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Norovirus (NV) is a major cause of gastroenteritis worldwide. Antivirals against such important pathogens are on demand. Among the viral proteins that orchestrate viral replication, RNA-dependent-RNA-polymerase (RdRp) is a promising drug development target. From an in silico-docking search focused on the RdRp active site, we selected the compound PPNDS, which showed low micromolar IC50vs. murine NV-RdRp in vitro. We report the crystal structure of the murine NV-RdRp/PPNDS complex showing that two molecules of the inhibitor bind in antiparallel stacking interaction, properly oriented to block exit of the newly synthesized RNA. Such inhibitor-binding mode mimics two stacked nucleotide-bases of the RdRp/ssRNA complex., (Copyright © 2014 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

Norovirus RNA-dependent RNA polymerase (RdRp) is a promising target enzyme for the development of new antiviral drugs. Starting from the crystal structure of norovirus RdRp, we had previously performed an in silico docking search using a library of low-molecular-weight compounds that enabled us to select molecules with predicted enzyme inhibitory activity. Among these, the polysulfonated naphthylurea suramin proved to inhibit in vitro both murine and human norovirus polymerases, with IC50 values in the low micromolar range. The negatively charged inhibitor, however, displayed poor cell permeability in cell-based experiments. Therefore, we produced different suramin-loaded liposome formulations and evaluated their activities in cell-based assays using murine norovirus cultivated in RAW 264.7 macrophages, as a model for norovirus genus. The results obtained show that suramin, when delivered through liposomes, can effectively inhibit murine norovirus replication., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Noroviruses (NV) are +ssRNA viruses responsible for severe gastroenteritis; no effective vaccines/antivirals are currently available. We previously identified Suramin (9) as a potent inhibitor of NV-RNA dependent RNA polymerase (NV-RdRp). Despite significant in vitro activities versus several pharmacological targets, Suramin clinical use is hampered by pharmacokinetics/toxicity problems. To improve Suramin access to NV-RdRp in vivo, a Suramin-derivative, 8, devoid of two sulphonate groups, was synthesized, achieving significant anti-human-NV-RdRp activity (IC50 = 28 nM); the compound inhibits also murine NV (mNV) RdRp. The synthesis process led to the isolation/characterization of lower molecular weight intermediates (3-7) hosting only one sulphonate head. The crystal structures of both hNV/mNV-RdRps in complex with 6, were analyzed, providing new knowledge on the interactions that a small fragment can establish with NV-RdRps, and establishing a platform for structure-guided optimization of potency, selectivity and drugability.

Noroviruses are members of the Caliciviridae family of positive sense RNA viruses. In humans Noroviruses cause rapid onset diarrhea and vomiting. Currently Norovirus infection is responsible for 21 million gastroenteritis yearly cases in the USA. Nevertheless, despite the obvious public health and socio-economic relevance, no effective vaccines/antivirals are yet available to treat Norovirus infection. Since the activity of RNA-dependent RNA polymerase (RdRp) plays a key role in genome replication and in the synthesis/amplification of subgenomic RNA, the enzyme is considered a promising target for antiviral drug development. In this context, following the identification of suramin and NF023 as Norovirus RdRp inhibitors, we analyzed the potential inhibitory role of naphthalene di-sulfonate (NAF2), a fragment derived from these two molecules. Although NAF2, tested in enzymatic polymerase inhibition assays, displayed low activity against RdRp (IC50=14μM), the crystal structure of human Norovirus RdRp revealed a thumb domain NAF2 binding site that differs from that characterized for NF023/suramin. To further map the new potential inhibitory site, we focused on the structurally related molecule pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) tetrasodium salt (PPNDS). PPNDS displayed below-micromolar inhibitory activity versus human Norovirus RdRp (IC50=0.45μM), similarly to suramin and NF023. Inspection of the crystal structure of the RdRp/PPNDS complex showed that the inhibitor bound to the NAF2 thumb domain site, highlighting the relevance of such new binding site for exploiting Norovirus RdRp inhibitors., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Caliciviridae are RNA viruses with a single-stranded, positively oriented polyadenylated genome, responsible for a broad spectrum of diseases such as acute gastroenteritis in humans. Recently, analyses on the structures and functionalities of the RNA-dependent RNA polymerase (RdRp) from several Caliciviruses have been reported. The RdRp is predicted to play a key role in genome replication, as well as in synthesis and amplification of additional subgenomic RNA. Starting from the crystal structures of human Norovirus (hNV) RdRp, we performed an in silico docking search to identify synthetic compounds with predicted high affinity for the enzyme active site. The best-ranked candidates were tested in vitro on murine Norovirus (MNV) and hNV RdRps to assay their inhibition of RNA polymerization. The results of such combined computational and experimental screening approach led to the identification of two high-potency inhibitors: Suramin and NF023, both symmetric divalent molecules hosting two naphthalene-trisulfonic acid heads. We report here the crystal structure of MNV RdRp alone and in the presence of the two identified inhibitors. Both inhibitory molecules occupy the same RdRp site, between the fingers and thumb domains, with one inhibitor head close to residue 42 and to the protein active site. To further validate the structural results, we mutated Trp42 to Ala in MNV RdRp and the corresponding residue (i.e., Tyr41 to Ala) in hNV RdRp. Both NF023 and Suramin displayed reduced inhibitory potency versus the mutated hNV RdRp, thus hinting at a conserved inhibitor binding mode in the two polymerases., (Copyright © 2012. Published by Elsevier Ltd.)

Norovirus (NV) is a leading cause of gastroenteritis worldwide and a major public health concern. So far, the replication strategy of NV remains poorly understood, mainly because of the lack of a cell system to cultivate the virus. In this study, the function and the structure of a key viral enzyme of replication, the RNA-dependent RNA polymerase (RdRp, NS7), was examined. The overall structure of the NV NS7 RdRp was determined by X-ray crystallography to a 2.3 A (0.23 nm) resolution (PDB ID 2B43), displaying a right-hand fold typical of the template-dependent polynucleotide polymerases. Biochemical analysis evidenced that NV NS7 RdRp is active as a homodimer, with an apparent K(d) of 0.649 microM and a positive cooperativity (Hill coefficient n(H)=1.86). Crystals of the NV NS7 homodimer displayed lattices containing dimeric arrangements with high shape complementarity statistics. This experimental data on the structure and function of the NV RdRp may set the cornerstone for the development of polymerase inhibitors to control the infection with NV, a medically relevant pathogen.

Protein translation in noroviruses requires translational processing of a polyprotein precursor by the viral protease. So far, the molecular mechanisms of catalytic cleavage by the viral protease are poorly understood. In this study, the catalytic activities and substrate specificities of the viral protease were examined in vitro by using synthetic peptides (11-15 residues) corresponding to the cleavage sites of the norovirus polyprotein. Both predicted forms of the viral protease, the 3C-like protease (3C(pro)) and the 3CD-like protease polymerase protein (3CD(propol)), displayed a specific trans cleavage activity of peptides bearing Gln-Gly at the scissile bond. In contrast, peptides bearing Glu-Gly at the scissile bond (p20/VPg and 3C(pro)/3D(pol) junctions) were resistant to trans-cleavage by 3C(pro) and 3CD(propol). Interestingly, the VPg/3C(pro) scissile bond (Glu-Ala) was cleaved only by 3CD(propol), and examination of relative cleavage efficiencies revealed significant differences in processing of peptides, indicating differential cleavage patterns for 3C(pro) and 3CD(propol).

Norovirus (NV) 3D(pol) is a non-structural protein predicted to play an essential role in the replication of the NV genome. In this study, the characteristics of NV 3D(pol) activity and initiation of RNA synthesis have been examined in vitro. Recombinant NV 3D(pol), as well as a 3D(pol) active-site mutant were expressed in Escherichia coli and purified. NV 3D(pol) was able to synthesize RNA in vitro and displayed flexibility with respect to the use of Mg(2+) or Mn(2+) as a cofactor. NV 3D(pol) yielded two different products when incubated with synthetic RNA in vitro: (i) a double-stranded RNA consisting of two single strands of opposite polarity or (ii) the single-stranded RNA template labelled at its 3' terminus by terminal transferase activity. Initiation of RNA synthesis occurred de novo rather than by back-priming, as evidenced by the fact that the two strands of the double-stranded RNA product could be separated, and by dissociation in time-course analysis of terminal transferase and RNA synthesis activities. In addition, RNA synthesis was not affected by blocking of the 3' terminus of the RNA template by a chain terminator, sustaining de novo initiation of RNA synthesis. NV 3D(pol) displays in vitro properties characteristic of RNA-dependent RNA polymerases, allowing the implementation of this in vitro enzymic assay for the development and validation of antiviral drugs against NV, a so far non-cultivated virus and an important human pathogen.

Noroviruses (Caliciviridae) are RNA viruses with a single-stranded, positive-oriented polyadenylated genome. To date, little is known about the replication strategy of norovirus, a so-far noncultivable virus. We have examined the initiation of replication of the norovirus genome in vitro, using the active norovirus RNA-dependent RNA polymerase (3D(pol)), homopolymeric templates, and synthetic subgenomic or antisubgenomic RNA. Initiation of RNA synthesis on homopolymeric templates as well as replication of subgenomic polyadenylated RNA was strictly primer dependent. In this context and as observed for other enteric RNA viruses, i.e., poliovirus, a protein-primed initiation of RNA synthesis after elongation of the VPg by norovirus 3D(pol) was postulated. To address this question, norovirus VPg was expressed in Escherichia coli and purified. Incubation of VPg with norovirus 3D(pol) generated VPg-poly(U), which primed the replication of subgenomic polyadenylated RNA. In contrast, replication of antisubgenomic RNA was not primer dependent, nor did it depend on a leader sequence, as evidenced by deletion analysis of the 3' termini of subgenomic and antisubgenomic RNA. On nonpolyadenylated RNA, i.e., antisubgenomic RNA, norovirus 3D(pol) initiated RNA synthesis de novo and terminated RNA synthesis by a poly(C) stretch. Interestingly, on poly(C) RNA templates, norovirus 3D(pol) initiated RNA synthesis de novo in the presence of high concentrations of GTP. We propose a novel model for initiation of replication of the norovirus genome by 3D(pol), with a VPg-protein-primed initiation of replication of polyadenylated genomic RNA and a de novo initiation of replication of antigenomic RNA.

Noroviruses are single-stranded RNA viruses with high genomic variability. They have emerged in the last decade as a major cause of acute gastroenteritis. It remains so far unclear whether norovirus evolution is driven by sequence mutation and/or recombination. In this study, we have assessed the occurrence of recombination in the norovirus capsid gene. For this purpose, 69 complete capsid sequences of norovirus strains accessible in GenBank as well as 25 complete capsid sequences generated from norovirus-positive clinical samples were examined. Unreported recombination was detected in about 8% of norovirus strains belonging to genetic clusters I/1 (n = 1), II/1 (n = 1), II/3 (n = 1), II/4 (n = 3), and II/5 (n = 1). Recombination breakpoints were mainly located at the interface of the putative P1-1 and P2 domains of the capsid protein and/or within the P2 domain. The recombination region displayed features such as length, sequence composition (upstream and downstream GC- and AU-rich sequences, respectively), and predicted RNA secondary structure that are characteristic of homologous recombination activators. Our results suggest that recombination in the norovirus capsid gene may naturally occur, involving capsid domains presumably exposed to immunological pressure.

A single-step multiplex reverse transcription-polymerase chain reaction (RT-PCR) assay that detects and identifies Norovirus, Astrovirus and Adenovirus in clinical stool samples is described. Four hundred sixty stool samples were tested from patients with non-rotavirus acute gastroenteritis, that were either stored at -80 degrees C and tested retrospectively, or tested immediately after viral nucleic acid extraction in a prospective manner, including outbreaks of gastroenteritis that occurred in Germany during the winter of 2003. The multiplex RT-PCR was validated against simplex RT-PCR with published primers for Norovirus (JV12/JV13 and p289/p290) and Astrovirus (Mon340/348), and against simplex PCR for Adenovirus. In both retrospective and prospective settings, the multiplex RT-PCR was equally sensitive and specific in detecting non-rotavirus infections compared with simplex RT-PCR/PCR. The specificity of the multiplex RT-PCR was assessed by sequencing of the amplicons that showed high nucleotide identities to Norovirus genogroup I/1, I/4, II/2, or II/4 clades, as well as to Astrovirus serotypes 1, 2, 4, or 8. The multiplex RT-PCR was also more sensitive than Astrovirus and Norovirus antigen enzyme immunoassays (IDEIA, Dako), as well as Astrovirus isolation in cell culture. This novel multiplex RT-PCR is an attractive technique for the rapid, specific, and cost-effective laboratory diagnosis of non-rotavirus acute gastroenteritis.