Identification of regions and residues in feline junctional adhesion molecule required for feline calicivirus binding and infection

Glycan Recognition in Human Norovirus Infections

Recognition of cell-surface glycans is an important step in the attachment of several viruses to susceptible host cells. The molecular basis of glycan interactions and their functional consequences are well studied for human norovirus (HuNoV), an important gastrointestinal pathogen. Histo-blood group antigens (HBGAs), a family of fucosylated carbohydrate structures that are present on the cell surface, are utilized by HuNoVs to initially bind to cells. In this review, we describe the discovery of HBGAs as genetic susceptibility factors for HuNoV infection and review biochemical and structural studies investigating HuNoV binding to different HBGA glycans. Recently, human intestinal enteroids (HIEs) were developed as a laboratory cultivation system for HuNoV. We review how the use of this novel culture system has confirmed that fucosylated HBGAs are necessary and sufficient for infection by several HuNoV strains, describe mechanisms of antibody-mediated neutralization of infection that involve blocking of HuNoV binding to HBGAs, and discuss the potential for using the HIE model to answer unresolved questions on viral interactions with HBGAs and other glycans.

Isolation and phylogenetic analysis of strains of feline calicivirus in Beijing, China

Feline calicivirus (FCV) is a contagious cat pathogen that causes oral ulceration and/or upper respiratory disease. In this study, we collected 61 samples from a pet hospital in Beijing and used PCR or RT-PCR to detect FCV and feline herpesvirus 1 (FHV-1). Approximately 44.3% (27/61) of the samples were FCV positive, and 23.0% (14/61) were coinfected with FCV and FHV-1. FCV was isolated from 15 samples. One isolate was from a cat with virulent systemic disease (VSD) signs, and 14 isolates were from cats with stomatitis or upper respiratory diseases. The range of genome sequence identity among these isolates was 76.1-100.0%. Four of the isolates were considered to be of the same strain, with sequence identity ranging from 99.5 to 99.7%, and two isolates, BJ-280 and BJ-288, had completely identical sequences. The genomic sequence identity ranged from 76.0 to 88.5% between the 15 isolates and several reference strains, including the F4 and F9 vaccine strains. These results demonstrate that many FCV strains are co-circulating in Beijing. Due to the diversity of FCV in Beijing, it is necessary to monitor the current prevalence of the virus. This study provides more information for the development of effective measures to control FCV.

Bile Goes Viral

Laboratory cultivation of viruses is critical for determining requirements for viral replication, developing detection methods, identifying drug targets, and developing antivirals. Several viruses have a history of recalcitrance towards robust replication in laboratory cell lines, including human noroviruses and hepatitis B and C viruses. These viruses have tropism for tissue components of the enterohepatic circulation system: the intestine and liver, respectively. The purpose of this review is to discuss how key enterohepatic signaling molecules, bile acids (BAs), and BA receptors are involved in the replication of these viruses and how manipulation of these factors was useful in the development and/or optimization of culture systems for these viruses. BAs have replication-promoting activities through several key mechanisms: (1) affecting cellular uptake, membrane lipid composition, and endocytic acidification; (2) directly interacting with viral capsids to influence binding to cells; and (3) modulating the innate immune response. Additionally, expression of the Na+-taurocholate cotransporting polypeptide BA receptor in continuous liver cell lines is critical for hepatitis B virus entry and robust replication in laboratory culture. Viruses are capable of hijacking normal cellular functions, and understanding the role of BAs and BA receptors, components of the enterohepatic system, is valuable for expanding our knowledge on the mechanisms of norovirus and hepatitis B and C virus replication.

Bile acids and ceramide overcome the entry restriction for GII.3 human norovirus replication in human intestinal enteroids

Human noroviruses (HuNoVs) cause sporadic and epidemic outbreaks of gastroenteritis in all age groups worldwide. We previously reported that stem cell-derived human intestinal enteroid (HIE) cultures support replication of multiple HuNoV strains and that some strains (e.g., GII.3) replicate only in the presence of bile. Heat- and trypsin-treatment of bile did not reduce GII.3 replication, indicating a nonproteinaceous component in bile functions as an active factor. Here we show that bile acids (BAs) are critical for GII.3 replication and replication correlates with BA hydrophobicity. Using the highly effective BA, glycochenodeoxycholic acid (GCDCA), we show BAs act during the early stage of infection, BA-dependent replication in HIEs is not mediated by detergent effects or classic farnesoid X receptor or Takeda G protein-coupled receptor 5 signaling but involves another G protein-coupled receptor, sphingosine-1-phosphate receptor 2, and BA treatment of HIEs increases particle uptake. We also demonstrate that GCDCA induces multiple cellular responses that promote GII.3 replication in HIEs, including enhancement of 1) endosomal uptake, 2) endosomal acidification and subsequent activity of endosomal/lysosomal enzyme acid sphingomyelinase (ASM), and 3) ceramide levels on the apical membrane. Inhibitors of endosomal acidification or ASM reduce GII.3 infection and exogenous addition of ceramide alone permits infection. Furthermore, inhibition of lysosomal exocytosis of ASM, which is required for ceramide production at the apical surface, decreases GII.3 infection. Together, our results support a model where GII.3 exploits rapid BA-mediated cellular endolysosomal dynamic changes and cellular ceramide to enter and replicate in jejunal HIEs.

Multiple Correspondence Analysis on Amino Acid Properties within the Variable Region of the Capsid Protein Shows Differences between Classical and Virulent Systemic Feline Calicivirus Strains

Feline calicivirus (FCV) is a widespread and highly prevalent pathogen of domestic cats, responsible for mild upper respiratory tract disease. Outbreaks of severe virulent systemic disease (VSD) associated with FCV infection have been reported worldwide. VSD FCV strains have a broader tropism and cause a systemic vascular compromise. Despite clear differences in the pathogenesis of VSD and oral respiratory infections, attempts to identify specific molecular markers of VSD strains on the major capsid protein VP1 have failed. Region E of VP1 is responsible for the interaction with the cell receptor Junctional Adhesion Molecule JAM-1 (FeJAM-1) and with VP2 minor capsid protein during the entry of the virus. We carried out an original analysis on the sequences from region E of VSD and classical strains. A Multiple Correspondence Analysis was performed on a Boolean matrix built by coding sequences on the basis of their amino acid properties. For the first time, this approach was able to differentiate VSD and classical FCV. Seven remarkable residue positions were shown to be statistically significant for pathotype differentiation, mainly located in the N-terminal hypervariable part of region E. As structural analysis suggested an interaction of these residues with FeJAM-1 or VP2, post-binding events, and specific conformational changes may explain the difference of pathogenesis between pathotypes.

Survivin Overexpression Has a Negative Effect on Feline Calicivirus Infection

It is known that levels of the anti-apoptotic protein survivin are reduced during Murine norovirus MNV-1 and Feline calicivirus (FCV) infection as part of the apoptosis establishment required for virus release and propagation in the host. Recently, our group has reported that overexpression of survivin causes a reduction of FCV protein synthesis and viral progeny production, suggesting that survivin may affect early steps of the replicative cycle. Using immunofluorescence assays, we observed that overexpression of survivin, resulted in the reduction of FCV infection not only in transfected but also in the neighboring nontransfected CrFK cells, thus suggesting autocrine and paracrine protective effects. Cells treated with the supernatants collected from CrFK cells overexpressing survivin showed a reduction in FCV but not MNV-1 protein production and viral yield, suggesting that FCV binding and/or entry were specifically altered. The reduced ability of FCV to bind to the surface of the cells overexpressing survivin, or treated with the supernatants collected from these cells, correlate with the reduction in the cell surface of the FCV receptor, the feline junctional adhesion molecule (fJAM) 1, while no effect was observed in the cells transfected with the pAm-Cyan vector or in cells treated with the corresponding supernatants. Moreover, the overexpression of survivin affects neither Vaccinia virus (VACV) production in CrFK cells nor MNV-1 virus production in RAW 267.4 cells, indicating that the effect is specific for FCV. All of these results taken together indicate that cells that overexpress survivin, or cell treatment with the conditioned medium from these cells, results in the reduction of the fJAM-1 molecule and, therefore, a specific reduction in FCV entry and infection.

Norovirus Attachment and Entry

Human norovirus is a major human pathogen causing the majority of cases of viral gastroenteritis globally. Viral entry is the first step of the viral life cycle and is a significant determinant of cell tropism, host range, immune interactions, and pathogenesis. Bile salts and histo-blood group antigens are key mediators of norovirus entry; however, the molecular mechanisms by which these molecules promote infection and the identity of a potential human norovirus receptor remain unknown. Recently, there have been several important advances in norovirus entry biology including the identification of CD300lf as the receptor for murine norovirus and of the role of the minor capsid protein VP2 in viral genome release. Here, we will review the current understanding about norovirus attachment and entry and highlight important future directions.

The Dynamic Capsid Structures of the Noroviruses

Noroviruses are responsible for almost a fifth of all cases of gastroenteritis worldwide. New strains evolve every 2–4 years by escaping herd immunity and cause worldwide epidemics. In the US alone, noroviruses are responsible for ~20 million cases and more than 70,000 hospitalizations of infected children, annually. Efforts towards a vaccine have been hindered by a lack of detailed structural information about antibody binding and the mechanisms of antibody escape. Caliciviruses have 180 copies of the major capsid protein (VP1; ~58 kDa), that is divided into the N-terminus (N), the shell (S) and C-terminal protruding (P) domains. The S domain forms a shell around the viral RNA genome, while the P domains dimerize to form protrusions on the capsid surface. The P domain is subdivided into P1 and P2 subdomains, with the latter containing the binding sites for cellular receptors and neutralizing antibodies. There is increasing evidence that these viruses are extremely dynamic and this flexibility is critical for viral replication. There are at least two modes of flexibility; the entire P domain relative to the shell and within the P domain itself. Here, the details and possible roles for this remarkable flexibility will be reviewed.

Virus recognition of glycan receptors

Attachment of viruses to cell-surface receptors is the initial step in infection. Many mammalian viruses have evolved to recognize receptors that are glycans on cell-surface glycoproteins or glycolipids. Although glycans are a ubiquitous component of mammalian cells, the types of terminal structures expressed vary among different cell-types and tissues, and even between comparable cells and tissues from different species, frequently leading to specific tissue and species tropisms as a direct consequence of glycan receptor recognition. Covering the majority of known virus families, this review provides an overview of mammalian viruses that use glycans as receptors, and their roles in determining in host recognition and tropism.

Fexaramine as an entry blocker for feline caliciviruses

Feline calicivirus (FCV) is a small non-enveloped virus containing a single-stranded, positive-sense RNA genome of approximately 7.7 kb. FCV is a highly infectious pathogen of cats and typically causes moderate, self-limiting acute oral and upper respiratory tract diseases or chronic oral diseases. In addition, in recent years, virulent, systemic FCV (vs-FCV) strains causing severe systemic diseases with a high mortality rate of up to 67% have been reported in cats. Although FCV vaccines are commercially available, their efficacy is limited due to antigenic diversity of FCV strains and short duration of immunity. In this study, we identified fexaramine as a potent inhibitor of FCV including vs-FCV strains in cell culture and demonstrated that fexaramine is a entry blocker for FCV by using various experiments including time-of-addition studies, generation of resistant viruses in cell culture and the reverse genetics system. A fexaramine resistant FCV mutant has a single amino acid change in the P2 domain of VP1 (the major capsid), and the importance of this mutation for conferring resistance was confirmed using the reverse genetics system. A comparative analysis of viral resistance was also performed using a peptidyl inhibitor (NPI52) targeting FCV 3C-like protease. Finally, the effects of combination treatment of fexaramine and NPI52 against FCV replication and emergence of resistant viruses were investigated in cell culture.

Conserved Surface Residues on the Feline Calicivirus Capsid Are Essential for Interaction with Its Receptor Feline Junctional Adhesion Molecule A (fJAM-A)

Host cell surface receptors are required for attachment, binding, entry, and infection by nonenveloped viruses. Receptor binding can induce conformational changes in the viral capsid and/or the receptor that couple binding with downstream events in the virus life cycle (intracellular signaling, endocytosis and trafficking, and membrane penetration). Virus-receptor interactions also influence viral spread and pathogenicity. The interaction between feline calicivirus (FCV) and its receptor, feline junctional adhesion molecule A (fJAM-A), on host cells is required for infection and induces irreversible, inactivating conformational changes in the capsid of some viral strains. Cryoelectron microscopy (cryo-EM) structures of FCV bound to fJAM-A showed several possible virus-receptor interactions. However, the specific residues on the viral capsid required for binding are not known. Capsid residues that may be involved in postbinding events have been implicated by isolation of soluble receptor-resistant (srr) mutants in which changes in the capsid protein sequence change the capacity of such srr mutants to be inactivated upon incubation with soluble fJAM-A. To clarify which residues on the surface of FCV are required for its interaction with fJAM-A and to potentially identify residues required for postreceptor binding events, we used the existing atomic-resolution structures of FCV and the FCV-fJAM-A cryo-EM structures to select 14 capsid residues for mutation and preparation of recombinant viral capsids. Using this approach, we identified residues on the FCV capsid that are required for fJAM-A binding and other residues that are not required for binding but are required for infection that are likely important for subsequent postbinding events.IMPORTANCE Feline calicivirus (FCV) is a common cause of mild upper respiratory disease in cats. Some FCV isolates can cause virulent systemic disease. The genetic determinants of virulence for FCV are unknown. We previously found that virulent FCV isolates have faster in vitro growth kinetics than less virulent isolates. Differences in viral growth in vitro may correlate with differences in virulence. Here, we investigated the roles of specific FCV capsid residues on the receptor-virus interaction and viral growth in vitro We show that the capsid protein genes of the virulent FCV-5 isolate determine its faster in vitro growth kinetics compared to those of the nonvirulent FCV-Urbana infectious clone. We also identified residues on the capsid VP1 protein that are important for receptor binding or for steps subsequent to receptor binding. Our data provide further insight into the specific molecular interactions between fJAM-A and the FCV capsid that regulate binding and infectious entry.

Identification of Human Junctional Adhesion Molecule 1 as a Functional Receptor for the Hom-1 Calicivirus on Human Cells

The Hom-1 vesivirus was reported in 1998 following the inadvertent transmission of the animal calicivirus San Miguel sea lion virus to a human host in a laboratory. We characterized the Hom-1 strain and investigated the mechanism by which human cells could be infected. An expression library of 3,559 human plasma membrane proteins was screened for reactivity with Hom-1 virus-like particles, and a single interacting protein, human junctional adhesion molecule 1 (hJAM1), was identified. Transient expression of hJAM1 conferred susceptibility to Hom-1 infection on nonpermissive Chinese hamster ovary (CHO) cells. Virus infection was markedly inhibited when CHO cells stably expressing hJAM were pretreated with anti-hJAM1 monoclonal antibodies. Cell lines of human origin were tested for growth of Hom-1, and efficient replication was observed in HepG2, HuH7, and SK-CO15 cells. The three cell lines (of hepatic or intestinal origin) were confirmed to express hJAM1 on their surface, and clustered regularly interspaced short palindromic repeats/Cas9-mediated knockout of the hJAM1 gene in each line abolished Hom-1 propagation. Taken together, our data indicate that entry of the Hom-1 vesivirus into these permissive human cell lines is mediated by the plasma membrane protein hJAM1 as a functional receptor.IMPORTANCE Vesiviruses, such as San Miguel sea lion virus and feline calicivirus, are typically associated with infection in animal hosts. Following the accidental infection of a laboratory worker with San Miguel sea lion virus, a related virus was isolated in cell culture and named Hom-1. In this study, we found that Hom-1 could be propagated in a number of human cell lines, making it the first calicivirus to replicate efficiently in cultured human cells. Screening of a library of human cell surface membrane proteins showed that the virus could utilize human junctional adhesion molecule 1 as a receptor to enter cells and initiate replication. The Hom-1 virus presents a new system for the study of calicivirus biology and species specificity.

Discovery of a proteinaceous cellular receptor for a norovirus

Noroviruses (NoVs) are a leading cause of gastroenteritis globally, yet the host factors required for NoV infection are poorly understood. We identified host molecules that are essential for murine NoV (MNoV)-induced cell death, including CD300lf as a proteinaceous receptor. We found that CD300lf is essential for MNoV binding and replication in cell lines and primary cells. Additionally, Cd300lf(-/-) mice are resistant to MNoV infection. Expression of CD300lf in human cells breaks the species barrier that would otherwise restrict MNoV replication. The crystal structure of the CD300lf ectodomain reveals a potential ligand-binding cleft composed of residues that are critical for MNoV infection. Therefore, the presence of a proteinaceous receptor is the primary determinant of MNoV species tropism, whereas other components of cellular machinery required for NoV replication are conserved between humans and mice.

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.

Nucleolin promotes in vitro translation of feline calicivirus genomic RNA

Feline calicivirus depends on host-cell proteins for its replication. We previously showed that knockdown of nucleolin (NCL), a phosphoprotein involved in ribosome biogenesis, resulted in the reduction of FCV protein synthesis and virus yield. Here, we found that NCL may not be involved in FCV binding and entry into cells, but it binds to both ends of the FCV genomic RNA, and stimulates its translation in vitro. AGRO100, an aptamer that specifically binds and inactivates NCL, caused a strong reduction in FCV protein synthesis. This effect could be reversed by the addition of full-length NCL but not by a ΔrNCL, lacking the N-terminal domain. Consistent with this, FCV infection of CrFK cells stably expressing ΔrNCL led to a reduction in virus protein translation. These results suggest that NCL is part of the FCV RNA translational complex, and that the N-terminal part of the protein is required for efficient FCV replication.

The role of cellular adhesion molecules in virus attachment and entry

As obligate intracellular parasites, viruses must traverse the host-cell plasma membrane to initiate infection. This presents a formidable barrier, which they have evolved diverse strategies to overcome. Common to all entry pathways, however, is a mechanism of specific attachment to cell-surface macromolecules or ‘receptors’. Receptor usage frequently defines viral tropism, and consequently, the evolutionary changes in receptor specificity can lead to emergence of new strains exhibiting altered pathogenicity or host range. Several classes of molecules are exploited as receptors by diverse groups of viruses, including, for example, sialic acid moieties and integrins. In particular, many cell-adhesion molecules that belong to the immunoglobulin-like superfamily of proteins (IgSF CAMs) have been identified as viral receptors. Structural analysis of the interactions between viruses and IgSF CAM receptors has not shown binding to specific features, implying that the Ig-like fold may not be key. Both proteinaceous and enveloped viruses exploit these proteins, however, suggesting convergent evolution of this trait. Their use is surprising given the usually occluded position of CAMs on the cell surface, such as at tight junctions. Nonetheless, the reason for their widespread involvement in virus entry most probably originates in their functional rather than structural characteristics.

Ceramide formation mediated by acid sphingomyelinase facilitates endosomal escape of caliciviruses

Our recent results demonstrated that bile acids facilitate virus escape from the endosomes into the cytoplasm for successful replication of porcine enteric calicivirus (PEC). We report a novel finding that bile acids can be substituted by cold treatment for endosomal escape and virus replication. This endosomal escape by cold treatment or bile acids is associated with ceramide formation by acid sphingomyelinase (ASM). ASM catalyzes hydrolysis of sphingomyelin into ceramide, which is known to destabilize lipid bilayer. Treatment of LLC-PK cells with bile acids or cold led to ceramide formation, and small molecule antagonists or siRNA of ASM blocked ceramide formation in the endosomes and significantly reduced PEC replication. Inhibition of ASM resulted in the retention of PEC, feline calicivirus or murine norovirus in the endosomes in correlation with reduced viral replication. These results suggest the importance of viral escape from the endosomes for the replication of various caliciviruses.

Assessment of the IFN-β response to four feline caliciviruses: Infection in CRFK cells

Feline calicivirus (FCV) is a highly contagious pathogen with a widespread distribution. Although the cat genome has been sequenced, little is known about innate immunity in cats, which limits the understanding of FCV pathogenesis. To investigate the IFN-β response during FCV infection in CRFK cells, we first cloned and identified the feline IFN-β promoter sequence and the positive regulatory domain (PRD) motifs, which shared a high similarity with human and porcine IFN-β promoters. Next, we found that infections with FCV strains F9, Bolin and HRB-SS at the 100 or 1000 TCID50 doses could not activate the IFN-β promoter at 12 and 24h post-infection. Only strain 2280 infection at a 1000 TCID50 dose could induce the IFN-β promoter mainly through IRF3 and partially through NF-κB, at 24h post-infection. However, the IFN response occurred much later and was smaller in magnitude compared with that following Sendai virus (SeV) infection. Further, we found that induction of the IFN-β promoter by FCV 2280 infection depended on dsRNA and not on viral proteins. Finally, we examined whether the IFN-β response had an antiviral effect against FCV replication. The over-expression of IFN-β before exposure to the virus reduced viral yields by a range of 2.2-3.2 log10TCID50, but its over-expression at 12h post-infection did not inhibit FCV replication. Our results indicate that some FCV strains cannot induce IFN-β expression in vitro, which may be a potential factor for FCV survival in cats. Whether this is important in evading the host interferon response in vivo must be investigated.

Expression pattern of immunoglobulin superfamily members in the silkworm, Bombyx mori

Immunoglobulin superfamily (IgSF) proteins are involved in cell adhesion, cell communication and immune functions. In this study, 152 IgSF genes containing at least one immunoglobulin (Ig) domain were predicted in the Bombyx mori silkworm genome. Of these, 145 were distributed on 25 chromosomes with no genes on chromosomes 16, 18 and 26. Multiple sequence alignments and phylogenetic evolution analysis indicated that IgSFs evolved rapidly. Gene ontology (GO) annotation indicated that IgSF members functioned as cellular components and in molecular functions and biological processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis suggested that IgSF proteins were involved in signal transduction, signaling molecules and interaction, and cell communication. Microarray-based expression data showed tissue expression for 136 genes in anterior silkgland, middle silkgland, posterior silkgland, testis, ovary, fat body, midgut, integument, hemocyte, malpighian tubule and head. Expression pattern of IgSF genes in the silkworm ovary and midgut was analyzed by RNA-Seq. Expression of 105 genes was detected in the ovary in strain Dazao. Expression in the midgut was detected for 74 genes in strain Lan5 and 75 genes in strain Ou17. Expression of 34 IgSF genes in the midgut relative to the actin A3 gene was significantly different between strains Lan5 and Ou17. Furthermore, 1 IgSF gene was upregulated and 1 IgSF gene was downregulated in strain Lan5, and 4 IgSF genes were upregulated and 2 IgSF genes were downregulated in strain Ou17 after silkworms were challenged with B. mori cypovirus (BmCPV), indicating potential involvement in the response to BmCPV-infection. These results provide an overview of IgSF family members in silkworms, and lay the foundation for further functional studies.

Histo-blood group antigens: a common niche for norovirus and rotavirus

Noroviruses (NoVs) and rotaviruses (RVs), the two most important causes of viral acute gastroenteritis, are found to recognise histo-blood group antigens (HBGAs) as receptors or ligands for attachment. Human HBGAs are highly polymorphic containing ABO, secretor and Lewis antigens. In addition, both NoVs and RVs are highly diverse in how they recognise these HBGAs. Structural analysis of the HBGA-binding interfaces of NoVs revealed a conserved central binding pocket (CBP) interacting with a common major binding saccharide (MaBS) of HBGAs and a variable surrounding region interacting with additional minor binding saccharides. The conserved CBP indicates a strong selection of NoVs by the host HBGAs, whereas the variable surrounding region explains the diverse recognition patterns of different HBGAs by NoVs and RVs as functional adaptations of the viruses to human HBGAs. Diverse recognition of HBGAs has also been found in bacterial pathogen Helicobacter pylori. Thus, exploratory research into whether such diverse recognitions also occur for other viral and bacterial pathogens that recognise HBGAs is warranted.

Virus inactivation on hard surfaces or in suspension by chemical disinfectants: systematic review and meta-analysis of norovirus surrogates

Norovirus (NoV) infections are the leading cause of foodborne illness in the United States. Effective disinfection is important for controlling outbreaks caused by this highly infectious virus but can be difficult to achieve because NoV is very resistant to many common disinfection protocols. The inability of human NoV to replicate in tissue culture complicates NoV research, generally necessitating genome copy quantification, the use of surrogate viruses, or the use of other substitutes such as virus-like particles. To date, comprehensive comparisons among NoV surrogates and between surrogates and human NoV are missing, and it is not clear how best to extrapolate information from surrogate data. We conducted a systematic review and meta-analysis of comparisons of NoV surrogates with regard to their susceptibility to disinfection on hard surfaces or in suspension. Restricting our analysis to those studies in which two or more virus surrogates were compared allowed us to improve the signal-to-noise ratio in our analysis, similar to the epidemiological concept of matching. Using meta-analysis methods, our results indicate that hepatitis A virus, murine norovirus 1, and phage MS2 are significantly more resistant to disinfection than is feline calicivirus, but average differences in viral titer reduction appeared to be modest, 1.5 log PFU or less in all cases. None of the studies that compared surrogates and human NoV met our inclusion criteria, precluding a direct comparison between human NoV and NoV surrogates in this study. For all surrogates with sufficient data available to permit subgroup analyses, we detected strong evidence that the type of disinfectant impacted the relative susceptibility of the surrogates. Therefore, extrapolation of results between surrogates or from surrogates to human NoV must consider the type of disinfectant studied.

A virus biosensor with single virus-particle sensitivity based on fluorescent vesicle labels and equilibrium fluctuation analysis

Biosensors allowing for the rapid and sensitive detection of viral pathogens in environmental or clinical samples are urgently needed to prevent disease outbreaks and spreading. We present a bioanalytical assay for the detection of whole viral particles with single virus sensitivity. Specifically, we focus on the detection of human norovirus, a highly infectious virus causing gastroenteritis. In our assay configuration, virus-like particles are captured onto a supported lipid bilayer containing a virus-specific glycolipid and detected after recognition by a glycolipid-containing fluorescent vesicle. Read-out is performed after illumination of the vesicle labels by total internal reflection fluorescence microscopy. This allows for visualization of individual vesicles and for recording of their binding kinetics under equilibrium conditions (equilibrium fluctuation analysis), as demonstrated previously. In this work we extend the concept and demonstrate that this simple assay setup can be used as a bioanalytical assay for the detection of virus particles at a limit of detection of 16 fM. Furthermore, we demonstrate how the analysis of the single vesicle-virus-like particle interaction dynamics can contribute to increase the accuracy and sensitivity of the assay by discriminating specific from non-specific binding events. This method is suggested to be generally applicable, provided that these events display different interaction kinetics.

The cryo-electron microscopy structure of feline calicivirus bound to junctional adhesion molecule A at 9-angstrom resolution reveals receptor-induced flexibility and two distinct conformational changes in the capsid protein VP1

Caliciviridae are small icosahedral positive-sense RNA-containing viruses and include the human noroviruses, a leading cause of infectious acute gastroenteritis and feline calicivirus (FCV), which causes respiratory illness and stomatitis in cats. FCV attachment and entry is mediated by feline junctional adhesion molecule A (fJAM-A), which binds to the outer face of the capsomere, inducing a conformational change in the capsid that may be important for viral uncoating. Here we present the results of our structural investigation of the virus-receptor interaction and ensuing conformational changes. Cryo-electron microscopy and three-dimensional image reconstruction were used to solve the structure of the virus decorated with a soluble fragment of the receptor at subnanometer resolution. In initial reconstructions, the P domains of the capsid protein VP1 and fJAM-A were poorly resolved. Sorting experiments led to improved reconstructions of the FCV-fJAM-A complex both before and after the induced conformational change, as well as in three transition states. These data showed that the P domain becomes flexible following fJAM-A binding, leading to a loss of icosahedral symmetry. Furthermore, two distinct conformational changes were seen; an anticlockwise rotation of up to 15° of the P domain was observed in the AB dimers, while tilting of the P domain away from the icosahedral 2-fold axis was seen in the CC dimers. A list of putative contact residues was calculated by fitting high-resolution coordinates for fJAM-A and VP1 to the reconstructed density maps, highlighting regions in both virus and receptor important for virus attachment and entry.

Alternative attachment factors and internalization pathways for GIII.2 bovine noroviruses

Bovine noroviruses belong to the family Caliciviridae, genus Norovirus. Two genotypes have been described and viruses genetically related to the Jena and Newbury2 strains have been classified into genotypes 1 and 2, respectively. In this study, virus-like particles (VLP) of the previously detected B309 Belgian strain, genetically related to genotype 2 bovine noroviruses, were used to investigate virus-host interactions in vitro. B309 VLP were shown to bind to several bovine cell lines. This binding was not affected by heparinase or chondroitinase treatment but was significantly inhibited by both sodium periodate, α-galactosidase, trypsin and phospholipase C treatment. Cell treatment by neuraminidase also moderately affected this binding. Taken together, these results show that, in addition to a galactosyl residue, sialic acid could also be involved in binding to susceptible cells. In addition, both the cholesterol-dependent pathway and macropinocytosis are used for B309 VLP internalization by Madin-Darby bovine kidney cells. The data increase the knowledge on bovine norovirus cell interactions.

Nested case-control study of feline calicivirus viruria, oral carriage, and serum neutralizing antibodies in cats with idiopathic cystitis

BACKGROUND

The epidemiology of feline calicivirus (FCV) infection in cats with idiopathic cystitis (FIC) has not been investigated by contemporary molecular biologic methods.

OBJECTIVES

To determine the prevalence of and evaluate risk factors for FCV viruria, oral carriage, and virus neutralizing (VN) antibodies in cats with and without FIC.

ANIMALS

Cats with nonobstructive FIC (n = 47), obstructive FIC (n = 22), and FCV upper respiratory tract infection (URI; n = 25), and healthy client-owned (n = 18) and colony-housed (n = 24) cats.

METHODS

Oropharyngeal secretions and urine were evaluated with a FCV p30 gene-based real-time reverse-transcriptase polymerase chain reaction (RT-PCR) assay. Serum VN antibody titers were determined by a modified microtiter assay. Associations of risk factors with log-transformed antibody titers were determined by multivariable generalized linear regression.

RESULTS

FCV viruria was detected in 4 (6%) and 3 (12%) cats with FIC and URI, respectively. In 3 FIC cats, viruria was unassociated with detectable oral virus carriage. Oral FCV carriage was detected in 7 (10%) FIC cats. Median antibody titers were significantly higher in cats with obstructive FIC (1 :256), nonobstructive FIC (1:128), and URI (1:512) compared with healthy client-owned (1:16) and colony-housed (1:4) cats (P < .001). Other than disease, multivariate analysis did not identify any other explanatory variables for increased titers in cats with FIC or URI.

CONCLUSIONS AND CLINICAL IMPORTANCE

FCV viruria was detected in cats with FIC and URI, however, its etiologic significance is uncertain. Serologic results suggest increased FCV exposure in FIC cats compared with controls. Further investigations are needed to clarify the potential role of FCV in FIC.

Distribution of the Feline Calicivirus Receptor Junctional Adhesion Molecule A in Feline Tissues

Junctional adhesion molecule A (JAM-A) is an immunoglobulin superfamily protein that plays an important role in the assembly and maintenance of tight junctions and the establishment of epithelial cell polarity. The feline JAM-A (fJAM-A) is a functional receptor for feline calicivirus (FCV). Among natural diseases associated with FCV infection, isolates that cause oral vesicular disease are detected in epithelial cells; however, isolates that cause systemic disease are detected in multiple cell types. The distribution of an FCV receptor or receptors in feline tissues is relevant to viral pathogenesis in that it should reflect the wide latitude of clinical sequelae associated with FCV infection. The authors examined the expression of feline JAM-A in the cat by using confocal immunofluorescence localization on normal tissues, with special regard to tissue targets of naturally occurring FCV. As described in the human and the mouse, fJAM-A was widely distributed in feline tissues, where it localized at cell–cell junctions of epithelial and endothelial cells. fJAM-A was highly expressed on feline platelets, with lower levels of expression on feline peripheral blood leukocytes. Additionally, FCV infection of a feline epithelial cell monolayer causes redistribution of fJAM-A to the cytosol of infected cells. It is reasonable to propose that the spectrum of lesions caused by FCV reflects disruption of intercellular junctions that rely on fJAM-A function and tight junctional integrity.

Glycosphingolipids as receptors for non-enveloped viruses

Glycosphingolipids are ubiquitous molecules composed of a lipid and a carbohydrate moiety. Their main functions are as antigen/toxin receptors, in cell adhesion/recognition processes, or initiation/modulation of signal transduction pathways. Microbes take advantage of the different carbohydrate structures displayed on a specific cell surface for attachment during infection. For some viruses, such as the polyomaviruses, binding to gangliosides determines the internalization pathway into cells. For others, the interaction between microbe and carbohydrate can be a critical determinant for host susceptibility. In this review, we summarize the role of glycosphingolipids as receptors for members of the non-enveloped calici-, rota-, polyoma- and parvovirus families.

Conformational changes in the capsid of a calicivirus upon interaction with its functional receptor

Nonenveloped viral capsids are metastable structures that undergo conformational changes during virus entry that lead to interactions of the capsid or capsid fragments with the cell membrane. For members of the Caliciviridae, neither the nature of these structural changes in the capsid nor the factor(s) responsible for inducing these changes is known. Feline junctional adhesion molecule A (fJAM-A) mediates the attachment and infectious viral entry of feline calicivirus (FCV). Here, we show that the infectivity of some FCV isolates is neutralized following incubation with the soluble receptor at 37 degrees C. We used this property to select mutants resistant to preincubation with the soluble receptor. We isolated and sequenced 24 soluble receptor-resistant (srr) mutants and characterized the growth properties and receptor-binding activities of eight mutants. The location of the mutations within the capsid structure of FCV was mapped using a new 3.6-A structure of native FCV. The srr mutations mapped to the surface of the P2 domain were buried at the protruding domain dimer interface or were present in inaccessible regions of the capsid protein. Coupled with data showing that both the parental FCV and the srr mutants underwent increases in hydrophobicity upon incubation with the soluble receptor at 37 degrees C, these findings indicate that FCV likely undergoes conformational change upon interaction with its receptor. Changes in FCV capsid conformation following its interaction with fJAM-A may be important for subsequent interactions of the capsid with cellular membranes, membrane penetration, and genome delivery.

The caliciviruses

The caliciviruses are by far the major cause of non-bacterial gastroenteritis, highly infectious, and have a rapid and severe onset of symptoms. Studies on this family of viruses have been hampered by the lack of animal model and tissue culture system. However, recent advances in protein expression systems and the development of a mouse norovirus animal model has led to rapid advances in our understanding of these viruses with regard to structure and the host immune response. Our current understanding of this important family of viruses is reviewed here.

Immunoglobulin superfamily is conserved but evolved rapidly and is active in the silkworm, Bombyx mori

Immunoglobulin superfamily (IgSF) proteins are known for their abilities to specifically recognize and adhere to cells. In this paper, we predicted the presence of 133 IgSF proteins in the silkworm (Bombyx mori) genome. Comparison with similar proteins in other model organisms (Caenorhabditis elegans, Drosophila melanogaster, Anopheles gambiae, Apis mellifera and Homo sapiens) indicated that IgSF proteins are conserved but have rapidly evolved from worms to human beings. However, these proteins are well conserved amongst insects. Silkworm microarray-based expression data showed tissue expression of 57 IgSF genes and microbe-induced differential expression of 37 genes. Based on the expression data, we can conclude that the silkworm IgSF is active.

Structure of reovirus sigma1 in complex with its receptor junctional adhesion molecule-A

Viral attachment to specific host receptors is the first step in viral infection and serves an essential function in the selection of target cells. Mammalian reoviruses are highly useful experimental models for studies of viral pathogenesis and show promise as vectors for oncolytics and vaccines. Reoviruses engage cells by binding to carbohydrates and the immunoglobulin superfamily member, junctional adhesion molecule-A (JAM-A). JAM-A exists at the cell surface as a homodimer formed by extensive contacts between its N-terminal immunoglobulin-like domains. We report the crystal structure of reovirus attachment protein σ1 in complex with a soluble form of JAM-A. The σ1 protein disrupts the JAM-A dimer, engaging a single JAM-A molecule via virtually the same interface that is used for JAM-A homodimerization. Thus, reovirus takes advantage of the adhesive nature of an immunoglobulin-superfamily receptor by usurping the ligand-binding site of this molecule to attach to the cell surface. The dissociation constant (K_D) of the interaction between σ1 and JAM-A is 1,000-fold lower than that of the homophilic interaction between JAM-A molecules, indicating that JAM-A strongly prefers σ1 as a ligand. Analysis of reovirus mutants engineered by plasmid-based reverse genetics revealed residues in σ1 required for binding to JAM-A and infectivity of cultured cells. These studies define biophysical mechanisms of reovirus cell attachment and provide a platform for manipulating reovirus tropism to enhance vector targeting.

Mammalian orthoreoviruses (reoviruses) are useful models for studies of virus–receptor interactions and viral pathogenesis. They are closely related in structure to adenoviruses and share similar mechanisms of cell attachment and entry. The receptor for reovirus, junctional adhesion molecule-A (JAM-A), is a component of cellular junctions and also used as a receptor by feline calicivirus. To better understand how viruses engage cellular receptors, we determined the structure of reovirus attachment protein σ1 bound to JAM-A. The structure provides an understanding of the biological function of the interaction and yields information that may enable targeting of reovirus to alternate receptors. Since the repertoire of receptors bound by a virus contributes importantly to determining which types of cells are infected, such targeting plays an essential role in gene delivery for vaccine or therapeutic applications. New cancer therapy approaches include the use of viruses, including reovirus, to lyse tumor cells. New knowledge about reovirus attachment to cellular receptors at an atomic level will help to harness the therapeutic potential of this virus.

Structural insights into calicivirus attachment and uncoating

The Caliciviridae family comprises positive-sense RNA viruses of medical and veterinary significance. In humans, caliciviruses are a major cause of acute gastroenteritis, while in animals respiratory illness, conjunctivitis, stomatitis, and hemorrhagic disease are documented. Investigation of virus-host interactions is limited by a lack of culture systems for many viruses in this family. Feline calicivirus (FCV), a member of the Vesivirus genus, provides a tractable model, since it may be propagated in cell culture. Feline junctional adhesion molecule 1 (fJAM-1) was recently identified as a functional receptor for FCV. We have analyzed the structure of this virus-receptor complex by cryo-electron microscopy and three-dimensional image reconstruction, combined with fitting of homology modeled high-resolution coordinates. We show that domain 1 of fJAM-1 binds to the outer face of the P2 domain of the FCV capsid protein VP1, inducing conformational changes in the viral capsid. This study provides the first structural view of a native calicivirus-protein receptor complex and insights into the mechanisms of virus attachment and uncoating.