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

Beta-integrin mediates WSSV infection

White Spot Syndrome Virus (WSSV) is a virulent and widespread dsDNA virus with a wide range of hosts. Although remarkable progress has been made on virus characterization, however, its mechanism of infection is poorly understood. In this study, by analyzing the phage display library of the WSSV genome, a WSSV envelope protein VP187 (wsv209) was found to interact with shrimp integrin. VP187 possesses the RGD motif. The interaction between integrin and VP187 was confirmed with coimmunoprecipitation. These results demonstrate for the first time an interaction between the WSSV envelope protein and a cell surface molecule. Soluble integrin, integrin-specific antibody and an RGD-containing peptide were found to block the WSSV infection in vivo and in vitro. Gene silencing using a sequence-specific dsRNA targeting beta-integrin effectively inhibited the virus infection. These findings suggest that beta-integrin may function as a cellular receptor for WSSV infection.

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.

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.

Heat shock enhances the susceptibility of BHK cells to rotavirus infection through the facilitation of entry and post-entry virus replication steps

Rotavirus infection is known to induce several cellular stress proteins, although their possible involvement in the replication cycle of the virus has not been studied. In addition, the heat shock cognate protein hsc70 has been shown to function as a post-attachment receptor during virus entry. In this work we have studied the effect of heat shock on the susceptibility of cells to rotavirus infection. BHK cells, which are largely refractory to the virus, became about 100-fold more susceptible when heat-treated, while the rotavirus highly susceptible MA104 cells did not significantly modified their susceptibility upon heat stress, suggesting that heat shock induces factors that are rate-limiting the replication of rotaviruses in BHK but not in MA104 cells. The heat treatment was shown to facilitate the rotavirus infection of BHK cells at the penetration and post-penetration levels, and each of these stages seems to contribute comparably to the overall observed 100-fold increase in infectivity. Since the binding of the virus to the cell surface was not affected, the caloric stress probably facilitates the penetration and/or uncoating of the virus. The pathway of virus entry into heat-shocked BHK cells seems to be similar to that used in MA104 cells, since treatments that affect MA104 cell infection also affected rotavirus infectivity in heat-treated BHK cells.

Cell Adhesion Molecules for Targeted Drug Delivery

Rapid advancement of the understanding of the structure and function of cell adhesion molecules (i.e., integrins, cadherins) has impacted the design and development of drugs (i.e., peptide, proteins) with the potential to treat cancer and heart and autoimmune diseases. For example, RGD peptides/peptidomimetics have been marketed as anti-thrombic agents and are being investigated for inhibiting tumor angiogenesis. Other cell adhesion peptides derived from ICAM-1 and LFA-1 sequences were found to block T-cell adhesion to vascular endothelial cells and epithelial cells; these peptides are being investigated for treating autoimmune diseases. Recent findings suggest that cell adhesion receptors such as integrins can internalize their peptide ligands into the intracellular space. Thus, many cell adhesion peptides (i.e., RGD peptide) were used to target drugs, particles, and diagnostic agents to a specific cell that has increased expression of cell adhesion receptors. This review is focused on the utilization of cell adhesion peptides and receptors in specific targeted drug delivery, diagnostics, and tissue engineering. In the future, more information on the mechanism of internalization and intracellular trafficking of cell adhesion molecules will be exploited for delivering drug molecules to a specific type of cell or for diagnosis of cancer and heart and autoimmune diseases.

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 peptide-binding and ATPase domains of recombinant hsc70 are required to interact with rotavirus and reduce its infectivity

The heat shock cognate protein hsc70 has been implicated as a postattachment cell receptor for rotaviruses. Here we show that hsc70 interacts specifically with rotaviruses through its peptide-binding domain, since a recombinant full-length hsc70 protein and its peptide-binding domain, but not its ATPase domain, bound triple-layered particles in a solid-phase assay, and known ligands of hsc70 competed this binding. The peptide ligands of hsc70 were also shown to block rotavirus infectivity when added to cells before virus infection, suggesting that hsc70 on the surface of MA104 cells also interacts with the virus through its peptide-binding domain and that this interaction is important for virus entry. When purified infectious virus was incubated with soluble hsc70 in the presence of the cochaperone hsp40 and ATP and then pelleted through a sucrose cushion, the recovered virus had lost 60% of its infectivity, even though hsc70 was not detected in the pellet fraction. The hsc70-treated virus showed slightly different reactivities with monoclonal antibodies and was more susceptible to heat and basic pHs than the untreated virus, suggesting that hsc70 induces a subtle conformational change in the virus that results in a reduction of its infectivity. The relevance of the ATPase activity of hsc70 for reducing virus infectivity was demonstrated by the finding that in the presence of a nonhydrolyzable analogue of ATP, virus infectivity was not affected, and a mutant protein lacking ATPase activity failed to reduce virus infection. Altogether, these results suggest that during cell infection, the interaction of the virus with hsc70 on the surface of MA104 cells results in a conformational change of virus particles that facilitates their entry into the cell cytoplasm.

Beta1 integrin mediates internalization of mammalian reovirus

Reovirus infection is initiated by interactions between the attachment protein sigma1 and cell surface carbohydrate and junctional adhesion molecule A (JAM-A). Expression of a JAM-A mutant lacking a cytoplasmic tail in nonpermissive cells conferred full susceptibility to reovirus infection, suggesting that cell surface molecules other than JAM-A mediate viral internalization following attachment. The presence of integrin-binding sequences in reovirus outer capsid protein lambda2, which serves as the structural base for sigma1, suggests that integrins mediate reovirus endocytosis. A beta1 integrin-specific antibody, but not antibodies specific for other integrin subunits, inhibited reovirus infection of HeLa cells. Expression of a beta1 integrin cDNA, along with a cDNA encoding JAM-A, in nonpermissive chicken embryo fibroblasts conferred susceptibility to reovirus infection. Infectivity of reovirus was significantly reduced in beta1-deficient mouse embryonic stem cells in comparison to isogenic cells expressing beta1. However, reovirus bound equivalently to cells that differed in levels of beta1 expression, suggesting that beta1 integrins are involved in a postattachment entry step. Concordantly, uptake of reovirus virions into beta1-deficient cells was substantially diminished in comparison to viral uptake into beta1-expressing cells. These data provide evidence that beta1 integrin facilitates reovirus internalization and suggest that viral entry occurs by interactions of reovirus virions with independent attachment and entry receptors on the cell surface.

Listeria monocytogenes invades the epithelial junctions at sites of cell extrusion

Listeria monocytogenes causes invasive disease by crossing the intestinal epithelial barrier. This process depends on the interaction between the bacterial surface protein Internalin A and the host protein E-cadherin, located below the epithelial tight junctions at the lateral cell-to-cell contacts. We used polarized MDCK cells as a model epithelium to determine how L. monocytogenes breaches the tight junctions to gain access to this basolateral receptor protein. We determined that L. monocytogenes does not actively disrupt the tight junctions, but finds E-cadherin at a morphologically distinct subset of intercellular junctions. We identified these sites as naturally occurring regions where single senescent cells are expelled and detached from the epithelium by extrusion. The surrounding cells reorganize to form a multicellular junction that maintains epithelial continuity. We found that E-cadherin is transiently exposed to the lumenal surface at multicellular junctions during and after cell extrusion, and that L. monocytogenes takes advantage of junctional remodeling to adhere to and subsequently invade the epithelium. In intact epithelial monolayers, an anti-E-cadherin antibody specifically decorates multicellular junctions and blocks L. monocytogenes adhesion. Furthermore, an L. monocytogenes mutant in the Internalin A gene is completely deficient in attachment to the epithelial apical surface and is unable to invade. We hypothesized that L. monocytogenes utilizes analogous extrusion sites for epithelial invasion in vivo. By infecting rabbit ileal loops, we found that the junctions at the cell extrusion zone of villus tips are the specific target for L. monocytogenes adhesion and invasion. Thus, L. monocytogenes exploits the dynamic nature of epithelial renewal and junctional remodeling to breach the intestinal barrier.

Cellular entry of Hantaan virus A9 strain: Specific interactions with β3 integrins and a novel 70kDa protein

Cellular entry of pathogenic hantaviruses had been shown to be mediated by beta3 integrins. However, no direct evidence exists that hantavirus binds to beta3 integrins, and integrin beta3 subunit is not expressed on some cells permissive to hantavirus infection. In this report, utilizing beta3-integrin-transfected CHO cells, we demonstrated that integrin beta3 subunit renders CHO cells susceptible to Chinese Hantaan virus (HTN) strain A9 (isolated in China), and the viral infection was correspondingly inhibited by antibodies to alphavbeta3, alphaIIbbeta3, beta3, and alphav integrins. Furthermore, virus overlay protein-binding assay and 'quarternary Western' analysis indicate that HTN A9 directly interacts with beta3 integrins and an unidentified 70kDa protein. These findings indicate that beta3 integrins play a crucial role in cellular entry of HTN A9 via specific interactions with the virus. In addition, a novel 70kDa protein may serves as a candidate receptor or alternative cellular component for interaction with HTN.

Rotaviruses interact with alpha4beta7 and alpha4beta1 integrins by binding the same integrin domains as natural ligands

Group A rotaviruses are major intestinal pathogens that express potential alpha4beta1 and alpha4beta7 integrin ligand sequences Leu-Asp-Val and Leu-Asp-Ile in their outer capsid protein VP7, and Ile-Asp-Ala in their spike protein VP4. Monkey rotavirus SA11 can use recombinant alpha4beta1 as a cellular receptor. In this study a new potential alpha4beta1, alpha4beta7 and alpha9beta1 integrin ligand sequence, Tyr-Gly-Leu, was identified in VP4. It was shown that several human and monkey rotaviruses bound alpha4beta1 and alpha4beta7, but not alpha9beta1. Binding to alpha4beta1 mediated the infectivity and growth of monkey rotaviruses, and binding to alpha4beta7 mediated their infectivity. A porcine rotavirus interacted with alpha4 integrins at a post-binding stage to facilitate infection. Activation of alpha4beta1 increased rotavirus infectivity. Cellular treatment with peptides containing the alpha4 integrin ligand sequences Tyr-Gly-Leu and Ile-Asp-Ala eliminated virus binding to alpha4 integrins and infectivity. In contrast, rotavirus recognition of alpha4 integrins was unaffected by a peptide containing the sequence Leu-Asp-Val or by a mutation in the VP7 Leu-Asp-Val sequence. VP4 involvement in rotavirus recognition of alpha4beta1 was demonstrated with rotavirus reassortants. Swapping and point mutagenesis of alpha4 surface loops showed that rotaviruses required the same alpha4 residues and domains for binding as the natural alpha4 integrin ligands: mucosal addressin cell adhesion molecule-1, fibronectin and vascular cell adhesion molecule-1. Several rotaviruses are able to use alpha4beta7 and alpha4beta1 for cell binding or entry, through the recognition of the same alpha4-subunit domains as natural alpha4 ligands.

Synthesis, in vitro, and in vivo characterization of an integrin alpha(v)beta(3)-targeted molecular probe for optical imaging of tumor

Integrin alpha(v)beta(3) is a widely-recognized target for the development of targeted molecular probes for imaging pathological conditions. alpha(v)beta(3) is a cell-surface receptor protein that is upregulated in various pathological conditions including osteoporosis, rheumatoid arthritis, macular degeneration, and cancer. The synthesis of an alpha(v)beta(3)-targeted optical probe 7 from compound 1, and its in vitro and in vivo characterization is described. A series of aliphatic carbamate derivatives of the potent non-peptide integrin antagonist 1 was synthesized and the binding affinity to alpha(v)beta(3) was determined in both enzyme linked immunosorbent assay (ELISA) and cell adhesion inhibition assays. The hydrophobic carbamate-linked appendages improved the binding affinity of the parent compound for alpha(v)beta(3) by 2-20 times. A Boc-protected neopentyl derivative in the series is shown to have the best binding affinity to alpha(v)beta(3) (IC(50)=0.72 nM) when compared to compound 1 as well as to c-RGDfV. Optical probe 7 utilizes the neopentyl linker and demonstrates increased binding affinity and significant tumor cell uptake in vitro as well as specific tumor accumulation and retention in vivo. These results illustrate the potential of employing integrin-targeted molecular probes based on 1 to image a multitude of diseases associated with alpha(v)beta(3) overexpression.

Bacterial viruses against viruses pathogenic for man?

In this review, we discuss possible models of bacteriophage-virus interactions. The first is based on the mechanism by which phages may interact indirectly with viruses. Its essence is that bacteriophage-derived nucleic acid may inhibit pathogenic virus infection. It seems that this phenomenon can be partly explained on the basis of interferon induction. We also discuss a study by Borecky's group (conducted over two decades ago) which provided some clinical data on the effectiveness of the application of native bacteriophage RNA in the treatment of viral infections. The second interaction model is based on the direct competition of bacteriophages and viruses for cellular receptors for viral cell-entry. The use of bacteriophages as inducers or displayers of antibodies with antiviral action is considered as the third model. In this part of the article, we also discuss other data and hypotheses on conceivable interactions between bacterial and animal viruses. As our current supply of antiviral drugs is quite limited, using natural agents such as bacteriophages as a weapon against pathogenic viruses could be an attractive and cost-efficient alternative, and further studies are urgently needed to test this possibility.

pH-induced conformational change of the rotavirus VP4 spike: implications for cell entry and antibody neutralization

The rotavirus spike protein, VP4, is a major determinant of infectivity and neutralization. Previously, we have shown that trypsin-enhanced infectivity of rotavirus involves a transformation of the VP4 spike from a flexible to a rigid bilobed structure. Here we show that at elevated pH the spike undergoes a drastic, irreversible conformational change and becomes stunted, with a pronounced trilobed appearance. These particles with altered spikes, at a normal pH of 7.5, despite the loss of infectivity and the ability to hemagglutinate, surprisingly exhibit sialic acid (SA)-independent cell binding in contrast to the SA-dependent cell binding exhibited by native virions. Remarkably, a neutralizing monoclonal antibody that remains bound to spikes throughout the pH changes (pH 7 to 11 and back to pH 7) completely prevents this conformational change, preserving the SA-dependent cell binding and hemagglutinating functions of the virion. A hypothesis that emerges from the present study is that high-pH treatment triggers a conformational change that mimics a post-SA-attachment step to expose an epitope recognized by a downstream receptor in the rotavirus cell entry process. This process involves sequential interactions with multiple receptors, and the mechanism by which the antibody neutralizes is by preventing this conformational change.

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.

An arthritogenic alphavirus uses the α1β1 integrin collagen receptor

Ross River (RR) virus is an alphavirus endemic to Australia and New Guinea and is the aetiological agent of epidemic polyarthritis or RR virus disease. Here we provide evidence that RR virus uses the collagen-binding alpha1beta1 integrin as a cellular receptor. Infection could be inhibited by collagen IV and antibodies specific for the beta1 and alpha1 integrin proteins, and fibroblasts from alpha1-integrin-/- mice were less efficiently infected than wild-type fibroblasts. Soluble alpha1beta1 integrin bound immobilized RR virus, and peptides representing the alpha1beta1 integrin binding-site on collagen IV inhibited virus binding to cells. We speculate that two highly conserved regions within the cell-receptor binding domain of E2 mimic collagen and provide access to cellular collagen-binding receptors.

The unique stacked rings in the nucleocapsid of the white spot syndrome virus virion are formed by the major structural protein VP664, the largest viral structural protein ever found

One unique feature of the shrimp white spot syndrome virus (WSSV) genome is the presence of a giant open reading frame (ORF) of 18,234 nucleotides that encodes a long polypeptide of 6,077 amino acids with a hitherto unknown function. In the present study, by applying proteomic methodology to analyze the sodium dodecyl sulfate-polyacrylamide gel electrophoresis profile of purified WSSV virions by liquid chromatography-mass spectrometry (LC-MS/MS), we found that this giant polypeptide, designated VP664, is one of the viral structural proteins. The existence of the corresponding 18-kb transcript was confirmed by sequencing analysis of reverse transcription-PCR products, which also showed that vp664 was intron-less. A time course analysis showed that this transcript was actively transcribed at the late stage, suggesting that this gene product should contribute primarily to the assembly and morphogenesis of the virion. Several polyclonal antisera against this giant protein were prepared, and one of them was successfully used for immunoelectron microscopy analysis to localize the protein in the virion. Immunoelectron microscopy with a gold-labeled secondary antibody showed that the gold particles were regularly distributed around the periphery of the nucleocapsid with a periodicity that matched the characteristic stacked ring subunits that appear as striations. From this and other evidence, we argue that this giant ORF in fact encodes the major WSSV nucleocapsid protein.

Multistep entry of rotavirus into cells: a Versaillesque dance

Rotavirus entry into a cell is a complex multistep process in which different domains of the rotavirus surface proteins interact with different cell surface molecules, which act as attachment and entry receptors. These recently described molecules include several integrins and a heat shock protein, which have been found to be associated with cell membrane lipid microdomains. The requirement during viral entry for several cell molecules, which might be required to be present and organized in a precise fashion, could explain the selective cell and tissue tropism of these viruses. This review focuses on recent data describing the virus-receptor interactions, the role of lipid microdomains in rotavirus infection and the mechanism of rotavirus cell entry.

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.

VP7 mediates the interaction of rotaviruses with integrin alphavbeta3 through a novel integrin-binding site

Rotavirus entry is a complex multistep process that depends on the trypsin cleavage of the virus spike protein VP4 into polypeptides VP5 and VP8 and on the interaction of these polypeptides and of VP7, the second viral surface protein, with several cell surface molecules, including integrin alphavbeta3. We characterized the effect of the trypsin cleavage of VP4 on the binding to MA104 cells of the sialic acid-dependent virus strain RRV and its sialic acid-independent variant, nar3. We found that, although the trypsin treatment did not affect the attachment of these viruses to the cell surface, their binding was qualitatively different. In contrast to the trypsin-treated viruses, which initially bound to the cell surface through VP4, the non-trypsin-treated variant nar3 bound to the cell through VP7. Amino acid sequence comparison of the surface proteins of rotavirus and hantavirus, both of which interact with integrin alphavbeta3 in an RGD-independent manner, identified a region shared by rotavirus VP7 and hantavirus G1G2 protein in which six of nine amino acids are identical. This region, which is highly conserved among the VP7 proteins of different rotavirus strains, mediates the binding of rotaviruses to integrin alphavbeta3 and probably represents a novel binding motif for this integrin.

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.

Rotavirus RRV associates with lipid membrane microdomains during cell entry

Rotavirus cell entry is a multistep process, not completely understood, which requires at least four interactions between the virus and cell surface molecules. In this work, we investigated the role of the sphingolipid- and cholesterol-enriched lipid microdomains (rafts) in the entry of rotavirus strain RRV to MA104 cells. We found that ganglioside GM1, integrin subunits alpha2 and beta3, and the heat shock cognate protein 70 (hsc70), all of which have been implicated as rotavirus receptors, are associated with TX-100 and Lubrol WX detergent-resistant membranes (DRMs). Integrin subunits alpha2 and beta3 were found to be particularly enriched in DRMs resistant to lysis by Lubrol WX. When purified RRV particles were incubated with cells at 4 degrees C, about 10% of the total infectious virus was found associated with DRMs, and the DRM-associated virus increased to 37% in Lubrol-resistant membrane domains after 60-min incubation at 37 degrees C. The virus was excluded from DRMs if the cells were treated with methyl-beta-cyclodextrin (MbetaCD). Immunoblot analysis of the viral proteins showed that the virus surface proteins became enriched in DRMs upon incubation at 37 degrees C, being almost exclusively localized in Lubrol-resistant DRMs after 60 min. These data suggest that detergent-resistant membrane domains play an important role in the cell entry of rotaviruses, which could provide a platform to facilitate the efficient interaction of the rotavirus receptors with the virus particle.

Emerging themes in rotavirus cell entry, genome organization, transcription and replication

Rotaviruses, causative agents of gastroenteritis in young animals and humans, are large icosahedral viruses with a complex architecture. The double-stranded RNA (dsRNA) genome composed of 11 segments, which codes for 6 structural and 6 non-structural proteins, is enclosed within three concentric capsid layers. In addition to facilitating host-specific interactions, the design of the capsid architecture in rotaviruses as in other dsRNA viruses should also be conducive to the requirement of transcribing the enclosed genome segments repeatedly and simultaneously within the capsid interior. Several non-structural proteins facilitate the subsequent processes of genome replication and packaging. Electron cryomicroscopy studies of intact virions, recombinant virus-like particles, functional complexes, together with recent X-ray crystallographic studies on rotavirus proteins have provided structural insights into the capsid architecture, genome organization, antibody interaction, cell entry, trypsin-enhanced infectivity, endogenous transcription and replication. These studies underscore contrasting features and unifying themes between rotavirus and other dsRNA viruses.

Discrete domains within the rotavirus VP5* direct peripheral membrane association and membrane permeability

Cleavage of the rotavirus spike protein, VP4, is required for rotavirus-induced membrane permeability and viral entry into cells. The VP5* cleavage product selectively permeabilizes membranes and liposomes and contains an internal hydrophobic domain that is required for membrane permeability. Here we investigate VP5* domains (residues 248 to 474) that direct membrane binding. We determined that expressed VP5 fragments containing residues 248 to 474 or 265 to 474, including the internal hydrophobic domain, bind to cellular membranes but are not present in Triton X-100-resistant membrane rafts. Expressed VP5 partitions into aqueous but not detergent phases of Triton X-114, suggesting that VP5 is not integrally inserted into membranes. Since high-salt or alkaline conditions eluted VP5 from membranes, our findings demonstrate that VP5 is peripherally associated with membranes. Interestingly, mutagenesis of residue 394 (W-->R) within the VP5 hydrophobic domain, which abolishes VP5-directed permeability, had no effect on VP5's peripheral membrane association. In contrast, deletion of N-terminal VP5 residues (residues 265 to 279) abolished VP5 binding to membranes. Alanine mutagenesis of two positively charged residues within this domain (residues 274R and 276K) dramatically reduced (>95%) binding of VP5 to membranes and suggested their potential interaction with polar head groups of the lipid bilayer. Mutations in either the VP5 hydrophobic or basic domain blocked VP5-directed permeability of cells. These findings indicate that there are at least two discrete domains within VP5* required for pore formation: an N-terminal basic domain that permits VP5* to peripherally associate with membranes and an internal hydrophobic domain that is essential for altering membrane permeability. These results provide a fundamental understanding of interactions between VP5* and the membrane, which are required for rotavirus 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.

The prospects of modifying the antimicrobial properties of milk

Milk contains a variety of substances, which inhibit the infection of pathogens. This is of benefit to the mother, safeguarding the integrity of the lactating mammary gland, but also of huge importance for protection of the suckling offspring. The antimicrobial substances in milk can be classified into two categories. First, nonspecific, broad-spectrum defense substances, which have evolved over long periods of time, and secondly, substances like antibodies, which are specifically directed against particular pathogens and have developed during the mother's lifetime. Substances in both categories may be targets for biological intervention and manipulation with the goal of improving the antimicrobial properties of milk. These alterations of milk composition have applications in human as well as in animal health.

Roles of outer capsid proteins as determinants of pathogenicity and host range restriction of avian rotaviruses in a suckling mouse model

We previously demonstrated that a pigeon rotavirus, PO-13, but not turkey strains Ty-3 and Ty-1 and a chicken strain, Ch-1, induced diarrhea in heterologous suckling mice. In this study, it was suggested that these avirulent strains, but not PO-13, were inactivated immediately in gastrointestinal tracts of suckling mice when they were orally inoculated. To determine which viral proteins contribute to the differences between the pathogenicitiy and the inactivation of PO-13 and Ty-3 in suckling mice, six PO-13 x Ty-3 reassortant strains that had the genes of the outer capsid proteins, VP4 and VP7, derived from the opposite strain were prepared and were orally inoculated to suckling mice. A single strain that had both PO-13 VP4 and VP7 with the genetic background of Ty-3 had an intermediate virulence for suckling mice. Three strains with Ty-3 VP7, regardless of the origin of VP4, rapidly disappeared from gastrointestinal tracts of suckling mice. These results indicated that the difference between the pathogenicity of PO-13 and that of Ty-3 was mainly dependent on both their VP4 and VP7. In particular, VP7 was found to be related to the inactivation of Ty-3 in gastrointestinal tracts of suckling mice.

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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.

Integrin-using rotaviruses bind alpha2beta1 integrin alpha2 I domain via VP4 DGE sequence and recognize alphaXbeta2 and alphaVbeta3 by using VP7 during cell entry

Integrins alpha2beta1, alphaXbeta2, and alphaVbeta3 have been implicated in rotavirus cell attachment and entry. The virus spike protein VP4 contains the alpha2beta1 ligand sequence DGE at amino acid positions 308 to 310, and the outer capsid protein VP7 contains the alphaXbeta2 ligand sequence GPR. To determine the viral proteins and sequences involved and to define the roles of alpha2beta1, alphaXbeta2, and alphaVbeta3, we analyzed the ability of rotaviruses and their reassortants to use these integrins for cell binding and infection and the effect of peptides DGEA and GPRP on these events. Many laboratory-adapted human, monkey, and bovine viruses used integrins, whereas all porcine viruses were integrin independent. The integrin-using rotavirus strains each interacted with all three integrins. Integrin usage related to VP4 serotype independently of sialic acid usage. Analysis of rotavirus reassortants and assays of virus binding and infectivity in integrin-transfected cells showed that VP4 bound alpha2beta1, and VP7 interacted with alphaXbeta2 and alphaVbeta3 at a postbinding stage. DGEA inhibited rotavirus binding to alpha2beta1 and infectivity, whereas GPRP binding to alphaXbeta2 inhibited infectivity but not binding. The truncated VP5* subunit of VP4, expressed as a glutathione S-transferase fusion protein, bound the expressed alpha2 I domain. Alanine mutagenesis of D308 and G309 in VP5* eliminated VP5* binding to the alpha2 I domain. In a novel process, integrin-using viruses bind the alpha2 I domain of alpha2beta1 via DGE in VP4 and interact with alphaXbeta2 (via GPR) and alphaVbeta3 by using VP7 to facilitate cell entry and infection.

Interaction of rotaviruses with Hsc70 during cell entry is mediated by VP5

Rotavirus infection seems to be a multistep process in which the viruses are required to interact with several cell surface molecules to enter the cell. The virus spike protein VP4, which is cleaved by trypsin into two subunits, VP5 and VP8, is involved in some of these interactions. We have previously shown that the neuraminidase-sensitive rotavirus strain RRV initially attaches to a sialic acid-containing cell molecule through the VP8 subunit of VP4 and subsequently interacts with integrin alpha2beta1 through VP5. After these initial contacts, the virus interacts with at least two additional proteins located at the cell surface, the integrin alphavbeta3 and the heat shock cognate protein Hsc70. In this work, we have shown that rotavirus RRV and its neuraminidase-resistant variant nar3 interact with Hsc70 through a VP5 domain located between amino acids 642 and 658 of the protein. This conclusion is based on the observation that a recombinant protein comprising the 300 carboxy-terminal amino acids of VP5 binds specifically to Hsc70 and a synthetic peptide containing amino acids 642 to 658 competes with the binding of the RRV and nar3 viruses to the heat shock protein. The VP5 peptide also competed with the binding to Hsc70 of the recombinant VP5 protein, and an antibody to Hsc70 reduced the binding of the recombinant protein to the surface of MA104 cells. The fact that the synthetic peptide blocks the infectivity of rotaviruses RRV and nar3 but not their binding to cells indicates that the interaction of VP5 with Hsc70 most probably occurs at a postattachment step during the virus entry process.

Interactions of rotavirus VP4 spike protein with the endosomal protein Rab5 and the prenylated Rab acceptor PRA1

Rotavirus spike protein VP4 is implicated in several important functions, such as cell attachment, penetration, hemagglutination, neutralization, virulence, and host range. It is present at the plasma membrane and colocalizes with the cytoskeleton in infected cells. We looked for cellular partners responsible for the localization of VP4 by two-hybrid screening of a monkey CV1 cell cDNA library. In the screen we isolated repeatedly three cDNAs encoding either two isoforms (a and c) of Rab5 protein or the prenylated Rab acceptor (PRA1). The small GTPase Rab5 is a molecule regulating the vesicular traffic and the motility of early endosomes along microtubules. Rab5 interacts with a large number of effectors, in particular with PRA1. Interactions of VP4 with both partners, Rab5 and PRA1, were confirmed by coimmunoprecipitation from infected- or transfected-cell lysates. Interaction of Rab5 and PRA1 was restricted to free VP4, since neither triple-layered particles nor NSP4-VP4-VP7 heterotrimeric complexes could be coprecipitated. Site-directed and deletion mutants of VP4 were used to map a VP4 domain(s) interacting with Rab5 or PRA1. Of the 10 mutants tested, 2 interacted exclusively with a single partner. In contrast, the domain extending from amino acids 560 to 722 of VP4 is essential for both interactions. These results suggest that Rab5 and PRA1 may be involved in the localization and trafficking of VP4 in infected 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.

Entry of viruses through the epithelial barrier: pathogenic trickery

Mucosal surfaces--such as the lining of the gut or the reproductive tract--are the main point of entry for viruses into the body. As such, almost all viruses interact with epithelial cells, and make use of the normal epithelial signalling and trafficking pathways of the host cell. In addition to protein receptors, carbohydrate chains of proteoglycans and epithelial-membrane glycosphingolipids have emerged as a new class of receptors for viral attachment to the host cell.

Specificity and affinity of sialic acid binding by the rhesus rotavirus VP8* core

Nuclear magnetic resonance spectroscopy demonstrates that the rhesus rotavirus hemagglutinin specifically binds alpha-anomeric N-acetylneuraminic acid with a K(d) of 1.2 mM. The hemagglutinin requires no additional carbohydrate moieties for binding, does not distinguish 3' from 6' sialyllactose, and has approximately tenfold lower affinity for N-glycolylneuraminic than for N-acetylneuraminic acid. The broad specificity and low affinity of sialic acid binding by the rotavirus hemagglutinin are consistent with this interaction mediating initial cell attachment prior to the interactions that determine host range and cell type specificity.

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.

Small bowel infections

Infections of the human gastrointestinal tract with enteric pathogens are among the leading causes of disease, suffering, and death worldwide. Enteric pathogens are ingested from contaminated food and water and pass through the entire gastrointestinal tract during establishment in the host and subsequent shedding and spread to new hosts. Nonetheless, each pathogen exploits a unique niche within the intestinal tract and has developed unique strategies to interact with different host cells and functions. The most important and prevalent infections of the small intestine are caused by diarrheagenic Escherichia coli, particularly enterotoxigenic and enteropathogenic E. coli, rotavirus, Giardia lamblia, and Cryptosporidium parvum. This review focuses on recent insights into the pathogenesis of infections with these pathogens and host defenses against them.

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.

Differential infection of polarized epithelial cell lines by sialic acid-dependent and sialic acid-independent rotavirus strains

Infection of epithelial cells by some animal rotaviruses, but not human or most animal rotaviruses, requires the presence of N-acetylneuraminic (sialic) acid (SA) on the cell surface for efficient infectivity. To further understand how rotaviruses enter susceptible cells, six different polarized epithelial cell lines, grown on permeable filter membrane supports containing 0.4-microm pores, were infected apically or basolaterally with SA-independent or SA-dependent rotaviruses. SA-independent rotaviruses applied apically or basolaterally were capable of efficiently infecting both sides of the epithelium of all six polarized cell lines tested, while SA-dependent rotaviruses only infected efficiently through the apical surface of five of the polarized cell lines tested. Regardless of the route of virus entry, SA-dependent and SA-independent rotaviruses were released almost exclusively from the apical domain of the plasma membrane of polarized cells before monolayer disruption or cell lysis. The transepithelial electrical resistance (TER) of cells decreased at the same time, irrespective of whether infection with SA-independent rotaviruses occurred apically or basolaterally. The TER of cells infected apically with SA-dependent rotaviruses decreased earlier than that of cells infected basolaterally. Rotavirus infection decreased TER before the appearance of cytopathic effect and cell death and resulted in an increase in the paracellular permeability to [(3)H]inulin as a function of loss of TER. The presence of SA residues on either the apical or basolateral side was determined using a Texas Red-conjugated lectin, wheat germ agglutinin (WGA), which binds SA residues. WGA bound exclusively to SA residues on the apical surface of the cells, confirming the requirement for SA residues on the apical cell membrane for efficient infectivity of SA-dependent rotaviruses. These results indicate that the rotavirus SA-independent cellular receptor is present on both sides of the epithelium, but SA-dependent and SA-independent rotavirus strains infect polarized epithelial cells by different mechanisms, which may be relevant for pathogenesis and selection of vaccine strains. Finally, rotavirus-induced alterations of the epithelial barrier and paracellular permeability suggest that common mechanisms of pathogenesis may exist between viral and bacterial pathogens of the intestinal tract.

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.

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.