Integrins alpha2beta1 and alpha4beta1 can mediate SA11 rotavirus attachment and entry into cells

Alpha4beta1 integrin acts as a cell receptor for murine polyomavirus at the postattachment level

The initial interaction of murine polyomavirus (Py) with host cells occurs through direct binding of the major capsid protein VP1 with cell membrane molecules containing terminal sialic acids; however, these Py receptor molecules have not yet been identified. Analysis of the capsid protein primary sequences of all murine strains revealed the presence of integrin ligand motifs in the DE and EF loops of VP1 (LDV and DLXXL, respectively) and at the N terminus of VP2 (DGE). We show that infectivity of the Py A2 strain in mouse Swiss 3T3 fibroblasts is significantly reduced only in the presence of natural integrin ligands carrying an LDV motif or antibodies directed against the alpha4 and beta1 integrin subunits. Furthermore, we demonstrate that expression of the alpha4 subunit in the alpha4-deficient BALB/c 3T3 cells increases viral infectivity. Addition of alpha4 function-blocking antibodies, prior to or after virus adsorption, blocks this increased infectivity without affecting virus binding to cells. Taken together, these data indicate that expression of alpha4 integrin enhances permissivity to Py, probably by acting as one of the postattachment receptors.

Growth of rotaviruses in primary pancreatic cells

Rotavirus infection in children at risk of developing type 1 diabetes has been temporally associated with development of pancreatic islet autoantibodies. In this study, nonobese diabetic mice were shown to be susceptible to rhesus rotavirus infection and pancreatic islets from nonobese diabetic mice, nonobese diabetes-resistant mice, fetal pigs, and macaque monkeys supported various degrees of rotavirus growth. Human rotaviruses replicated in monkey islets only. This islet susceptibility shows that rotavirus infection of the pancreas in vivo might be possible.

Molecular biology of 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. The entry of rotaviruses into epithelial cells appears to be a multistep process during which at least three contacts between the virus and cell receptors occur. Different rotavirus strains display different requirements to infect cells. Some strains depend on the presence of sialic acid on the cell surface; however, interaction with a sialic acid-containing receptor does not seem to be essential, because variants that no longer need sialic acid to infect the cells can be isolated from sialic acid-dependent strains. Comparative characterization of the sialic acid-dependent rotavirus strain RRV, its neuraminidase-resistant variant nar3, and the human rotavirus strain Wa have allowed to show that alpha2beta1 integrin is used by nar3 as its primary cell attachment site, and by RRV in a second interaction subsequent to its initial contact with a sialic acid-containing cell receptor. These first two interactions are mediated by the virus spike protein VP4. After attaching to the cell, all three strains interact with integrin alphaVbeta3 and protein hsc70, interactions perhaps important for the virus to penetrate into the cell's interior. The cell molecules proposed to serve as rotavirus receptors have been found associated with cholesterol and glycosphingolipid-enriched lipid microdomains, and disorganization of these domains greatly inhibits rotavirus infectivity. We propose that the functional rotavirus receptor is a complex of several cell molecules most likely immersed in plasma membrane lipid microdomains.

Antibodies to rotavirus outer capsid glycoprotein VP7 neutralize infectivity by inhibiting virion decapsidation

The rotavirus capsid is composed of three concentric protein layers. Proteins VP4 and VP7 comprise the outer layer. VP4 forms spikes, is the viral attachment protein, and is cleaved by trypsin into VP8* and VP5*. VP7 is a glycoprotein and the major constituent of the outer protein layer. Both VP4 and VP7 induce neutralizing and protective antibodies. To gain insight into the virus neutralization mechanisms, the effects of neutralizing monoclonal antibodies (MAbs) directed against VP8*, VP5*, and VP7 on the decapsidation process of purified OSU and RRV virions were studied. Changes in virion size were followed in real time by 90 degrees light scattering. The transition from triple-layered particles to double-layered particles induced by controlled low calcium concentrations was completely inhibited by anti-VP7 MAbs but not by anti-VP8* or anti-VP5* MAbs. The inhibitory effect of the MAb directed against VP7 was concentration dependent and was abolished by papain digestion of virus-bound antibody under conditions that generated Fab fragments but not under conditions that generated F(ab')(2) fragments. Electron microscopy showed that RRV virions reacted with an anti-VP7 MAb stayed as triple-layered particles in the presence of excess EDTA. Furthermore, the infectivity of rotavirus neutralized via VP8*, but not that of rotavirus neutralized via VP7, could be recovered by lipofection of neutralized particles into MA-104 cells. These data are consistent with the notion that antibodies directed at VP8* neutralize by inhibiting binding of virus to the cell. They also indicate that antibodies directed at VP7 neutralize by inhibiting virus decapsidation, in a manner that is dependent on the bivalent binding of the antibody.

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.

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.

Influence of calcium on the early steps of rotavirus infection

The structure of rotaviruses and many steps of their replication cycle depend on the concentration of calcium in the microenvironment. In this work, to learn about the role of calcium during the early steps of the infection, we characterized the effect of increasing the calcium concentration in the medium on the infectivity of rotaviruses. We found that a fivefold increase in the calcium concentration of the cell culture medium results in an increased viral titer in all rotavirus strains tested. The effect of this divalent ion seems to be mainly on the viral particle and not on the surface of the cell. Analysis of the intrinsic fluorescence spectra of purified triple-layered particles revealed that changes in the environment of tryptophan residues occurred as calcium concentration increased, suggesting that conformational changes in the viral particle might be responsible for the effect of this ion on the viral infectivity.

The rhesus rotavirus VP4 sialic acid binding domain has a galectin fold with a novel carbohydrate binding site

Cell attachment and membrane penetration are functions of the rotavirus outer capsid spike protein, VP4. An activating tryptic cleavage of VP4 produces the N-terminal fragment, VP8*, which is the viral hemagglutinin and an important target of neutralizing antibodies. We have determined, by X-ray crystallography, the atomic structure of the VP8* core bound to sialic acid and, by NMR spectroscopy, the structure of the unliganded VP8* core. The domain has the beta-sandwich fold of the galectins, a family of sugar binding proteins. The surface corresponding to the galectin carbohydrate binding site is blocked, and rotavirus VP8* instead binds sialic acid in a shallow groove between its two beta-sheets. There appears to be a small induced fit on binding. The residues that contact sialic acid are conserved in sialic acid-dependent rotavirus strains. Neutralization escape mutations are widely distributed over the VP8* surface and cluster in four epitopes. From the fit of the VP8* core into the virion spikes, we propose that VP4 arose from the insertion of a host carbohydrate binding domain into a viral membrane interaction protein.

Integrin alpha3beta1 (CD 49c/29) is a cellular receptor for Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) entry into the target cells

Human herpesvirus-8 (HHV-8) is implicated in the pathogenesis of Kaposi's sarcoma. HHV-8 envelope glycoprotein B possesses the RGD motif known to interact with integrin molecules, and HHV-8 infectivity was inhibited by RGD peptides, antibodies against RGD-dependent alpha3 and beta1 integrins, and by soluble alpha3beta1 integrin. Expression of human alpha3 integrin increased the infectivity of virus for Chinese hamster ovary cells. Anti-gB antibodies immunoprecipitated the virus-alpha3 and -beta1 complexes, and virus binding studies suggest a role for alpha3beta1 in HHV-8 entry. Further, HHV-8 infection induced the integrin-mediated activation of focal adhesion kinase (FAK). These findings implicate a role for alpha3beta1 integrin and the associated signaling pathways in HHV-8 entry into the target cells.

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.

Localization of membrane permeabilization and receptor binding sites on the VP4 hemagglutinin of rotavirus: implications for cell entry

The surface of rotavirus is decorated with 60 spike-like projections, each composed of a dimer of VP4, the viral hemagglutinin. Trypsin cleavage of VP4 generates two fragments, VP8*, which binds sialic acid (SA), and VP5*, containing an integrin binding motif and a hydrophobic region that permeabilizes membranes and is homologous to fusion domains. Although the mechanism for cell entry by this non-enveloped virus is unclear, it is known that trypsin cleavage enhances viral infectivity and facilitates viral entry. We used electron cryo-microscopy and difference map analysis to localize the binding sites for two neutralizing monoclonal antibodies, 7A12 and 2G4, which are directed against the SA-binding site within VP8* and the membrane permeabilization domain within VP5*, respectively. Fab 7A12 binds at the tips of the dimeric heads of VP4, and 2G4 binds in the cleft between the two heads of the spike. When these binding results are combined with secondary structure analysis, we predict that the VP4 heads are composed primarily of beta-sheets in VP8* and that VP5* forms the body and base primarily in beta-structure and alpha-helical conformations, respectively. Based on these results and those of others, a model is proposed for cell entry in which VP8* and VP5* mediate receptor binding and membrane permeabilization, and uncoating occurs during transfer across the lipid bilayer, thereby generating the transcriptionally active particle.

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.

Structural Analysis of the α2 Integrin I Domain/Procollagenase-1 (Matrix Metalloproteinase-1) Interaction

Previous studies have established that ligation of keratinocyte alpha(2)beta(1) integrin by type I collagen induces expression of matrix metalloproteinase-1 (MMP-1) and that MMP-1 activity is required for the alpha(2)beta(1) integrin-dependent migration of primary keratinocytes across collagenous matrices. We now present evidence that MMP-1 binds the alpha(2)beta(1) integrin via the I domain of the alpha(2) integrin subunit. Using an enzyme-linked immunosorbent assay with purified human MMP-1 and recombinant alpha(2) integrin I domain, we showed that the alpha(2) integrin I domain specifically bound in a divalent cation-dependent manner to both the pro and active forms of MMP-1, but not to MMP-3 or MMP-13. Although both the I domain and MMP-1 bind divalent cations, MMP-1 bound, in a divalent cation-dependent manner, to alpha(2) integrin I domains containing metal ion-dependent adhesion sites motif mutations that prevent divalent cation binding to the I domain, demonstrating that the metal ion dependence is a function of MMP-1. Using a series of MMP-1-MMP-3 and MMP-1-MMP-13 chimeras, we determined that both the linker domain and the hemopexin-like domain of MMP-1 were required for optimal binding to the I domain. The alpha(2) integrin/MMP-1 interaction described here extends an emerging paradigm in matrix biology involving anchoring of proteinases to the cell surface to regulate their biological activities.

Trypsin cleavage stabilizes the rotavirus VP4 spike

Trypsin enhances rotavirus infectivity by an unknown mechanism. To examine the structural basis of trypsin-enhanced infectivity in rotaviruses, SA11 4F triple-layered particles (TLPs) grown in the absence (nontrypsinized rotavirus [NTR]) or presence (trypsinized rotavirus [TR]) of trypsin were characterized to determine the structure, the protein composition, and the infectivity of the particles before and after trypsin treatment. As expected, VP4 was not cleaved in NTR particles and was cleaved into VP5(*) and VP8(*) in TR particles. However, surprisingly, while the VP4 spikes were clearly visible and well ordered in the electron cryomicroscopy reconstructions of TR TLPs, they were totally absent in the reconstructions of NTR TLPs. Biochemical analysis with radiolabeled particles indicated that the stoichiometry of the VP4 in NTR particles was the same as that in TR particles and that the VP8(*) portion of NTR, but not TR, particles is susceptible to further proteolysis by trypsin. Taken together, these structural and biochemical data show that the VP4 spikes in the NTR TLPs are icosahedrally disordered and that they are conformationally different. Structural studies on the NTR TLPs after trypsin treatment showed that spike structure could be partially recovered. Following additional trypsin treatment, infectivity was enhanced for both NTR and TR particles, but the infectivity of NTR remained 2 logs lower than that of TR particles. Increased infectivity in these particles corresponded to additional cleavages in VP5(*), at amino acids 259, 583, and putatively 467, which are conserved in all P serotypes of human and animal group A rotaviruses and also corresponded with a structural change in VP7. These biochemical and structural results show that trypsin cleavage imparts order to VP4 spikes on de novo synthesized virus particles, and these ordered spikes make virus entry into cells more efficient.

RhoA is activated during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is an important human pathogen that can cause severe and life-threatening respiratory infections in infants and immunocompromised adults. We have recently shown the RSV F glycoprotein, which mediates viral fusion and entry, interacts with the cellular protein RhoA in two-hybrid and in vitro binding assays. Whether this interaction occurs in living cells remains an open question. However, because RhoA signaling is associated with many cellular functions relevant to RSV pathogenesis such as actin cytoskeleton organization, expression of proinflammatory cytokines, and smooth muscle contraction, we asked whether RhoA activation occurred during RSV infection of HEp-2 cells. We found that the amount of isoprenylated and membrane-bound RhoA in RSV-infected cultures was increased. Further evidence of RhoA activation was demonstrated by downstream signaling activity mediated by RhoA. There was an increase in p130(cas) phosphorylation during RSV infection, which was prevented by Y-27632, a specific inhibitor of Rho kinase, or lovastatin, an HMG-CoA reductase inhibitor that reduces the synthesis of groups needed for isoprenylation. In addition, RSV infection of HEp-2 cells resulted in an increase in the formation of actin stress fibers. Pretreatment of HEp-2 cells with Clostridium botulinum C3 exotoxin, an enzyme that specifically ADP-ribosylates and inactivates RhoA, prevented RSV-induced stress fiber formation. These observations indicate that RhoA and subsequent downstream signaling events are activated during RSV infection, which has implications for RSV pathogenesis.

RGD tripeptide of bluetongue virus VP7 protein is responsible for core attachment to Culicoides cells

Bluetongue virus (BTV) is an arthropod-borne virus transmitted by Culicoides species to vertebrate hosts. The double-capsid virion is infectious for Culicoides vector and mammalian cells, while the inner core is infectious for only Culicoides-derived cells. The recently determined crystal structure of the BTV core has revealed an accessible RGD motif between amino acids 168 to 170 of the outer core protein VP7, whose structure and position would be consistent with a role in cell entry. To delineate the biological role of the RGD sequence within VP7, we have introduced point mutations in the RGD tripeptide and generated three recombinant baculoviruses, each expressing a mutant derivative of VP7 (VP7-AGD, VP7-ADL, and VP7-AGQ). Each expressed mutant protein was purified, and the oligomeric nature and secondary structure of each was compared with those of the wild-type (wt) VP7 molecule. Each mutant VP7 protein was used to generate empty core-like particles (CLPs) and were shown to be biochemically and morphologically identical to those of wt CLPs. However, when mutant CLPs were used in an in vitro cell binding assay, each showed reduced binding to Culicoides cells compared to wt CLPs. Twelve monoclonal antibodies (MAbs) was generated using purified VP7 or CLPs as a source of antigen and were utilized for epitope mapping with available chimeric VP7 molecules and the RGD mutants. Several MAbs bound to the RGD motif on the core, as shown by immunogold labeling and cryoelectron microscopy. RGD-specific MAb H1.5, but not those directed to other regions of the core, inhibited the binding activity of CLPs to the Culicoides cell surface. Together, these data indicate that the RGD motif present on BTV VP7 is responsible for Culicoides cell binding activity.

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 and calicivirus infections of the gastrointestinal tract

Virus infections of the gastrointestinal tract, leading to gastroenteritis, are a common problem in both developed and developing countries. Rotavirus and Norwalk-like viruses are the most common agents responsible for clinically severe disease in humans, and this paper focuses on new information about the mechanisms of pathogenesis and epidemiology of these two pathogens. Rotavirus-induced disease involves a viral enterotoxin and activation of the enteric nervous system, as well as malabsorption, suggesting that common mechanisms of pathogenesis may exist between viral and bacterial pathogens. Each gastrointestinal virus possesses unique molecular properties that can be exploited to discover new information about responses of cells of the gastrointestinal tract. Work continues toward making vaccines for rotavirus and Norwalk-like viruses.

Analysis of anti-rotavirus activity of extract from Stevia rebaudiana

Anti-human rotavirus (HRV) activity of hot water extracts from Stevia rebaudiana (SE) was examined. SE inhibited the replication of all four serotypes of HRV in vitro. This inhibitory effect of SE was not reduced on the prior exposure of SE to HCl for 30 min at pH 2. Binding assay with radiolabeled purified viruses indicated that the inhibitory mechanism of SE is the blockade of virus binding. The SE inhibited the binding of anti-VP7 monoclonal antibody to HRV-infected MA104 cells. The inhibitory components of SE were found to be heterogeneous anionic polysaccharides with different ion charges. The component analyses suggested that the purified fraction named as Stevian with the highest inhibitory activity consists of the anionic polysaccharide with molecular weight of 9800, and contains Ser and Ala as amino acids. Analyses of sugar residues suggest uronic acid(s) as sugar components. It did not contain amino and neutral sugars and sulfate residues. These findings suggest that SE may bind to 37 kD VP7 and interfere with the binding of VP7 to the cellular receptors by steric hindrance, which results in the blockade of the virus attachment to cells.

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.

Integrin alpha2beta1 mediates the cell attachment of the rotavirus neuraminidase-resistant variant nar3

It was previously reported that integrins alpha2beta1, alpha4beta1, and alphaXbeta2 are involved in rotavirus cell infection. In this work we studied the role of integrin subunits alpha2, alpha4, and beta2 on the attachment of rotaviruses RRV and nar3 to MA104 cells. Integrin alpha2beta1 was found to serve as the binding receptor for the neuraminidase-resistant virus nar3, whereas the neuraminidase-sensitive strain RRV interacted with this integrin at a postattachment step. It was shown that nar3 binds alpha2beta1 through the DGE integrin-recognition motif located in the virus surface protein VP5. Integrin subunits alpha4 and beta2 do not seem to be involved in the initial cell binding of either virus.

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.

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.

Characterization of a monoclonal antibody directed to the surface of MA104 cells that blocks the infectivity of rotaviruses

Rhesus rotavirus (RRV) binds to sialic acid residues on the surface of target cells, and treatment of these cells with neuraminidase greatly reduces virus binding with the consequent reduction of infectivity. Variants that can efficiently infect neuraminidase-treated cells have been isolated, indicating that attachment to sialic acid is not an essential step for animal rotaviruses to infect cells. To identify and characterize the neuraminidase-resistant receptor for rotaviruses, we have isolated a hybridoma that secrets a monoclonal antibody (MAb) (2D9) that specifically blocks the infectivity of wild-type (wt) RRV and of its sialic acid-independent variant nar3, in untreated as well as in neuraminidase-treated cells. The infectivity of a human rotavirus was also inhibited, although to a lesser extent. MAb 2D9 blocks the binding of the variant to MA104 cells, while not affecting the binding of wt RRV; in addition, this MAb blocked the attachment of a recombinant glutathione S-transferase (GST)-VP5 fusion protein, but did not affect the binding of GST-VP8. Altogether these results suggest that MAb 2D9 is directed to the neuraminidase-resistant receptor. This receptor seems to mediate the direct attachment of the variant to the cell, through VP5, while the receptor is used by wt RRV for a secondary interaction, after its initial binding to sialic acid, through VP8. MAb 2D9 interacts specifically with the cell surface by indirect immunofluorescence, immunoelectron microscopy, and FACS. By a solid-phase immunoisolation technique, MAb 2D9 was found to react with three proteins of ca. 47, 55, and 220 kDa, which might form a complex.