS-CoV), and efficient inhibition was achieved in primary human epithelia cultures representing the entry port of human coronavirus infection. Collectively, this study proposes an evolutionary conserved step in the life cycle of positive-stranded RNA viruses, the recruitment of cellular membranes for viral replication, as vulnerable and, most importantly, druggable target for antiviral intervention. We expect this mode of action to serve as a paradigm for the development of potent antiviral drugs to combat many animal and human virus infections.

Self-replicating Replicon-RNA Delivery to Dendritic Cells by Chitosan-nanoparticles for Translation In Vitro and In Vivo.

Self-amplifying replicon RNA (RepRNA) possesses high potential for increasing antigen load within dendritic cells (DCs). The major aim of the present work was to define how RepRNA delivered by biodegradable, chitosan-based nanoparticulate delivery vehicles (nanogel-alginate (NGA)) interacts with DCs, and whether this could lead to translation of the RepRNA in the DCs. Although studies employed virus replicon particles (VRPs), there are no reports on biodegradable, nanoparticulate vehicle delivery of RepRNA. VRP studies employed cytopathogenic agents, contrary to DC requirements-slow processing and antigen retention. We employed noncytopathogenic RepRNA with NGA, demonstrating for the first time the efficiency of RepRNA association with nanoparticles, NGA delivery to DCs, and RepRNA internalization by DCs. RepRNA accumulated in vesicular structures, with patterns typifying cytosolic release. This promoted RepRNA translation, in vitro and in vivo. Delivery and translation were RepRNA concentration-dependent, occurring in a kinetic manner. Including cationic lipids with chitosan during nanoparticle formation enhanced delivery and translation kinetics, but was not required for translation of immunogenic levels in vivo. This work describes for the first time the characteristics associated with chitosan-nanoparticle delivery of self-amplifying RepRNA to DCs, leading to translation of encoded foreign genes, namely influenza virus hemagglutinin and nucleoprotein.

Pestiviral E(rns) blocks TLR-3-dependent IFN synthesis by LL37 complexed RNA

The ribonuclease activity of the soluble glycoprotein E(rns) of pestiviruses represents a unique mechanism to circumvent the host's innate immune system by blocking interferon type-I synthesis in response to extracellularly added single- (ss) and double-stranded (ds) RNA. However, the reason why pestiviruses encode a ribonuclease in addition to the abundant serum RNases remained elusive. Here, we show that the 5' UTR and NS5B regions of various strains of the RNA genome of the pestivirus bovine viral diarrhea virus (BVDV) are resistant to serum RNases and are potent TLR-3 agonists. Inhibitory activity of E(rns) was restricted to cleavable RNA products, and did not extend to the synthetic TLR-7/8 agonist R-848. RNA complexed with the antimicrobial peptide LL37 was protected from degradation by E(rns)in vitro but was fully inhibited by E(rns) in its ability to induce IFN in cell cultures, suggesting that the viral protein is mainly active in cleaving RNA in an intracellular compartment. We propose that secreted E(rns) represents a potent IFN antagonist, which degrades viral RNA that is resistant to the ubiquitous host RNases in the extracellular space. Thus, the viral RNase prevents its own pathogen-associated molecular pattern (PAMP) to inadvertently activate the IFN response that might break innate immunotolerance required for persistent pestivirus infections.

Electrospray tandem mass spectrometry of mixed-sequence RNA/DNA oligonucleotides

The fragmentation of electrospray-generated multiply deprotonated RNA and mixed-sequence RNA/DNA pentanucleotides upon low-energy collision-induced dissociation (CID) in a hybrid quadrupole time-of-flight mass spectrometer was investigated. The goal of unambiguous sequence identification of mixed-sequence RNA/DNA oligonucleotides requires detailed understanding of the gas-phase dissociation of this class of compounds. The two major dissociation events, base loss and backbone fragmentation, are discussed and the unique fragmentation behavior of oligoribonucleotides is demonstrated. Backbone fragmentation of the all-RNA pentanucleotides is characterized by abundant c-ions and their complementary y-ions as the major sequence-defining fragment ion series. In contrast to the dissociation of oligodeoxyribonucleotides, where backbone fragmentation is initiated by the loss of a nucleobase which subsequently leads to the formation of the w- and [a-base]-ions, backbone dissociation of oligoribonucleotides is essentially decoupled from base loss. The different behavior of RNA and DNA oligonucleotides is related to the presence of the 2'-hydroxyl substituent, which is the only structural alteration between the DNA and RNA pentanucleotides studied. CID of mixed-sequence RNA/DNA pentanucleotides results in a combination of the nucleotide-typical backbone fragmentation products, with abundant w-fragment ions generated by cleavage of the phosphodiester backbone adjacent to the deoxy building blocks, whereas backbone cleavage adjacent to ribonucleotides induces the formation of c- and y-ions. (C) 2002 American Society for Mass Spectrometry.

Stability and kinetics of nucleic acid triplexes with chimaeric DNA/RNA third strands

A series of chimaeric DNA/RNA triplex-forming oligonucleotides (TFOs) with identical base-sequence but varying sequential composition of the sugar residues were prepared. The structural, kinetic and thermodynamic properties of triplex formation with their corresponding double-helical DNA target were investigated by spectroscopic methods. Kinetic and thermodynamic data were obtained from analysis of non-equilibrium UV-melting- and annealing curves in the range of pH 5.1 to 6.7 in a 10 mM citrate/phosphate buffer containing 0.1M NaCl and 1 mM EDTA. It was found that already single substitutions of ribo- for deoxyribonucleotides in the TFOs greatly affect stability and kinetics of triplex formation in a strongly sequence dependent manner. Within the sequence context investigated, triplex stability was found to increase when deoxyribonucleotides were present at the 5'-side and ribonucleotides in the center of the TFO. Especially the substitution of thymidines for uridines in the TFO was found to accelerate both, the association and dissociation process, in a strongly position-dependent way. Differential structural information on triplexes and TFO single-strands was obtained from CD-spectroscopy and gel mobility experiments. Only minor changes were observed in the CD spectra of the triplexes at all pH values investigated, and the electrophoretic mobility was nearly identical in all cases, indicating a high degree of structural similarity. In contrast, the single-stranded TFOs showed high structural variability as determined in the same way. The results are discussed in the context of the design of TFOs for therapeutic or biochemical applications.