Antiviral strategies to control calicivirus infections

Nonstructural protein 4 of human norovirus self-assembles into various membrane-bridging multimers

Single-stranded, positive-sense RNA ((+)RNA) viruses replicate their genomes in virus-induced intracellular membrane compartments. (+)RNA viruses dedicate a significant part of their small genomes (a few thousands to a few tens of thousands of bases) to the generation of these compartments by encoding membrane-interacting proteins and/or protein domains. Noroviruses are a very diverse genus of (+)RNA viruses including human and animal pathogens. Human noroviruses are the major cause of acute gastroenteritis worldwide, with genogroup II genotype 4 (GII.4) noroviruses accounting for the vast majority of infections. Three viral proteins encoded in the N terminus of the viral replication polyprotein direct intracellular membrane rearrangements associated with norovirus replication. Of these three, nonstructural protein 4 (NS4) seems to be the most important, although its exact functions in replication organelle formation are unknown. Here, we produce, purify, and characterize GII.4 NS4. AlphaFold modeling combined with experimental data refines and corrects our previous crude structural model of NS4. Using simple artificial liposomes, we report an extensive characterization of the membrane properties of NS4. We find that NS4 self-assembles and thereby bridges liposomes together. Cryo-EM, NMR, and membrane flotation show formation of several distinct NS4 assemblies, at least two of them bridging pairs of membranes together in different fashions. Noroviruses belong to (+)RNA viruses whose replication compartment is extruded from the target endomembrane and generates double-membrane vesicles. Our data establish that the 21-kDa GII.4 human norovirus NS4 can, in the absence of any other factor, recapitulate in tubo several features, including membrane apposition, that occur in such processes.

Isolation and molecular characteristics of a recombinant feline calicivirus from Qingdao, China

Feline calicivirus (FCV) is a highly contagious pathogen seriously affecting the upper respiratory tract and producing oral diseases in the feline. Despite widespread vaccination, the prevalence of FCV remains high. In this study, the FCV qingdao (qd)/2019/china was isolated from a domestic feline oropharyngeal swab collected from Qingdao, China. The virus was purified using the plaque assay and identified using the Polymerase chain reaction and indirect immunofluorescence assay methods, the capsid amino acid, VP1 of qd/2019/china, showed sequence identity with the other isolates ranging between 83.90% (ym3/2001/jp) and 91.10% (CH-JL4). The sequence of the capsid amino acid revealed qd/2019/china to be closely related to CH-JL4 and clustered with CH-JL4 in the phylogenetic tree. The phylo-genetic analysis indicated that the complete genomes (GenBank® accession No. MZ322896) of qd/2019/china and CH-JL4 were also classified into the same cluster. The recombination analysis with Simplot indicated that the qd/2019/china originated from the recombination of CH-JL4 and HRB-SS, and the region 3,821 - 5,301 nt originated from HRB-SS. Further, the region 3,821 - 5,301 nt were found to belong to the protease-polymerase (PP) of HRB-SS. Here, we isolated a new recombinant virus, FCV qd/2019/china. Therefore, these results would be beneficial for better understanding of the evolution and epidemiology of FCV.

Strategies for managing marine disease

The incidence of emerging infectious diseases (EIDs) has increased in wildlife populations in recent years and is expected to continue to increase with global environmental change. Marine diseases are relatively understudied compared with terrestrial diseases but warrant parallel attention as they can disrupt ecosystems, cause economic loss, and threaten human livelihoods. Although there are many existing tools to combat the direct and indirect consequences of EIDs, these management strategies are often insufficient or ineffective in marine habitats compared with their terrestrial counterparts, often due to fundamental differences between marine and terrestrial systems. Here, we first illustrate how the marine environment and marine organism life histories present challenges and opportunities for wildlife disease management. We then assess the application of common disease management strategies to marine versus terrestrial systems to identify those that may be most effective for marine disease outbreak prevention, response, and recovery. Finally, we recommend multiple actions that will enable more successful management of marine wildlife disease emergencies in the future. These include prioritizing marine disease research and understanding its links to climate change, improving marine ecosystem health, forming better monitoring and response networks, developing marine veterinary medicine programs, and enacting policy that addresses marine and other wildlife diseases. Overall, we encourage a more proactive rather than reactive approach to marine wildlife disease management and emphasize that multidisciplinary collaborations are crucial to managing marine wildlife health.

Nanobody-Mediated Neutralization Reveals an Achilles Heel for Norovirus

Human norovirus frequently causes outbreaks of acute gastroenteritis. Although discovered more than five decades ago, antiviral development has, until recently, been hampered by the lack of a reliable human norovirus cell culture system. Nevertheless, a lot of pathogenesis studies were accomplished using murine norovirus (MNV), which can be grown routinely in cell culture. In this study, we analyzed a sizeable library of nanobodies that were raised against the murine norovirus virion with the main purpose of developing nanobody-based inhibitors. We discovered two types of neutralizing nanobodies and analyzed the inhibition mechanisms using X-ray crystallography, cryo-electron microscopy (cryo-EM), and cell culture techniques. The first type bound on the top region of the protruding (P) domain. Interestingly, this nanobody binding region closely overlapped the MNV receptor-binding site and collectively shared numerous P domain-binding residues. In addition, we showed that these nanobodies competed with the soluble receptor, and this action blocked virion attachment to cultured cells. The second type bound at a dimeric interface on the lower side of the P dimer. We discovered that these nanobodies disrupted a structural change in the capsid associated with binding cofactors (i.e., metal cations/bile acid). Indeed, we found that capsids underwent major conformational changes following addition of Mg²⁺ or Ca²⁺ Ultimately, these nanobodies directly obstructed a structural modification reserved for a postreceptor attachment stage. Altogether, our new data show that nanobody-based inhibition could occur by blocking functional and structural capsid properties.IMPORTANCE This research discovered and analyzed two different types of MNV-neutralizing nanobodies. The top-binding nanobodies sterically inhibited the receptor-binding site, whereas the dimeric-binding nanobodies interfered with a structural modification associated with cofactor binding. Moreover, we found that the capsid contained a number of vulnerable regions that were essential for viral replication. In fact, the capsid appeared to be organized in a state of flux, which could be important for cofactor/receptor-binding functions. Blocking these capsid-binding events with nanobodies directly inhibited essential capsid functions. Moreover, a number of MNV-specific nanobody binding epitopes were comparable to human norovirus-specific nanobody inhibitors. Therefore, this additional structural and inhibition information could be further exploited in the development of human norovirus antivirals.

The Adenosine Analogue NITD008 has Potent Antiviral Activity against Human and Animal Caliciviruses

The widespread nature of calicivirus infections globally has a substantial impact on the health and well-being of humans and animals alike. Currently, the only vaccines approved against caliciviruses are for feline and rabbit-specific members of this group, and thus there is a growing effort towards the development of broad-spectrum antivirals for calicivirus infections. In this study, we evaluated the antiviral activity of the adenosine analogue NITD008 in vitro using three calicivirus model systems namely; feline calicivirus (FCV), murine norovirus (MNV), and the human norovirus replicon. We show that the nucleoside analogue (NA), NITD008, has limited toxicity and inhibits calicivirus replication in all three model systems with EC₅₀ values of 0.94 μM, 0.91 µM, and 0.21 µM for MNV, FCV, and the Norwalk replicon, respectively. NITD008 has a similar level of potency to the most well-studied NA 2′-C-methylcytidine in vitro. Significantly, we also show that continual NITD008 treatment effectively cleared the Norwalk replicon from cells and treatment with 5 µM NITD008 was sufficient to completely prevent rebound. Given the potency displayed by NITD008 against several caliciviruses, we propose that this compound should be interrogated further to assess its effectiveness in vivo. In summary, we have added a potent NA to the current suite of antiviral compounds and provide a NA scaffold that could be further modified for therapeutic use against calicivirus infections.

Targeting the Viral Polymerase of Diarrhea-Causing Viruses as a Strategy to Develop a Single Broad-Spectrum Antiviral Therapy

Viral gastroenteritis is an important cause of morbidity and mortality worldwide, being particularly severe for children under the age of five. The most common viral agents of gastroenteritis are noroviruses, rotaviruses, sapoviruses, astroviruses and adenoviruses, however, no specific antiviral treatment exists today against any of these pathogens. We here discuss the feasibility of developing a broad-spectrum antiviral treatment against these diarrhea-causing viruses. This review focuses on the viral polymerase as an antiviral target, as this is the most conserved viral protein among the diverse viral families to which these viruses belong to. We describe the functional and structural similarities of the different viral polymerases, the antiviral effect of reported polymerase inhibitors and highlight common features that might be exploited in an attempt of designing such pan-polymerase inhibitor.

Therapeutics and Immunoprophylaxis Against Noroviruses and Rotaviruses: The Past, Present, and Future

Background:

Noroviruses and rotaviruses are important viral etiologies of severe gastroenteritis. Noroviruses are the primary cause of nonbacterial diarrheal outbreaks in humans, whilst rotaviruses are a major cause of childhood diarrhea. Although both enteric pathogens substantially impact human health and economies, there are no approved drugs against noroviruses and rotaviruses so far. On the other hand, whilst the currently licensed rotavirus vaccines have been successfully implemented in over 100 countries, the most advanced norovirus vaccine has recently completed phase-I and II trials.

Methods:

We performed a structured search of bibliographic databases for peer-reviewed research litera-ture on advances in the fields of norovirus and rotavirus therapeutics and immunoprophylaxis.

Results:

Technological advances coupled with a proper understanding of viral morphology and replication over the past decade has facilitated pioneering research on therapeutics and immunoprophylaxis against noroviruses and rotaviruses, with promising outcomes in human clinical trials of some of the drugs and vaccines. This review focuses on the various developments in the fields of norovirus and rotavirus thera-peutics and immunoprophylaxis, such as potential antiviral drug molecules, passive immunotherapies (oral human immunoglobulins, egg yolk and bovine colostral antibodies, llama-derived nanobodies, and anti-bodies expressed in probiotics, plants, rice grains and insect larvae), immune system modulators, probiot-ics, phytochemicals and other biological substances such as bovine milk proteins, therapeutic nanoparti-cles, hydrogels and viscogens, conventional viral vaccines (live and inactivated whole virus vaccines), and genetically engineered viral vaccines (reassortant viral particles, virus-like particles (VLPs) and other sub-unit recombinant vaccines including multi-valent viral vaccines, edible plant vaccines, and encapsulated viral particles).

Conclusions:

This review provides important insights into the various approaches to therapeutics and im-munoprophylaxis against noroviruses and rotaviruses..

In silico screening for human norovirus antivirals reveals a novel non-nucleoside inhibitor of the viral polymerase

Human norovirus causes approximately 219,000 deaths annually, yet there are currently no antivirals available. A virtual screening of commercially available drug-like compounds (~300,000) was performed on the suramin and PPNDS binding-sites of the norovirus RNA-dependent RNA polymerase (RdRp). Selected compounds (n = 62) were examined for inhibition of norovirus RdRp activity using an in vitro transcription assay. Eight candidates demonstrated RdRp inhibition (>25% inhibition at 10 µM), which was confirmed using a gel-shift RdRp assay for two of them. The two molecules were identified as initial hits and selected for structure-activity relationship studies, which resulted in the synthesis of novel compounds that were examined for inhibitory activity. Five compounds inhibited human norovirus RdRp activity (>50% at 10 µM), with the best candidate, 54, demonstrating an IC₅₀ of 5.6 µM against the RdRp and a CC₅₀ of 62.8 µM. Combinational treatment of 54 and the known RdRp site-B inhibitor PPNDS revealed antagonism, indicating that 54 binds in the same binding pocket. Two RdRps with mutations (Q414A and R419A) previously shown to be critical for the binding of site-B compounds had no effect on inhibition, suggesting 54 interacts with distinct site-B residues. This study revealed the novel scaffold 54 for further development as a norovirus antiviral.

Broad-spectrum non-nucleoside inhibitors for caliciviruses

Viruses of the Caliciviridae cause significant and sometimes lethal diseases, however despite substantial research efforts, specific antivirals are lacking. Broad-spectrum antivirals could combat multiple viral pathogens, offering a rapid solution when no therapies exist. The RNA-dependent RNA polymerase (RdRp) is an attractive antiviral target as it is essential for viral replication and lacks mammalian homologs. To focus the search for pan-Caliciviridae antivirals, the RdRp was probed with non-nucleoside inhibitors (NNIs) developed against hepatitis C virus (HCV) to reveal both allosteric ligands for structure-activity relationship enhancement, and highly-conserved RdRp pockets for antiviral targeting. The ability of HCV NNIs to inhibit calicivirus RdRp activities was assessed using in vitro enzyme and murine norovirus cell culture assays. Results revealed that three NNIs which bound the HCV RdRp Thumb I (TI) site also inhibited transcriptional activities of six RdRps spanning the Norovirus, Sapovirus and Lagovirus genera of the Caliciviridae. These NNIs included JTK-109 (RdRp inhibition range: IC₅₀ 4.3-16.6 μM), TMC-647055 (IC₅₀ range: 18.8-45.4 μM) and Beclabuvir (IC₅₀ range: 23.8->100 μM). In silico studies and site-directed mutagenesis indicated the JTK-109 binding site was within the calicivirus RdRp thumb domain, in a pocket termed Site-B, which is highly-conserved within all calicivirus RdRps. Additionally, RdRp inhibition assays revealed that JTK-109 was antagonistic with the previously reported RdRp inhibitor pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) tetrasodium salt (PPNDS), that also binds to Site-B. Moreover, like JTK-109, PPNDS was also a potent inhibitor of polymerases from six viruses spanning the three Caliciviridae genera tested (IC₅₀ range: 0.1-2.3 μM). Together, this study demonstrates the potential for de novo development of broad-spectrum antivirals that target the highly-conserved RdRp thumb pocket, Site-B. We also revealed three broad-spectrum HCV NNIs that could be used as antiviral scaffolds for further development against caliciviruses and other viruses.

Antiviral effect of theaflavins against caliciviruses

Caliciviruses are contagious pathogens of humans and various animals. They are the most common cause of viral gastroenteritis in humans, and can cause lethal diseases in domestic animals such as cats, rabbits and immunocompromised mice. In this study, we conducted cytopathic effect-based screening of 2080 selected compounds from our in-house library to find antiviral compounds against three culturable caliciviruses: feline calicivirus, murine norovirus (MNV) and porcine sapovirus (PoSaV). We identified active six compounds, of which two compounds, both related to theaflavins, showed broad antiviral activities against all three caliciviruses; three compounds (abamectin, a mixture of avermectin B1a and B1b; avermectin B1a; and (-)-epigallocatechin gallate hydrate) were effective against PoSaV only; and a heterocyclic carboxamide derivative (BFTC) specifically inhibited MNV infectivity in cell cultures. Further studies of the antiviral mechanism and structure-activity relationship of theaflavins suggested the following: (1) theaflavins worked before the viral entry step; (2) the effect of theaflavins was time- and concentration-dependent; and (3) the hydroxyl groups of the benzocycloheptenone ring were probably important for the anti-calicivirus activity of theaflavins. Theaflavins could be used for the calicivirus research, and as potential disinfectants and antiviral reagents to prevent and control calicivirus infections in animals and humans.

Norovirus drug candidates that inhibit viral capsid attachment to human histo-blood group antigens

Human noroviruses are the leading causative agents of epidemic and sporadic viral gastroenteritis and childhood diarrhoea worldwide. Human histo-blood group antigens (HBGA) serve as receptors for norovirus capsid protein attachment and play a critical role in infection. This makes HBGA-norovirus binding a promising target for drug development. Recently solved crystal structures of norovirus bound to HBGA have provided a structural basis for identification of potential anti-norovirus drugs and subsequently performed in silico and in vitro drug screens have identified compounds that block norovirus binding and may thereby serve as structural templates for design of therapeutic norovirus inhibitors. This review explores norovirus therapeutic options based on the strategy of blocking norovirus-HBGA binding.

Current tools for norovirus drug discovery

INTRODUCTION

Rapid transmission of norovirus often occurs due to its low infectious dosage, high genetic diversity and its short incubation time. The viruses cause acute gastroenteritis and may lead to death. Presently, no effective vaccine or selective drugs accepted by the United States Food and Drug Administration (FDA) are available for the treatment of norovirus. Advances in the development of norovirus replicon cell lines, GII.4-Sydney HuNoV strain human B cells, and murine and gnotobiotic pig norovirus models have facilitated the discovery of effective small molecule inhibitors in vitro and in vivo.

AREAS COVERED

This review gives a brief discussion of the biology and replication of norovirus before highlighting the discovery of anti-norovirus molecules. The article coverage includes: an overview of the current state of norovirus drug discovery, the targeting of the norovirus life cycle, the inhibition of structural and nonstructural proteins of norovirus such as proteases and polymerase, and the blockage of virus entry into host cells. Finally, anti-norovirus drugs in the clinical development stage are described.

EXPERT OPINION

The current approach for the counteraction of norovirus focuses on the inhibition of viral RNA polymerase, norovirus 3C-like protease and the structural proteins VP1 as well as the blockade of norovirus entry. Broad-spectrum anti-norovirus molecules, based on the inhibition of 3C-like protease, have been developed. Other host factors and ways to overcome the development of resistance through mutation are also being examined. A dual approach in targeting viral and host factors may lead to an effective counteraction of norovirus infection. Current successes in developing norovirus replicon harboring cells and norovirus infected human cells, as well as murine norovirus models and other animal models such as piglets have facilitated the discovery of effective drugs and helped our understanding of its mechanism of action.

Translational Control during Calicivirus Infection

In this review, we provide an overview of the strategies developed by caliciviruses to subvert or regulate the host protein synthesis machinery to their advantage. As intracellular obligate parasites, viruses strictly depend on the host cell resources to produce viral proteins. Thus, many viruses have developed strategies that regulate the function of the host protein synthesis machinery, often leading to preferential translation of viral mRNAs. Caliciviruses lack a 5′ cap structure but instead have a virus-encoded VPg protein covalently linked to the 5′ end of their mRNAs. Furthermore, they encode 2–4 open reading frames within their genomic and subgenomic RNAs. Therefore, they use alternative mechanisms for translation whereby VPg interacts with eukaryotic initiation factors (eIFs) to act as a proteinaceous cap-substitute, and some structural proteins are produced by reinitiation of translation events. This review discusses our understanding of these key mechanisms during caliciviruses infection as well as recent insights into the global regulation of eIF4E activity.

In vitro antiviral effect of germacrone on feline calicivirus

Feline calicivirus (FCV) often causes respiratory tract and oral disease in cats and is a highly contagious virus. Widespread vaccination does not prevent the spread of FCV. Furthermore, the low fidelity of the RNA-dependent RNA polymerase of FCV leads to the emergence of new variants, some of which show increased virulence. Currently, few effective anti-FCV drugs are available. Here, we found that germacrone, one of the main constituents of volatile oil from rhizoma curcuma, was able to effectively reduce the growth of FCV strain F9 in vitro. This compound exhibited a strong anti-FCV effect mainly in the early phase of the viral life cycle. The antiviral effect depended on the concentration of the drug. In addition, germacrone treatment had a significant inhibitory effect against two other reference strains, 2280 and Bolin, and resulted in a significant reduction in the replication of strains WZ-1 and HRB-SS, which were recently isolated in China. This is the first report of antiviral effects of germacrone against a calicivirus, and extensive in vivo research is needed to evaluate this drug as an antiviral therapeutic agent for FCV.

Antiviral effects of black raspberry (Rubus coreanus) seed extract and its polyphenolic compounds on norovirus surrogates

Black raspberry seeds, a byproduct of wine and juice production, contain large quantities of polyphenolic compounds. The antiviral effects of black raspberry seed extract (RCS) and its fraction with molecular weight less than 1 kDa (RCS-F1) were examined against food-borne viral surrogates, murine norovirus-1 (MNV-1) and feline calicivirus-F9 (FCV-F9). The maximal antiviral effect was achieved when RCS or RCS-F1 was added simultaneously to cells with MNV-1 or FCV-F9, reaching complete inhibition at 0.1-1 mg/mL. Transmission electron microscopy (TEM) images showed enlarged viral capsids or disruption (from 35 nm to up to 100 nm) by RCS-F1. Our results thus suggest that RCS-F1 can interfere with the attachment of viral surface protein to host cells. Further, two polyphenolic compounds derived from RCS-F1, cyanidin-3-glucoside (C3G) and gallic acid, identified by liquid chromatography-tandem mass spectrometry, showed inhibitory effects against the viruses. C3G was suggested to bind to MNV-1 RNA polymerase and to enlarge viral capsids using differential scanning fluorimetry and TEM, respectively.

Identification of novel rabbit hemorrhagic disease virus B-cell epitopes and their interaction with host histo-blood group antigens

Rabbit haemorrhagic disease, caused by rabbit hemorrhagic disease virus (RHDV), results in the death of millions of adult rabbits worldwide, with a mortality rate that exceeds 90%. The sole capsid protein, VP60, is divided into shell (S) and protruding (P) domains, and the more exposed P domain likely contains determinants for cell attachment and antigenic diversity. Nine mAbs against VP60 were screened and identified. To map antigenic epitopes, a set of partially overlapping and consecutive truncated proteins spanning VP60 were expressed. The minimal determinants of the linear B-cell epitopes of VP60 in the P domain, N(326)PISQV(331), D(338)MSFV(342) and K(562)STLVFNL(569), were recognized by one (5H3), four (1B8, 3D11, 4C2 and 4G2) and four mAbs (1D4, 3F7, 5G2 and 6B2), respectively. Sequence alignment showed epitope D(338)MSFV(342) was conserved among all RHDV isolates. Epitopes N(326)PISQV(331) and K(562)STLVFNL(569) were highly conserved among RHDV G1-G6 and variable in RHDV2 strains. Previous studies demonstrated that native viral particles and virus-like particles (VLPs) of RHDV specifically bound to synthetic blood group H type 2 oligosaccharides. We established an oligosaccharide-based assay to analyse the binding of VP60 and epitopes to histo-blood group antigens (HBGAs). Results showed VP60 and its epitopes (aa 326-331 and 338-342) in the P2 subdomain could significantly bind to blood group H type 2. Furthermore, mAbs 1B8 and 5H3 could block RHDV VLP binding to synthetic H type 2. Collectively, these two epitopes might play a key role in the antigenic structure of VP60 and interaction of RHDV and HBGA.

In vitro inhibition of field isolates of feline calicivirus with short interfering RNAs (siRNAs)

Feline calicivirus (FCV) is a common infection of domestic cats. Most infections are mild and self-limiting; however more severe disease manifestations, such as FCV-associated virulent systemic disease, may be associated with significant morbidity and mortality. There is currently a lack of effective antiviral treatments for these disease manifestations. In this study, a panel of eight siRNAs were designed to target four conserved regions of the FCV genome. siRNAs were screened for in vitro antiviral efficacy against the reference strain FCV F9 by determination of extracellular virus titres and morphological assessment of protection from cytopathic effect. Three of the siRNA (FCV3.7, FCV4.1, and FCV4.2) demonstrated a marked antiviral effect with a greater than 99% reduction in extracellular viral titre. Titration of these effective siRNAs demonstrated a clear concentration-response relationship, with IC50 values of approximately 1 nM, and combination treatment with multiple siRNAs demonstrated additive or synergistic effects. To assess the potential usefulness of the compounds in a clinical setting, siRNAs were screened against a panel of six recent Australian FCV isolates from cats with FCV-related disease. The siRNAs shown to be effective against the reference strain FCV F9 were active against the majority of the isolates tested, although some variability was noted. Taken together these data suggest potential therapeutic application of antiviral RNAi for treating FCV-associated disease in cats.

Effects of Oils and Essential Oils from Seeds of Zanthoxylum schinifolium against Foodborne Viral Surrogates

Human noroviruses are the most frequent cause of foodborne viral disease and are responsible for the vast majority of nonbacterial gastroenteritis. However, no specific therapies are available for the efficient control or prevention of foodborne viral disease. Here, we determined the antiviral activities of oils from seeds of Zanthoxylum schinifolium (ZSO) against foodborne viral surrogates, feline calicivirus-F9 (FCV-F9), and murine norovirus-1 (MNV-1), using plaque assay. Time-of-addition experiments were designed to determine the antiviral mechanism of action of ZSO against the surrogates. Maximal antiviral effect was observed upon pretreatment of FCV-F9 or MNV-1 with ZSO, which comprised oleic acid, linoleic acid, palmitic acid, and linolenic acid as the major fatty acids. FCV-F9 was more sensitive to ZSO than MNV-1, and the 50% effective concentration of ZSO against pretreatment of FCV-F9 was 0.0007%. However, essential oils from Z. schinifolium (ZSE), which comprised 42% estragole, showed no inhibitory effects against FCV-F9 and MNV-1. These results suggest that the inhibitory activities of ZSO were exerted by direct interaction of FCV-F9 or MNV-1 virion with ZSO, which may be a food material candidate for control of foodborne viral disease.

Norovirus infection in immunocompromised hosts

Acute gastroenteritis caused by noroviruses often has a duration of 2-3 days and is characteristically self-limiting. In contrast, chronic infection caused by noroviruses in immunocompromised individuals can last from weeks to years, making clinical management difficult. The mechanisms by which noroviruses establish persistent infection, and the role of immunocompromised hosts as a reservoir for noroviruses in the general human population, are not known. However, study of this patient cohort may lead to new insights into norovirus biology and approaches to treatment.

ISG15 functions as an interferon-mediated antiviral effector early in the murine norovirus life cycle

Human noroviruses (HuNoV) are the leading cause of nonbacterial gastroenteritis worldwide. Similar to HuNoV, murine noroviruses (MNV) are enteric pathogens spread via the fecal-oral route and have been isolated from numerous mouse facilities worldwide. Type I and type II interferons (IFN) restrict MNV-1 replication; however, the antiviral effectors impacting MNV-1 downstream of IFN signaling are largely unknown. Studies using dendritic cells, macrophages, and mice deficient in free and conjugated forms of interferon-stimulated gene 15 (ISG15) revealed that ISG15 conjugation contributes to protection against MNV-1 both in vitro and in vivo. ISG15 inhibited a step early in the viral life cycle upstream of viral genome transcription. Directly transfecting MNV-1 RNA into IFN-stimulated mouse embryonic fibroblasts (MEFs) and bone marrow-derived dendritic cells (BMDC) lacking ISG15 conjugates bypassed the antiviral activity of ISG15, further suggesting that ISG15 conjugates restrict the MNV-1 life cycle at the viral entry/uncoating step. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of early stages of MNV-1 replication.

IMPORTANCE

Type I IFNs are important in controlling murine norovirus 1 (MNV-1) infections; however, the proteins induced by IFNs that restrict viral growth are largely unknown. This report reveals that interferon-stimulated gene 15 (ISG15) mitigates MNV-1 replication both in vitro and in vivo. In addition, it shows that ISG15 inhibits MNV-1 replication by targeting an early step in the viral life cycle, MNV-1 entry and/or uncoating. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of viral entry/uncoating.

Treatment of norovirus infections: moving antivirals from the bench to the bedside

Noroviruses (NV) are the most common cause of acute gastrointestinal illness in the United States and worldwide. The development of specific antiviral countermeasures has lagged behind that of other viral pathogens, primarily because norovirus disease has been perceived as brief and self-limiting and robust assays suitable for drug discovery have been lacking. The increasing recognition that NV illness can be life-threatening, especially in immunocompromised patients who often require prolonged hospitalization and intensive supportive care, has stimulated new research to develop an effective antiviral therapy. Here, we propose a path forward for evaluating drug therapy in norovirus-infected immunocompromised individuals, a population at high risk for serious and prolonged illness. The clinical and laboratory features of norovirus illness in immunocompromised patients are reviewed, and potential markers of drug efficacy are defined. We discuss the potential design of clinical trials in these patients and how an antiviral therapy that proves effective in immunocompromised patients might also be used in the setting of acute outbreaks, especially in confined settings such as nursing homes, to block the spread of infection and reduce the severity of illness. We conclude by reviewing the current status of approved and experimental compounds that might be evaluated in a hospital setting.

Nonnucleoside inhibitors of norovirus RNA polymerase: scaffolds for rational drug design

Norovirus (NoV) is the leading cause of acute gastroenteritis worldwide, causing over 200,000 deaths a year. NoV is nonenveloped, with a single-stranded RNA genome, and is primarily transmitted person to person. The viral RNA-dependent RNA polymerase (RdRp) is critical for the production of genomic and subgenomic RNA and is therefore a prime target for antiviral therapies. Using high-throughput screening, nearly 20,000 "lead-like" compounds were tested for inhibitory activity against the NoV genogroup II, genotype 4 (GII.4) RdRp. The four most potent hits demonstrated half-maximal inhibitory concentrations (IC50s) between 5.0 μM and 9.8 μM against the target RdRp. Compounds NIC02 and NIC04 revealed a mixed mode of inhibition, while NIC10 and NIC12 were uncompetitive RdRp inhibitors. When examined using enzymes from related viruses, NIC02 demonstrated broad inhibitory activity while NIC04 was the most specific GII.4 RdRp inhibitor. The antiviral activity was examined using available NoV cell culture models; the GI.1 replicon and the infectious GV.1 murine norovirus (MNV). NIC02 and NIC04 inhibited the replication of the GI.1 replicon, with 50% effective concentrations (EC50s) of 30.1 μM and 71.1 μM, respectively, while NIC10 and NIC12 had no observable effect on the NoV GI.1 replicon. In the MNV model, NIC02 reduced plaque numbers, size, and viral RNA levels in a dose-dependent manner (EC50s between 2.3 μM and 4.8 μM). The remaining three compounds also reduced MNV replication, although with higher EC50s, ranging from 32 μM to 38 μM. In summary, we have identified novel nonnucleoside inhibitor scaffolds that will provide a starting framework for the development and future optimization of targeted antivirals against NoV.

Delivery of suramin as an antiviral agent through liposomal systems

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.

Activation of dendritic cells by the novel Toll-like receptor 3 agonist RGC100

Toll-like receptor (TLR) 3 agonists emerged as attractive candidates for vaccination strategies against tumors and pathogens. An important mechanism of action of such agonists is based on the activation of TLR3-expressing dendritic cells (DCs), which display a unique capacity to induce and stimulate T-cell responses. In this context, it has been demonstrated that targeting of TLR3 by double-stranded RNA such as poly(I:C) results in potent activation of DCs. Major disadvantages of poly(I:C) comprise its undefined chemical structure and very poor homogeneity, with subsequent unpredictable pharmacokinetics and high toxicity. In the present study, we evaluated the physicochemical properties and biological activity of the novel TLR3 agonist RGC100. RGC100 has a defined chemical structure, with a defined length (100 bp) and molecular weight (64.9 KDa) and a good solubility. RGC100 is stable in serum and activates myeloid DCs through TLR3 targeting, as evidenced by gene silencing experiments. Activation of mouse and human myeloid CD1c⁺ DCs by RGC100 leads to secretion of several proinflammatory cytokines. In addition, RGC100 improves the ability of CD1c⁺ DCs to stimulate T-cell proliferation. Due to its physicochemical properties and its immunostimulatory properties, RGC100 may represent a promising adjuvant for prophylactic and therapeutic vaccination strategies.

Progress towards the prevention and treatment of norovirus infections

Noroviruses are now recognized as the major cause of acute gastroenteritis in the developed world, yet our ability to prevent and control infection is limited. Recent work has highlighted that, while typically an acute infection in the population, immunocompromised patients often experience long-term infections that may last many years. This cohort of patients and those regularly exposed to infectious material, for example, care workers and others, would benefit greatly from the development of a vaccine or antiviral therapy. While a licensed vaccine or antiviral has yet to be developed, work over the past 10 years in this area has intensified and trials with a vaccine candidate have proven promising. Numerous antiviral targets and small molecule inhibitors that have efficacy in cell culture have now been identified; however, further studies in this area are required in order to make these suitable for clinical use.

Norovirus capsid proteins self-assemble through biphasic kinetics via long-lived stave-like intermediates

The self-assembly kinetics for a norovirus capsid protein were probed by time-resolved small-angle X-ray scattering and then analyzed by singular value decomposition and global fitting. Only three species contribute to the total scattering intensities: dimers, intermediates comprising some 11 dimers, and icosahedral T = 3 capsids made up of 90 dimers. Three-dimensional reconstructions of the intermediate robustly show a stave-like shape consistent with an arrangement of two pentameric units connected by an interstitial dimer. Upon triggering of self-assembly, the biphasic kinetics consist of a fast step in which dimers are assembled into intermediates, followed by a slow step in which intermediates interlock into capsids. This simple kinetic model reproduces experimental data with an excellent agreement over 6 decades in time and with nanometer resolution. The extracted form factors are robust against changes in experimental conditions. These findings challenge and complement currently accepted models for the assembly of norovirus capsids.

Atomic model of rabbit hemorrhagic disease virus by cryo-electron microscopy and crystallography

Rabbit hemorrhagic disease, first described in China in 1984, causes hemorrhagic necrosis of the liver. Its etiological agent, rabbit hemorrhagic disease virus (RHDV), belongs to the Lagovirus genus in the family Caliciviridae. The detailed molecular structure of any lagovirus capsid has yet to be determined. Here, we report a cryo-electron microscopic (cryoEM) reconstruction of wild-type RHDV at 6.5 Å resolution and the crystal structures of the shell (S) and protruding (P) domains of its major capsid protein, VP60, each at 2.0 Å resolution. From these data we built a complete atomic model of the RHDV capsid. VP60 has a conserved S domain and a specific P2 sub-domain that differs from those found in other caliciviruses. As seen in the shell portion of the RHDV cryoEM map, which was resolved to ∼5.5 Å, the N-terminal arm domain of VP60 folds back onto its cognate S domain. Sequence alignments of VP60 from six groups of RHDV isolates revealed seven regions of high variation that could be mapped onto the surface of the P2 sub-domain and suggested three putative pockets might be responsible for binding to histo-blood group antigens. A flexible loop in one of these regions was shown to interact with rabbit tissue cells and contains an important epitope for anti-RHDV antibody production. Our study provides a reliable, pseudo-atomic model of a Lagovirus and suggests a new candidate for an efficient vaccine that can be used to protect rabbits from RHDV infection.

Rabbit hemorrhagic disease (RHD), first described in China in 1984, causes hemorrhagic necrosis of the liver within three days after infection and with a mortality rate that exceeds 90%. RHD has spread to large parts of the world and threatens the rabbit industry and related ecology. Its etiological agent, rabbit hemorrhagic disease virus (RHDV), belongs to the Lagovirus genus in the family Caliciviridae. Currently, the absence of a high-resolution model of any lagovirus impedes our understanding of its molecular interactions with hosts and successful design of an efficient anti-RHDV vaccine. Here, we use hybrid structural approaches to construct a pseudo-atomic model of RHDV that reveals significant differences in the P2 sub-domain of the major capsid protein compared to that seen in other caliciviruses. We identified seven regions of high sequence variation in this sub-domain that dictate the binding specificities of histo-blood group antigens. In one of these regions, we identified an antigenic peptide that interacts with rabbit tissue cells and elicits a significant immune response in rabbits and, hence, protects them from RHDV infection. Our pseudo-atomic model provides a structural framework for developing new anti-RHDV vaccines and will also help guide use of the RHDV capsid as a vehicle to display human tumor antigens as part of anti-tumor therapy.

Reliability of non-culturable virus monitoring by PCR-based detection methods in environmental waters containing various concentrations of target RNA

Owing to the lack of practical cell culture system for human noroviruses (HuNoV), various detection methods based on conventional reverse transcription-PCR (RT-PCR) and the quantitative real-time PCR have been major tools for monitoring environmental water safety. In this study, we showed that the proportion of water sample concentrates used for one-step RT-PCR significantly influences false-negative findings of the non-culturable viruses. In total, 59 archived samples of previously analyzed water concentrates were reexamined for HuNoV RNA by the one-step RT-PCR and semi-nested PCR. Using new aliquots for RNA extraction for every trial, up to 20 PCR trials were performed for each archive to determine whether the crosscheck results supported the previous determinations. We reconfirmed that 27.6% (8/29) of the samples were HuNoV-positive samples: 6.7% (1/15) from groundwater, 33.3% (3/9) from river water, and 80% (4/5) from treated sewage effluent (TSE). These results corresponded to the ratio of previously negative HuNoV samples now identified as positive (8/30): 6.7% (1/15) from groundwater, 20% (1/5) from river water, and 60% (6/10) from TSE. To elucidate the cause of these results, 16 different concentrations of murine norovirus (MNV) RNA (from 2×10(2) to 8×10(3) copies, divided into 10 tubes for each concentration) were subjected to one-step RT-PCR. The detection frequency and reproducibility decreased sharply when the number of MNV RNA copies fell below threshold levels. These observations suggest that the proportion of water concentrate used for PCR-based detection should be considered carefully when deciding viral presence in certain types of environmental water, particularly in regard with legal controls.

Identification of RNA-protein interaction networks involved in the norovirus life cycle

Human noroviruses are one of the major causes of acute gastroenteritis in the developed world, yet our understanding of their molecular mechanisms of genome translation and replication lags behind that for many RNA viruses. Due to the nonculturable nature of human noroviruses, many related members of the Caliciviridae family of small RNA viruses are often used as model systems to dissect the finer details of the norovirus life cycle. Murine norovirus (MNV) has provided one such system with which to study the basic mechanisms of norovirus translation and replication in cell culture. In this report we describe the use of riboproteomics to identify host factors that interact with the extremities of the MNV genome. This network of RNA-protein interactions contains many well-characterized host factors, including PTB, La, and DDX3, which have been shown to play a role in the life cycle of other RNA viruses. By using RNA coimmunoprecipitation, we confirmed that a number of the factors identified using riboproteomics are associated with the viral RNA during virus replication in cell culture. We further demonstrated that RNA inhibition-mediated knockdown of the intracellular levels of a number of these factors inhibits or slows norovirus replication in cell culture, allowing identification of new intracellular targets for this important group of pathogens.

Antiviral effects of black raspberry (Rubus coreanus) juice on foodborne viral surrogates

Abstract Human noroviruses (HuNoVs) are the most frequent cause of foodborne viral gastroenteritis, causing approximately 90% of non-bacterial epidemic outbreaks around the world. Rubus coreanus is a species of black raspberry, rich in polyphenols, and known to exert anti-inflammatory, antibacterial, and antiviral activities. In the present study, the antiviral effects of R. coreanus juice (black raspberry [BRB] juice) on foodborne viral surrogates, murine norovirus-1 (MNV-1) and feline calicivirus-F9 (FCV-F9), were compared with those of cranberry juice, grape juice, and orange juice by plaque assays. Among the four juices tested, BRB juice was the most effective in reducing plaques formation of these viruses. Time-of-addition experiments were designed to determine the mechanism of action of BRB juice on MNV-1 and FCV-F9. The maximal antiviral effect of BRB juice against MNV-1 was observed when it was added to RAW 264.7 cells (mouse leukemic monocyte macrophage cell line) simultaneously with the virus. Pre-treatment of either Crandell Reese Feline Kidney cells or FCV-F9 with BRB juice exhibited significant antiviral activity. The inhibition of viral infection by BRB juice on MNV-1 and FCV-F9 probably occurs at the internalization of virions into the cell or the attachment of the viral surface protein to the cellular receptor. The polyphenol components in BRB (i.e., gallic acid and quercetin), however, did not show any activity against these viruses. Our data provide great promise for the utilization of BRB in the prevention of foodborne viral outbreaks.

Virucidal effects of bleach activators, sodium alkyl acyloxybenzene sulfonate and acyloxybenzoic acid, against Feline calicivirus

Noroviruses (NVs) are major causative pathogens of gastroenteritis. The disinfection of contaminated clothing during common household washing is desirable. The virucidal effects of 2 bleach activators, sodium alkyl acyloxybenzene sulfonate (OBS) and alkyl acyloxybenzoic acid (OBC), were studied using Feline calicivirus (FCV) as a surrogate for NVs. FCV was added to solutions containing either OBS or OBC and sodium percarbonate at various temperatures and for varying lengths of time. OBS and OBC, which generate long carbon chain peroxy acids, enhanced the virucidal effect of sodium percarbonate (PC). In particular, sodium lauroyloxybenzene sulfonate (OBS-12) and decanoyloxybenzoic acid (OBC-10) showed superior virucidal effects. Although the virucidal effect of 38-200 mg/L OBS-12 was maintained with 2-5% (v/v) horse serum, there was less of an effect with the same concentration of available chlorine. OBS and OBC have been used as ingredients in some laundry products to increase bleaching activity. It is expected that the use of OBS and OBC is also effective for the inactivation of NVs under common household washing conditions.

Rabbit haemorrhagic disease (RHD) and rabbit haemorrhagic disease virus (RHDV): a review

Rabbit haemorrhagic disease virus (RHDV) is a calicivirus of the genus Lagovirus that causes rabbit haemorrhagic disease (RHD) in adult European rabbits (Oryctolagus cuniculus). First described in China in 1984, the virus rapidly spread worldwide and is nowadays considered as endemic in several countries. In Australia and New Zealand where rabbits are pests, RHDV was purposely introduced for rabbit biocontrol. Factors that may have precipitated RHD emergence remain unclear, but non-pathogenic strains seem to pre-date the appearance of the pathogenic strains suggesting a key role for the comprehension of the virus origins. All pathogenic strains are classified within one single serotype, but two subtypes are recognised, RHDV and RHDVa. RHD causes high mortality in both domestic and wild adult animals, with individuals succumbing between 48-72 h post-infection. No other species has been reported to be fatally susceptible to RHD. The disease is characterised by acute necrotising hepatitis, but haemorrhages may also be found in other organs, in particular the lungs, heart, and kidneys due to disseminated intravascular coagulation. Resistance to the disease might be explained in part by genetically determined absence or weak expression of attachment factors, but humoral immunity is also important. Disease control in rabbitries relies mainly on vaccination and biosecurity measures. Such measures are difficult to be implemented in wild populations. More recent research has indicated that RHDV might be used as a molecular tool for therapeutic applications. Although the study of RHDV and RHD has been hampered by the lack of an appropriate cell culture system for the virus, several aspects of the replication, epizootology, epidemiology and evolution have been disclosed. This review provides a broad coverage and description of the current knowledge on the disease and the virus.

Potent norovirus inhibitors based on the acyclic sulfamide scaffold

The development of small molecule therapeutics to combat norovirus infection is of considerable interest from a public health perspective because of the highly contagious nature of noroviruses. A series of amino acid-derived acyclic sulfamide-based norovirus inhibitors has been synthesized and evaluated using a cell-based replicon system. Several compounds were found to display potent anti-norovirus activity, low toxicity, and good aqueous solubility. These compounds are suitable for further optimization of pharmacological and ADMET properties.

Inhibition of noroviruses by piperazine derivatives

There is currently an unmet need for the development of small-molecule therapeutics for norovirus infection. The piperazine scaffold, a privileged structure embodied in many pharmacological agents, was used to synthesize an array of structurally-diverse derivatives which were screened for anti-norovius activity in a cell-based replicon system. The studies described herein demonstrate for the first time that functionalized piperazine derivatives possess anti-norovirus activity. Furthermore, these studies have led to the identification of two promising compounds (6a and 9l) that can be used as a launching pad for the optimization of potency, cytotoxicity, and drug-like characteristics.

Cyclosulfamide-based derivatives as inhibitors of noroviruses

An optimization campaign focused on improving pharmacological activity and physicochemical properties of a recently-identified class of cyclosulfamide-based norovirus inhibitors has been carried out. Dimeric compound 4 was found to be a ∼10-fold more potent norovirus inhibitor (ED(50) 0.4 μM) compared to the original hit, however, isonipecotic acid ester derivatives 7e and 10a were shown to have superior therapeutic indices.

Potent inhibition of Norwalk virus by cyclic sulfamide derivatives

A new class of compounds that exhibit anti-norovirus activity in a cell-based system and embody in their structure a cyclosulfamide scaffold has been identified. The structure of the initial hit (compound 2a, ED(50) 4 μM, TD(50) 50 μM) has been prospected by exploiting multiple points of diversity and generating appropriate structure-activity relationships.

The VIZIER project: overview; expectations; and achievements

VIZIER is an acronym for a research project entitled “Comparative Structural Genomics of Viral Enzymes Involved in Replication” funded by the European Commission between November 1st, 2004 and April 30th, 2009. It involved 25 partners from 12 countries. In this paper, we describe the organization of the project and the culture created by its multidisciplinary essence. We discuss the main thematic sections of the project and the strategy adopted to optimize the integration of various scientific fields into a common objective: to obtain crystal structures of the widest variety of RNA virus replication enzymes documented and validated as potential drug targets. We discuss the thematic sections and their overall organization, their successes and bottlenecks around the protein production pipeline, the “low hanging fruit” strategy, and measures directed to problem solving. We discuss possible future options for such large-scale projects in the area of antiviral drug design. In a series of accompanying papers in Antiviral Research, the project and its achievements are presented for each virus family.