Entry of rotaviruses is a multistep process

Bacterial-derived sialidases inhibit porcine rotavirus OSU replication by interfering with the early steps of infection

Rotavirus infections in suckling and weaning piglets cause severe dehydration and death, resulting in significant economic losses in the pig breeding industry. With the continuous emergence of porcine rotavirus (PoRV) variants and poor vaccine cross-protection among various genotypes, there is an urgent need to develop alternative strategies such as seeking effective antiviral products from nature, microbial metabolites and virus-host protein interaction. Sialidases play a crucial role in various physiopathological processes and offer a promising target for developing antivirus drugs. However, the effect of bacterial-derived sialidases on the infection of PoRVs remains largely unknown. Herein, we investigated the impact of bacterial-derived sialidases (sialidase Cp and Vc) on PoRV strain OSU(Group A) infection, using differentiated epithelial monkey kidney cells (MA104) as a model. Our results indicated that the pretreatment of MA104 with exogenous sialidases effectively suppressed PoRV OSU in a concentration-dependent manner. Notably, even at a concentration of 0.01 μU/mL, sialidases significantly inhibited the virus (MOI = 0.01). Meanwhile, we found that sialidase Vc pretreatment sharply reduced the binding rate of PoRV OSU. Last, we demonstrated that PoRV OSU might recognize α-2,3-linked sialic acid as the primary attachment factor in MA104. Our findings provide new insights into the underlying mechanism of PoRV OSU infections, shedding lights on the development of alternative antivirus approaches based on bacteria-virus interaction.

Rotavirus Interactions With Host Intestinal Epithelial Cells

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

Heat shock protein 70, glutamate dehydrogenase, and angiotensin-converting enzyme of Bombyx mori mediate the cell attachment of Cypovirus 1

Dendrolimus punctatus causes great damage to pine forests worldwide. Dendrolimus punctatus cypovirus 1 (DpCPV-1) is an important pathogen of D. punctatus. However, the mechanism of DpCPV-1 cell entry has not been elucidated. In this study, we revealed that both GTase and MTase domains of VP3 (B-spike) and VP4 (A-spike) of DpCPV-1 interacted with the midgut proteins of Bombyx mori. Binding and competition assays revealed that GTase, MTase and VP4 played roles as viral attachment proteins. Far-Western blotting and LC-MS/MS analyses identified that heat shock protein 70 (BmHSP70), glutamate dehydrogenase (BmGDH), and angiotensin-converting enzyme (BmACE) in the midgut proteins as ligand candidates of the viral attachment proteins, and this was further verified by co-immunoprecipitation and fluorescence co-localization assays. Viral binding to the host midgut in vitro was inhibited by pre-treating B. mori midgut proteins with anti-BmHSP70, anti-BmGDH, anti-BmACE antibodies singly and in combination. Incubating DpCPV-1 virions with prokaryotically expressed BmHSP70, BmGDH, and BmACE also decreased viral attachment to the host midgut. In vivo bioassays revealed that viral infection in Helicoverpa armigera was partially neutralized by BmHSP70, BmGDH, and BmACE. Taking together, we concluded that HSP70, GDH, and ACE mediate DpCPV attachment and entry via binding to the viral attachment proteins, VP3 and VP4. The findings provide foundation for further understanding the entry mechanisms of cypoviruses.

Mechanisms of Cell Entry by dsRNA Viruses: Insights for Efficient Delivery of dsRNA and Tools for Improved RNAi-Based Pest Control

While RNAi is often heralded as a promising new strategy for insect pest control, a major obstacle that still remains is the efficient delivery of dsRNA molecules within the cells of the targeted insects. However, it seems overlooked that dsRNA viruses already have developed efficient strategies for transport of dsRNA molecules across tissue barriers and cellular membranes. Besides protecting their dsRNA genomes in a protective shell, dsRNA viruses also display outer capsid layers that incorporate sophisticated mechanisms to disrupt the plasma membrane layer and to translocate core particles (with linear dsRNA genome fragments) within the cytoplasm. Because of the perceived efficiency of the translocation mechanism, it is well worth analyzing in detail the molecular processes that are used to achieve this feat. In this review, the mechanism of cell entry by dsRNA viruses belonging to the Reoviridae family is discussed in detail. Because of the large amount of progress in mammalian versus insect models, the mechanism of infections of reoviruses in mammals (orthoreoviruses, rotaviruses, orbiviruses) will be treated as a point of reference against which infections of reoviruses in insects (orbiviruses in midges, plant viruses in hemipterans, insect-specific cypoviruses in lepidopterans) will be compared. The goal of this discussion is to uncover the basic principles by which dsRNA viruses cross tissue barriers and translocate their cargo to the cellular cytoplasm; such knowledge subsequently can be incorporated into the design of dsRNA virus-based viral-like particles for optimal delivery of RNAi triggers in targeted insect pests.

Rotavirus Replication: Gaps of Knowledge on Virus Entry and Morphogenesis

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

An Effective Platform for Exploring Rotavirus Receptors by Bacterial Surface Display System

Rotavirus (RV) is a major foodborne pathogen. For RV prevention and control, it is a key to uncover the interaction mechanism between virus and its receptors. However, it is hard to specially purify the viral receptors, including histo-blood group antigens (HBGAs). Previously, the protruding domain protein (P protein) of human norovirus (genotype II.4) was displayed on the surface of Escherichia coli, and it specifically recognized and captured the viral ligands. In order to further verify the feasibility of the system, P protein was replaced by VP8* of RV (G9P[8]) in this study. In the system, VP8* could be correctly released by thrombin treatment with antigenicity retaining, which was confirmed by Western blot and Enzyme-Linked Immunosorbent Assays. Type A HBGAs from porcine gastric mucin (PGM) were recognized and captured by this system. From saliva mixture, the captured viral receptor bound with displayed VP8* was confirmed positive with monoclonal antibody against type A HBGAs. It indicated that the target ligands could be easily separated from the complex matrix. These results demonstrate that the bacterial surface display system will be an effective platform to explore viral receptors/ligands from cell lines or food matrix.

Synthesis of asymmetrical multiantennary human milk oligosaccharides

Despite mammalian glycans typically having highly complex asymmetrical multiantennary architectures, chemical and chemoenzymatic synthesis has almost exclusively focused on the preparation of simpler symmetrical structures. This deficiency hampers investigations into the biology of glycan-binding proteins, which in turn complicates the biomedical use of this class of biomolecules. Herein, we describe an enzymatic strategy, using a limited number of human glycosyltransferases, to access a collection of 60 asymmetric, multiantennary human milk oligosaccharides (HMOs), which were used to develop a glycan microarray. Probing the array with several glycan-binding proteins uncovered that not only terminal glycoepitopes but also complex architectures of glycans can influence binding selectivity in unanticipated manners. N- and O-linked glycans express structural elements of HMOs, and thus, the reported synthetic principles will find broad applicability.

Rotavirus replication is correlated with S/G2 interphase arrest of the host cell cycle

In infected cells rotavirus (RV) replicates in viroplasms, cytosolic structures that require a stabilized microtubule (MT) network for their assembly, maintenance of the structure and perinuclear localization. Therefore, we hypothesized that RV could interfere with the MT-breakdown that takes place in mitosis during cell division. Using synchronized RV-permissive cells, we show that RV infection arrests the cell cycle in S/G2 phase, thus favoring replication by improving viroplasms formation, viral protein translation, and viral assembly. The arrest in S/G2 phase is independent of the host or viral strain and relies on active RV replication. RV infection causes cyclin B1 down-regulation, consistent with blocking entry into mitosis. With the aid of chemical inhibitors, the cytoskeleton network was linked to specific signaling pathways of the RV-induced cell cycle arrest. We found that upon RV infection Eg5 kinesin was delocalized from the pericentriolar region to the viroplasms. We used a MA104-Fucci system to identify three RV proteins (NSP3, NSP5, and VP2) involved in cell cycle arrest in the S-phase. Our data indicate that there is a strong correlation between the cell cycle arrest and RV replication.

Drebrin restricts rotavirus entry by inhibiting dynamin-mediated endocytosis

Despite the wide administration of several effective vaccines, rotavirus (RV) remains the single most important etiological agent of severe diarrhea in infants and young children worldwide, with an annual mortality of over 200,000 people. RV attachment and internalization into target cells is mediated by its outer capsid protein VP4. To better understand the molecular details of RV entry, we performed tandem affinity purification coupled with high-resolution mass spectrometry to map the host proteins that interact with VP4. We identified an actin-binding protein, drebrin (DBN1), that coprecipitates and colocalizes with VP4 during RV infection. Importantly, blocking DBN1 function by siRNA silencing, CRISPR knockout (KO), or chemical inhibition significantly increased host cell susceptibility to RV infection. Dbn1 KO mice exhibited higher incidence of diarrhea and more viral antigen shedding in their stool samples compared with the wild-type littermates. In addition, we found that uptake of other dynamin-dependent cargos, including transferrin, cholera toxin, and multiple viruses, was also enhanced in DBN1-deficient cells. Inhibition of cortactin or dynamin-2 abrogated the increased virus entry observed in DBN1-deficient cells, suggesting that DBN1 suppresses dynamin-mediated endocytosis via interaction with cortactin. Our study unveiled an unexpected role of DBN1 in restricting the entry of RV and other viruses into host cells and more broadly to function as a crucial negative regulator of diverse dynamin-dependent endocytic pathways.

Polarized rotavirus entry and release from differentiated small intestinal cells

Rotaviruses infect mature enterocytes from small intestine, however most data about their cellular entry are from studies carried out in non-intestinal polarized or non-polarized cell lines. In this work the entry of porcine rotavirus YM strain into small intestinal cell line IPEC-J2 was studied. It was found that YM and the human rotavirus Wa strain infect preferentially from the basolateral cell surface. Cell infection from the apical and basolateral surfaces was dependent on the presence of cholesterol. The treatment with neuraminidase, sucrose, and bafilomycin suggests that there are differences in the receptor usage and entry mechanism of the virus from the apical and basolateral surface. While cell entry is more efficient from basolateral surface, the viruses egressed mainly from the apical cell side.

MYH9 is an Essential Factor for Porcine Reproductive and Respiratory Syndrome Virus Infection

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is an important swine disease worldwide. PRRSV has a limited tropism for certain cells, which may at least in part be attributed to the expression of the necessary cellular molecules serving as the virus receptors or factors on host cells for virus binding or entry. However, these molecules conferring PRRSV infection have not been fully characterized. Here we show the identification of non-muscle myosin heavy chain 9 (MYH9) as an essential factor for PRRSV infection using the anti-idiotypic antibody specific to the PRRSV glycoprotein GP5. MYH9 physically interacts with the PRRSV GP5 protein via its C-terminal domain and confers susceptibility of cells to PRRSV infection. These findings indicate that MYH9 is an essential factor for PRRSV infection and provide new insights into PRRSV-host interactions and viral entry, potentially facilitating development of control strategies for this important swine disease.

Solar and temperature treatments affect the ability of human rotavirus wa to bind to host cells and synthesize viral RNA

Rotavirus, the leading cause of diarrheal diseases in children under the age of five, is often resistant to conventional wastewater treatment and thus can remain infectious once released into the aquatic environment. Solar and heat treatments can inactivate rotavirus, but it is unknown how these treatments inactivate the virus on a molecular level. To answer this question, our approach was to correlate rotavirus inactivation with the inhibition of portions of the virus life cycle as a means to identify the mechanisms of solar or heat inactivation. Specifically, the integrity of the rotavirus NSP3 gene, virus-host cell interaction, and viral RNA synthesis were examined after heat (57°C) or solar treatment of rotavirus. Only the inhibition of viral RNA synthesis positively correlated with a loss of rotavirus infectivity; 57°C treatment of rotavirus resulted in a decrease of rotavirus RNA synthesis at the same rate as rotavirus infectivity. These data suggest that heat treatment neutralized rotaviruses primarily by targeting viral transcription functions. In contrast, when using solar disinfection, the decrease in RNA synthesis was responsible for approximately one-half of the decrease in infectivity, suggesting that other mechanisms, including posttranslational, contribute to inactivation. Nevertheless, both solar and heat inactivation of rotaviruses disrupted viral RNA synthesis as a mechanism for inactivation.

Targeting of rotavirus VP6 to DEC-205 induces protection against the infection in mice

Rotavirus (RV) is the primary etiologic agent of severe gastroenteritis in human infants. Although two attenuated RV-based vaccines have been licensed to be applied worldwide, they are not so effective in low-income countries, and the induced protection mechanisms have not been clearly established. Thus, it is important to develop new generation vaccines that induce long lasting heterotypic immunity. VP6 constitutes the middle layer protein of the RV virion. It is the most conserved protein and it is the target of protective T-cells; therefore, it is a potential candidate antigen for a new generation vaccine against the RV infection. We determined whether targeting the DEC-205 present in dendritic cells (DCs) with RV VP6 could induce protection at the intestinal level. VP6 was cross-linked to a monoclonal antibody (mAb) against murine DEC-205 (αDEC-205:VP6), and BALB/c mice were inoculated subcutaneously (s.c.) twice with the conjugated containing 1.5 μg of VP6 in the presence of polyinosinic-polycytidylic acid (Poly I:C) as adjuvant. As controls and following the same protocol, mice were immunized with ovalbumin (OVA) cross-linked to the mAb anti-DEC-205 (αDEC-205:OVA), VP6 cross-linked to a control isotype mAb (Isotype:VP6), 3 μg of VP6 alone, Poly I:C or PBS. Two weeks after the last inoculation, mice were orally challenged with a murine RV. Mice immunized with α-DEC-205:VP6 and VP6 alone presented similar levels of serum Abs to VP6 previous to the virus challenge. However, after the virus challenge, only α-DEC-205:VP6 induced up to a 45% IgA-independent protection. Memory T-helper (Th) cells from the spleen and the mesenteric lymph node (MLN) showed a Th1-type response upon antigen stimulation in vitro. These results show that when VP6 is administered parenterally targeting DEC-205, it can induce protection at the intestinal level at a very low dose, and this protection may be Th1-type cell dependent.

Structural correlates of rotavirus cell entry

Cell entry by non-enveloped viruses requires translocation into the cytosol of a macromolecular complex--for double-strand RNA viruses, a complete subviral particle. We have used live-cell fluorescence imaging to follow rotavirus entry and penetration into the cytosol of its ∼ 700 Å inner capsid particle ("double-layered particle", DLP). We label with distinct fluorescent tags the DLP and each of the two outer-layer proteins and track the fates of each species as the particles bind and enter BSC-1 cells. Virions attach to their glycolipid receptors in the host cell membrane and rapidly become inaccessible to externally added agents; most particles that release their DLP into the cytosol have done so by ∼ 10 minutes, as detected by rapid diffusional motion of the DLP away from residual outer-layer proteins. Electron microscopy shows images of particles at various stages of engulfment into tightly fitting membrane invaginations, consistent with the interpretation that rotavirus particles drive their own uptake. Electron cryotomography of membrane-bound virions also shows closely wrapped membrane. Combined with high resolution structural information about the viral components, these observations suggest a molecular model for membrane disruption and DLP penetration.

Relative roles of GM1 ganglioside, N-acylneuraminic acids, and α2β1 integrin in mediating rotavirus infection. J Virol. 2014;88:4558-4571. [PMID: 24501414 DOI: 10.1128/jvi.03431-13]

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.

Waterborne Viral Gastroenteritis: An Introduction to Common Agents

Acute gastroenteritis is among the most common illnesses of human beings, and its associated morbidity and mortality are greatest among those at the extremes of age; children and elderly. During the 1970s, several viruses were associated with this syndrome, which are now known to be caused mainly by viruses belonging to four distinct families—rotaviruses, caliciviruses, astroviruses, and adenoviruses. Other viruses, such as the toroviruses, picobirnaviruses, coronavirus, and enterovirus 22, may play a role as well. Transmission by food or water has been documented for astroviruses, caliciviruses, rotaviruses, and norovirus. In developing countries, gastroenteritis is a common cause of death in children <5 years, while deaths from diarrhea are less common, much illness leads to hospitalization or doctor visits. Laboratory confirmation of waterborne illness is based on demonstration of virus particles or antigen in stool, detection of viral nucleic acid in stool, or demonstration of a rise in specific antibody to the virus. Newer methods for syndrome surveillance of acute viral gastroenteritis are being developed like multiplex real-time reverse transcriptase PCRs. Application of these more sensitive methods to detect and characterize individual agents is just beginning, but has already opened up new avenues to reassess their disease burden, examine their molecular epidemiology, and consider new directions for their prevention and control through vaccination, improvements in water quality, and sanitary practices.

Characterization of human astrovirus cell entry

Human astroviruses (HAstV) are a frequent cause of gastroenteritis in young children and immunocompromised patients. To understand the early steps of HAstV infection in the highly permissive Caco-2 cell line, the binding and entry processes of the virus were characterized. The half-time of virus binding to the cell surface was about 10 min, while virus decapsidation took around 130 min. Drugs affecting clathrin-mediated endocytosis, endosome acidification, and actin filament polymerization, as well as those that reduce the presence of cholesterol in the cell membrane, decreased the infectivity of the virus. The infection was also reduced by silencing the expression of the clathrin heavy chain (CHC) by RNA interference or by overexpression of dominant-negative mutants of dynamin 2 and Eps15. Furthermore, the entry of HAstV apparently depends on the maturation of endosomes, since the infection was reduced by silencing the expression of Rab7, a small GTPase involved in the early- to late-endosome maturation. Altogether, our results suggest that HAstV enters Caco-2 cells using a clathrin-dependent pathway and reaches late endosomes to enter cells. Here, we have characterized the mechanism used by human astroviruses, important agents of gastroenteritis in children, to gain entry into their host cells. Using a combination of biochemical and genetic tools, we found that these viruses enter Caco-2 cells using a clathrin-dependent endocytic pathway, where they most likely need to travel to late endosomes to reach the cytoplasm and begin their replication cycle.

Implication of Hsc70, PDI and integrin αvβ3 involvement during entry of the murine rotavirus ECwt into small-intestinal villi of suckling mice

In the present study, a homologous rotavirus, ECwt, infecting small intestinal villi isolated from ICR and BALB/c mice were used as a model for identifying cell-surface molecules involved in rotavirus entry. Small-intestinal villi were treated with anti-Hsc70, anti-PDI, anti-integrin β3 or anti-ERp57 antibodies or their corresponding F(ab')2 fragments before inoculation with rotavirus ECwt, RRV or Wa. Pretreatment of villi decreased virus infectivity by about 50-100 % depending of the rotavirus strain, antibody structure and detection assay used. Similar results were obtained by treating viral inocula with purified proteins Hsc70, PDI or integrin β3 before inoculation of untreated villi. Rotavirus infection of villi proved to be sensitive to membrane-impermeant thiol/disulfide inhibitors such as DTNB and bacitracin, suggesting the involvement of a redox reaction in infection. The present results suggest that PDI, Hsc70 and integrin β3 are used by both homologous and heterologous rotaviruses during infection of isolated mouse villi.

18β-glycyrrhetinic acid inhibits rotavirus replication in culture

BACKGROUND

Glycyrrhizin (GA) and primary metabolite 18β-glycyrrhetinic acid (GRA) are pharmacologically active components of the medicinal licorice root, and both have been shown to have antiviral and immunomodulatory properties. Although these properties are well established, the mechanisms of action are not completely understood. In this study, GA and GRA were tested for the ability to inhibit rotavirus replication in cell culture, toward a long term goal of discovering natural compounds that may complement existing vaccines.

METHODS

Epithelial cells were treated with GA or GRA various times pre- or post-infection and virus yields were measured by immunofluorescent focus assay. Levels of viral proteins VP2, VP6, and NSP2 in GRA treated cells were measured by immunoblot to determine if there was an effect of GRA treatment on the accumulation of viral protein.

RESULTS

GRA treatment reduced rotavirus yields by 99% when added to infected cultures post-- virus adsorption, whereas virus yields in GA treated cultures were similar to mock treated controls. Time of addition experiments indicated that GRA-mediated replication inhibition likely occurs at a step or steps subsequent to virus entry. The amounts of VP2, VP6 and NSP2 were substantially reduced when GRA was added to cultures up to two hours post-entry.

CONCLUSIONS

GRA, but not GA, has significant antiviral activity against rotavirus replication in vitro, and studies to determine whether GRA attenuates rotavirus replication in vivo are underway.

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

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

Different rotavirus strains enter MA104 cells through different endocytic pathways: the role of clathrin-mediated endocytosis. J Virol. 2010;84:9161-9169. [PMID: 20631149 DOI: 10.1128/jvi.00731-10]

Rotaviruses, the single most important agents of acute severe gastroenteritis in children, are nonenveloped viruses formed by a three-layered capsid that encloses a genome formed by 11 segments of double-stranded RNA. The mechanism of entry of these viruses into the host cell is not well understood. The best-studied strain, RRV, which is sensitive to neuraminidase (NA) treatment of the cells, uses integrins alpha2 beta1 and alphav beta3 and the heat shock protein hsc70 as receptors and enters MA104 cells through a non-clathrin-, non-caveolin-mediated pathway that depends on a functional dynamin and on the presence of cholesterol on the cell surface. In this work, using a combination of pharmacological, biochemical, and genetic approaches, we compared the entry characteristics of four rotavirus strains known to have different receptor requirements. We chose four rotavirus strains that represent all phenotypic combinations of NA resistance or sensitivity and integrin dependence or independence. We found that even though all the strains share their requirements for hsc70, dynamin, and cholesterol, three of them differ from the simian strain RRV in the endocytic pathway used. The human strain Wa, porcine strain TFR-41, and bovine strain UK seem to enter the cell through clathrin-mediated endocytosis, since treatments that inhibit this pathway block their infectivity; consistent with this entry route, these strains were sensitive to changes in the endosomal pH. The inhibition of other endocytic mechanisms, such as macropinocytosis or caveola-mediated uptake, had no effect on the internalization of the rotavirus strains tested here.

Rotavirus cell entry

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

Dissecting the role of integrin subunits alpha 2 and beta 3 in rotavirus cell entry by RNA silencing

Several cell surface molecules have been implicated in rotavirus cell entry, however, their individual relevance during this process is unknown. In this work, the expression of integrins alpha2, beta2, and alpha v beta 3, the heat shock cognate protein 70, and of ganglioside GM1 in different cell lines of human and simian origin was correlated with the infectivity of four rotavirus strains. We observed that different combinations of receptor expression correlated with the infectivity of rotavirus strains, suggesting that the participation of several receptors is important for rotavirus infection. To characterize the relevance of integrins alpha2 and alpha v beta 3 in more detail, their expression was silenced using RNA interference. About 80% decrease in the cell content of integrins resulted in 15-30% decrease of infectivity of strains RRV and Wa when measured by a focus-forming assay, while there was no decrease of infectivity when measured by flow cytometry in integrin-deficient cells. Altogether these data suggest that integrins alpha2 and alpha v beta 3 do not play a major role in the rotavirus entry process.

Sialic acid dependence in rotavirus host cell invasion. Nat Chem Biol. 2009;5:91-93. [PMID: 19109595 DOI: 10.1038/nchembio.134]

We used NMR spectroscopy, molecular modeling and infectivity competition assays to investigate the key interactions between the spike protein (VP8(*)) from 'sialidase-insensitive' human Wa and 'sialidase-sensitive' porcine CRW-8 rotaviruses and the glycans of gangliosides G(M1) and G(D1a). Our data provide strong evidence that N-acetylneuraminic acid is a key determinant for binding of these rotaviruses. This is in contrast to the widely accepted paradigm that sialic acids are irrelevant in host cell recognition by sialidase-insensitive rotaviruses.

Amino acid domains 280–297 of VP6 and 531–554 of VP4 are implicated in heat shock cognate protein hsc70-mediated rotavirus infection

The rotavirus infection mechanism seems to be a multi-step process which is still not fully understood. The heat shock cognate protein hsc70 has been proposed as being a co-receptor molecule for rotavirus entry into susceptible cells. In this work, an attempt was made to determine the existence of possible domains for VP4 and VP6 binding to hsc70. We selected amino acid sequences 531-554 from VP4 and 280-297 from VP6 on the basis of already recognized sequences for binding to hsc70. This study determined that DLPs and synthetic peptides from VP6 (aa 280-297) and VP4 (aa 531-554), individually or in combination, inhibited rotavirus RRV, YM and WA entry into MA104 and Caco-2 cells in an additive and dose-dependent manner. Hyperimmune sera against these synthetic peptides blocked infection by infectious TLPs. Capture ELISA results showed that DLPs interact with hsc70, probably through VP6 as the specific interaction between hcs70 and DLPs was disrupted by a VP6 peptide. These results suggest that VP6 takes part during rotavirus cell entry by binding to hsc70. This, as well as previous work, provides insight concerning the function of hsc70 within a multi-step model of rotavirus entry.

Lectin binding profile of the small intestine of five-week-old pigs in response to the use of chlortetracycline as a growth promotant and under gnotobiotic conditions

Antibiotics have traditionally been used for growth promotion in the pork industry; however, their use in animal feed has recently been limited because of human health concerns. The intestinal microbiota plays an important role in mediating many physiological functions such as digestion and animal growth. It was hypothesized that use of antibiotics as growth promotants and subsequent variations in intestinal microbiota induce significant changes in the intestinal glycoconjugate composition, which ultimately affects animal growth and disease susceptibility. The aim of this study was to characterize the lectin binding profiles of the ileum of weanling pigs in response to the absence of intestinal microbiota and to the use of the antibiotic chlortetracycline as growth promotant. Eighteen half-sib piglets obtained by cesarean section were divided into 3 treatment groups (n = 6) and maintained as control, antibiotic-fed, and gnotobiotic piglets until 5 wk of age. The glycoconjugate composition of the ileal tissues was examined by lectin histochemistry. Lycopersicon esculentum lectin, Jacalin, Pisum sativum agglutinin, Lens culinaris agglutinin (LCA), and Sambucus nigra lectin showed significant differences (P < 0.05) in binding intensities on the dome and villous epithelium between the treatment groups. Griffonia simplicifolia lectin I, Glycine maxi agglutinin, and Arachis hypogea agglutinin exhibited differences (P < 0.05) between treatment groups in lectin binding on goblet cells. Triticum vulgaris agglutinin, Pisum sativum agglutinin, and LCA showed significant differences (P < 0.05) in binding intensities on dome, corona, and follicular regions of the ileum among treatment groups of animals. Overall, ileal tissues from gnotobiotic piglets expressed significantly weaker (P < 0.05) lectin binding for many lectins compared with control and antibiotic groups. This suggests that the intestinal microbiota plays an important role in the expression of sugar moieties in the intestine. Lectins LCA, Phaseolus vulgaris Leucoagglutinin, and Maackia amurensis lectin II showed significant differences (P < 0.05) in lectin bindings between control and antibiotic-fed piglets. This indicates that chlortetracycline as a growth promotant induces biologically relevant changes in the lectin binding profile of the ileum. These findings will help in further understanding the role of the gut microbiota and the mechanisms of action of antibiotics as growth promotants in pigs.

Early Steps in Rotavirus Cell Entry

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

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

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

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

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

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

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

Attachment factors

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

Selection of single-chain antibodies against the VP8* subunit of rotavirus VP4 outer capsid protein and their expression in Lactobacillus casei

Single-chain antibodies (scFv) recognizing the VP8* fraction of rotavirus outer capsid and blocking rotavirus infection in vitro were isolated by phage display. Vectors for the extracellular expression in Lactobacillus casei of one of the scFv were constructed. L. casei was able to secrete active scFv to the growth medium, showing the potential of probiotic bacteria to be engineered to express molecules suitable for in vivo antirotavirus therapies.

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.

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.

Characterization of rotavirus cell entry

While recently we have learned much about the viral and cellular proteins involved in the initial attachment of rotaviruses to MA104 cells, the mechanism by which these viruses reach the interior of the cell is poorly understood. For this study, we observed the effects of drugs and of dominant-negative mutants, known to impair clathrin-mediated endocytosis and endocytosis mediated by caveolae, on rotavirus cell infection. Rotaviruses were able to enter cells in the presence of compounds that inhibit clathrin-mediated endocytosis as well as cells overexpressing a dominant-negative form of Eps15, a protein crucial for the assembly of clathrin coats. We also found that rotaviruses infected cells in which caveolar uptake was blocked; treatment with the cholesterol binding agents nystatin and filipin, as well as transfection of cells with dominant-negative caveolin-1 and caveolin-3 mutants, had no effect on rotavirus infection. Interestingly, cells treated with methyl-beta-cyclodextrin, a drug that sequesters cholesterol from membranes, and cells expressing a dominant-negative mutant of the large GTPase dynamin, which is known to function in several membrane scission events, were not infected by rotaviruses, indicating that cholesterol and dynamin play a role in the entry of rotaviruses.

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.

Characterization and biological activity of gangliosides in buffalo milk

Gangliosides (GS) were evaluated in Swiss cow's milk (SCM), Italian buffalo milk (IBM) and its serum, Pakistan buffalo colostrum (PBC), Pakistan buffalo mature milk (PBM), and Pakistan buffalo milk from rice-growing areas (PBR). Dairy GS were obtained from the Folch's upper (hydrophilic) and lower (lipophilic) extraction phases, respectively, and determined as lipid-bound sialic acid (LBSA) by colorimetry. Molar ratios of LBSA in the hydro- and lipophilic GS fractions were 52:48 to 79:21. Mature buffalo milk types had 40-100% more LBSA in the lipophilic GS fraction compared to SCM. Liquid PBC was higher in LBSA (24 nmol/g) compared to mature milk types (8-11 nmol/g). Thin-layer chromatography (TLC) and scanning densitometry showed distinct profiles of hydrophilic and lipophilic GS fractions. Lipophilic GS (but importantly not hydrophilic GS) from IBM and its serum decreased prostaglandin series 2 production by 75-80% in cultured human colonic epithelial cells exposed to tumor necrosis factor alpha (TNFalpha). Hydrophilic GD(3) and lipophilic GM(3) selectively bound rotavirus particles prepared from a rhesus strain and its mutant. A GS fraction in IBM showed a GM(1)-specific binding to cholera toxin subunit B (CTB). IBM serum (IBMS) was a rich source of LBSA (420 nmol/g proteins). In summary, improved methodology led to increased LBSA recovery and isolation of additional and bioactive milk GS. Human and Italian buffalo milk had similar CTB binding, and both had increased polysialo-GS compared to cows milk. The toxin binding properties of buffalo milk GS, and the anti-inflammatory activity of the lipophilized GS fraction could be important for developing innovative food applications, as well as the subject of future research.

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.

Sialic acid functions in enterovirus 70 binding and infection

The interaction of viruses with host cell receptors is the initial step in viral infection and is an important determinant of virus host range, tissue tropism, and pathogenesis. The complement regulatory protein decay-accelerating factor (DAF/CD55) is an attachment receptor for enterovirus 70 (EV70), a member of the Picornaviridae, commonly associated with an eye infection in humans known as acute hemorrhagic conjunctivitis. In early work, the EV70 receptor on erythrocytes, responsible for its hemagglutinating activity, was shown to be sensitive to neuraminidase, implying an essential role for sialic acid in virus attachment. Here, we extend these results to show that cell surface sialic acid is required for EV70 binding to nucleated cells susceptible to virus infection and that sialic acid binding is important in productive infection. Through the use of site-directed mutagenesis to eliminate the single N-linked glycosylation site of DAF and of a chimeric receptor protein in which the O-glycosylated domain of DAF was replaced by a region of the HLA-B44 molecule, a role in EV70 binding for the sialic acid residues of DAF was excluded, suggesting the existence of at least one additional, sialylated EV70-binding factor at the cell surface. Treatment of cells with metabolic inhibitors of glycosylation excluded a role for the N-linked oligosaccharides of glycoproteins but suggested that O-linked glycosylation is important for EV70 binding.

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.

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

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

Heat shock cognate protein 70 is involved in rotavirus cell entry

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

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.

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.

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

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

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

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

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

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