Cross-reactivity among sapovirus recombinant capsid proteins

Collision induced unfolding and molecular dynamics simulations of norovirus capsid dimers reveal strain-specific stability profiles

Collision induced unfolding (CIU) is a method used with ion mobility mass spectrometry to examine protein structures and their stability. Such experiments yield information about higher order protein structures, yet are unable to provide details about the underlying processes. That information can however be provided using molecular dynamics simulations. Here, we investigate the gas-phase unfolding of norovirus capsid dimers from the Norwalk and Kawasaki strains by employing molecular dynamics simulations over a range of temperatures, representing different levels of activation, together with CIU experiments. The dimers have highly similar structures, but their CIU reveals different stability that can be explained by the different dynamics that arises in response to the activation seen in the simulations, including a part of the sequence with previously observed strain-specific dynamics in solution. Our findings show how similar protein variants can be examined using mass spectrometric techniques in conjunction with atomistic molecular dynamics simulations to reveal differences in stability as well as differences in how and where unfolding takes place upon activation.

Fucose Binding Cancels out Mechanical Differences between Distinct Human Noroviruses

The majority of nonbacterial gastroenteritis in humans and livestock is caused by noroviruses. Like most RNA viruses, frequent mutations result in various norovirus variants. The strain-dependent binding profiles of noroviruses to fucose are supposed to facilitate norovirus infection. It remains unclear, however, what the molecular mechanism behind strain-dependent functioning is. In this study, by applying atomic force microscopy (AFM) nanoindentation technology, we studied norovirus-like particles (noroVLPs) of three distinct human norovirus variants. We found differences in viral mechanical properties even between the norovirus variants from the same genogroup. The noroVLPs were then subjected to fucose treatment. Surprisingly, after fucose treatment, the previously found considerable differences in viral mechanical properties among these variants were diminished. We attribute a dynamic switch of the norovirus P domain upon fucose binding to the reduced differences in viral mechanical properties across the tested norovirus variants. These findings shed light on the mechanisms used by norovirus capsids to adapt to environmental changes and, possibly, increase cell infection. Hereby, a new step towards connecting viral mechanical properties to viral prevalence is taken.

Trends on Human Norovirus Virus-like Particles (HuNoV-VLPs) and Strategies for the Construction of Infectious Viral Clones toward In Vitro Replication

Most acute gastroenteritis (AGE) outbreaks and sporadic cases in developing countries are attributable to infection by human norovirus (HuNoV), the enteric virus mainly transmitted via fecal-contaminated water. However, it has been challenging to study HuNoV due to the lack of suitable systems to cultivate and replicate the virus, hindering the development of treatments and vaccines. Researchers have been using virus-like particles (VLPs) and infectious viral clones to overcome this challenge as alternatives to fresh virus isolates in various in vitro and ex vivo models. VLPs are multiprotein structures that mimic the wild-type virus but cannot replicate in host cells due to the lack of genetic materials for replication, limiting downstream analysis of the virus life cycle and pathogenesis. The development of in vitro cloning systems has shown promise for HuNoV replication studies. This review discusses the approaches for constructing HuNoV-VLPs and infectious viral clones, the techniques involved, and the challenges faced. It also highlights the relationship between viral genes and their protein products and provides a perspective on technical considerations for producing efficient HuNoV-VLPs and infectious viral clones, which could substitute for native human noroviruses in future studies.

Timing and genotype distribution of symptomatic and asymptomatic sapovirus infections and re-infections in a Nicaraguan birth cohort

OBJECTIVES

To characterize the timing and genotype distribution of symptomatic and asymptomatic sapovirus infections and re-infections in a Nicaraguan birth cohort.

METHODS

Infants (N = 444) were enrolled at 10-14 days of life and observed weekly until 2 years of age. Stool samples were collected for each acute gastroenteritis (AGE) episode, and routine stool samples were collected monthly. Stool samples were tested for sapovirus using RT-qPCR, and positive samples were genotyped.

RESULTS

A total of 348 children completed 2 years of AGE weekly surveillance; 93 (26.7%) of them experienced sapovirus AGE. Most infections occurred after 5 months of age and mainly during the second year of life (62.4%, 58/93) and early in the rainy season. Sapovirus screening in all stools from a subset of 67 children who consistently provided samples showed sapovirus infections in 91 of 330 (27.6%) AGE episodes and in 39 of 1350 (2.9%) routine stools. In this subset, the median age at the first sapovirus AGE was 11.2 months (95% CI, 9.3-15.9 months); 38 of 67 (57%) children experienced re-infections, 19 symptomatic and 19 asymptomatic. On average, sapovirus re-infections were reported 7.2 months after symptomatic and 5.3 months after asymptomatic infections. Genogroup GI (64%, 69/108) was the most common detected. Sapovirus GI.1 was more frequently detected in AGE stool samples than in routine stool samples (47.2%, 43/91 vs. 25.6%, 10/39; p 0.005), and re-infection with the same genotype was uncommon.

DISCUSSION

The first sapovirus infections occurred at approximately 11 months of age, whereas the median time to symptomatic re-infection was 7.2 months. Re-infections with the same sapovirus genotype were rare during 2 years of life suggesting genotype-specific protection after natural infection.

Structural Basis for Rabbit Hemorrhagic Disease Virus Antibody Specificity

Isolated RHDV antibodies have been used for decades to distinguish between antigenic variants, monitor temporal capsid evolution, and examine neutralizing capacities. In this study, we provided the structural basis for an RHDV GI.2 specific diagnostic antibody (2D9) binding and reveal that a small number of amino acid substitutions at the binding site could differentiate between RHDV GI.2 and GI.1b.

High Genetic Diversity of Porcine Sapovirus From Diarrheic Piglets in Yunnan Province, China

As one of the most important enteric viruses, sapovirus (SaV) can infect humans and a variety of animals. Until now, 19 SaV genogroups have been identified, among which 4 from human (GI, GII, GIV, and GV) and 8 from swine (GIII, GV–GXI). Porcine sapovirus (PoSaV) GIII has been prevalent in China; however, the status of PoSaV infection in Yunnan province remains unknown. In this study, 202 fecal samples were collected from piglets associated with outbreaks of acute diarrhea in Yunnan between January and May 2020. PoSaV detection revealed that the total PoSaV infection rate in Yunnan was 35.2%, with 21 PoSaV strains determined and phylogenetically analyzed. The phylogenetic tree analyses demonstrated that twenty PoSaV strains belonged to GIII and fell into five genotypes, whereas one PoSaV strain (YNQB) belonged to GV. Sequence alignments revealed deletions in VP2 region in 10 of the 20 GIII strains, as well as deletions and insertions in VP1 region of the GV strain (YNQB). Furthermore, genomic recombination analyses showed that two GIII strains (YNAN and YNJD) were recombinants, closely related to reference sequences MK965898 and LC215880, MK965898 and FJ387164, respectively. In summary, PoSaV-GIII strains were identified in Yunnan in 2020, and for the first time, a PoSaV-GV strain was identified from China, whereas the comprehensive analyses illustrated high genetic diversity of Yunnan PoSaV strains. This study may shed new light on the current PoSaV infections in Yunnan and pave the way toward further control of the PoSaV infections in China.

N-terminal VP1 Truncations Favor T = 1 Norovirus-Like Particles

Noroviruses cause immense sporadic gastroenteritis outbreaks worldwide. Emerging genotypes, which are divided based on the sequence of the major capsid protein VP1, further enhance this public threat. Self-assembling properties of the human norovirus major capsid protein VP1 are crucial for using virus-like particles (VLPs) for vaccine development. However, there is no vaccine available yet. Here, VLPs from different variants produced in insect cells were characterized in detail using a set of biophysical and structural tools. We used native mass spectrometry, gas-phase electrophoretic mobility molecular analysis, and proteomics to get clear insights into particle size, structure, and composition, as well as stability. Generally, noroviruses have been known to form mainly T = 3 particles. Importantly, we identified a major truncation in the capsid proteins as a likely cause for the formation of T = 1 particles. For vaccine development, particle production needs to be a reproducible, reliable process. Understanding the underlying processes in capsid size variation will help to produce particles of a defined capsid size presenting antigens consistent with intact virions. Next to vaccine production itself, this would be immensely beneficial for bio-/nano-technological approaches using viral particles as carriers or triggers for immunological reactions.

Human sapovirus propagation in human cell lines supplemented with bile acids

Human sapoviruses (HuSaVs) cause acute gastroenteritis similar to human noroviruses. Although HuSaVs were discovered four decades ago, no HuSaV has been grown in vitro, which has significantly impeded the understanding of viral biology and the development of antiviral strategies. In this study, we identified two susceptible human cell lines, that originated from testis and duodenum, that support HuSaV replication and found that replication requires bile acids. HuSaVs replicated more efficiently in the duodenum cell line, and viral RNA levels increased up to ∼6 log₁₀-fold. We also detected double-stranded RNA, viral nonstructural and structural proteins in the cell cultures, and intact HuSaV particles. We confirmed the infectivity of progeny viruses released into the cell culture supernatants by passaging. These results indicate the successful growth of HuSaVs in vitro. Additionally, we determined the minimum infectious dose and tested the sensitivities of HuSaV GI.1 and GII.3 to heat and ultraviolet treatments. This system is inexpensive, scalable, and reproducible in different laboratories, and can be used to investigate mechanisms of HuSaV replication and to evaluate antivirals and/or disinfection methods for HuSaVs.

Genomic and phylodynamic analysis of sapoviruses isolated in Henan Province, China

In this study, we determined the near-complete and partial genome sequences of ten SaV isolates. Phylogenetic analysis based on full-length VP1 and RdRp nucleotide sequences indicated that nine isolates were of GI.1 and one was GII.3. Evolutionary dynamics analysis indicated that GI.1 and GII.3 SaVs evolved at different rates, the latter evolving more rapidly. Cluster analysis indicated that distantly related GI.1 SaVs were more similar in their amino acid compositions than were GII.3 SaVs. The data provided in this study may facilitate studies on SaV genomic diversity and epidemiological patterns in China and worldwide.

Structural heterogeneity of a human norovirus vaccine candidate

Human norovirus virus-like particles (VLPs) are assumed to be morphologically and antigenically similar to virion particles. The norovirus virion is assembled from 180 copies of the capsid protein (VP1) and exhibits T = 3 icosahedral symmetry. In this study, we showed that the vaccine candidate GII.4c VP1 formed T = 1 and T = 3 VLPs, but mainly assembled into T = 4 icosahedral particles that were composed of 240 VP1 copies. In contrast, another clinically important genotype, GII.17, almost exclusively folded into T = 3 VLPs. Interestingly, the GII.4c T = 1 particles had higher binding capacities to norovirus-specific Nanobodies than to GII.4c T = 3 and T = 4 particles. Our data indicated that the occluded Nanobody-binding epitopes on the T = 1 particles were more accessible compared to the larger T = 3 and T = 4 particles. Overall, this new data revealed that GII.4c VLPs had a preference for forming the T = 4 icosahedral symmetry and future studies with varied sized norovirus VLPs should take caution when examining antigenicity.

Heterologous expression of human norovirus GII.4 VP1 leads to assembly of T=4 virus-like particles

Human noroviruses are a leading cause of acute gastroenteritis, yet there are still no vaccines or antivirals available. Expression of the norovirus capsid protein (VP1) in insect cells typically results in the formation of virus-like particles (VLPs) that are morphologically and antigenically comparable to native virions. Indeed, several different norovirus VLP candidates are currently used in clinical trials. So far, structural analysis of norovirus VLPs showed that the capsid has a T = 3 icosahedral symmetry and is composed of 180 copies of VP1 that are folded into three quasi-equivalent subunits (A, B, and C). In this study, the VLP structures of two norovirus GII.4 genetic variants that were identified in 1974 and 2012 were determined using cryo-EM. Surprisingly, we found that greater than 95% of these GII.4 VLPs were larger than virions and 3D reconstruction showed that these VLPs exhibited T = 4 icosahedral symmetry. We also discovered that the T = 4 VLPs presented several novel structural features. The T = 4 particles assembled from 240 copies of VP1 that adopted four quasi-equivalent conformations (A, B, C, and D) and formed two distinct dimers, A/B and C/D. The protruding domains were elevated ∼21 Å off the capsid shell, which was ∼7 Å more than in the previously studied GII.10 T = 3 VLPs. A small cavity and flap-like structure at the icosahedral two-fold axis disrupted the contiguous T = 4 shell. Overall, our findings indicated that GII.4 VP1 sequences assemble into T = 4 VLPs and these larger particles might have important consequences for VLP-based vaccine development.

Structural Basis of Nanobodies Targeting the Prototype Norovirus

The discovery of vulnerable regions on norovirus particles is instrumental in the development of effective inhibitors, particularly for GI noroviruses that are genetically diverse. Analysis of these GI.1-specific Nanobodies has shown that similar to GII norovirus particles, the GI particles have vulnerable regions. The only known cofactor region, the HBGA binding pocket, represents the main target for inhibition. With a combination treatment, i.e., the addition of Nano-7 or Nano-94 with 2′FL, the effect of inhibition was increased. Therefore, combination drug treatments might offer a better approach to combat norovirus infections, especially since the GI genotypes are highly diverse and are continually changing the capsid landscape, and few conserved epitopes have so far been identified.

Human norovirus infections are a major disease burden. In this study, we analyzed three new norovirus-specific Nanobodies that interacted with the prototype human norovirus (i.e., genogroup I genotype 1 [GI.1]). We showed that the Nanobodies bound on the side (Nano-7 and Nano-62) and top (Nano-94) of the capsid-protruding (P) domain using X-ray crystallography. Nano-7 and Nano-62 bound at a similar region on the P domain, but the orientations of these two Nanobodies clashed with the shell (S) domain and neighboring P domains on intact particles. This finding suggested that the P domains on the particles should shift in order for Nano-7 and Nano-62 to bind to intact particles. Interestingly, both Nano-7 and Nano-94 were capable of blocking norovirus virus-like particles (VLPs) from binding to histo-blood group antigens (HBGAs), which are important cofactors for norovirus infection. Previously, we showed that the GI.1 HBGA pocket could be blocked with the soluble human milk oligosaccharide 2-fucosyllactose (2′FL). In the current study, we showed that a combined treatment of Nano-7 or Nano-94 with 2′FL enhanced the blocking potential with an additive (Nano-7) or synergistic (Nano-94) effect. We also found that GII Nanobodies with 2′FL also enhanced inhibition. The Nanobody inhibition likely occurred by different mechanisms, including particle aggregation or particle disassembly, whereas 2′FL blocked the HBGA binding site. Overall, these new data showed that the positive effect of the addition of 2′FL was not limited to a single mode of action of Nanobodies or to a single norovirus genogroup.

IMPORTANCE The discovery of vulnerable regions on norovirus particles is instrumental in the development of effective inhibitors, particularly for GI noroviruses that are genetically diverse. Analysis of these GI.1-specific Nanobodies has shown that similar to GII norovirus particles, the GI particles have vulnerable regions. The only known cofactor region, the HBGA binding pocket, represents the main target for inhibition. With a combination treatment, i.e., the addition of Nano-7 or Nano-94 with 2′FL, the effect of inhibition was increased. Therefore, combination drug treatments might offer a better approach to combat norovirus infections, especially since the GI genotypes are highly diverse and are continually changing the capsid landscape, and few conserved epitopes have so far been identified.

Prevalence of Human Sapovirus in Low and Middle Income Countries

Background

Sapovirus (SV) infection is a public health concern which plays an important role in the burden of diarrhoeal diseases, causing acute gastroenteritis in people of all ages in both outbreaks and sporadic cases worldwide.

Objective/Study Design

The purpose of this report is to summarise the available data on the detection of human SV in low and middle income countries. A systematic search on PubMed and ScienceDirect database for SV studies published between 2004 and 2017 in low and middle income countries was done. Studies of SV in stool and water samples were part of the inclusion criteria.

Results

From 19 low and middle income countries, 45 published studies were identified. The prevalence rate for SV was 6.5%. A significant difference (P=0) in SV prevalent rate was observed between low income and middle income countries. Thirty-three (78.6%) of the studies reported on children and 8 (19%) studies reported on all age groups with diarrhoea. The majority (66.7%) of studies reported on hospitalised patients with acute gastroenteritis. Sapovirus GI was shown as the dominant genogroup, followed by SV-GII.

Conclusion

The detection of human SV in low and middle income countries is evident; however the reports on its prevalence are limited. There is therefore a need for systematic surveillance of the circulation of SV, and their role in diarrhoeal disease and outbreaks, especially in low and middle income countries.

Seroprevalence of sapovirus in dogs using baculovirus-expressed virus-like particles

Caliciviruses of the Sapovirus genus have been recently detected in dogs. Canine sapoviruses (SaVs) have been identified in the stools of young or juvenile animals with gastro-enteric disease at low prevalence (2.0-2.2%), but whether they may have a role as enteric pathogens and to which extent dogs are exposed to SaVs remains unclear. Here, we report the expression in a baculovirus system of virus like-particles (VLPs) of a canine SaV strain, the prototype virus Bari/4076/2007/ITA. The recombinant antigen was used to develop an enzyme-linked immunosorbent assay (ELISA). By screening an age-stratified collection of serum samples from 516 dogs in Italy, IgG antibodies specific for the canine SaV VLPs were detected in 40.3% (208/516) of the sera. Also, as observed for SaV infection in humans, we observed a positive association between seropositivity and age, with the highest prevalence rates in dogs older than 4 years of age.

Norovirus-like VP1 particles exhibit isolate dependent stability profiles

Noroviruses are the main cause of viral gastroenteritis with new variants emerging frequently. There are three norovirus genogroups infecting humans. These genogroups are divided based on the sequence of their major capsid protein, which is able to form virus-like particles (VLPs) when expressed recombinantly. VLPs of the prototypical GI.1 Norwalk virus are known to disassemble into specific capsid protein oligomers upon alkaline treatment. Here, native mass spectrometry and electron microscopy on variants of GI.1 and of GII.17 were performed, revealing differences in terms of stability between these groups. Beyond that, these experiments indicate differences even between variants within a genotype. The capsid stability was monitored in different ammonium acetate solutions varying both in ionic strength and pH. The investigated GI.1 West Chester isolate showed comparable disassembly profiles to the previously studied GI.1 Norwalk virus isolate. However, differences were observed with the West Chester being more sensitive to alkaline pH. In stark contrast to that, capsids of the variant belonging to the currently prevalent genogroup GII were stable in all tested conditions. Both variants formed smaller capsid particles already at neutral pH. Certain amino acid substitutions in the S domain of West Chester relative to the Norwalk virus potentially result in the formation of these T  =  1 capsids.

Short communication: Immune responses in sows induced by porcine sapovirus virus-like particles reduce viral shedding in suckled piglets

Porcine sapovirus (PoSaV) is a potential threat to public health owing to its capacity for reassortment with human sapovirus strains. However, there is still no vaccine available for the prevention and control of this infectious disease. In this study, we developed PoSaV virus-like particles (VLPs) using a baculovirus expression system. Immunization with PoSaV VLPs induced high titers of serum antibody specific for VP1 in sows. The results of our challenge study demonstrated that maternally-derived antibodies (MDA) induced by VLP immunization dramatically reduced viral shedding of PoSaV in the feces of next generation piglets. Therefore, the results of this study indicate that the immune responses of sows elicited by PoSaV VLPs can inhibit in vivo viral replication in their offspring and represent a promising strategy for developing vaccines against PoSaV.

Nanobodies targeting norovirus capsid reveal functional epitopes and potential mechanisms of neutralization

Norovirus is the leading cause of gastroenteritis worldwide. Despite recent developments in norovirus propagation in cell culture, these viruses are still challenging to grow routinely. Moreover, little is known on how norovirus infects the host cells, except that histo-blood group antigens (HBGAs) are important binding factors for infection and cell entry. Antibodies that bind at the HBGA pocket and block attachment to HBGAs are believed to neutralize the virus. However, additional neutralization epitopes elsewhere on the capsid likely exist and impeding the intrinsic structural dynamics of the capsid could be equally important. In the current study, we investigated a panel of Nanobodies in order to probe functional epitopes that could trigger capsid rearrangement and/ or interfere with HBGA binding interactions. The precise binding sites of six Nanobodies (Nano-4, Nano-14, Nano-26, Nano-27, Nano-32, and Nano-42) were identified using X-ray crystallography. We showed that these Nanobodies bound on the top, side, and bottom of the norovirus protruding domain. The impact of Nanobody binding on norovirus capsid morphology was analyzed using electron microscopy and dynamic light scattering. We discovered that distinct Nanobody epitopes were associated with varied changes in particle structural integrity and assembly. Interestingly, certain Nanobody-induced capsid morphological changes lead to the capsid protein degradation and viral RNA exposure. Moreover, Nanobodies employed multiple inhibition mechanisms to prevent norovirus attachment to HBGAs, which included steric obstruction (Nano-14), allosteric interference (Nano-32), and violation of normal capsid morphology (Nano-26 and Nano-85). Finally, we showed that two Nanobodies (Nano-26 and Nano-85) not only compromised capsid integrity and inhibited VLPs attachment to HBGAs, but also recognized a broad panel of norovirus genotypes with high affinities. Consequently, Nano-26 and Nano-85 have a great potential to function as novel therapeutic agents against human noroviruses.

We determined the binding sites of six novel human norovirus specific Nanobodies (Nano-4, Nano-14, Nano-26, Nano-27, Nano-32, and Nano-42) using X-ray crystallography. The unique Nanobody recognition epitopes were correlated with their potential neutralizing capacities. We showed that one Nanobody (Nano-26) bound numerous genogroup II genotypes and interacted with highly conserved capsid residues. Four Nanobodies (Nano-4, Nano-26, Nano-27, and Nano-42) bound to occluded regions on the intact particles and impaired normal capsid morphology and particle integrity. One Nanobody (Nano-14) bound contiguous to the HBGA pocket and interacted with several residues involved in binding HBGAs. We found that the Nanobodies delivered multiple inhibition mechanisms, which included steric obstruction, allosteric interference, and disruption of the capsid stability. Our data suggested that the HBGA pocket might not be an ideal target for drug development, since the surrounding region is highly variable and inherently suffers from lack of conservation among the genetically diverse genotypes. Instead, we showed that the capsid contained other highly susceptible regions that could be targeted for virus inhibition.

Human norovirus inhibition by a human milk oligosaccharide

Human noroviruses are the leading cause of outbreaks of acute gastroenteritis. Norovirus interactions with histo-blood group antigens (HBGAs) are known to be important for an infection. In this study, we identified the HBGA binding pocket for an emerging GII genotype 17 (GII.17) variant using X-ray crystallography. The GII.17 variant bound the HBGA with an equivalent set of residues as the leading pandemic GII.4 variants. These structural data highlights the conserved nature of HBGA binding site between prevalent GII noroviruses. Noroviruses also interact with human milk oligosaccharides (HMOs), which mimic HBGAs and may function as receptor decoys. We previously showed that HMOs inhibited the binding of rarely detected GII.10 norovirus to HBGAs. We now found that an HMO, 2'-fucosyllactose (2'FL), additionally blocked both the GI.1 and GII.17 noroviruses from binding to HBGAs. Together, these findings provide evidence that 2'FL might function as a broadly reactive antiviral against multiple norovirus genogroups.

Antigenic and Cryo-Electron Microscopy Structure Analysis of a Chimeric Sapovirus Capsid

The capsid protein (VP1) of all caliciviruses forms an icosahedral particle with two principal domains, shell (S) and protruding (P) domains, which are connected via a flexible hinge region. The S domain forms a scaffold surrounding the nucleic acid, while the P domains form a homodimer that interacts with receptors. The P domain is further subdivided into two subdomains, termed P1 and P2. The P2 subdomain is likely an insertion in the P1 subdomain; consequently, the P domain is divided into the P1-1, P2, and P1-2 subdomains. In order to investigate capsid antigenicity, N-terminal (N-term)/S/P1-1 and P2/P1-2 were switched between two sapovirus genotypes GI.1 and GI.5. The chimeric VP1 constructs were expressed in insect cells and were shown to self-assemble into virus-like particles (VLPs) morphologically similar to the parental VLPs. Interestingly, the chimeric VLPs had higher levels of cross-reactivities to heterogeneous antisera than the parental VLPs. In order to better understand the antigenicity from a structural perspective, we determined an intermediate-resolution (8.5-Å) cryo-electron microscopy (cryo-EM) structure of a chimeric VLP and developed a VP1 homology model. The cryo-EM structure revealed that the P domain dimers were raised slightly (∼5 Å) above the S domain. The VP1 homology model allowed us predict the S domain (67-229) and P1-1 (229-280), P2 (281-447), and P1-2 (448-567) subdomains. Our results suggested that the raised P dimers might expose immunoreactive S/P1-1 subdomain epitopes. Consequently, the higher levels of cross-reactivities with the chimeric VLPs resulted from a combination of GI.1 and GI.5 epitopes.

IMPORTANCE

We developed sapovirus chimeric VP1 constructs and produced the chimeric VLPs in insect cells. We found that both chimeric VLPs had a higher level of cross-reactivity against heterogeneous VLP antisera than the parental VLPs. The cryo-EM structure of one chimeric VLP (Yokote/Mc114) was solved to 8.5-Å resolution. A homology model of the VP1 indicated for the first time the putative S and P (P1-1, P2, and P1-2) domains. The overall structure of Yokote/Mc114 contained features common among other caliciviruses. We showed that the P2 subdomain was mainly involved in the homodimeric interface, whereas a large gap between the P1 subdomains had fewer interactions.

Comprehensive review of human sapoviruses

Sapoviruses cause acute gastroenteritis in humans and animals. They belong to the genus Sapovirus within the family Caliciviridae. They infect and cause disease in humans of all ages, in both sporadic cases and outbreaks. The clinical symptoms of sapovirus gastroenteritis are indistinguishable from those caused by noroviruses, so laboratory diagnosis is essential to identify the pathogen. Sapoviruses are highly diverse genetically and antigenically. Currently, reverse transcription-PCR (RT-PCR) assays are widely used for sapovirus detection from clinical specimens due to their high sensitivity and broad reactivity as well as the lack of sensitive assays for antigen detection or cell culture systems for the detection of infectious viruses. Sapoviruses were first discovered in 1976 by electron microscopy in diarrheic samples of humans. To date, sapoviruses have also been detected from several animals: pigs, mink, dogs, sea lions, and bats. In this review, we focus on genomic and antigenic features, molecular typing/classification, detection methods, and clinical and epidemiological profiles of human sapoviruses.

Characterization of St-Valerien-like virus genome detected in Japan

A novel calicivirus, St-Valerien-like virus (SVV), has been identified in asymptomatic swine in Canada, Italy and the U.S.A. In this study, we characterized a new SVV strain (NUP-24/JP) detected in fecal samples of swine in Japan. The NUP-24/JP genome had 6,409 nucleotides and 2 open reading frames (ORF1 and ORF2). ORF1 and ORF2 consist of 5,940 and 453 nucleotides, respectively. Phylogenetic analysis revealed that NUP-24/JP was closely related to other SVV strains, particularly to U.S.A. strain NC-WGS93C/US. This finding suggests that SVV is prevalent in swine worldwide. Using a baculovirus expression system, we successfully produced virus-like particles, which would be useful for seroepidemiological studies of SVV.

Novel monoclonal antibodies broadly reactive to human recombinant sapovirus-like particles

Sapovirus (SaV), a member of the family Caliciviridae, is an important cause of acute epidemic gastroenteritis in humans. Human SaV is genetically and antigenically diverse and can be classified into four genogroups (GI, GII, GIV, and GV) and 16 genotypes (7 GI [GI.1-7], 7 GII, [GII.1-7], 1 GIV and 1 GV), based on capsid sequence similarities. Monoclonal antibodies (MAbs) are powerful tools for examining viruses and proteins. PAI myeloma cells were fused with spleen cells from mice immunized with a single type of recombinant human SaV virus-like particles (VLPs) (GI.1, GI.5, GI.6, GII.3, GIV, or GV). Sixty-five hybrid clones producing MAbs were obtained. Twenty-four MAbs were characterized by ELISA, according to their cross-reactivity to each VLP (GI.1, GI.5, GI.6, GII.2, GII.3, GII.4, GII.7, GIV, and GV). The MAbs were classified by this method into: (i) MAbs broadly cross-reactive to all GI, GII, GIV and GV strains; (ii) those reactive in a genogroup-specific; and (iii) those reactive in a genotype-specific manner. Further analysis of three broadly cross-reactive MAbs with a competitive ELISA demonstrated that at least two different common epitopes are located on the capsid protein of human SaVs in the four genogroups. The MAbs generated and characterized in this study will be useful tools for further study of the antigenic and structural topography of the human SaV virion and for developing new diagnostic assays for human SaV.

A confirmation of sapovirus re-infection gastroenteritis cases with different genogroups and genetic shifts in the evolving sapovirus genotypes, 2002-2011

Sapovirus (SaV) is an important pathogen that causes acute gastroenteritis in humans. Human SaV is highly diverse genetically and is classified into multiple genogroups and genotypes. At present, there is no clear evidence for gastroenteritis cases caused by re-infection with SaV. We found that two individuals were sequentially infected with SaVs of two different genogroups and had gastroenteritis after each infection, although in one of the subsequent cases, both SaV and norovirus were detected. We also found a genetic shift in SaVs from gastroenteritis outpatients in the same geographical location. Our results suggest that protective immunity may be at least genogroup-specific for SaV.

Binding of Norovirus virus-like particles (VLPs) to human intestinal Caco-2 cells and the suppressive effect of pasteurized bovine colostrum on this VLP binding

Noroviruses (NoVs), which cannot be grown in cell culture, are a major infectious agent of gastroenteritis. An in vitro assay system was established for the evaluation of NoV binding to enterocytes using virus-like particles (VLPs) produced in a baculovirus system expressing a NoV VP1 capsid protein. After confirmation of the purity by MS analysis, VLPs were incubated with human intestinal Caco-2 cells. NoV VLPs were detected clearly by confocal laser microscopy only on a certain population of Caco-2 cells, and were semi-quantified by immunoblotting of cell lysates. Then the suppressive effect of pasteurized bovine colostrum was analyzed on the VLP binding to Caco-2 cells by immunoblotting. The colostrum reduced VLP binding in a dose-dependent manner, at about 50% suppression with 12.5 microg of the colostral proteins. Furthermore, the colostrum contained IgG antibodies reacting to VLPs, suggesting that cross-reactive antibodies in the bovine colostrums block human NoV binding to intestinal cells.

Distinct genotype and antigenicity among genogroup II sapoviruses

SaV, a pathogen of acute gastroenteritis, is divided into five genogroups, GI to GV. However, the relation between SaV antigenicity and genetic clusters is not fully understood. We have recently identified two GII SaV strains, Mc10 and C12, which are grouped into the same cluster based on the polymerase but are grouped into distinct clusters based on the capsid. To evaluate the difference in antigenicity between these two strains, VLP were expressed in mammalian cells. An antigen ELISA demonstrated for the first time that strains in the same GII SaV genogroup, but within different clusters, have distinct antigenicities.

Self-assembly of sapovirus recombinant virus-like particles from polyprotein in mammalian cells

The SaV genome is a positive-sense, non-segmented single-strand RNA molecule of approximately 7.5 kb that is polyadenylated at its 3' terminus. The major capsid (VP1) of SaV is thought to be produced as the ORF1 polyprotein followed by cleavage, or translation from subgenomic RNA (3'-coterminal with the virus genome), or both. We have recently reported the formation of SaV VLP from subgenomic-like RNA in mammalian cells. In the present study, we demonstrated that the VP1 cleaved from a part of ORF1 polyprotein self-assembled into VLP in mammalian cells when a transient expression system using a recombinant vaccinia virus encoding T7 RNA polymerase was used.

Sapovirus-like particles derived from polyprotein

We expressed full-length sapovirus genome constructs in insect cells and analyzed their products. The capsid protein was cleaved from the ORF1 polyprotein from a native-like genome construct and two full-length genome constructs with mutations in an active polymerase motif, whereas the capsid protein was not cleaved from a full-length genome construct with a mutation in an active protease motif. Our results showed that the sapovirus protease-polymerase precursor protein cleaved the capsid protein from the polyprotein at the putative conserved capsid start. Importantly, the cleaved capsid protein formed empty virus-like particles that were morphologically and antigenically similar to native sapovirus.

Antigenic diversity of human sapoviruses

Sapovirus (SaV) is a causative agent of gastroenteritis. On the basis of capsid protein (VP1) nucleotide sequences, SaV can be divided into 5 genogroups (GI-GV), of which the GI, GII, GIV, and GV strains infect humans. SaV is uncultivable, but expression of recombinant VP1 in insect cells results in formation of viruslike particles (VLPs) that are antigenically similar to native SaV. In this study, we newly expressed SaV GII and GIV VLPs to compare genetic and antigenic relationships among all human SaV genogroups. Hyperimmune antiserum samples against VLPs reacted strongly with homologous VLPs. However, several antiserum samples weakly cross-reacted against heterologous VLPs in an antibody ELISA. Conversely, an antigen ELISA showed that VLPs of SaV in all human genogroups were antigenically distinct. These findings indicate a likely correspondence between SaV antigenicity and VP1 genogrouping and genotyping.

Human sapoviruses: genetic diversity, recombination, and classification

The family Caliciviridae contains four genera Sapovirus, Norovirus, Lagovirus and Vesivirus, which include Sapporo virus (SaV), Norwalk virus (NoV), Rabbit hemorrhagic disease virus (RHDV) and Feline calicivirus (FCV), respectively. SaV is a causative agent of gastroenteritis in children and adults. SaV can be divided into five genogroups (GI-GV), among which GI, GII, GIV and GV are known to infect humans, whereas SaV GIII infects porcine species. Detection methods include ELISA, RT-PCR and real-time RT-PCR. Since few SaV studies have been conducted, it is difficult to draw correlations between or conclusions about rates of incidence, detection and overall prevalence. Nevertheless, most studies agree that SaV infection is more frequent in young children than adults and that infection in children almost always occurs by 5 years of age. In addition, children at day-care centres and institutions are at greatest risk of SaV-associated infection and transmission. Recently, a number of important findings concerning human SaV were discovered. SaV strains were detected in water samples, which included untreated wastewater specimens, treated wastewater samples and river samples. SaV strains were also detected in shellfish samples destined for human consumption, and recombinant SaV strains were identified in a number of different countries. The purpose of this review was to highlight the current knowledge of human SaV, which appears to be an increasingly important virus causing gastroenteritis in humans.

Outbreak of gastroenteritis due to sapovirus

An outbreak of gastroenteritis occurred at a kindergarten in Yokote City, Japan, between February 2006 and March 2006. Sapovirus was identified in 19 of 26 stool specimens by reverse transcription-PCR. A high viral shedding pattern was found for this strain, which was shown to be antigenically distinct from other genogroups.

Porcine enteric caliciviruses: genetic and antigenic relatedness to human caliciviruses, diagnosis and epidemiology

Porcine enteric caliciviruses include sapoviruses and noroviruses. Porcine sapoviruses infect pigs of all ages and cause diarrhea in young pigs, whereas porcine noroviruses were detected exclusively from adult pigs without clinical signs. Importantly, certain porcine norovirus strains were genetically and antigenically related to human noroviruses. This raises public health concerns that pigs may be reservoirs for emergence of epidemic human norovirus strains. This article reviews the discovery of porcine noroviruses and sapoviruses, their classification, diagnosis, epidemiology and genetic and antigenic relatedness to human caliciviruses.

Genetic variability in the sapovirus capsid protein

Sapovirus (SV), which causes gastroenteritis in humans, is composed of genetically divergent viruses classified into 5 genogroups. In this study, 2.2-kb nucleotide sequences of the 3' terminus of the genome of 15 SV strains detected in Japan were determined. The 15 SV strains could be classified into four genogroups (GI, GII, GIV and GV), and in two of these, GI and GII, 10 genotypes were identified. The amino acid sequences of the central variable region of the capsid protein showed less than 81% identity when strains belonging to different genotypes were compared. It was therefore supposed that antigenic variety exists between different genotypes. These results will be useful for further genetic and antigenic analyses of SV.

Binding activity of norovirus and sapovirus to histo-blood group antigens

Noroviruses (NoVs) and sapoviruses (SaVs) are causative agents of human gastroenteritis. There is increasing evidence that certain human NoV strains bind to histo-blood group antigens (HBGAs). We found that several NoV virus-like particles (VLPs) showed binding activity to HBGAs, while neither SaV genogroup I (GI) VLP nor SaV GV VLP showed such activity.

Development of an enzyme immunoassay for detection of sapovirus-specific antibodies and its application in a study of seroprevalence in children

Sapoviruses (SVs) are an important cause of acute pediatric gastroenteritis. Due to the lack of appropriate diagnostic methods, the epidemiology of SV-associated illness remains poorly understood. Baculovirus and Escherichia coli expression systems were evaluated for the development of antibody detection enzyme immunoassays (EIA). Age-related antibody prevalence in children was studied using the new EIA. Because of the low yield of the baculovirus system, the E. coli-expressed SV capsid proteins were used to develop the EIA. The antigenic specificities of the E. coli-expressed SV capsid proteins were demonstrated using hyperimmune antisera raised in animals and sera collected from patients. A high prevalence (>90%) of antibodies to both SV (strain Mex340) and norovirus (strain VA387) was observed in children involved in a birth cohort at 20 to 24 months of age; however, at 1 to 3 months of age, <25% of the children possessed anti-SV antibodies versus >90% with anti-NV antibodies. The E. coli-derived SV proteins are an excellent source of antigens for the EIA. SV infection is common in the first 2 years of life. The low prevalence of maternal antibodies detected in Mexican children against SVs in this study is unique and needs to be addressed in future studies.

Identification of the cleavage sites of sapovirus open reading frame 1 polyprotein

Sapovirus (SaV), a member of the family Caliciviridae, is a causative agent of acute gastroenteritis in humans and swine and is currently divided into five genogroups, GI–GV. The proteolytic processing of the SaV open reading frame 1 (ORF1) polyprotein with a human GII SaV Mc10 strain has recently been determined and the products are arranged in the following order: NH2–p11–p28–p35 (NTPase)–p32–p14 (VPg)–p70 (Pro–Pol)–p60 (VP1)–COOH. The cleavage site between p14 (VPg) and p70 (Pro–Pol) was identified as E1055/A1056 by N-terminal amino acid sequencing. To identify other cleavage sites, a series of GII SaV Mc10 full-length clones containing disrupted potential cleavage sites in the ORF1 polyprotein were constructed and used to generate linear DNA templates for in vitro coupled transcription–translation. The translation products were analysed by SDS-PAGE or by immunoprecipitation with region-specific antibodies. N-terminal amino acid sequencing with Escherichia coli-expressed recombinant proteins was also used to identify the cleavage site between p32 and p14. These approaches enabled identification of the six cleavage sites of the Mc10 ORF1 polyprotein as E69/G70, Q325/G326, Q666/G667, E940/A941, E1055/A1056 and E1722/G1723. The alignment of the SaV full-length ORF1 amino acid sequences indicated that the dipeptides used for the cleavage sites were either E or Q at the P1 position and A, G or S at the P1′ position, which were conserved in the GI, GII, GIII, GIV and GV SaV ORF1 polyprotein.

Enhancement of sapovirus recombinant capsid protein expression in insect cells

Human sapovirus (SaV) is uncultivable, but expression of the recombinant capsid protein (rVP1) in insect cells results in the formation of virus-like particles (VLPs) that are morphologically similar to the native viruses. However, the SaV rVP1 expression levels are considerably low. We have found that inclusions of short foreign nucleotide sequences inserted directly upstream from the predicted rVP1 AUG start codon lead to increased yield of VLPs. This method allowed us to express a SaV rVP1, which could not have been expressed to measurable or practical levels otherwise.

Intergenogroup recombination in sapoviruses

This first report of intergenogroup recombination for any calicivirus highlights a possible route of zoonoses.

Sapovirus, a member of the family Caliciviridae, is an etiologic agent of gastroenteritis in humans and pigs. Analyses of the complete genome sequences led us to identify the first sapovirus intergenogroup recombinant strain. Phylogenetic analysis of the nonstructural region (i.e., genome start to capsid start) grouped this strain into genogroup II, whereas the structural region (i.e., capsid start to genome end) grouped this strain into genogroup IV. We found that a recombination event occurred at the polymerase and capsid junction. This is the first report of intergenogroup recombination for any calicivirus and highlights a possible route of zoonoses because sapovirus strains that infect pig species belong to genogroup III.

Development of an antigen ELISA to detect sapovirus in clinical stool specimens

Human sapovirus (SaV) strains are etiological agents of mild and/or acute gastroenteritis in children and adults. In this study, we describe the development of a novel antigen enzyme-linked immunosorbent assay (ELISA) detection system that was based on hyperimmune rabbit and guinea pig antisera raised against SaV genogroup I (GI) virus-like particles. The ELISA had 100% specificity, and sensitivities of 60% and 25% when compared to single-round PCR and nested PCR, respectively. Our results have shown the ELISA was useful in detecting SaV GI antigens in clinical stool specimens collected two days after the onset of illness.

Expression of sapovirus virus-like particles in mammalian cells

Sapovirus (SaV) is an etiological agent of acute gastroenteritis in human and swine. SaV can be divided into five genogroups, GI to GV. Virus-like particles (VLPs) morphologically similar to native SaV have been expressed for GI, GII, GIII and GV strains in insect cells, although only low expression levels were observed for GII strains. In this study, we report the successful expression of SaV GII VLPs using cultured mammalian COS-7 and 293T cells. Our results demonstrated that this mammalian expression system was able to express and form SaV VLPs.

Deletion analysis of the sapovirus VP1 gene for the assembly of virus-like particles

Human sapovirus (SaV) strains are agents of gastroenteritis. They cannot be grown in cell culture. In this study, constructs containing SaV N- and C-terminal-deleted recombinant capsid proteins (rVP1) were expressed in a baculovirus expression system to allow us to better understand the sequence requirements for the formation of virus-like particles (VLPs). Only proteins derived from N-terminal-deleted rVP1 constructs that began 49 nucleotides downstream assembled into VLPs, which included both small and native-size VLPs. Our results were similar to those reported in a rabbit hemorrhagic disease virus (RHDV) N- and C-terminal-deleted rVP1 expression study but were distinct from those reported in a norovirus N- and C-terminal-deleted rVP1 expression study, suggesting that SaV and RHDV may have similar expression requirements.

Cleavage activity of the sapovirus 3C-like protease in Escherichia coli

We recently determined the ORF1 cleavage map of Mc10, a human sapovirus (SaV) strain, as follows: NH2-p11-p28-p35(NTPase)-p32-p14(VPg)-p70(Pro-Pol)-p60(VP1)-COOH. This cleavage was dependent on the viral encoded 3C-like protease. To identify the cleavage site of SaV ORF1, putative p70 (Pro-Pol) and p14-p70 (VPg-Pro-Pol) were expressed as N-terminal GST and C-terminal 6 x His-tag fusion proteins in Escherichia coli, and the expressed products were analyzed by SDS-PAGE and Western blotting. Our results indicated that the efficient proteolytic cleavage occurred between p14 (VPg) and p70 (Pro-Pol), and N-terminal amino acid sequencing revealed that the cleavage site was between E(1055) and A(1056). In contrast, the p70 (Pro-Pol) was not further cleaved. We also found that SaV protease cleaved the Q/G site within the rhinovirus 3C protease recognition site. Site-directed mutagenesis in a conserved GDCG motif of the protease completely abolished these proteolytic activities. This is the first report to identify the cleavage site of the SaV ORF1 polyprotein.

Mutational study of sapovirus expression in insect cells

Human sapovirus (SaV), an agent of human gastroenteritis, cannot be grown in cell culture, but expression of the recombinant capsid protein (rVP1) in a baculovirus expression system results in the formation of virus-like particles (VLPs). In this study we compared the time-course expression of two different SaV rVP1 constructs. One construct had the native sequence (Wt construct), whereas the other had two nucleotide point mutations in which one mutation caused an amino acid substitution and one was silent (MEG-1076 construct). While both constructs formed VLPs morphologically similar to native SaV, Northern blot analysis indicated that the MEG-1076 rVP1 mRNA had increased steady-state levels. Furthermore, Western blot analysis and an antigen enzyme-linked immunosorbent assay showed that the MEG-1076 construct had increased expression levels of rVP1 and yields of VLPs. Interestingly, the position of the mutated residue was strictly conserved residue among other human SaV strains, suggesting an important role for rVP1 expression.