Cell surface ligands for rotavirus: mouse intestinal glycolipids and synthetic carbohydrate analogs

Ceramide and Related Molecules in Viral Infections

Ceramide is a lipid messenger at the heart of sphingolipid metabolism. In concert with its metabolizing enzymes, particularly sphingomyelinases, it has key roles in regulating the physical properties of biological membranes, including the formation of membrane microdomains. Thus, ceramide and its related molecules have been attributed significant roles in nearly all steps of the viral life cycle: they may serve directly as receptors or co-receptors for viral entry, form microdomains that cluster entry receptors and/or enable them to adopt the required conformation or regulate their cell surface expression. Sphingolipids can regulate all forms of viral uptake, often through sphingomyelinase activation, and mediate endosomal escape and intracellular trafficking. Ceramide can be key for the formation of viral replication sites. Sphingomyelinases often mediate the release of new virions from infected cells. Moreover, sphingolipids can contribute to viral-induced apoptosis and morbidity in viral diseases, as well as virus immune evasion. Alpha-galactosylceramide, in particular, also plays a significant role in immune modulation in response to viral infections. This review will discuss the roles of ceramide and its related molecules in the different steps of the viral life cycle. We will also discuss how novel strategies could exploit these for therapeutic benefit.

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.

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.

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.

Evolution of cell recognition by viruses: a source of biological novelty with medical implications

The picture beginning to form from genome analyses of viruses, unicellular organisms, and multicellular organisms is that viruses have shared functional modules with cells. A process of coevolution has probably involved exchanges of genetic information between cells and viruses for long evolutionary periods. From this point of view present-day viruses show flexibility in receptor usage and a capacity to alter through mutation their receptor recognition specificity. It is possible that for the complex DNA viruses, due to a likely limited tolerance to generalized high mutation rates, modifications in receptor specificity will be less frequent than for RNA viruses, albeit with similar biological consequences once they occur. It is found that different receptors, or allelic forms of one receptor, may be used with different efficiency and receptor affinities are probably modified by mutation and selection. Receptor abundance and its affinity for a virus may modulate not only the efficiency of infection, but also the capacity of the virus to diffuse toward other sites of the organism. The chapter concludes that receptors may be shared by different, unrelated viruses and that one virus may use several receptors and may expand its receptor specificity in ways that, at present, are largely unpredictable.

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.

VLA-2 (alpha2beta1) integrin promotes rotavirus entry into cells but is not necessary for rotavirus attachment

In an attempt to identify the rotavirus receptor, we tested 46 cell lines of different species and tissue origins for susceptibility to infection by three N-acetyl-neuraminic (sialic) acid (SA)-dependent and five SA-independent rotavirus strains. Susceptibility to SA-dependent or SA-independent rotavirus infection varied depending on the cell line tested and the multiplicity of infection (MOI) used. Cells of renal or intestinal origin and transformed cell lines derived from breast, stomach, bone, or lung were all susceptible to rotavirus infection, indicating a wider host tissue range than previously appreciated. Chinese hamster ovary (CHO), baby hamster kidney (BHK-21), guinea pig colon (GPC-16), rat small intestine (Rie1), and mouse duodenum (MODE-K) cells were found to support only limited rotavirus replication even at MOIs of 100 or 500, but delivery of rotavirus particles into the cytoplasm by lipofection resulted in efficient rotavirus replication. The rotavirus cell attachment protein, the outer capsid spike protein VP4, contains the sequence GDE(A) recognized by the VLA-2 (alpha2beta1) integrin, and to test if VLA-2 is involved in rotavirus attachment and entry, we measured infection in CHO cells that lack VLA-2 and CHO cells transfected with the human alpha2 subunit (CHOalpha2) or with both the human alpha2 and beta1 subunits (CHOalpha2beta1) of VLA-2. Infection by SA-dependent or SA-independent rotavirus strains was 2- to 10-fold more productive in VLA-2-expressing CHO cells than in parental CHO cells, and the increased susceptibility to infection was blocked with anti-VLA-2 antibody. However, the levels of binding of rotavirus to CHO, CHOalpha2, and CHOalpha2beta1 cells were equivalent and were not increased over binding to susceptible monkey kidney (MA104) cells or human colonic adenocarcinoma (Caco-2, HT-29, and T-84) cells, and binding was not blocked by antibody to the human alpha2 subunit. Although the VLA-2 integrin promotes rotavirus infection in CHO cells, it is clear that the VLA-2 integrin alone is not responsible for rotavirus cell attachment and entry. Therefore, VLA-2 is not involved in the initial attachment of rotavirus to cells but may play a role at a postattachment level.

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.

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.

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.

Sialic acid dependence and independence of group A rotaviruses

We have found (1), in contrast to previous reports, the human rotavirus Wa strain is sialic acid-dependent for binding to and infectivity of MA-104 cells and (2), a dual carbohydrate binding specificity is associated with both human Wa and Porcine OSU rotaviruses. One carbohydrate binding activity is associated with triple-layered virus particles (TLP) and the other with double-layered virus particles (DLP). In binding and infectivity studies, we found that gangliosides were the most potent inhibitors of both the human and procine rotavirus TLP. Furthermore, glycosylation mutant cells deficient in sialylation or neuraminidase-treated MA104 cells, did not bind rotavirus TLP from either strain. Our results show that human Wa binding and infectivity cannot be distinguished from the porcine OSU strain and appears to be sialic acid-dependent. Direct binding of human or porcine TLP to a variety of intact gangliosides was demonstrated in an thin-layer chromatographic (TLC) overlay assay. Human or porcine rotavirus DLP did not bind to any of the intact gangliosides but surprisingly bound asialogangliosides. This binding was abolished by prior treatment of the glycolipids with ceramide glycanase suggesting the intact asialoglycolipid was required for DLP binding. After treatment of either human or porcine TLP with EDTA to remove the outer shell, virus particles bound only to the immobilized asialogangliosides. These results suggest that rotavirus sugar binding specificity can be interpreted either as sialic acid-dependent or independent based on whether the virus preparation consists primarily of triple-layered or double-layered particles. Of perhaps greater interest is the possibility that sialic acid-independent carbohydrate binding activity plays a role in virus maturation or assembly.

Entry of rotaviruses is a multistep process

The infection of epithelial cells by some animal rotavirus strains requires the presence of sialic acid (SA) on the cell surface. Recently, we isolated rhesus rotavirus variants, named nar, whose infectivity, like that of human rotaviruses, is not dependent on SA. In this work, we have determined the binding properties of these SA-dependent and -independent rotavirus strains to MA104 cells. The half-time of attachment of the SA-dependent porcine rotavirus YM and reassortant virus DS1xRRV was found to be about 10 times longer in neuraminidase-treated cells than in untreated cells. On the other hand, human rotaviruses Wa and DS1, and the variant nar3, bound to cells two to three times more rapidly in the absence of SA. To investigate whether the SA-independent cellular structure recognized by the variant and human rotaviruses was the same, we used an infection assay designed to detect competition for cell surface molecules at both attachment and post-attachment steps. In this assay, human rotavirus Wa efficiently competed the infectivity of YM in untreated cells and that of the variant nar3 in untreated, as well as neuraminidase-treated, cells. This competition was nonreciprocal, since YM and nar3 did not compete, but rather increased three- to fivefold the infectivity of Wa. In contrast, a two-direction competition between the variant nar3 and DS1xRRV was found. Similar results were obtained when psoralen-inactivated viruses were used as competitors, indicating that the competition observed was during the early stages of infection. Altogether, these results suggest the existence of multiple interactions between rotaviruses and the cell surface and revealed the existence of common steps during the entry of human and animal rotavirus strains.

Animal virus receptors

The term 'receptor' is generally accepted as the cell-surface component that participates in virus binding and facilitates subsequent viral infection. Recent advances in technology have permitted the identification of several virus receptors, increasing our understanding of the significance of this initial virus-cell and virus-host interaction. Virus binding was previously considered to involve simple recognition and attachment to a single cell surface molecule by virus attachment proteins. The classical concept of these as single entities that participate in a lock-and-key-type process has been superseded by new data indicating that binding can be a multistep process, often involving different virus-attachment proteins and more than one host-cell receptor.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Sulphatide binds to human and animal influenza A viruses, and inhibits the viral infection

We found, by using a virus overlay assay, that influenza A virus isolates bind to sulphatide (HSO3-Gal beta 1-->1'Cer), which has no sialic acid residue, and that the infection of Madin-Darby canine kidney cells with the human influenza virus A/Memphis/1/71 (H3N2) is inhibited by sulphatide. A/Memphis/1/71 (H3N2) causes obvious haemagglutination and low-pH haemolysis of asialoerythrocytes reconstituted with sulphatide. All influenza A virus isolates from the species of animals so far tested bound to sulphatide. The sulphatide-binding specificity of the isolates was different from the viral sialyl-linkage specificity. Influenza A virus isolates also bound to galactosyl ceramide (GalCer; Gal beta 1-->1'Cer), as well as sulphatide, in the virus overlay assays. In contrast, the influenza virus did not bind to N-deacyl, a derivative of sulphatide, glucosyl ceramide or the other neutral glycolipids tested. These results indicate that the linkage of galactose, or sulphated galactose, to ceramide is important for viral binding.

Aglycone modulation of glycolipid receptor function

The aglycone has been largely ignored in consideration of glycoconjugate function. Evidence is reviewed which suggests that the role of the lipid in glycolipid carbohydrate function may be particularly significant. The lipid moiety can promote or reduce carbohydrate exposure of membrane glycolipids. Theoretical calculation has indicated that the plane of the plasma membrane can restrict the permitted conformations of a given glycolipid oligosaccharide. Thus the lipid moiety may influence the relative conformation of such carbohydrate sequences. Evidence of ceramide regulation of glycolipid function can be found in studies of enzyme substrate specificity, antiglycolipid recognition and bacterial/host cell interactions. Studies of verotoxin binding to its glycolipid receptor globotriaosyl ceramide indicate that modulation of receptor function by glycolipid fatty acid content plays an important role in in vitro binding assays, cell cytotoxicity and intracellular routing.

Genetic mapping indicates that VP4 is the rotavirus cell attachment protein in vitro and in vivo

To identify the rotavirus protein which mediates attachment to cells in culture, viral reassortants between the simian rotavirus strain RRV and the murine strains EHP and EW or between the simian strain SA-11 and the human strain DS-1 were isolated. These parental strains differ in the requirement for sialic acid to bind and infect cells in culture. Infectivity and binding assays with the parental and reassortant rotaviruses indicate that gene 4 encodes the rotavirus protein which mediates attachment to cells in culture for both sialic acid-dependent and -independent strains. Using ligated intestinal segments of newborn mice and reassortants obtained between the murine strain EW and RRV, we developed an in vivo infectivity assay. In this system, the infectivity of EW was not affected by prior treatment of the enterocytes with neuraminidase, while neuraminidase treatment reduced the infectivity of a reassortant carrying gene 4 from RRV on an EW background more than 80% relative to the controls. Thus, VP4 appears to function as the cell attachment protein in vivo as well as in vitro.

Characterization of SA-11 rotavirus receptorial structures on human colon carcinoma cell line HT-29

The involvement of different cell membrane components in the receptor structures for SA-11 rotavirus was investigated. As experimental model, the human enterocyte-like HT-29 cell line, was used because of its closer resemblance to the in vivo viral cellular target as compared to other in vitro systems. Rotavirus was incubated with whole membranes or their separated protein and lipid fractions before infection. Either isolated cell membranes or lipid components were capable of binding to the virus and to prevent infection, whereas proteins did not show any inhibitory activity. Among lipids, the glycolipid fraction was shown to impede rotaviral antigen synthesis with a dose-dependent relationship, whereas phospholipids failed to prevent viral infection. To confirm these findings, membranes and target cells were subjected to different enzymatic treatments prior to infection. In addition, HT-29 cells were also incubated with different lectins before infection. The blocking activity of membranes was inhibited by treatment with ceramide glycanase, neuraminidase, and beta-galactosidase but not by treatment with proteases or heat (100 degrees C). Viral infection was prevented by preincubation of target cells with lectins specific for sialic acid and galactose or with ceramide glycanase, neuraminidase, and beta-galactosidase, whereas protease treatments were not active. The results of these experimental procedures indicate that glycolipids containing specific carbohydrate moieties, such as sialic acid and galactose, contribute to the SA-11 rotavirus receptor structure on HT-29 cells.

The rotavirus genus

Human rotaviruses, discovered nearly 20 years ago, have been proven to be major cause of paediatric diarrhoeal disease morbidity and mortality. The clinical significance of these viruses stimulated basic studies on their biology, molecular and antigenic properties and epidemiology. General features, clinical relevance, epidemiologic pattern and laboratory diagnosis of human rotavirus infections are here reviewed.

Assay for evaluation of rotavirus-cell interactions: identification of an enterocyte ganglioside fraction that mediates group A porcine rotavirus recognition

A virus-host cell-binding assay was developed and used to investigate specific binding between group A porcine rotavirus and MA-104 cells or porcine enterocytes. A variety of glycoconjugates and cellular components were screened for their ability to block rotavirus binding to cells. During these experiments a crude ganglioside mixture was observed to specifically block rotavirus binding. On the basis of these results, enterocytes were harvested from susceptible piglets and a polar lipid fraction was isolated by solvent extraction and partitioning. Throughout subsequent purification of this fraction by Sephadex partition, ion-exchange, silicic acid, and thin-layer chromatography, blocking activity behaved as a monosialoganglioside (GMX) that displayed a thin-layer chromatographic mobility between those of GM2 and GM3. The blocking activity of GMX was inhibited by treatment with neuraminidase and ceramide glycanase but not by treatment with protease or heat (100 degrees C). Further purification of GMX by high-pressure liquid chromatography resulted in the resolution of two monosialogangliosides, GMX and a band which comigrated with GM1 on thin-layer chromatography. These data suggest that a cell surface monosialoganglioside or family of monosialogangliosides may function as an in vivo relevant receptor for group A porcine rotavirus and that sialic acid is a required epitope for virus-binding activity.

Binding to sialic acids is not an essential step for the entry of animal rotaviruses to epithelial cells in culture

The infection of target cells by animal rotaviruses requires the presence of sialic acids on the cell surface. Treatment of the cells with neuraminidases or incubation of the viruses with some sialoglycoproteins, such as glycophorin A, greatly reduces virus binding, with the consequent reduction of viral infectivity. In this work, we report the isolation of animal rotavirus variants whose infectivity is no longer dependent on the presence of sialic acids on the cell surface. In addition, although these variants bind to glycophorin A as efficiently as the wild-type virus, this interaction no longer inhibit viral infectivity. These observations indicate that the initial interaction of the mutants with the cell occurs at a site different from the sialic acid-binding site located on VP8, the smaller trypsin cleavage product of VP4. Reassortant analysis showed that the mutant phenotype segregates with the VP4 gene. Neutralizing monoclonal antibodies directed to VP4 and VP7 were tested for their ability to neutralize the variants. Antibodies to VP7 and VP5, the larger trypsin cleavage product of VP4, neutralized the mutants as efficiently as the wild-type virus. In contrast, although antibodies to VP8 were able to bind to the mutants, they showed little or no neutralizing activity. The implications of these findings in rotavirus attachment to and penetration of epithelial cells in culture are discussed.