Comparison of human, simian, and bovine rotaviruses for requirement of sialic acid in hemagglutination and cell adsorption

Milk lactose protects against porcine group A rotavirus infection

Rotavirus A (RVA) is an important pathogen causing acute gastroenteritis in animals and humans. Attachment to the host receptor is a crucial step for virus replication. The VP8* domain is the distal terminal region of the RVA spike protein VP4 (expressed by the P gene) and is important for rotavirus binding and infectivity. Recent studies have indicated a role for non-sialylated glycans, including mucin core 2 and histo-blood group antigens (HBGAs), in the infectivity of human and animal group A rotaviruses. In the present study, we determined if porcine rotavirus-derived recombinant VP8* of the endemic strains 14R103 G5P[6], 13R054 G5P[7], 12R010 G4P[13], 12R046 G9P[23], and 12R022 G2P[27] interact with hitherto uncharacterized glycans. We successfully produced five recombinant GST-VP8* proteins of genotype P[6], P[7], P[13], P[23], and P[27]. The hemagglutination assay showed genotypes P[7] and P[23] hemagglutinate porcine and human red blood cells. In an array screen of > 300 glycans, recombinant VP8* of rotavirus genotype P[6], P[7], and P[13] showed specific binding to glycans with a Gal-β-1,4-Glc (β-lactose) motif, which forms the core structure of HBGAs. The specificity of glycan-binding was confirmed through an ELISA-based oligosaccharide binding assay. Further, 13R054 G5P[7] and 12R046 G9P[23] infectivity was significantly reduced by β-lactose in MA104 cells and primary porcine enterocytes. These data suggest that lactose, the main natural sugar in milk, plays an important role in protecting piglets from enteric viral replication and diarrhea.

Rotavirus Interactions With Host Intestinal Epithelial Cells

Rotavirus (RV) is the foremost enteric pathogen associated with severe diarrheal illness in young children (<5years) and animals worldwide. RV primarily infects mature enterocytes in the intestinal epithelium causing villus atrophy, enhanced epithelial cell turnover and apoptosis. Intestinal epithelial cells (IECs) being the first physical barrier against RV infection employs a range of innate immune strategies to counteract RVs invasion, including mucus production, toll-like receptor signaling and cytokine/chemokine production. Conversely, RVs have evolved numerous mechanisms to escape/subvert host immunity, seizing translation machinery of the host for effective replication and transmission. RV cell entry process involve penetration through the outer mucus layer, interaction with cell surface molecules and intestinal microbiota before reaching the IECs. For successful cell attachment and entry, RVs use sialic acid, histo-blood group antigens, heat shock cognate protein 70 and cell-surface integrins as attachment factors and/or (co)-receptors. In this review, a comprehensive summary of the existing knowledge of mechanisms underlying RV-IECs interactions, including the role of gut microbiota, during RV infection is presented. Understanding these mechanisms is imperative for developing efficacious strategies to control RV infections, including development of antiviral therapies and vaccines that target specific immune system antagonists within IECs.

Structural Basis of Glycan Recognition of Rotavirus

Rotavirus (RV) is an important pathogen causing acute gastroenteritis in young humans and animals. Attachment to the host receptor is a crucial step for the virus infection. The recent advances in illustrating the interactions between RV and glycans promoted our understanding of the host range and epidemiology of RVs. VP8*, the distal region of the RV outer capsid spike protein VP4, played a critical role in the glycan recognition. Group A RVs were classified into different P genotypes based on the VP4 sequences and recognized glycans in a P genotype-dependent manner. Glycans including sialic acid, gangliosides, histo-blood group antigens (HBGAs), and mucin cores have been reported to interact with RV VP8*s. The glycan binding specificities of VP8*s of different RV genotypes have been studied. Here, we mainly discussed the structural basis for the interactions between RV VP8*s and glycans, which provided molecular insights into the receptor recognition and host tropism, offering new clues to the design of RV vaccine and anti-viral agents.

Advances in the Development of Antiviral Compounds for Rotavirus Infections

Group A rotaviruses (RVAs) are the major cause of severe acute gastroenteritis (AGE) in children under 5 years of age, annually resulting in nearly 130,000 deaths worldwide. Social conditions in developing countries that contribute to decreased oral rehydration and vaccine efficacy and the lack of approved antiviral drugs position RVA as a global health concern. In this minireview, we present an update in the field of antiviral compounds, mainly in relation to the latest findings in RVA virion structure and the viral replication cycle. In turn, we attempt to provide a perspective on the possible treatments for RVA-associated AGE, with special focus on novel approaches, such as those representing broad-spectrum therapeutic options. In this context, the modulation of host factors, lipid droplets, and the viral polymerase, which is highly conserved among AGE-causing viruses, are analyzed as possible drug targets.

Rotavirus Replication: Gaps of Knowledge on Virus Entry and Morphogenesis

In 1973, rotaviruses A (RVAs) were discovered as major causative agents of acute gastroenteritis in infants and young children worldwide. The infectious RV virion is an icosahedral particle composed of three concentric protein layers surrounding the 11 double-stranded (dsRNA) segments. An in vitro replication system for RVs in permanent cell lines was developed in 1982 and expanded to replication in intestinal organoids in 2015. However, the details of rotavirus (RV) entry into cells and particle maturation mechanisms at the molecular level remain incompletely understood. Slowing down human RVA replication in cell culture on ice allowed morphological visualization of virus particle entry and the assembly of triple-layered particles (virion). Although RVAs are non-enveloped viruses, after virus attachment to the cell membrane, the virus enters the cell by perforating the plasma membrane by a fusion mechanism involving VP5* of the cleaved VP4 protein, as the alternative virus entry route besides the receptor-mediated endocytosis which is generally accepted. After assembling double-layered particles (DLPs) in viroplasm or cytoplasm, they appear to be connected with the endoplasmic reticulum (ER) membrane and become coated with outer capsid proteins (VP4 and VP7) in a coating process. The perforation of the ER membrane is caused by an unknown mechanism following interaction between non-structural protein 4 (NSP4) and the inner capsid protein VP6 of the DLPs. The coating process is closely related to the formation of a hetero-oligomeric complex (NSP4, VP4 and VP7). These lines of evidence suggest the existence of novel mechanisms of RV morphogenesis.

The Symmetry of Viral Sialic Acid Binding Sites-Implications for Antiviral Strategies

Virus infections are initiated by the attachment of the viral particle to protein or carbohydrate receptors on the host cell. Sialic acid-bearing glycan structures are prominently displayed at the cell surface, and, consequently, these structures can function as receptors for a large number of diverse viruses. Structural biology research has helped to establish the molecular bases for many virus–sialic acid interactions. Due to the icosahedral 532 point group symmetry that underlies many viral capsids, the receptor binding sites are frequently arranged in a highly symmetric fashion and linked by five-fold, three-fold, or two-fold rotation axes. For the inhibition of viral attachment, one emerging strategy is based on developing multivalent sialic acid-based inhibitors that can simultaneously engage several of these binding sites, thus binding viral capsids with high avidity. In this review, we will evaluate the structures of non-enveloped virus capsid proteins bound to sialylated glycan receptors and discuss the potential of these structures for the development of potent antiviral attachment inhibitors.

Glycosphingolipid-Protein Interaction in Signal Transduction

Glycosphingolipids (GSLs) are a class of ceramide-based glycolipids essential for embryo development in mammals. The synthesis of specific GSLs depends on the expression of distinctive sets of GSL synthesizing enzymes that is tightly regulated during development. Several reports have described how cell surface receptors can be kept in a resting state or activate alternative signalling events as a consequence of their interaction with GSLs. Specific GSLs, indeed, interface with specific protein domains that are found in signalling molecules and which act as GSL sensors to modify signalling responses. The regulation exerted by GSLs on signal transduction is orthogonal to the ligand–receptor axis, as it usually does not directly interfere with the ligand binding to receptors. Due to their properties of adjustable production and orthogonal action on receptors, GSLs add a new dimension to the control of the signalling in development. GSLs can, indeed, dynamically influence progenitor cell response to morphogenetic stimuli, resulting in alternative differentiation fates. Here, we review the available literature on GSL–protein interactions and their effects on cell signalling and development.

Diversity in Rotavirus-Host Glycan Interactions: A "Sweet" Spectrum

Interaction with cellular glycans is a critical initial step in the pathogenesis of many infectious agents. Technological advances in glycobiology have expanded the repertoire of studies delineating host glycan-pathogen interactions. For rotavirus, the VP8* domain of the outer capsid spike protein VP4 is known to interact with cellular glycans. Sialic acid was considered the key cellular attachment factor for rotaviruses for decades. Although this is true for many rotavirus strains causing infections in animals, glycan array screens show that many human rotavirus strains bind nonsialylated glycoconjugates, called histo-blood group antigens, in a strain-specific manner. The expression of histo-blood group antigens is determined genetically and is regulated developmentally. Variations in glycan binding between different rotavirus strains are biologically relevant and provide new insights into multiple aspects of virus pathogenesis such as interspecies transmission, host range restriction, and tissue tropism. The genetics of glycan expression may affect susceptibility to different rotavirus strains and vaccine viruses, and impact the efficacy of rotavirus vaccination in different populations. A multidisciplinary approach to understanding rotavirus-host glycan interactions provides molecular insights into the interaction between microbial pathogens and glycans, and opens up new avenues to translate findings from the bench to the human population.

Expanding diversity of glycan receptor usage by rotaviruses

Rotaviruses are major etiologic agents of severe gastroenteritis in human and animals, infecting the mature intestinal epithelium. Their attachment to host cell glycans is mediated through the virion spike protein. This is considered to be crucial for successful host cell invasion by rotaviruses. Recent studies have greatly expanded our understanding of the diversity of glycans commonly recognized by rotaviruses, to include the ganglioside GM1a and histo-blood group antigens. Here, these new findings are integrated with advances in knowledge of spike protein structure, rotavirus entry mechanisms and innate intestinal immunity to provide an overview of the variety of rotavirus glycan receptors and their roles in cell penetration, host tropism and pathogenesis.

Carbohydrate–Protein Interactions: A 3D View by NMR

NMR spectroscopy is a key tool for carbohydrate research. In studies with complex oligosaccharides there are limits to the amount of relevant structural information provided by these observables due to problems of signal overlapping, strong coupling and/or the scarcity of the key NOE information. Thus, there is an increasing need for additional parameters with structural information, such as residual dipolar couplings (RDCs), paramagnetic relaxation enhancements (PREs) or pseudo contact shifts (PCSs). Carbohydrates are rather flexible molecules. Therefore, NMR observables do not always correlate with a single conformer but with an ensemble of low free-energy conformers that can be accessed by thermal fluctuations. Depending on the system under study, different NMR approaches can be followed to characterize protein–carbohydrate interactions: the standard methodologies can usually be classified as “ligand-based” or “receptor-based”. The selection of the proper methodology is usually determined by the size of the receptor, the dissociation constant of the complex (KD), the availability of the labelled protein (15N, 13C) and the access to soluble receptors at enough concentration for NMR measurements.

Investigating virus-glycan interactions using glycan microarrays

While all viruses must transit the plasma membrane of mammalian cells to initiate infection, we know little about the complex processes involved in viral attachment, which commonly involve recognition of glycans by viral proteins. Glycan microarrays derived from both synthetic glycans and natural glycans isolated through shotgun glycomics approaches provide novel platforms for interrogating diverse glycans as potential viral receptors. Recent studies with influenza and rotaviruses using such glycan microarrays provide examples of their utility in exploring the challenging questions raised in efforts to define the complex mechanistic protein-glycan interactions that regulate virus attachment to host cells.

Relative roles of GM1 ganglioside, N-acylneuraminic acids, and α2β1 integrin in mediating rotavirus infection

N-acetyl- and N-glycolylneuraminic acids (Sia) and α2β1 integrin are frequently used by rotaviruses as cellular receptors through recognition by virion spike protein VP4. The VP4 subunit VP8*, derived from Wa rotavirus, binds the internal N-acetylneuraminic acid on ganglioside GM1. Wa infection is increased by enhanced internal Sia access following terminal Sia removal from main glycan chains with sialidase. The GM1 ligand cholera toxin B (CTB) reduces Wa infectivity. Here, we found sialidase treatment increased cellular GM1 availability and the infectivity of several other human (including RV-3) and animal rotaviruses, typically rendering them susceptible to methyl α-d-N-acetylneuraminide treatment, but did not alter α2β1 usage. CTB reduced the infectivity of these viruses. Aceramido-GM1 inhibited Wa and RV-3 infectivity in untreated and sialidase-treated cells, and GM1 supplementation increased their infectivity, demonstrating the importance of GM1 for infection. Wa recognition of α2β1 and internal Sia were at least partially independent. Rotavirus usage of GM1 was mapped to VP4 using virus reassortants, and RV-3 VP8* bound aceramido-GM1 by saturation transfer difference nuclear magnetic resonance (STD NMR). Most rotaviruses recognizing terminal Sia did not use GM1, including RRV. RRV VP8* interacted minimally with aceramido-GM1 by STD NMR. Unusually, TFR-41 rotavirus infectivity depended upon terminal Sia and GM1. Competition of CTB, Sia, and/or aceramido-GM1 with cell binding by VP8* from representative rotaviruses showed that rotavirus Sia and GM1 preferences resulted from VP8*-cell binding. Our major finding is that infection by human rotaviruses of commonly occurring VP4 serotypes involves VP8* binding to cell surface GM1 glycan, typically including the internal N-acetylneuraminic acid.

IMPORTANCE

Rotaviruses, the major cause of severe infantile gastroenteritis, recognize cell surface receptors through virus spike protein VP4. Several animal rotaviruses are known to bind sialic acids at the termini of main carbohydrate chains. Conversely, only a single human rotavirus is known to bind sialic acid. Interestingly, VP4 of this rotavirus bound to sialic acid that forms a branch on the main carbohydrate chain of the GM1 ganglioside. Here, we use several techniques to demonstrate that other human rotaviruses exhibit similar GM1 usage properties. Furthermore, binding by VP4 to cell surface GM1, involving branched sialic acid recognition, is shown to facilitate infection. In contrast, most animal rotaviruses that bind terminal sialic acids did not utilize GM1 for VP4 cell binding or infection. These studies support a significant role for GM1 in mediating host cell invasion by human rotaviruses.

Rotavirus VP8*: phylogeny, host range, and interaction with histo-blood group antigens

The distal portion of rotavirus (RV) VP4 spike protein (VP8*) is implicated in binding to cellular receptors, thereby facilitating viral attachment and entry. While VP8* of some animal RVs engage sialic acid, human RVs often attach to and enter cells in a sialic acid-independent manner. A recent study demonstrated that the major human RVs (P[4], P[6], and P[8]) recognize human histo-blood group antigens (HBGAs). In this study, we performed a phylogenetic analysis of RVs and showed further variations of RV interaction with HBGAs. On the basis of the VP8* sequences, RVs are grouped into five P genogroups (P[I] to P[V]), of which P[I], P[IV], and P[V] mainly infect animals, P[II] infects humans, and P[III] infects both animals and humans. The sialic acid-dependent RVs (P[1], P[2], P[3], and P[7]) form a subcluster within P[I], while all three major P genotypes of human RVs (P[4], P[6], and P[8]) are clustered in P[II]. We then characterized three human RVs (P[9], P[14], and P[25]) in P[III] and observed a new pattern of binding to the type A antigen which is distinct from that of the P[II] RVs. The binding was demonstrated by hemagglutination and saliva binding assay using recombinant VP8* and native RVs. Homology modeling and mutagenesis study showed that the locations of the carbohydrate binding interfaces are shared with the sialic acid-dependent RVs, although different amino acids are involved. The P[III] VP8* proteins also bind the A antigens of the porcine and bovine mucins, suggesting the A antigen as a possible factor for cross-species transmission of RVs. Our study suggests that HBGAs play an important role in RV infection and evolution.

Spike protein VP8* of human rotavirus recognizes histo-blood group antigens in a type-specific manner

Rotaviruses (RVs), an important cause of severe diarrhea in children, have been found to recognize sialic acid as receptors for host cell attachment. While a few animal RVs (of P[1], P[2], P[3], and P[7]) are sialidase sensitive, human RVs and the majority of animal RVs are sialidase insensitive. In this study, we demonstrated that the surface spike protein VP8* of the major P genotypes of human RVs interacts with the secretor histo-blood group antigens (HBGAs). Strains of the P[4] and P[8] genotypes shared reactivity with the common antigens of Lewis b (Le(b)) and H type 1, while strains of the P[6] genotype bound the H type 1 antigen only. The bindings between recombinant VP8* and human saliva, milk, or synthetic HBGA oligosaccharides were demonstrated, which was confirmed by blockade of the bindings by monoclonal antibodies (MAbs) specific to Le(b) and/or H type 1. In addition, specific binding activities were observed when triple-layered particles of a P[8] (Wa) RV were tested. Our results suggest that the spike protein VP8* of RVs is involved in the recognition of human HBGAs that may function as ligands or receptors for RV attachment to host cells.

Murine noroviruses bind glycolipid and glycoprotein attachment receptors in a strain-dependent manner

Human norovirus infections are the most common cause of acute nonbacterial gastroenteritis in humans worldwide, and glycan binding plays an important role in the susceptibility to these infections. However, due to the lack of an efficient cell culture system or small animal model for human noroviruses, little is known about the biological role of glycan binding during infection. Murine noroviruses (MNV) are also enteric viruses that bind to cell surface glycans, but in contrast to their human counterparts, they can be grown in tissue culture and a small animal host. In this study, we determined glycan-binding specificities of the MNV strains MNV-1 and CR3 in vitro, identified molecular determinants of glycan binding, and analyzed infection in vivo. We showed that unlike MNV-1, CR3 binding to murine macrophages was resistant to neuraminidase treatment and glycosphingolipid depletion. Both strains depended on N-linked glycoproteins for binding, while only MNV-1 attachment to macrophages was sensitive to O-linked glycoprotein depletion. In vivo, CR3 showed differences in tissue tropism compared to MNV-1 by replicating in the large intestine. Mapping of a glycan-binding site in the MNV-1 capsid by reverse genetics identified a region topologically similar to the histo-blood group antigen (HBGA)-binding sites of the human norovirus strain VA387. The recombinant virus showed distinct changes in tissue tropism compared to wild-type virus. Taken together, our data demonstrate that MNV strains evolved multiple strategies to bind different glycan receptors on the surface of murine macrophages and that glycan binding contributes to tissue tropism in vivo.

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.

Rotavirus cell entry

Infecting nearly every child by age five, rotaviruses are the major causative agents of severe gastroenteritis in young children. While much is known about the structure of these nonenveloped viruses and their components, the exact mechanism of viral cell entry is still poorly understood. A consensus opinion that appears to be emerging from recent studies is that rotavirus cell entry involves a series of complex and coordinated events following proteolytic priming of the virus. Rotaviruses attach to the cell through sialic acid containing receptors, with integrins and Hsc70 acting as postattachment receptors, all localized on lipid rafts. Unlike other endocytotic mechanisms, this internalization pathway appears to be independent of clathrin or caveola. Equally complex and coordinated is the fascinating structural gymnastics of the VP4 spikes that are implicated in facilitating optimal interface between viral and host components. While these studies only begin to capture the basic cellular, molecular, and structural mechanisms of cell entry, the unusual features they have uncovered and many intriguing questions they have raised undoubtedly will prompt further investigations.

Effects on sialic acid recognition of amino acid mutations in the carbohydrate-binding cleft of the rotavirus spike protein

The rotavirus spike protein VP4 mediates attachment to host cells and subsequent membrane penetration. The VP8(*) domain of VP4 forms the spike tips and is proposed to recognize host-cell surface glycans. For sialidase-sensitive rotaviruses such as rhesus (RRV), this recognition involves terminal sialic acids. We show here that the RRV VP8(*)(64-224) protein competes with RRV infection of host cells, demonstrating its relevance to infection. In addition, we observe that the amino acids revealed by X-ray crystallography to be in direct contact with the bound sialic acid derivative methyl alpha-D-N-acetylneuraminide, and that are highly conserved amongst sialidase-sensitive rotaviruses, are residues that are also important in interactions with host-cell carbohydrates. Residues Arg101 and Ser190 of the RRV VP8(*) carbohydrate-binding site were mutated to assess their importance for binding to the sialic acid derivative and their competition with RRV infection of host cells. The crystallographic structure of the Arg(101)Ala mutant crystallized in the presence of the sialic acid derivative was determined at 295 K to a resolution of 1.9 A. Our multidisciplinary study using X-ray crystallography, saturation transfer difference nuclear magnetic resonance spectroscopy, isothermal titration calorimetry, and competitive virus infectivity assays to investigate RRV wild-type and mutant VP8(*) proteins has provided the first evidence that the carbohydrate-binding cavity in RRV VP8(*) is used for host-cell recognition, and this interaction is not only with the sialic acid portion but also with other parts of the glycan structure.

Rotavirus-neutralizing antibodies inhibit virus binding to integrins alpha 2 beta 1 and alpha 4 beta 1

Rotavirus outer capsid proteins VP5(*), VP8(*) and VP7 elicit neutralizing, protective antibodies. The alpha 2 beta 1 integrin is a cellular receptor for rotavirus that is bound by VP5(*). Some rotaviruses also recognize the alpha 4 beta 1 integrin. In this study, the effects of antibodies to rotavirus on virus binding to recombinant alpha 2 beta 1 and alpha 4 beta 1 expressed on K562 cells were determined. All neutralizing monoclonal antibodies to VP5(*) tested (YO-2C2, 2G4, 1A10) and two to VP7 (RV-3:2, RV-4:2) inhibited rotavirus binding to alpha 2 beta 1. Rotavirus binding to alpha 4 beta 1 was reduced by 2G4 and neutralizing antibody F45:2, directed to VP7. However, a neutralizing antibody to VP8(*) (RV-5:2) and one to VP7 (RV-3:1) did not affect rotavirus binding to these integrins. Virus-cell binding was unaffected by non-neutralizing antibody RVA to the rotavirus inner capsid protein VP6. The attachment of human rotavirus strain Wa to these integrins was inhibited by infection sera with neutralizing activity collected from two children hospitalised with severe rotavirus gastroenteritis. A negative reference serum did not affect rotavirus-cell attachment. As the binding of rotaviruses to alpha 2 beta 1 and alpha 4 beta 1 is inhibited by neutralizing antibodies to VP5(*) and VP7, and serum from children with rotavirus disease, rotavirus recognition of these integrins may be important for host infection.

Insight into host cell carbohydrate-recognition by human and porcine rotavirus from crystal structures of the virion spike associated carbohydrate-binding domain (VP8*)

Rotavirus infection leads to the death of half a million children annually. The exact specifics of interaction between rotavirus particles and host cells enabling invasion and infection have remained elusive. Host cell oligosaccharides are critical components, and their involvement aids the virus in cell-recognition and attachment, as well as dictation of the remarkable host-specificity that rotaviruses demonstrate. Interaction between the rotavirus spike-protein carbohydrate-binding domain (VP8*) and cell surface oligosaccharides facilitate virus recognition of host cells and attachment. Rotaviruses are considered, controversially, to recognise vastly different carbohydrate structures and either with incorporation of terminal sialic acid or without, as assessed by their ability to infect cells that have been pre-treated with sialidases. Herein, the X-ray crystallographic structures of VP8* from the sialidase insensitive Wa and the sialidase sensitive CRW-8 rotavirus strains that cause debilitating gastroenteritis in human and pig are reported. Striking differences are apparent regarding recognition of the sialic acid derivative methyl alpha-D-N-acetylneuraminide, presenting the first experimental evidence of the inability of the human rotavirus strain to bind this monosaccharide, that correlates with Wa and CRW-8 recognising sialidase-resistant and sialidase-sensitive receptors, respectively. Identified are structural features that provide insight in attainment of substrate specificity exhibited by porcine strains as compared to rhesus rotavirus. Revealed in the CRW-8 VP8* structure is an additional bound ligand that intriguingly, is within a cleft located equivalent to the carbohydrate-binding region of galectins, and is suggestive of a new region for interaction with cell-surface carbohydrates. This novel result and detailed comparison of our representative sialidase-sensitive CRW-8 and insensitive Wa VP8* structures with those reported leads to our hypothesis that this groove is used for binding carbohydrates, and that for the human strains, as for other sialidase-insensitive strains could represent a major oligosaccharide-binding region.

Role of sialic acids in rotavirus infection

Rotaviruses are the leading cause of childhood diarrhea. The entry of rotaviruses into the host cell is a complex process that includes several interactions of the outer layer proteins of the virus with different cell surface molecules. The fact that neuraminidase treatment of the cells, or preincubation of the virus with sialic acid-containing compounds decrease the infectivity of some rotavirus strains, suggested that these viruses interact with sialic acid on the cell surface. The infectivity of some other rotavirus strains is not affected by neuraminidase treatment of the cells, and therefore they are considered neuraminidase-resistant. However, the current evidence suggests that even these neuraminidase-resistant strains might interact with sialic acids located in context different from that of the sialic acids used by the neuraminidase-sensitive strains. This review summarizes our current knowledge of the rotavirus-sialic acid interaction, its structural basis, the specificity with which distinct rotavirus isolates interact with sialic acid-containing compounds, and also the potential use of these compounds as therapeutic agents.

Early Steps in Rotavirus Cell Entry

Rotaviruses, the leading cause of severe dehydrating diarrhea in infants and young children worldwide, are non-enveloped viruses formed by three concentric layers of protein that enclose a genome of double-stranded RNA. These viruses have a specific cell tropism in vivo, infecting primarily the mature enterocytes of the villi of the small intestine. It has been found that rotavirus cell entry is a complex multistep process, in which different domains of the rotavirus surface proteins interact sequentially with different cell surface molecules, which act as attachment and entry receptors. These recently described molecules include integrins (alpha2beta1, alphavbeta3, and alphaxbeta2) and a heat shock protein (hsc70), and have been found to be associated with cell membrane lipid microdomains. The requirement for several cell molecules, which might need to be present and organized in a precise fashion, could explain the cell and tissue tropism of these viruses. This review focuses on recent data describing the interactions between the virus and its receptors, the role of lipid microdomains in rotavirus infection, and the possible mechanism of rotavirus cell entry.

STD NMR spectroscopy and molecular modeling investigation of the binding of N-acetylneuraminic acid derivatives to rhesus rotavirus VP8* core

The VP8* subunit of rotavirus spike protein VP4 contains a sialic acid (Sia)-binding domain important for host cell attachment and infection. In this study, the binding epitope of the N-acetylneuraminic acid (Neu5Ac) derivatives has been characterized by saturation transfer difference (STD) nuclear magnetic resonance (NMR) spectroscopy. From this STD NMR data, it is proposed that the VP8* core recognizes an identical binding epitope in both methyl alpha-D-N-acetylneuraminide (Neu5Acalpha2Me) and the disaccharide methyl S-(alpha-D-N-acetylneuraminosyl)-(2-->6)-6-thio-beta-D-galactopyranoside (Neu5Ac-alpha(2,6)-S-Galbeta1Me). In the VP8*-disaccharide complex, the Neu5Ac moiety contributes to the majority of interaction with the protein, whereas the galactose moiety is solvent-exposed. Molecular dynamics calculations of the VP8*-disaccharide complex indicated that the galactose moiety is unable to adopt a conformation that is in close proximity to the protein surface. STD NMR experiments with methyl 9-O-acetyl-alpha-D-N-acetylneuraminide (Neu5,9Ac(2)alpha2Me) in complex with rhesus rotavirus (RRV) VP8* revealed that both the N-acetamide and 9-O-acetate moieties are in close proximity to the Sia-binding domain, with the N-acetamide's methyl group being saturated to a larger extent, indicating a closer association with the protein. RRV VP8* does not appear to significantly recognize the unsaturated Neu5Ac derivative [2-deoxy-2,3-didehydro-D-N-acetylneuraminic acid (Neu5Ac2en)]. Molecular modeling of the protein-Neu5Ac2en complex indicates that key interactions between the protein and the unsaturated Neu5Ac derivative when compared with Neu5Acalpha2Me would not be sustained. Neu5Acalpha2Me, Neu5Ac-alpha(2,6)-S-Galbeta1Me, Neu5,9Ac(2)alpha2Me, and Neu5Ac2en inhibited rotavirus infection of MA104 cells by 61%, 35%, 30%, and 0%, respectively, at 10 mM concentration. NMR spectroscopic, molecular modeling, and infectivity inhibition results are in excellent agreement and provide valuable information for the design of inhibitors of rotavirus infection.

Rotavirus spike protein VP5* binds alpha2beta1 integrin on the cell surface and competes with virus for cell binding and infectivity

Rotaviruses recognize several cell-surface molecules, including the alpha2beta1 integrin, and the processes of rotavirus cell attachment and entry appear to be multifactorial. The VP5* subunit of the rotavirus spike protein VP4 contains the alpha2beta1 ligand sequence Asp-Gly-Glu at residues 308-310. Binding to alpha2beta1 and infectivity of monkey rotavirus strain RRV and human rotavirus strain Wa, but not porcine rotavirus strain CRW-8, are inhibited by peptides containing Asp-Gly-Glu. Asp308 and Gly309 are necessary for the binding of RRV VP5* (aa 248-474) to expressed I domain of the alpha2 integrin subunit. Here, the ability of RRV VP5* to bind cells and affect rotavirus-integrin interactions was determined. Interestingly, VP5* bound to cells at 4 and 37 degrees C, both via alpha2beta1 and independently of this integrin. Prior VP5* binding at 37 degrees C eliminated RRV binding to cellular alpha2beta1 and reduced RRV and Wa infectivity in MA104 cells by 38-46 %. VP5* binding did not affect the infectivity of CRW-8. VP5* binding at 4 degrees C did not affect permissive-cell infection by RRV, indicating an energy requirement for VP5* competition with virus for infectivity. Mutagenesis of VP5* Asp308 and Gly309 eliminated VP5* binding to alpha2beta1 and the VP5* inhibition of rotavirus cell binding and infection, but not alpha2beta1-independent cell binding by VP5*. These studies show for the first time that expressed VP5* binds cell-surface alpha2beta1 using Asp308 and Gly309 and inhibits the infection of homologous and heterologous rotaviruses that use alpha2beta1 as a receptor.

The synthesis and biological evaluation of lactose-based sialylmimetics as inhibitors of rotaviral infection

Rotaviruses are the most significant cause of gastroenteritis in young children and are responsible for over 600,000 infant deaths annually. The rotaviral haemagglutinin protein (VP8*) of some strains has been implicated in early recognition and binding events of host cell-surface sialoglycoconjugates, and is therefore an attractive target for potential therapeutic intervention. Since N-acetylneuraminic acid alpha(2,3)-linked to galactose is believed to be the minimum binding epitope of rotavirus to host cells, we report here our development of an efficient and flexible synthetic route to a range of lactose-based sialylmimetics of alpha(2,3)-linked thiosialosides. These compounds were biologically evaluated as inhibitors of rotaviral infection using an in vitro neutralisation assay. The results suggest that these lactose-based sialylmimetics are not inhibitors of the rhesus rotavirus strain; however, they do exhibit modest inhibition of the human (Wa) strain, presumably through inhibition of the rotaviral adhesion process.

Attachment factors

As obligate intracellular parasites, viruses must bind to, and enter, permissive host cells in order to gain access to the cellular machinery that is required for their replication. The very large number of mammalian viruses identified to date is reflected in the fact that almost every human and animal cell type is a target for infection by one, or commonly more than one, species of virus. As viruses have adapted to target certain cell types for their propagation, there is exquisite specificity in cellular tropism. This specificity is frequently, but not always, mediated by the first step in the viral replication cycle: attachment of viral surface proteins to receptors expressed on susceptible cells. Viral receptors may be protein, carbohydrate, and/or lipid. Many viruses can use more than one attachment receptor, and indeed may sequentially engage multiple receptors to infect a cell. Thus, it is useful to differentiate between attachment receptors, that simply allow viruses a foothold at the limiting membrane of a cell, and entry receptors that mediate delivery the viral genome into the cytoplasm. For some viruses the attachment factors that promote binding to permissive cells are very well defined, but the sequence of events that triggers viral entry is only now beginning to be understood. For other viruses, despite many efforts, the receptors remain elusive. In this chapter we will confine our review to viruses that infect mammals, with particular focus on human pathogens. We do not intend that this will be an exhaustive overview of viral attachment receptors; instead we will take a number of examples of well-characterized virus-receptor interactions, discuss supporting evidence, and highlight any controversies and uncertainties in the field. We will then conclude with a reflection on general principles of viral attachment, consider some exceptions to these principles, and make some suggestion for future research.

Cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of the VP8* carbohydrate-binding protein of the human rotavirus strain Wa

Rotaviruses exhibit host-specificity and the first crystallographic information on a rotavirus strain that infects humans is reported here. Recognition and attachment to host cells, leading to invasion and infection, is critically linked to the function of the outer capsid spike protein of the rotavirus particle. In some strains the VP8* component of the spike protein is implicated in recognition and binding of sialic-acid-containing cell-surface carbohydrates, thereby enabling infection by the virus. The cloning, expression, purification, crystallization and initial X-ray diffraction analysis of the VP8* core from human Wa rotavirus is reported. Two crystal forms (trigonal P3(2)21 and monoclinic P2(1)) have been obtained and X-ray diffraction data have been collected, enabling determination of the VP8*(64-223) structure by molecular replacement.

Characterization of neuraminidase-resistant mutants derived from rotavirus porcine strain OSU

Infection by some rotavirus strains requires the presence of sialic acid on the cell surface, its infectivity being reduced in cells treated with neuraminidase. A neuraminidase treatment-resistant mutant was isolated from the porcine rotavirus strain OSU. In reassortant strains, the neuraminidase-resistant phenotype segregated with the gene coding for VP4. The mutant retained its capacity to bind to sialic acid. The VP4 sequence of the mutant differed from that of the parental OSU strain in an Asp-to-Asn substitution at position 100. Neutralization escape mutants selected from an OSU neuraminidase-sensitive clone by monoclonal antibodies that failed to recognize the neuraminidase-resistant mutant strain carried the same mutation at position 100 and were also neuraminidase resistant. Neuraminidase sensitivity was restored when the mutation at position 100 was compensated for by a second mutation (Gln to Arg) at position 125. Molecular mechanics simulations suggest that the neuraminidase-resistant phenotype associated with mutation of OSU residue 100 from Asp to Asn reflects the conformational changes of the sialic acid cleft that accompany sialic acid binding.

Evaluation of specificity and effects of monoclonal antibodies submitted to the Eighth Human Leucocyte Differentiation Antigen Workshop on rotavirus-cell attachment and entry

Rotavirus infection of permissive cells is a multi-step process that requires interaction with several cell surface receptors. Integrins alpha2beta1, alpha4beta1, alphaXbeta2, and alphavbeta3 are involved in the attachment and entry into permissive cells for many rotavirus strains. However, possible roles of known partners of these integrins in this process have not been studied. Here, the specificities of new monoclonal antibodies directed to beta1 and beta2 integrins were determined using integrin-transfected cells. The ability of monoclonal antibodies to integrin partners CD82, CD151, CD321, and CD322 to bind rotavirus-permissive cell lines (MA104, Caco-2, and RD) and K562 cells expressing or lacking alpha4beta1 also was investigated. CD82 and CD151 were expressed on K562, alpha4-K562, and RD cells. CD321-specific antibodies bound K562, alpha4-K562, MA104, and Caco-2 cells. CD322 expression was detected on MA104 but not Caco-2 cells. Antibodies to CD82, CD151, CD321, and CD322 that bound these cells were investigated for their ability to inhibit cellular attachment and entry by rotaviruses. Antibody blockade of these integrin-associated proteins did not affect cell attachment or entry of the integrin-using rhesus rotavirus RRV or porcine rotavirus CRW-8, which uses alpha4beta1 integrin for infection. Antibody blockade of CD322 did not alter cell attachment or infectivity by human rotavirus strain RV-3, so RV-3 infection was independent of CD322. Overall, these studies indicate that CD82, CD151, CD321, and CD322 are unlikely to play a role in rotavirus-cell binding or entry.

Crystallization and preliminary X-ray diffraction analysis of the sialic acid-binding domain (VP8*) of porcine rotavirus strain CRW-8

Rotavirus recognition and attachment to host cells involves interaction with the spike protein VP4 that projects outwards from the surface of the virus particle. An integral component of these spikes is the VP8* domain, which is implicated in the direct recognition and binding of sialic acid-containing cell-surface carbohydrates and facilitates subsequent invasion by the virus. The expression, purification, crystallization and preliminary X-ray diffraction analysis of VP8* from porcine CRW-8 rotavirus is reported. Diffraction data have been collected to 2.3 A resolution, enabling the determination of the VP8* structure by molecular replacement.

The cytokine osteopontin modulates the severity of rotavirus diarrhea

Osteopontin (OPN) is a sialated phosphoprotein found in tissues and secreted into body fluids. It is an integrin ligand with pleiotropic functions as an extracellular matrix protein in mineralized tissues and a cytokine that is active in cell signaling (A. B. Tuck, C. Hota, S. M. Wilson, and A. F. Chambers, Oncogene 22:1198-1205, 2003). To determine whether OPN may be important in mucosal defense against viral pathogens, we evaluated the OPN response to rotavirus infection and the extent of diarrhea manifested by infected opn null mutant (opn-/-) mice. Reverse transcription-PCR, Northern and Western blots, and immunohistochemical studies of the HT-29 intestinal epithelial cell line and murine intestine were used to evaluate OPN mRNA and product. Intestinal closed loops and diarrheal observations determined disease severity and duration. OPN mRNA levels increased after infection of HT-29 cells, peaking in 4 to 6 h. Infected cultures contained 925 microg of OPN/ml, while for controls the levels were below detection (50 microg/ml). Infection increased OPN mRNA levels in intestinal tissue between 2 and 24 h postinoculation and increased OPN protein in intestinal fluid. The cellular localization of OPN was supranuclear and apical, and responding cells were diffusely distributed on the villus surface. Three days after infection, closed intestinal loops from opn-/- mice contained more fluid than loops from controls, although secretion levels at the onset of illness were similar. Null mutant mice experienced more intense and prolonged diarrhea than controls. Rotavirus infection of intestinal epithelial cells and murine intestine caused marked increases in OPN mRNA levels and secreted OPN protein. OPN-deficient mice suffered prolonged disease.

Effects on rotavirus cell binding and infection of monomeric and polymeric peptides containing alpha2beta1 and alphaxbeta2 integrin ligand sequences

Integrin-using rotaviruses bind MA104 cell surface alpha2beta1 integrin via the Asp-Gly-Glu (DGE) sequence in virus spike protein VP4 and interact with alphaxbeta2 integrin during cell entry through outer capsid protein VP7. Infection is inhibited by the alpha2beta1 ligand Asp-Gly-Glu-Ala (DGEA) and the alphaxbeta2 ligand Gly-Pro-Arg-Pro (GPRP), and virus-alpha2beta1 binding is increased by alpha2beta1 activation. In this study, we analyzed the effects of monomers and polymers containing DGEA-, GPRP-, and DGEA-related peptides on rotavirus binding and infection in intestinal (Caco-2) and kidney (MA104) cells and virus binding to recombinant alpha2beta1. Blockade of rotavirus-cell binding and infection by peptides and anti-alpha2 antibody showed that Caco-2 cell entry is dependent on virus binding to alpha2beta1 and interaction with alphaxbeta2. At up to 0.5 mM, monomeric DGEA and DGAA inhibited binding to alpha2beta1 and infection. At higher concentrations, DGEA and DGAA showed a reduced ability to inhibit virus-cell binding and infection that depended on virus binding to alpha2beta1 but occurred without alteration in cell surface expression of alpha2, beta2, or alphavbeta3 integrin. This loss of DGEA activity was abolished by genistein treatment and so was dependent on tyrosine kinase signaling. It is proposed that this signaling activated existing cell surface alpha2beta1 to increase virus-cell attachment and entry. Polymeric peptides containing DGEA and GPRP or GPRP only were inhibitory to SA11 infection at approximately 10-fold lower concentrations than peptide monomers. As polymerization can improve peptide inhibition of virus-receptor interactions, this approach could be useful in the development of inhibitors of receptor recognition by other viruses.

The rotavirus surface protein VP8 modulates the gate and fence function of tight junctions in epithelial cells

Rotaviruses constitute a major cause of diarrhea in young mammals. Rotaviruses utilize different integrins as cell receptors, therefore upon their arrival to the intestinal lumen their integrin receptors will be hidden below the tight junction (TJ), on the basolateral membrane. Here we have studied whether the rotavirus outer capsid proteins are capable of opening the paracellular space sealed by the TJ. From the outermost layer of proteins of the rotavirus, 60 spikes formed of protein VP4 are projected. VP4 is essential for virus-cell interactions and is cleaved by trypsin into peptides VP5 and VP8. Here we found that when these peptides are added to confluent epithelial monolayers (Madin-Darby canine kidney cells), VP8 is capable of diminishing in a dose dependent and reversible manner the transepithelial electrical resistance. VP5 exerted no effect. VP8 can also inhibit the development of newly formed TJs in a Ca-switch assay. Treatment with VP8 augments the paracellular passage of non-ionic tracers, allows the diffusion of a fluorescent lipid probe and the apical surface protein GP135, from the luminal to the lateral membrane, and triggers the movement of the basolateral proteins Na+-K+-ATPase, alphanubeta3 integrin and beta1 integrin subunit, to the apical surface. VP8 generates a freeze-fracture pattern of TJs characterized by the appearance of loose end filaments, that correlates with an altered distribution of several TJ proteins. VP8 given orally to diabetic rats allows the enteral administration of insulin, thus indicating that it can be employed to modulate epithelial permeability.

Attachment and infection to MA104 cells of avian rotaviruses require the presence of sialic acid on the cell surface

To determine the characters of receptors on target cells for avian rotaviruses, the receptors on MA104 cells for the pigeon rotavirus PO-13, the turkey rotaviruses Ty-1 and Ty-3, and the chicken rotavirus Ch-1 were analyzed. Pretreatment of MA104 cells with neuraminidase greatly reduced the infection by all of the four avian rotavirus strains. Binding of the cell-attachment protein, purified VP8 expressed in bacteria, of strain PO-13 to MA104 cells was also inhibited by pretreatment of cells with neuraminidase. These findings suggest that avian rotaviruses primarily utilize sialic acid-containing molecules as receptors on MA 104 cells.

Monkey rotavirus binding to alpha2beta1 integrin requires the alpha2 I domain and is facilitated by the homologous beta1 subunit

Rotaviruses utilize integrins during virus-cell interactions that lead to infection. Cell binding and infection by simian rotavirus SA11 were inhibited by antibodies (Abs) to the inserted (I) domain of the alpha2 integrin subunit. To determine directly which integrins or other proteins bind rotaviruses, cell surface proteins precipitated by rotaviruses were compared with those precipitated by anti-alpha2beta1 Abs. Two proteins precipitated by SA11 and rhesus rotavirus RRV from MA104 and Caco-2 cells migrated indistinguishably from alpha2beta1 integrin, and SA11 precipitated beta1 from alpha2beta1-transfected CHO cells. These viruses specifically precipitated two MA104 cell proteins only, but an additional 160- to 165-kDa protein was precipitated by SA11 from Caco-2 cells. The role of the alpha2 I domain in rotavirus binding, infection, and growth was examined using CHO cell lines expressing wild-type or mutated human alpha2 or alpha2beta1. Infectious SA11 and RRV, but not human rotavirus Wa, specifically bound CHO cell-expressed human alpha2beta1 and, to a lesser extent, human alpha2 combined with hamster beta1. Binding was inhibited by anti-alpha2 I domain monoclonal Abs (MAbs), but not by non-I domain MAbs to alpha2, and required the presence of the alpha2 I domain. Amino acid residues 151, 221, and 254 in the metal ion-dependent adhesion site of the alpha2 I domain that are necessary for type I collagen binding to alpha2beta1 were not essential for rotavirus binding. Rotavirus-alpha2beta1 binding led to increased virus infection and RRV growth. SA11 and RRV require the alpha2 I domain for binding to alpha2beta1, and their binding to this integrin is distinguishable from that of collagen.

Gene expression pattern in Caco-2 cells following rotavirus infection

Rotaviruses are recognized as the leading cause of severe dehydrating diarrhea in infants and young children worldwide. Preventive and therapeutic strategies are urgently needed to fight this pathogen. In tissue culture and in vivo, rotavirus induces structural and functional alterations in the host cell. In order to better understand the molecular mechanisms involved in the events after rotavirus infection, we identified host cellular genes whose mRNA levels changed after infection. For this analysis, we used microarrays containing more than 38,000 human cDNAs to study the transcriptional response of the human intestinal cell line Caco-2 to rotavirus infection. We found that 508 genes were differentially regulated >2-fold at 16 h after rotavirus infection, and only one gene was similarly regulated at 1 h postinfection. Of these transcriptional changes, 73% corresponded to the upregulation of genes, with the majority of them occurring late, at 12 or more hours postinfection. Some of the regulated genes were classified according to known biological function and included genes encoding integral membrane proteins, interferon-regulated genes, transcriptional and translational regulators, and calcium metabolism-related genes. A new picture of global transcriptional regulation in the infected cell is presented and families of genes which may be involved in viral pathogenesis are discussed.

Initial interaction of rotavirus strains with N-acetylneuraminic (sialic) acid residues on the cell surface correlates with VP4 genotype, not species of origin

We examined 41 human and animal rotavirus strains representative of all known P genotypes for their dependency on cellular N-acetylneuraminic (sialic) acid (SA) residues for infectivity. Our results showed that all rotaviruses studied, whether of animal or human origin, belonging to P genotypes [1], [2], [3], and [7] depended on SA residues on the cell surface for efficient infectivity but that all human and animal rotavirus strains representative of the remaining known P genotypes were SA independent. The SA residue requirement for efficient infectivity did not change for reassortant rotavirus strains with altered VP4-VP7 combinations. The initial interaction of rotavirus strains with SA residues on the cell surface correlated with VP4 genotype specificity, not with species of origin or VP7 G serotype specificity (P = 0.001; r2 = 1.00, Pearson's correlation coefficient). In addition to being a requirement for infectivity, the presence of SA residues on the cell surface is a requirement for efficient growth in cell culture; recognition of the association of specific P genotypes with the binding of rotavirus to SA residues will facilitate our understanding of the molecular basis of the early events of rotavirus-cell interactions in cell culture models and of pathogenicity in vivo.

Heat shock cognate protein 70 is involved in rotavirus cell entry

In this work, we have identified the heat shock cognate protein (hsc70) as a receptor candidate for rotaviruses. hsc70 was shown to be present on the surface of MA104 cells, and antibodies to this protein blocked rotavirus infectivity, while not affecting the infectivity of reovirus and poliovirus. Preincubation of the hsc70 protein with the viruses also inhibited their infectivity. Triple-layered particles (mature virions), but not double-layered particles, bound hsc70 in a solid-phase assay, and this interaction was blocked by monoclonal antibodies to the virus surface proteins VP4 and VP7. Rotaviruses were shown to interact with hsc70 at a postattachment step, since antibodies to hsc70 and the protein itself did not inhibit the virus attachment to cells. We propose that the functional rotavirus receptor is a complex of several cell surface molecules that include, among others, hsc70.

Involvement of bovine lactoferrin metal saturation, sialic acid and protein fragments in the inhibition of rotavirus infection

Although the antiviral activity of lactoferrin is one of the major biological functions of this iron binding protein, the mechanism of action is still under debate. We have investigated the role of metal binding, of sialic acid and of tryptic fragments of bovine lactoferrin (bLf) in the activity towards rotavirus (intestinal pathogen naked virus) infecting enterocyte-like cells. The antiviral activity of bLf fully saturated with manganese or zinc was slightly decreased compared to that observed for apo- or iron-saturated bLf. The antiviral activity of differently metal-saturated bLf towards rotavirus was exerted during and after the virus attachment step. The removal of sialic acid enhanced the anti-rotavirus activity of bLf. Among all the peptidic fragments obtained by tryptic digestion of bLf and characterised by advanced mass spectrometric methodologies, a large fragment (86-258) and a small peptide (324-329: YLTTLK) were able to inhibit rotavirus even if at lower extent than undigested bLf.

Selection of rotavirus VP4 cell receptor binding domains for MA104 cells using a phage display library

Rotavirus infection of host cells, like other viruses, is a complex process that has not been fully elucidated, and much attention has been focused on the regions of the viral attachment protein, VP4, that are involved in binding to the cellular receptor. In this study, phage display technology was employed to generate a g3p VP4 gene-targeted phage display peptide library using the porcine rotavirus strain CRW8, and a method was optimised for panning this library on adherent MA104 cells to identify receptor binding domains. Recombinant phage that displayed expressed peptides from both the rotavirus VP4 trypsin cleavage products VP8* and VP5* were selected, and while some of the phage clones contained insert sequences from regions of VP4 implicated previously in cell binding and infection, new domains were also identified. In all, four regions within VP8* and six regions of VP5* were selected by panning. To our knowledge, this paper is the first description of using a gene-targeted phage display library to identify receptor binding domains on viral proteins.

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

Rotaviruses cause severe gastroenteritis in infants and are estimated to be responsible for over 600 000 deaths annually, primarily in developing countries. The development of potential inhibitors of this virus is therefore of great interest, particularly since the safety and efficacy of rotaviral vaccines has recently been questioned. This study describes the synthesis of a variety of compounds that can be considered as mimetics of N-acetylneuraminic acid thioglycosides and the subsequent in vitro biological evaluation of these sialylmimetics as inhibitors of rotaviral infection. Our results show that readily accessible carbohydrate-based compounds have the potential to act as inhibitors of rotaviral replication in vitro, presumably through inhibition of the rotaviral adhesion process.

Reovirus binding to cell surface sialic acid potentiates virus-induced apoptosis

Reovirus induces apoptosis in cultured cells and in vivo. Genetic studies indicate that the efficiency with which reovirus strains induce apoptosis is determined by the viral S1 gene, which encodes attachment protein sigma1. However, the biochemical properties of sigma1 that influence apoptosis induction are unknown. To determine whether the capacity of sigma1 to bind cell surface sialic acid determines the magnitude of the apoptotic response, we used isogenic reovirus mutants that differ in the capacity to engage sialic acid. We found that T3SA+, a virus capable of binding sialic acid, induces high levels of apoptosis in both HeLa cells and L cells. In contrast, non-sialic-acid-binding strain T3SA- induces little or no apoptosis in these cell types. Differences in the capacity of T3SA- and T3SA+ to induce apoptosis are not due to differences in viral protein synthesis or production of viral progeny. Removal of cell surface sialic acid with neuraminidase abolishes the capacity of T3SA+ to induce apoptosis. Similarly, incubation of T3SA+ with sialyllactose, a trisaccharide comprised of lactose and sialic acid, blocks apoptosis. These findings demonstrate that reovirus binding to cell surface sialic acid is a critical requirement for the efficient induction of apoptosis and suggest that virus receptor utilization plays an important role in regulating cell death.

Glycosphingolipid binding specificities of rotavirus: identification of a sialic acid-binding epitope

The glycosphingolipid binding specificities of neuraminidase-sensitive (simian SA11 and bovine NCDV) and neuraminidase-insensitive (bovine UK) rotavirus strains were investigated using the thin-layer chromatogram binding assay. Both triple-layered and double-layered viral particles of SA11, NCDV, and UK bound to nonacid glycosphingolipids, including gangliotetraosylceramide (GA1; also called asialo-GM1) and gangliotriaosylceramide (GA2; also called asialo-GM2). Binding to gangliosides was observed with triple-layered particles but not with double-layered particles. The neuraminidase-sensitive and neuraminidase-insensitive rotavirus strains showed distinct ganglioside binding specificities. All three strains bound to sialylneolactotetraosylceramide and GM2 and GD1a gangliosides. However, NeuAc-GM3 and the GM1 ganglioside were recognized by rotavirus strain UK but not by strains SA11 and NCDV. Conversely, NeuGc-GM3 was bound by rotaviruses SA11 and NCDV but not by rotavirus UK. Thus, neuraminidase-sensitive strains bind to external sialic acid residues in gangliosides, while neuraminidase-insensitive strains recognize gangliosides with internal sialic acids, which are resistant to neuraminidase treatment. By testing a panel of gangliosides with triple-layered particles of SA11 and NCDV, the terminal sequence sialyl-galactose (NeuGc/NeuAcalpha3-Galbeta) was identified as the minimal structural element required for the binding of these strains. The binding of triple-layered particles of SA11 and NCDV to NeuGc-GM3, but not to NeuAc-GM3, suggested that the sequence NeuGcalpha3Galbeta is preferred to NeuAcalpha3Galbeta. Further dissection of this binding epitope showed that the carboxyl group and glycerol side chain of sialic acid played an important role in the binding of such triple-layered particles.

Integrin alpha(v)beta(3) mediates rotavirus cell entry

Rotavirus strains differ in their need for sialic acid (SA) for initial binding to the cell surface; however, the existence of a postattachment cell receptor, common to most, if not all, rotavirus strains, has been proposed. In the present study, antibodies to the alpha(v) and beta(3) integrin subunits, and the alpha(v)beta(3) ligand, vitronectin, efficiently blocked the infectivity of the SA-dependent rhesus rotavirus RRV, its SA-independent variant nar3, and the neuraminidase-resistant human rotavirus strain Wa. Vitronectin and anti-beta(3) antibodies, however, did not block the binding of virus to cells, indicating that rotaviruses interact with alpha(v)beta(3) at a postbinding step, probably penetration. This interaction was shown to be independent of the tripeptide motif arginine-glycine-aspartic acid present in the natural ligands of this integrin. Transfection of CHO cells with alpha(v)beta(3) genes significantly increased their permissiveness to all three rotavirus strains, and the increment of virus infectivity was reverted by incubation of these cells either with antibodies to beta(3) or with vitronectin. These findings implicate alpha(v)beta(3) integrin as a cellular receptor common to neuraminidase-sensitive and neuraminidase-resistant rotaviruses, and support the hypothesis that this integrin could determine, at least in part, the cellular susceptibility to rotaviruses.

Biochemical characterization of rotavirus receptors in MA104 cells

We have tested the effect of metabolic inhibitors, membrane cholesterol depletion, and detergent extraction of cell surface molecules on the susceptibility of MA104 cells to infection by rotaviruses. Treatment of cells with tunicamycin, an inhibitor of protein N glycosylation, blocked the infectivity of the SA-dependent rotavirus RRV and its SA-independent variant nar3 by about 50%, while the inhibition of O glycosylation had no effect. The inhibitor of glycolipid biosynthesis d, l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) blocked the infectivity of RRV, nar3, and the human rotavirus strain Wa by about 70%. Sequestration of cholesterol from the cell membrane with beta-cyclodextrin reduced the infectivity of the three viruses by more than 90%. The involvement of N-glycoproteins, glycolipids, and cholesterol in rotavirus infection suggests that the virus receptor(s) might be forming part of lipid microdomains in the cell membrane. MA104 cells incubated with the nonionic detergent octyl-beta-glucoside (OG) showed a ca. 60% reduction in their ability to bind rotaviruses, the same degree to which they became refractory to infection, suggesting that OG extracts the potential virus receptor(s) from the cell surface. Accordingly, when preincubated with the viruses, the OG extract inhibited the virus infectivity by more than 95%. This inhibition was abolished when the extract was treated with either proteases or heat but not when it was treated with neuraminidase, indicating the protein nature of the inhibitor. Two protein fractions of around 57 and 75 kDa were isolated from the extract, and these fractions were shown to have rotavirus-blocking activity. Also, antibodies to these fractions efficiently inhibited the infectivity of the viruses in untreated as well as in neuraminidase-treated cells. Five individual protein bands of 30, 45, 57, 75, and 110 kDa, which exhibited virus-blocking activity, were finally isolated from the OG extract. These proteins are good candidates to function as rotavirus receptors.

Rotavirus infection of MA104 cells is inhibited by Ricinus lectin and separately expressed single binding domains

Various lectins were tested for blocking rotavirus infection of MA104 cells and it was observed that galactose-specific lectins were the most inhibitory. Of these Ricinus agglutinin was able to inhibit infection (by human and animal strains) at concentrations as low as 10(-9) M. In addition, in a virus overlay protein blot assay Ricinus agglutinin competed with simian rotavirus SA11 for binding to solubilized MA104 proteins. Amino acid sequence comparisons revealed similarity between the ricin toxin B subunit (which contains two separate carbohydrate-binding motifs: single binding domains (SBD) 1 and 2) and rotavirus spike protein VP4. A filamentous phage display system was used to independently express the two binding domains and while SBD1 inhibited infection of MA104 cells by CRW8, NCDV, and to a lesser extent Wa, SBD2 blocked only CRW8 and NCDV infection. Furthermore inhibition of CRW8 infection was a direct result of phage inhibiting virus attachment to cells. When amino acid 248 within SBD2 was mutated from the ricin toxin to the Ricinus agglutinin sequence this phage clone showed reduced binding to galactose and was no longer able to inhibit virus infection. Thus, rotavirus recognizes galactose as an important component of the receptor on MA104 cells.

Growth of rotaviruses in continuous human and monkey cell lines that vary in their expression of integrins

Rotavirus replication occurs in vivo in intestinal epithelial cells. Cell lines fully permissive to rotavirus include kidney epithelial (MA104), colonic (Caco-2) and hepatic (HepG2) types. Previously, it has been shown that cellular integrins alpha 2 beta 1, alpha 4 beta 1 and alpha X beta 2 are involved in rotavirus cell entry. As receptor usage is a major determinant of virus tropism, the levels of cell surface expression of these integrins have now been investigated by flow cytometry on cell lines of human (Caco-2, HepG2, RD, K562) and monkey (MA104, COS-7) origin in relation to cellular susceptibility to infection with monkey and human rotaviruses. Cells supporting any replication of human rotaviruses (RD, HepG2, Caco-2, COS-7 and MA104) expressed alpha 2 beta 1 and (when tested) alpha X beta 2, whereas the non-permissive K562 cells did not express alpha 2 beta 1, alpha 4 beta 1 or alpha X beta 2. Only RD cells expressed alpha 4 beta 1. Although SA11 grew to higher titres in RD, HepG2, Caco-2, COS-7 and MA104 cells, this virus still replicated at a low level in K562 cells. In all cell lines tested, SA11 replicated to higher titres than did human strains, consistent with the ability of SA11 to use sialic acids as alternative receptors. Levels of cell surface alpha 2 integrin correlated with levels of rotavirus growth. The alpha 2 integrin relative linear median fluorescence intensity on K562, RD, COS-7, MA104 and Caco-2 cells correlated linearly with the titre of SA11 produced in these cells at 20 h after infection at a multiplicity of 0.1, and the data best fitted a sigmoidal dose-response curve (r(2)=1.00, P=0.005). Thus, growth of rotaviruses in these cell lines correlates with their surface expression of alpha 2 beta 1 integrin and is consistent with their expression of alpha X beta 2 and alpha 4 beta 1 integrins.