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

Interaction of species A rotavirus VP4 with the cellular proteins vimentin and actin related protein 2 discovered by a proximity interactome assay

IMPORTANCE

Rotavirus (RV) is an important zoonosis virus, which can cause severe diarrhea and extra-intestinal infection. To date, some proteins or carbohydrates have been shown to participate in the attachment or internalization of RV, including HGBAs, Hsc70, and integrins. This study attempted to indicate whether there were other proteins that would participate in the entry of RV; thus, the RV VP4-interacting proteins were identified by proximity labeling. After analysis and verification, it was found that VIM and ACTR2 could significantly promote the proliferation of RV in intestinal cells. Through further viral binding assays after knockdown, antibody blocking, and recombinant protein overexpression, it was revealed that both VIM and ACTR2 could promote RV replication.

A Point Mutation in the Rhesus Rotavirus VP4 Protein Generated through a Rotavirus Reverse Genetics System Attenuates Biliary Atresia in the Murine Model

Rotavirus infection is one of the most common causes of diarrheal illness in humans. In neonatal mice, rhesus rotavirus (RRV) can induce biliary atresia (BA), a disease resulting in inflammatory obstruction of the extrahepatic biliary tract and intrahepatic bile ducts. We previously showed that the amino acid arginine (R) within the sequence SRL (amino acids 445 to 447) in the RRV VP4 protein is required for viral binding and entry into biliary epithelial cells. To determine if this single amino acid (R) influences the pathogenicity of the virus, we generated a recombinant virus with a single amino acid mutation at this site through a reverse genetics system. We demonstrated that the RRV mutant (RRV^VP4-R446G) produced less symptomatology and replicated to lower titers both in vivo and in vitro than those seen with wild-type RRV, with reduced binding in cholangiocytes. Our results demonstrate that a single amino acid change in the RRV VP4 gene influences cholangiocyte tropism and reduces pathogenicity in mice.IMPORTANCE Rotavirus is the leading cause of diarrhea in humans. Rhesus rotavirus (RRV) can also lead to biliary atresia (a neonatal human disease) in mice. We developed a reverse genetics system to create a mutant of RRV (RRV^VP4-R446G) with a single amino acid change in the VP4 protein compared to that of wild-type RRV. In vitro, the mutant virus had reduced binding and infectivity in cholangiocytes. In vivo, it produced fewer symptoms and lower mortality in neonatal mice, resulting in an attenuated form of biliary atresia.

The SRL peptide of rhesus rotavirus VP4 protein governs cholangiocyte infection and the murine model of biliary atresia

Biliary atresia (BA) is a neonatal obstructive cholangiopathy that progresses to end-stage liver disease, often requiring transplantation. The murine model of BA, employing rhesus rotavirus (RRV), parallels human disease and has been used to elucidate mechanistic aspects of a virus induced biliary cholangiopathy. We previously reported that the RRV VP4 gene plays an integral role in activating the immune system and induction of BA. Using rotavirus binding and blocking assays, this study elucidated how RRV VP4 protein governs cholangiocyte susceptibility to infection both in vitro and in vivo in the murine model of BA. We identified the amino acid sequence on VP4 and its cholangiocyte binding protein, finding that the sequence is specific to those rotavirus strains that cause obstructive cholangiopathy. Pretreatment of murine and human cholangiocytes with this VP4-derived peptide (TRTRVSRLY) significantly reduced the ability of RRV to bind and infect cells. However, the peptide did not block cholangiocyte binding of TUCH and Ro1845, strains that do not induce murine BA. The SRL sequence within TRTRVSRLY is required for cholangiocyte binding and viral replication. The cholangiocyte membrane protein bound by SRL was found to be Hsc70. Inhibition of Hsc70 by small interfering RNAs reduced RRV's ability to infect cholangiocytes. This virus-cholangiocyte interaction is also seen in vivo in the murine model of BA, where inoculation of mice with TRTRVSRLY peptide significantly reduced symptoms and mortality in RRV-injected mice.

CONCLUSION

The tripeptide SRL on RRV VP4 binds to the cholangiocyte membrane protein Hsc70, defining a novel binding site governing VP4 attachment. Investigations are underway to determine the cellular response to this interaction to understand how it contributes to the pathogenesis of BA. (Hepatology 2017;65:1278-1292).

Innate immune responses to rotavirus infection in macrophages depend on MAVS but involve neither the NLRP3 inflammasome nor JNK and p38 signaling pathways

Rotavirus infection is a major cause of life-threatening infantile gastroenteritis. The innate immune system provides an immediate mechanism of suppressing viral replication and is necessary for an effective adaptive immune response. Innate immunity involves host recognition of viral infection and establishment of a powerful antiviral state through the expression of pro-inflammatory cytokines such as type-1 interferon (IFN). Macrophages, the front-line cells of innate immunity, produce IFN and other cytokines in response to viral infection. However, the role of macrophages during rotavirus infection is not well defined. We demonstrate here that RRV rotavirus triggers the production of proinflammatory cytokines from mouse bone marrow-derived macrophages. IFN and antiviral cytokine production was abolished in rotavirus-infected MAVS (-/-) macrophages. This indicates that rotavirus triggers innate immunity in macrophages through RIG-I and/or MDA5 viral recognition, and MAVS signaling is essential for cytokine responses in macrophages. Rotavirus induced IFN expression in both wild type and MDA5 (-/-) macrophages, showing that MDA5 is not essential for IFN secretion following infection, and RIG-I and MDA5 may act redundantly in promoting rotavirus recognition. Interestingly, rotavirus neither stimulated mitogen-activated protein kinases p38 and JNK nor activated the NLRP3 inflammasome, demonstrating that these components might not be involved in innate responses to rotavirus infection in macrophages. Our results indicate that rotavirus elicits intracellular signaling in macrophages, resulting in the induction of IFN and antiviral cytokines, and advance our understanding of the involvement of these cells in innate responses against rotavirus.

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

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

IMPORTANCE

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

Determinants of the specificity of rotavirus interactions with the alpha2beta1 integrin

The human α2β1 integrin binds collagen and acts as a cellular receptor for rotaviruses and human echovirus 1. These ligands require the inserted (I) domain within the α2 subunit of α2β1 for binding. Previous studies have identified the binding sites for collagen and echovirus 1 in the α2 I domain. We used CHO cells expressing mutated α2β1 to identify amino acids involved in binding to human and animal rotaviruses. Residues where mutation affected rotavirus binding were located in several exposed loops and adjacent regions of the α2 I domain. Binding by all rotaviruses was eliminated by mutations in the activation-responsive αC-α6 and αF helices. This is a novel feature that distinguishes rotavirus from other α2β1 ligands. Mutation of residues that co-ordinate the metal ion (Ser-153, Thr-221, and Glu-256 in α2 and Asp-130 in β1) and nearby amino acids (Ser-154, Gln-215, and Asp-219) also inhibited rotavirus binding. The importance of most of these residues was greatest for binding by human rotaviruses. These mutations inhibit collagen binding to α2β1 (apart from Glu-256) but do not affect echovirus binding. Overall, residues where mutation affected both rotavirus and collagen recognition are located at one side of the metal ion-dependent adhesion site, whereas those important for collagen alone cluster nearby. Mutations eliminating rotavirus and echovirus binding are distinct, consistent with the respective preference of these viruses for activated or inactive α2β1. In contrast, rotavirus and collagen utilize activated α2β1 and show an overlap in α2β1 residues important for binding.

Cholangiocyte expression of alpha2beta1-integrin confers susceptibility to rotavirus-induced experimental biliary atresia

Inoculation of BALB/c mice with rhesus rotavirus (RRV) in the newborn period results in biliary epithelial cell (cholangiocyte) infection and the murine model of biliary atresia. Rotavirus infection of a cell requires attachment, which is governed in part by cell-surface expression of integrins such as alpha2beta1. We hypothesized that cholangiocytes were susceptible to RRV infection because they express alpha2beta1. RRV attachment and replication was measured in cell lines derived from cholangiocytes and hepatocytes. Flow cytometry was performed on these cell lines to determine whether alpha2beta1 was present. Cholangiocytes were blocked with natural ligands, a monoclonal antibody, or small interfering RNA against the alpha2-subunit and were infected with RRV. The extrahepatic biliary tract of newborn mice was screened for the expression of the alpha2beta1-integrin. Newborn mice were pretreated with a monoclonal antibody against the alpha2-subunit and were inoculated with RRV. RRV attached and replicated significantly better in cholangiocytes than in hepatocytes. Cholangiocytes, but not hepatocytes, expressed alpha2beta1 in vitro and in vivo. Blocking assays led to a significant reduction in attachment and yield of virus in RRV-infected cholangiocytes. Pretreatment of newborn pups with an anti-alpha2 monoclonal antibody reduced the ability of RRV to cause biliary atresia in mice. Cell-surface expression of the alpha2beta1-integrin plays a role in the mechanism that confers cholangiocyte susceptibility to RRV infection.

Integrins alpha1beta1 and alpha2beta1 are receptors for the rotavirus enterotoxin

Rotavirus NSP4 is a viral enterotoxin capable of causing diarrhea in neonatal mice. This process is initiated by the binding of extracellular NSP4 to target molecule(s) on the cell surface that triggers a signaling cascade leading to diarrhea. We now report that the integrins alpha1beta1 and alpha2beta1 are receptors for NSP4. NSP4 specifically binds to the alpha1 and alpha2 I domains with apparent K(d) = 1-2.7 muM. Binding is mediated by the I domain metal ion-dependent adhesion site motif, requires Mg(2+) or Mn(2+), is abolished with EDTA, and an NSP4 point mutant, E(120)A, fails to bind alpha2 integrin I domain. NSP4 has two distinct integrin interaction domains. NSP4 amino acids 114-130 are essential for binding to the I domain, and NSP4 peptide 114-135 blocks binding of the natural ligand, collagen I, to integrin alpha2. NSP4 amino acids 131-140 are not associated with the initial binding to the I domain, but elicit signaling that leads to the spreading of attached C2C12-alpha2 cells, mouse myoblast cells stably expressing the human alpha2 integrin. NSP4 colocalizes with integrin alpha2 on the basolateral surface of rotavirus-infected polarized intestinal epithelial (Caco-2) cells as well as surrounding noninfected cells. NSP4 mutants that fail to bind or signal through integrin alpha2 were attenuated in diarrhea induction in neonatal mice. These results indicate that NSP4 interaction with integrin alpha1 and alpha2 is an important component of enterotoxin function and rotavirus pathogenesis, further distinguishing this viral virulence factor from other microbial enterotoxins.

Rotavirus replication in intestinal cells differentially regulates integrin expression by a phosphatidylinositol 3-kinase-dependent pathway, resulting in increased cell adhesion and virus yield

Changes in the interactions between intestinal cells and their surrounding environment during virus infection have not been well documented. The growth and survival of intestinal epithelial cells, the main targets of rotavirus infection, are largely dependent on the interaction of cell surface integrins with the extracellular matrix. In this study, we detected alterations in cellular integrin expression following rotavirus infection, identified the signaling components required, and analyzed the subsequent effects on cell binding to the matrix component collagen. After rotavirus infection of intestinal cells, expression of alpha2beta1 and beta2 integrins was up-regulated, whereas that of alphaVbeta3, alphaVbeta5, and alpha5beta1 integrins, if present, was down-regulated. This differential regulation of integrins was reflected at the transcriptional level. It was unrelated to the use of integrins as rotavirus receptors, as both integrin-using and integrin-independent viruses induced integrin regulation. Using pharmacological agents that inhibit kinase activity, integrin regulation was shown to be dependent on phosphatidylinositol 3-kinase (PI3K) but independent of the activities of the mitogen-activated protein kinases p38 and ERK1/2, and cyclooxygenase-2. Replication-dependent activation of the PI3K/Akt pathway was observed following infection of intestinal and nonintestinal cell lines. Rotavirus activation of PI3K was important for regulation of alpha2beta1 expression. Blockade of integrin regulation by PI3K inhibition led to decreased adherence of infected intestinal cells to collagen and a concomitant decrease in virus titer. These findings indicate that rotavirus-induced PI3K activation causes regulation of integrin expression in intestinal cells, leading to prolonged adherence of infected cells to collagen and increased virus production.

Induction of nitric oxide synthase by rotavirus enterotoxin NSP4: implication for rotavirus pathogenicity

Rotavirus non-structural protein (NSP) 4 can induce aqueous secretion in the gastrointestinal tract of neonatal mice through activation of an age- and Ca(2+)-dependent plasma membrane anion permeability. Accumulating evidence suggests that nitric oxide (NO) plays a role in the modulation of aqueous secretion and the barrier function of intestinal cells. This study investigated transcriptional changes in inducible NO synthase (iNOS), an enzyme responsible for NO production, after rotavirus infection in mice and after treatment of intestinal cells with NSP4. Diarrhoea was observed in 5-day-old CD-1 mice from days 1 to 3 after inoculation with 10(7) focus-forming units of different rotavirus strains. Ileal iNOS mRNA expression was induced as early as 6 h post-inoculation, before the onset of clinical diarrhoea in infected mice, and was upregulated during the course of rotavirus-induced diarrhoea. Ex vivo treatment of ilea excised from CD-1 suckling mice with NSP4 resulted in upregulation of ileal iNOS mRNA expression within 4 h. Furthermore, NSP4 was able to induce iNOS expression and NO production in murine peritoneal macrophages and RAW264.7 cells. The specificity of NSP4 inducibility was confirmed by the inhibitory effect of anti-NSP4 serum. Using a series of truncated NSP4s, the domain responsible for iNOS induction in macrophages was mapped to the reported enterotoxin domain, aa 109-135. Thus, rotavirus infection induces ileal iNOS expression in vivo and rotavirus NSP4 also induces iNOS expression in the ileum and macrophages. Together, these findings suggest that NO plays a role in rotavirus-induced diarrhoea.

Rotavirus infection of infant and young adult nonobese diabetic mice involves extraintestinal spread and delays diabetes onset

Rotaviruses have been implicated as a possible viral trigger for exacerbations in islet autoimmunity, suggesting they might modulate type 1 diabetes development. In this study, the ability of rotavirus strain RRV to infect the pancreas and affect insulitis and diabetes was examined in nonobese diabetic (NOD) mice, an experimental model of type 1 diabetes. Mice were inoculated either orally or intraperitoneally as infants or young adults. In infant mice inoculated orally, rotavirus antigen was detected in pancreatic macrophages outside islets and infectious virus was found in blood cells, pancreas, spleen, and liver. Extraintestinal RRV spread and pancreatic presence of infectious virus also occurred in intraperitoneally inoculated infant and adult mice. The initiation of insulitis was unaltered by infection. The onset of diabetes was delayed in infant mice inoculated orally and infant and adult mice inoculated intraperitoneally. In contrast, adult mice inoculated orally showed no evidence of pancreatic RRV, the lowest rate of detectable RRV replication, and no diabetes modulation. Thus, the ability of RRV infection to modulate diabetes development in infant and young adult NOD mice was related to the overall extent of detectable virus replication and the presence of infectious virus extraintestinally, including in the pancreas. These studies show that RRV infection of infant and young adult NOD mice provides significant protection against diabetes. As these findings do not support the hypothesis that rotavirus triggers autoimmunity related to type 1 diabetes, further research is needed to resolve this issue.

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.

Rotavirus activates JNK and p38 signaling pathways in intestinal cells, leading to AP-1-driven transcriptional responses and enhanced virus replication

Rotavirus infection is known to regulate transcriptional changes in many cellular genes. The transcription factors NF-kappaB and AP-1 are activated by rotavirus infection, but the upstream processes leading to these events are largely unidentified. We therefore studied the activation state during rotavirus infection of c-Jun NH2-terminal kinase (JNK) and p38, which are kinases known to activate AP-1. As assessed by Western blotting using phospho-specific antibodies, infection with rhesus rotavirus (RRV) or exposure to UV-psoralen-inactivated RRV (I-RRV) resulted in the activation of JNK in HT-29, Caco-2, and MA104 cells. Activation of p38 during RRV infection was observed in Caco-2 and MA104 cells but not in HT-29 cells, whereas exposure to I-RRV did not lead to p38 activation in these cell lines. Rotavirus strains SA11, CRW-8, Wa, and UK also activated JNK and p38. Consistent with the activation of JNK, a corresponding increase in the phosphorylation of the AP-1 component c-Jun was shown. The interleukin-8 (IL-8) and c-jun promoters contain AP-1 binding sequences, and these genes have been shown previously to be transcriptionally up-regulated during rotavirus infection. Using specific inhibitors of JNK (SP600125) and p38 (SB203580) and real-time PCR, we showed that maximal RRV-induced IL-8 and c-jun transcription required JNK and p38 activity. This highlights the importance of JNK and p38 in RRV-induced, AP-1-driven gene expression. Significantly, inhibition of p38 or JNK in Caco-2 cells reduced RRV growth but not viral structural antigen expression, demonstrating the potential importance of JNK and p38 activation for optimal rotavirus replication.

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.

Attachment factors

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

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.

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.

Identity of the amino acid residues involved in C3bi binding to the I-domain supports a mosaic model to explain the broad ligand repertoire of integrin alpha M beta 2

Interactions between the complement degradation product C3bi and leukocyte integrin alpha(M)beta(2) are critical for host defense against foreign pathogens and in tumor cell surveillance. To gain insight into the mechanism by which the alpha(M)I-domain of the integrin interacts with C3bi, detailed mapping of the C3bi binding site was undertaken. Previous mutagenesis studies had implicated five small structural segments within the alpha(M)I-domain in recognition of this ligand. Sets of three amino acids within the five implicated segments were mutated to the corresponding alpha(L)I-domain residues. Then, within the affected mutants, single point mutations were introduced to precisely define the requisite residues. Ultimately, H148, F150, Q204, L205, R208, T211, T213, I256, P257 were identified as being critical for C3bi binding. A synthetic peptide approach confirmed the involvement of the specified residues with the complex midsegment, Q204-I215, in C3bi recognition. Furthermore, the alpha(D)I-domain, which has a low intrinsic affinity for C3bi, acquired high affinity for the ligand when the implicated residues were inserted. The residues necessary to engage C3bi reside on or adjacent to the cation binding MIDAS site of the alpha(M)I-domain. The amino acids involved in C3bi binding are distinct from those involved in interaction of previously mapped ligands with the alpha(M)I-domain. This divergence supports a mosaic model, in which different ligands engage different amino acids to bind to alpha(M)I-domain, accounting for the broad recognition capacity of integrin alpha(M)beta(2).

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.

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.