Interaction between respiratory syncytial virus and glycosaminoglycans, including heparan sulfate

Melittin-Related Peptides Interfere with Sandfly Fever Naples Virus Infection by Interacting with Heparan Sulphate

Emerging viruses pose an important global public health challenge, and early action is needed to control their spread. The Bunyaviridae family contains a great number of arboviruses which are potentially pathogenic for humans. For example, phleboviruses affect a large range of hosts, including humans and animals. Some infections usually have an asymptomatic course, but others lead to severe complications, such as Toscana virus, which is able to cause meningitis and encephalitis. Unfortunately, to date, no vaccines or antiviral treatments have been found. In the present study, we evaluated the effect of melittin-related peptides, namely the frog-derived RV-23 and AR-23, on sandfly fever Naples virus infection in vitro. Both peptides exhibited a strong antiviral activity by targeting the viral particles and blocking the virus-cell interaction. Their action was directed to an early phase of SFNV infection, in particular at viral adsorption on host cells, by interfering with the binding of common glycosaminoglycan receptors. Given the better antimicrobial behavior of AR-23 and RV-23 compared to melittin in terms of selectivity, our studies expand our understanding of the potential of these peptides as antimicrobials and stimulate further investigations in the direction of novel antiviral strategies against phlebovirus infection.

Effect of hot water extracts of Arthrospira maxima (spirulina) against respiratory syncytial virus

BACKGROUND

Spirulina (Arthrospira maxima) hot water extracts such as calcium spirulan (Ca-SP) have demonstrated antiviral effects against herpes simplex virus (HSV), human immunodeficiency virus-1 (HIV-1), and influenza virus infections. There is no prior evidence suggesting the anti-viral activity of the spirulina hot water extract against respiratory syncytial virus (RSV).

PURPOSE

There are currently no effective antivirals available to treat RSV infection. Therefore, the development of safe and novel anti-RSV drugs is urgent and necessary. The aim of this work was to demonstrate the anti-RSV activity of spirulina hot water extracts and determine the potential mechanism of action.

METHODS

Cytotoxicity and anti-RSV activity of spirulina hot water extracts were measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and neutralization assays, respectively. Potential mechanisms and components were assessed using time of addition, attachment, internalization, pull-down assays, and composition analysis.

RESULTS

The polysaccharide-enriched high-molecular weight fraction (>100 kDa, SHD1) had a high total sugar content, with rhamnose accounting for approximately 60 mol% of total monosaccharides. The main glycosyl linkages included 3-, 4-, and 2,3-rhamnopyranosyl linkages. All spirulina hot water extracts showed no toxicity toward human epithelial type 2 (HEp-2) cells but demonstrated anti-RSV activity. The SHD1 had a half maximal effective concentration (EC₅₀) of 0.0915 mg/ml and a selective index (SI) of >261.5 against RSV. SHD1 significantly reduced viral yield in a dose-dependent manner during the RSV attachment stage. SHD1 disrupted RSV internalization and inhibited RSV attachment (G) protein binding to heparan sulfate receptors on the host cell surface, thus preventing RSV attachment and entry.

CONCLUSION

SHD1 serves as an effective candidate for novel drug development against RSV infection.

Nucleolin: a cell portal for viruses, bacteria, and toxins

The main localization of nucleolin is the nucleolus, but this protein is present in multiple subcellular sites, and it is unconventionally secreted. On the cell surface, nucleolin acts as a receptor for various viruses, some bacteria, and some toxins. Aim of this review is to discuss the characteristics that make nucleolin able to act as receptor or co-receptor of so many and different pathogens. The important features that emerge are its multivalence, and its role as a bridge between the cell surface and the nucleus. Multiple domains, short linear motifs and post-translational modifications confer and modulate nucleolin ability to interact with nucleic acids, with proteins, but also with carbohydrates and lipids. This modular multivalence allows nucleolin to participate in different types of biomolecular condensates and to move to various subcellular locations, where it can act as a kind of molecular glue. It moves from the nucleus to the cell surface and can accompany particles in the reverse direction, from the cell surface into the nucleus, which is the destination of several pathogens to manipulate the cell in their favour.

CMV Seropositive Status Increases Heparanase SNPs Regulatory Activity, Risk of Acute GVHD and Yield of CD34+ Cell Mobilization

Heparanase is an endo-β-glucuronidase that is best known for its pro-cancerous effects but is also implicated in the pathogenesis of various viruses. Activation of heparanase is a common strategy to increase viral spread and trigger the subsequent inflammatory cascade. Using a Single Nucleotide Polymorphisms (SNP)-associated approach we identified enhancer and insulator regions that regulate HPSE expression. Although a role for heparanase in viral infection has been noticed, the impact of HPSE functional SNPs has not been determined. We investigated the effect of cytomegalovirus (CMV) serostatus on the involvement of HPSE enhancer and insulator functional SNPs in the risk of acute graft versus host disease (GVHD) and granulocyte-colony stimulating factor related CD34⁺ mobilization. A significant correlation between the C alleles of insulator rs4364254 and rs4426765 and CMV seropositivity was found in healthy donors and patients with hematological malignancies. The risk of developing acute GVHD after hematopoietic stem cell transplantation was identified only in CMV-seropositive patients. A significant correlation between the enhancer rs4693608 and insulator rs28649799 and CD34⁺ cell mobilization was demonstrated in the CMV-seropositive donors. It is thus conceivable that latent CMV infection modulates heparanase regulatory regions and enhances the effect of functional SNPs on heparanase function in normal and pathological processes.

Small Non-coding RNA Expression Following Respiratory Syncytial Virus or Measles Virus Infection of Neuronal Cells

Respiratory syncytial virus (RSV) or measles virus (MeV) infection modifies host responses through small non-coding RNA (sncRNA) expression. We show that RSV or MeV infection of neuronal cells induces sncRNAs including various microRNAs and transfer RNA fragments (tRFs). We show that these tRFs originate from select tRNAs (GCC and CAC for glycine, CTT and AAC for Valine, and CCC and TTT for Lysine). Some of the tRNAs are rarely used by RSV or MeV as indicated by relative synonymous codon usage indices suggesting selective cleavage of the tRNAs occurs in infected neuronal cells. The data implies that differentially expressed sncRNAs may regulate host gene expression via multiple mechanisms in neuronal cells.

CX3CR1 Is a Receptor for Human Respiratory Syncytial Virus in Cotton Rats

Respiratory syncytial virus (RSV) has been reported to use CX3CR1 in vitro as a receptor on cultured primary human airway epithelial cultures. To evaluate CX3CR1 as the receptor for RSV in vivo, we used the cotton rat animal model because of its high permissiveness for RSV infection. Sequencing the cotton rat CX3CR1 gene revealed 91% amino acid similarity to human CX3CR1. Previous work found that RSV binds to CX3CR1 via its attachment glycoprotein (G protein) to infect primary human airway cultures. To determine whether CX3CR1-G protein interaction is necessary for RSV infection, recombinant RSVs containing mutations in the CX3CR1 binding site of the G protein were tested in cotton rats. In contrast to wild-type virus, viral mutants did not grow in the lungs of cotton rats. When RSV was incubated with an antibody blocking the CX3CR1 binding site of G protein and subsequently inoculated intranasally into cotton rats, no virus was found in the lungs 4 days postinfection. In contrast, growth of RSV was not affected after preincubation with heparan sulfate (the receptor for RSV on immortalized cell lines). A reduction in CX3CR1 expression in the cotton rat lung through the use of peptide-conjugated morpholino oligomers led to a 10-fold reduction in RSV titers at day 4 postinfection. In summary, these results indicate that CX3CR1 functions as a receptor for RSV in cotton rats and, in combination with data from human airway epithelial cell cultures, strongly suggest that CX3CR1 is a primary receptor for naturally acquired RSV infection. IMPORTANCE The knowledge about a virus receptor is useful to better understand the uptake of a virus into a cell and potentially develop antivirals directed against either the receptor molecule on the cell or the receptor-binding protein of the virus. Among a number of potential receptor proteins, human CX3CR1 has been demonstrated to act as a receptor for respiratory syncytial virus (RSV) on human epithelial cells in tissue culture. Here, we report that the cotton rat CX3CR1, which is similar to the human molecule, acts as a receptor in vivo. This study strengthens the argument that CX3CR1 is a receptor molecule for RSV.

Binding of the SARS-CoV-2 spike protein to glycans

The pandemic of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a high number of deaths in the world. To combat it, it is necessary to develop a better understanding of how the virus infects host cells. Infection normally starts with the attachment of the virus to cell-surface glycans like heparan sulfate (HS) and sialic acid-containing glycolipids/glycoproteins. In this study, we examined and compared the binding of the subunits and spike (S) proteins of SARS-CoV-2, SARS-CoV, and Middle East respiratory disease (MERS)-CoV to these glycans. Our results revealed that the S proteins and subunits can bind to HS in a sulfation-dependent manner and no binding with sialic acid residues was detected. Overall, this work suggests that HS binding may be a general mechanism for the attachment of these coronaviruses to host cells, and supports the potential importance of HS in infection and in the development of antiviral agents against these viruses.

Heparan Sulfate Proteoglycans in Viral Infection and Treatment: A Special Focus on SARS-CoV-2

Heparan sulfate proteoglycans (HSPGs) encompass a group of glycoproteins composed of unbranched negatively charged heparan sulfate (HS) chains covalently attached to a core protein. The complex HSPG biosynthetic machinery generates an extraordinary structural variety of HS chains that enable them to bind a plethora of ligands, including growth factors, morphogens, cytokines, chemokines, enzymes, matrix proteins, and bacterial and viral pathogens. These interactions translate into key regulatory activity of HSPGs on a wide range of cellular processes such as receptor activation and signaling, cytoskeleton assembly, extracellular matrix remodeling, endocytosis, cell-cell crosstalk, and others. Due to their ubiquitous expression within tissues and their large functional repertoire, HSPGs are involved in many physiopathological processes; thus, they have emerged as valuable targets for the therapy of many human diseases. Among their functions, HSPGs assist many viruses in invading host cells at various steps of their life cycle. Viruses utilize HSPGs for the attachment to the host cell, internalization, intracellular trafficking, egress, and spread. Recently, HSPG involvement in the pathogenesis of SARS-CoV-2 infection has been established. Here, we summarize the current knowledge on the molecular mechanisms underlying HSPG/SARS-CoV-2 interaction and downstream effects, and we provide an overview of the HSPG-based therapeutic strategies that could be used to combat such a fearsome virus.

Roles of extracellular matrix components in Tiger frog virus attachment to fathead minnow (Pimephales promelas) cells

Tiger frog virus (TFV) belongs to the genus Ranavirus (family Iridoviridae) and causes significant harm in cultured frogs, resulting in substantial losses in ecological and economic field in Southern China. Attachment is the first step in viral life cycle, which is dependent on the interactions of virions with extracellular matrix (ECM) components. Studying this process will help in understanding virus infection and controlling viral diseases. In this study, the roles of primary ECM components in TFV attachment were investigated. The results on the kinetics of virus attachment showed TFV successful attachment to the cell surface as a relatively rapid process after TFV was used to inoculate cells for 10 min at 4 °C. Western blot and quantitative PCR analyses results showed that soluble fibronectin, collagen IV, laminin, or hyaluronic acid treatment with TFV caused no significant effect on virus attachment. Soluble heparin, heparan sulfate and chondroitin sulfate A/B could inhibit TFV attachment in a dose-dependent manner. Enzymic digestion by cell surface heparin/heparan sulfate using heparinase I, II, and III could significantly prevent TFV attachment, suggesting that heparan sulfate plays an important role in TFV attachment. Furthermore, the binding assays of heparin-agarose beads and virion showed that TFV virions specifically bound with heparin in a dose-dependent manner. Given that heparin is a structural analogue of heparan sulfate, the above results suggest that heparan sulfate might serve as an attachment factor of TFV infection. Our work would be beneficial to understand the mechanisms of TFV attachment and the interactions of TFV with cellular receptor(s).

CX3CR1 as a respiratory syncytial virus receptor in pediatric human lung

BACKGROUND

Data on the host factors that contribute to infection of young children by respiratory syncytial virus (RSV) are limited. The human chemokine receptor, CX3CR1, has recently been implicated as an RSV receptor. Here we evaluate a role for CX3CR1 in pediatric lung RSV infections.

METHODS

CX3CR1 transcript levels in the upper and lower pediatric airways were assessed. Tissue localization and cell-specific expression was confirmed using in situ hybridization and immunohistochemistry. The role of CX3CR1 in RSV infection was also investigated using a novel physiological model of pediatric epithelial cells.

RESULTS

Low levels of CX3CR1 transcript were often, but not always, expressed in both upper (62%) and lower airways (36%) of pediatric subjects. CX3CR1 transcript and protein expression was detected in epithelial cells of normal human pediatric lung tissues. CX3CR1 expression was readily detected on primary cultures of differentiated pediatric/infant human lung epithelial cells. RSV demonstrated preferential infection of CX3CR1-positive cells, and blocking CX3CR1/RSV interaction significantly decreased viral load.

CONCLUSION

CX3CR1 is present in the airways of pediatric subjects where it may serve as a receptor for RSV infection. Furthermore, CX3CR1 appears to play a mechanistic role in mediating viral infection of pediatric airway epithelial cells in vitro.

Respiratory syncytial virus (RSV): a scourge from infancy to old age

Respiratory syncytial virus (RSV) is the most common single cause of respiratory hospitalisation of infants and is the second largest cause of lower respiratory infection mortality worldwide. In adults, RSV is an under-recognised cause of deterioration in health, particularly in frail elderly persons. Infection rates typically rise in late autumn and early winter causing bronchiolitis in infants, common colds in adults and insidious respiratory illness in the elderly. Virus detection methods optimised for use in children have low detection rate in adults, highlighting the need for better diagnostic tests. There are many vaccines under development, mostly based on the surface glycoprotein F which exists in two conformations (prefusion and postfusion). Much of the neutralising antibody appears to be to the prefusion form. Vaccines being developed include live attenuated, subunit, particle based and live vectored agents. Different vaccine strategies may be appropriate for different target populations: at-risk infants, school-age children, adult caregivers and the elderly. Antiviral drugs are in clinical trial and may find a place in disease management. RSV disease is one of the major remaining common tractable challenges in infectious diseases and the era of vaccines and antivirals for RSV is on the near horizon.

Implication of the Whitefly, Bemisia tabaci, Collagen Protein in Begomoviruses Acquisition and Transmission

Begomoviruses are the largest group of plant viruses transmitted exclusively by the whitefly, Bemisia tabaci (Gennadius), in a persistent, circulative, and nonpropagative manner. Begomoviruses in association with B. tabaci cause enormous loss to world agricultural crops. Transmission, retention, and circulation of begomovirus in B. tabaci are facilitated by its interaction with several proteins of the insect and its endosymbionts. However, very few such proteins have been identified from B. tabaci that are involved in this specific interaction. Here, we have performed yeast two-hybrid assay between B. tabaci complementary DNA expression library and the coat protein (CP) of tomato leaf curl New Delhi virus (ToLCNDV) and cotton leaf curl Rajasthan virus (CLCuV). Collagen was the common protein found to be interacting with both of the viruses. The collagen protein was found to be localized in gut layers of B. tabaci. Additionally, pull-down and dot-blot assays confirmed the association of endogenous collagen with ToLCNDV CP. Immunolocalization analysis also showed colocalization of ToLCNDV particles and collagen within insect gut. Finally, B. tabaci fed on anticollagen antibody and exhibited ∼46% reduction in ToLCNDV transmission, suggesting a supportive role for collagen in virus transmission.

The Effect of Dextran Sulfate-as Model Glycosaminoglycan Analogue-on Membrane Lipids: DPPC, Cholesterol, and DPPC-Cholesterol Mixture. The Monolayer Study

Glycosaminoglycans (GAGs) are essential components of the extracellular matrices (ECMs) located on the outer surface of cellular membranes. They belong to the group of polysaccharides involved in diverse biological processes acting on the surface and across natural lipid membranes. Recently, particular attention has been focused on possible role of GAGs in the amyloid deposits. The amyloid formation is related to a disorder in protein folding, causing that soluble-in normal conditions-peptides become deposited extracellularly as insoluble fibrils, impairing tissue structure and its function. One of the hypothesis holds that GAGs may inhibit amyloid formation by interacting with the lipid membrane by blocking the accumulation of protein aggregates on the membrane surface. Although the biophysical properties of GAGs are described rather well, little is known about the nature of association between these polysaccharides and components of natural cell membranes. Therefore, a study of GAGs influence on membrane lipids is of particular importance. The aim of the present work is to get insight into the effect of hydrophilic dextran sulfate (DS)-that can be considered as GAG analogue-on membrane lipids organization. This study was based on examining interactions between DS sodium salt of molecular weight equal to about 40 kDa (DS40), dissolved in water subphase, and a model membrane, mimicked as Langmuir monolayer, formed by representative natural membrane lipids: cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as well as their mixtures. Due to the fact that calcium ions in excess may accumulate in the lipid membrane, attracting high molecular weight molecules to their surface, the influence of calcium ions present in the subphase on the DS40 activity has also been examined. It has been found that negatively charged DS, forming a sublayer underneath the monolayer, barely interacts with membrane lipids; however, in the presence of calcium ions the electrostatic interactions between DS40 and lipid membrane are significantly enhanced, leading to the formation of network-like crystalline structures at the surface of model membrane, which can prevent incorporation and interaction with other extracellular molecules, e.g., proteins.

Host cytoskeleton in respiratory syncytial virus assembly and budding

Respiratory syncytial virus (RSV) is one of the major pathogens responsible for lower respiratory tract infections (LRTI) in young children, the elderly, and the immunosuppressed. Currently, there are no antiviral drugs or vaccines available that effectively target RSV infections, proving a significant challenge in regards to prevention and treatment. An in-depth understanding of the host-virus interactions that underlie assembly and budding would inform new targets for antiviral development.Current research suggests that the polymerised form of actin, the filamentous or F-actin, plays a role in RSV assembly and budding. Treatment with cytochalasin D, which disrupts F-actin, has been shown to inhibit virus release. In addition, the actin cytoskeleton has been shown to interact with the RSV matrix (M) protein, which plays a central role in RSV assembly. For this reason, the interaction between these two components is hypothesised to facilitate the movement of viral components in the cytoplasm and to the budding site. Despite increases in our knowledge of RSV assembly and budding, M-actin interactions are not well understood. In this review, we discuss the current literature on the role of actin cytoskeleton during assembly and budding of RSV with the aim to integrate disparate studies to build a hypothetical model of the various molecular interactions between actin and RSV M protein that facilitate RSV assembly and budding.

Respiratory Syncytial Virus Uses CX3CR1 as a Receptor on Primary Human Airway Epithelial Cultures

Respiratory syncytial virus (RSV) is the most frequent cause of lower respiratory disease in infants, but no vaccine or effective therapy is available. The initiation of RSV infection of immortalized cells is largely dependent on cell surface heparan sulfate (HS), a receptor for the RSV attachment (G) glycoprotein in immortalized cells. However, RSV infects the ciliated cells in primary well differentiated human airway epithelial (HAE) cultures via the apical surface, but HS is not detectable on this surface. Here we show that soluble HS inhibits infection of immortalized cells, but not HAE cultures, confirming that HS is not the receptor on HAE cultures. Conversely, a “non-neutralizing” monoclonal antibody against the G protein that does not block RSV infection of immortalized cells, does inhibit infection of HAE cultures. This antibody was previously shown to block the interaction between the G protein and the chemokine receptor CX3CR1 and we have mapped the binding site for this antibody to the CX3C motif and its surrounding region in the G protein. We show that CX3CR1 is present on the apical surface of ciliated cells in HAE cultures and especially on the cilia. RSV infection of HAE cultures is reduced by an antibody against CX3CR1 and by mutations in the G protein CX3C motif. Additionally, mice lacking CX3CR1 are less susceptible to RSV infection. These findings demonstrate that RSV uses CX3CR1 as a cellular receptor on HAE cultures and highlight the importance of using a physiologically relevant model to study virus entry and antibody neutralization.

Respiratory syncytial virus (RSV) is the second most common infectious cause of infant death worldwide. Despite this great clinical impact, no effective antivirals or vaccines against RSV are available. Here we find that the RSV attachment (G) glycoprotein uses CX3CR1 as a receptor on primary human airway epithelial (HAE) cultures, an excellent model of RSV infection of the human lung. The G protein contains a CX3C motif and we find that this region is critical for its role in infection of HAE cultures, but not of immortalized cells. Furthermore, we find that antibodies against the G protein neutralize RSV infection of HAE cultures differently from immortalized cells. These insights suggest that HAE cultures should be used to quantify neutralizing antibodies, including during vaccine development, that the CX3CR1 interaction with the RSV G protein could be a target for antiviral drug development, and that the G protein should be considered for inclusion in vaccines.

Respiratory Syncytial Virus Infection Upregulates NLRC5 and Major Histocompatibility Complex Class I Expression through RIG-I Induction in Airway Epithelial Cells

Respiratory syncytial virus (RSV) is the leading cause of acute respiratory tract viral infection in infants, causing bronchiolitis and pneumonia. The host antiviral response to RSV acts via retinoic acid-inducible gene I (RIG-I). We show here that RSV infection upregulates major histocompatibility complex class I (MHC-I) expression through the induction of NLRC5, a NOD-like, CARD domain-containing intracellular protein that has recently been identified as a class I MHC transactivator (CITA). RSV infection of A549 cells promotes upregulation of NLRC5 via beta interferon (IFN-β) production, since the NLRC5-inducing activity in a conditioned medium from RSV-infected A549 cells was removed by antibody to IFN-β, but not by antibody to IFN-γ. RSV infection resulted in RIG-I upregulation and induction of NLRC5 and MHC-I. Suppression of RIG-I induction significantly blocked NLRC5, as well as MHC-I, upregulation and diminished IRF3 activation. Importantly, Vero cells deficient in interferon production still upregulated MHC-I following introduction of the RSV genome by infection or transfection, further supporting a key role for RIG-I. A model is therefore proposed in which the host upregulates MHC-I expression during RSV infection directly via the induction of RIG-I and NLRC5 expression. Since elevated expression of MHC-I molecules can sensitize host cells to T lymphocyte-mediated cytotoxicity or immunopathologic damage, the results have significant implications for the modification of immunity in RSV disease.

IMPORTANCE

Human respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and pneumonia in infants and young children worldwide. Infection early in life is linked to persistent wheezing and allergic asthma in later life, possibly related to upregulation of major histocompatibility class I (MHC-I) on the cell surface, which facilitates cytotoxic T cell activation and antiviral immunity. Here, we show that RSV infection of lung epithelial cells induces expression of RIG-I, resulting in induction of a class I MHC transactivator, NLRC5, and subsequent upregulation of MHC-I. Suppression of RIG-I induction blocked RSV-induced NLRC5 expression and MHC-I upregulation. Increased MHC-I expression may exacerbate the RSV disease condition due to immunopathologic damage, linking the innate immune response to RSV disease.

Recombinant subgroup B human respiratory syncytial virus expressing enhanced green fluorescent protein efficiently replicates in primary human cells and is virulent in cotton rats

Human respiratory syncytial virus (HRSV) is the most important viral cause of severe respiratory tract disease in infants. Two subgroups (A and B) have been identified, which cocirculate during, or alternate between, yearly epidemics and cause indistinguishable disease. Existing in vitro and in vivo models of HRSV focus almost exclusively on subgroup A viruses. Here, a recombinant (r) subgroup B virus (rHRSV(B05)) was generated based on a consensus genome sequence obtained directly from an unpassaged clinical specimen from a hospitalized infant. An additional transcription unit containing the gene encoding enhanced green fluorescent protein (EGFP) was introduced between the phosphoprotein and matrix genes (position 5) of the genome to generate rHRSV(B05)EGFP(5). The recombinant viruses replicated efficiently in both HEp-2 cells and in well-differentiated normal human bronchial cells grown at air-liquid interface. Intranasal infection of cotton rats (Sigmodon hispidus) resulted in high numbers of EGFP(+) cells in epithelia of the nasal septum and conchae. When administered in a relatively large inoculum volume, the virus also replicated efficiently in bronchiolar epithelial cells and spread extensively in both the upper and lower respiratory tracts. Virus replication was not observed in ciliated epithelial cells of the trachea. This is the first virulent rHRSV strain with the genetic composition of a currently circulating wild-type virus. In vivo tracking of infected cells by means of EGFP fluorescence in the absence of cytopathic changes increases the sensitivity of virus detection in HRSV pathogenesis studies.

IMPORTANCE

Virology as a discipline has depended on monitoring cytopathic effects following virus culture in vitro. However, wild-type viruses isolated from patients often do not cause significant changes to infected cells, necessitating blind passage. This can lead to genetic and phenotypic changes and the generation of high-titer, laboratory-adapted viruses with diminished virulence in animal models of disease. To address this, we determined the genome sequence of an unpassaged human respiratory syncytial virus from a sample obtained directly from an infected infant, assembled a molecular clone, and recovered a wild-type recombinant virus. Addition of a gene encoding enhanced green fluorescent protein allowed this wild-type virus to be tracked in primary human cells and living animals in the absence of significant cytopathic effects. Imaging of fluorescent cells proved to be a highly valuable tool for monitoring the spread of virus and may help improve assays for evaluating novel intervention strategies.

Polyclonal and monoclonal antibodies specific for the six-helix bundle of the human respiratory syncytial virus fusion glycoprotein as probes of the protein post-fusion conformation

Human respiratory syncytial virus (hRSV) has two major surface glycoproteins (G and F) anchored in the lipid envelope. Membrane fusion promoted by hRSV_F occurs via refolding from a pre-fusion form to a highly stable post-fusion state involving large conformational changes of the F trimer. One of these changes results in assembly of two heptad repeat sequences (HRA and HRB) into a six-helix bundle (6HB) motif. To assist in distinguishing pre- and post-fusion conformations of hRSV_F, we have prepared polyclonal (α-6HB) and monoclonal (R145) rabbit antibodies specific for the 6HB. Among other applications, these antibodies were used to explore the requirements of 6HB formation by isolated protein segments or peptides and by truncated mutants of the F protein. Site-directed mutagenesis and electron microscopy located the R145 epitope in the post-fusion hRSV_F at a site distantly located from previously mapped epitopes, extending the repertoire of antibodies that can decorate the F molecule.

Screening and evaluation of anti-respiratory syncytial virus compounds in cultured cells

Respiratory syncytial virus (RSV) is a highly contagious pathogen that infects mainly ciliated cells of respiratory epithelium and type 1 pneumocytes in the alveoli frequently causing serious respiratory disease in infants, elderly, and immunocompromised patients. At present, prevention/treatment of RSV infection is limited to the use of specific anti-RSV antibody or an aerosol formulation of ribavirin, a drug of suboptimal efficacy and low safety profile. There is an urgent need for development of novel anti-RSV drugs and virucides. Here we describe the cell culture-based methods used in our laboratory in identification of novel inhibitors of RSV including the P13 fusion inhibitor, and the PG545 virucide. Protocols for antiviral screening, evaluation of anti-RSV potency, and elucidation of mode of antiviral activity of test compounds are described.

The comparative genomics of human respiratory syncytial virus subgroups A and B: genetic variability and molecular evolutionary dynamics

Genomic variation and related evolutionary dynamics of human respiratory syncytial virus (RSV), a common causative agent of severe lower respiratory tract infections, may affect its transmission behavior. RSV evolutionary patterns are likely to be influenced by a precarious interplay between selection favoring variants with higher replicative fitness and variants that evade host immune responses. Studying RSV genetic variation can reveal both the genes and the individual codons within these genes that are most crucial for RSV survival. In this study, we conducted genetic diversity and evolutionary rate analyses on 36 RSV subgroup B (RSV-B) whole-genome sequences. The attachment protein, G, was the most variable protein; accordingly, the G gene had a higher substitution rate than other RSV-B genes. Overall, less genetic variability was found among the available RSV-B genome sequences than among RSV-A genome sequences in a comparable sample. The mean substitution rates of the two subgroups were, however, similar (for subgroup A, 6.47 × 10(-4) substitutions/site/year [95% credible interval {CI 95%}, 5.56 × 10(-4) to 7.38 × 10(-4)]; for subgroup B, 7.76 × 10(-4) substitutions/site/year [CI 95%, 6.89 × 10(-4) to 8.58 × 10(-4)]), with the time to their most recent common ancestors (TMRCAs) being much lower for RSV-B (19 years) than for RSV-A (46.8 years). The more recent RSV-B TMRCA is apparently the result of a genetic bottleneck that, over longer time scales, is still compatible with neutral population dynamics. Whereas the immunogenic G protein seems to require high substitution rates to ensure immune evasion, strong purifying selection in conserved proteins such as the fusion protein and nucleocapsid protein is likely essential to preserve RSV viability.

Syndecan-1 serves as the major receptor for attachment of hepatitis C virus to the surfaces of hepatocytes

Our recent studies demonstrated that apolipoprotein E mediates cell attachment of hepatitis C virus (HCV) through interactions with the cell surface heparan sulfate (HS). HS is known to covalently attach to core proteins to form heparan sulfate proteoglycans (HSPGs) on the cell surface. The HSPG core proteins include the membrane-spanning syndecans (SDCs), the lycosylphosphatidylinositol-linked glypicans (GPCs), the basement membrane proteoglycan perlecan (HSPG2), and agrin. In the present study, we have profiled each of the HSPG core proteins in HCV attachment. Substantial evidence derived from our studies demonstrates that SDC1 is the major receptor protein for HCV attachment. The knockdown of SDC1 expression by small interfering RNA (siRNA)-induced gene silence resulted in a significant reduction of HCV attachment to Huh-7.5 cells and stem cell-differentiated human hepatocytes. The silence of SDC2 expression also caused a modest decrease of HCV attachment. In contrast, the siRNA-mediated knockdown of other SDCs, GPCs, HSPG2, and agrin had no effect on HCV attachment. More importantly, ectopic expression of SDC1 was able to completely restore HCV attachment to Huh-7.5 cells in which the endogenous SDC1 expression was silenced by specific siRNAs. Interestingly, mouse SDC1 is also fully functional in mediating HCV attachment when expressed in the SDC1-deficient cells, consistent with recent reports that mouse hepatocytes are also susceptible to HCV infection when expressing other key HCV receptors. Collectively, our findings demonstrate that SDC1 serves as the major receptor protein for HCV attachment to cells, providing another potential target for discovery and development of antiviral drugs against HCV.

Filoviruses utilize glycosaminoglycans for their attachment to target cells

Filoviruses are the cause of severe hemorrhagic fever in human and nonhuman primates. The envelope glycoprotein (GP), responsible for both receptor binding and fusion of the virus envelope with the host cell membrane, has been demonstrated to interact with multiple molecules in order to enhance entry into host cells. Here we have demonstrated that filoviruses utilize glycosaminoglycans, and more specifically heparan sulfate proteoglycans, for their attachment to host cells. This interaction is mediated by GP and does not require the presence of the mucin domain. Both the degree of sulfation and the structure of the carbohydrate backbone play a role in the interaction with filovirus GPs. This new step of filovirus interaction with host cells can potentially be a new target for antiviral strategies. As such, we were able to inhibit filovirus GP-mediated infection using carrageenan, a broad-spectrum microbicide that mimics heparin, and also using the antiviral dendrimeric peptide SB105-A10, which interacts with heparan sulfate, antagonizing the binding of the virus to cells.

Structure and function of respiratory syncytial virus surface glycoproteins

The two major glycoproteins on the surface of the respiratory syncytial virus (RSV) virion, the attachment glycoprotein (G) and the fusion glycoprotein (F), control the initial phases of infection. G targets the ciliated cells of the airways, and F causes the virion membrane to fuse with the target cell membrane. The F protein is the major target for antiviral drug development, and both G and F glycoproteins are the antigens targeted by neutralizing antibodies induced by infection. In this chapter, we review the structure and function of the RSV surface glycoproteins, including recent X-ray crystallographic data of the F glycoprotein in its pre- and postfusion conformations, and discuss how this information informs antigen selection and vaccine development.

Cholesterol-rich microdomains as docking platforms for respiratory syncytial virus in normal human bronchial epithelial cells

Respiratory syncytial virus (RSV) is one of the major causes of respiratory infections in children, and it is the main pathogen causing bronchiolitis in infants. The binding and entry mechanism by which RSV infects respiratory epithelial cells has not yet been determined. In this study, the earliest stages of RSV infection in normal human bronchial epithelial cells were probed by tracking virions with fluorescent lipophilic dyes in their membranes. Virions colocalized with cholesterol-containing plasma membrane microdomains, identified by their ability to bind cholera toxin subunit B. Consistent with an important role for cholesterol in RSV infection, cholesterol depletion profoundly inhibited RSV infection, while cholesterol repletion reversed this inhibition. Merger of the outer leaflets of the viral envelope and the cell membrane appeared to be triggered at these sites. Using small-molecule inhibitors, RSV infection was found to be sensitive to Pak1 inhibition, suggesting the requirement of a subsequent step of cytoskeletal reorganization that could involve plasma membrane rearrangements or endocytosis. It appears that RSV entry depends on its ability to dock to cholesterol-rich microdomains (lipid rafts) in the plasma membrane where hemifusion events begin, assisted by a Pak1-dependent process.

Potent anti-respiratory syncytial virus activity of a cholestanol-sulfated tetrasaccharide conjugate

A number of different viruses including respiratory syncytial virus (RSV) initiate infection of cells by binding to cell surface glycosaminoglycans and sulfated oligo- and polysaccharide mimetics of these receptors exhibit potent antiviral activity in cultured cells. We investigated whether the introduction of different lipophilic groups to the reducing end of sulfated oligosaccharides would modulate their anti-RSV activity. Our results demonstrate that the cholestanol-conjugated tetrasaccharide (PG545) exhibited ∼5- to 16-fold enhanced anti-RSV activity in cultured cells compared with unmodified sulfated oligosaccharides. Furthermore, PG545 displayed virus-inactivating (virucidal) activity, a feature absent in sulfated oligosaccharides. To inhibit RSV infectivity PG545 had to be present during the initial steps of viral infection of cells. The anti-RSV activity of PG545 was due to both partial inhibition of the virus attachment to cells and a more profound interference with some post-attachment steps as PG545 efficiently neutralized infectivity of the cell-adsorbed virus. The anti-RSV activity of PG545 was reduced when tested in the presence of human nasal secretions. Serial passages of RSV in the presence of increasing concentrations of PG545 selected for weakly resistant viral variants that comprised the F168S and the P180S amino acid substitutions in the viral G protein. Altogether we identified a novel and potent inhibitor of RSV, which unlike sulfated oligo- and polysaccharide compounds, could irreversibly inactivate RSV infectivity.

Residues in domain III of the dengue virus envelope glycoprotein involved in cell-surface glycosaminoglycan binding

The dengue virus (DENV) envelope (E) protein mediates virus entry into cells via interaction with a range of cell-surface receptor molecules. Cell-surface glycosaminoglycans (GAGs) have been shown to play an early role in this interaction, and charged oligosaccharides such as heparin bind to the E protein. We have examined this interaction using site-directed mutagenesis of a recombinant form of the putative receptor-binding domain III of the DENV-2E protein expressed as an MBP (maltose-binding protein)-fusion protein. Using an ELISA-based GAG-binding assay, cell-based binding analysis and antiviral-activity assays, we have identified two critical residues, K291 and K295, that are involved in GAG interactions. These studies have also demonstrated differential binding between mosquito and human cells.

Identification of nucleolin as a cellular receptor for human respiratory syncytial virus

Human respiratory syncytial virus (RSV) causes a large burden of disease worldwide. There is no effective vaccine or therapy, and the use of passive immunoprophylaxis with RSV-specific antibodies is limited to high-risk patients. The cellular receptor (or receptors) required for viral entry and replication has yet to be described; its identification will improve understanding of the pathogenesis of infection and provide a target for the development of novel antiviral interventions. Here we show that RSV interacts with host-cell nucleolin via the viral fusion envelope glycoprotein and binds specifically to nucleolin at the apical cell surface in vitro. We observed decreased RSV infection in vitro in neutralization experiments using nucleolin-specific antibodies before viral inoculation, in competition experiments in which virus was incubated with soluble nucleolin before inoculation of cells, and upon RNA interference (RNAi) to silence cellular nucleolin expression. Transfection of nonpermissive Spodoptera frugiperda Sf9 insect cells with human nucleolin conferred susceptibility to RSV infection. RNAi-mediated knockdown of lung nucleolin was associated with a significant reduction in RSV infection in mice (P = 0.0004), confirming that nucleolin is a functional RSV receptor in vivo.

Propagation of classical swine fever virus in vitro circumventing heparan sulfate-adaptation

Amplification of natural virus isolates in permanent cell lines can result in adaptation, in particular enhanced binding to heparan sulfate (HS)-containing glycosaminoglycans present on most vertebrate cells. This has been reported for several viruses, including the pestivirus classical swine fever virus (CSFV), the causative agent of a highly contagious hemorrhagic disease in pigs. Propagation of CSFV in cell culture is essential in virus diagnostics and research. Adaptation of CSFV to HS-binding has been related to amino acid changes in the viral E(rns) glycoprotein, resulting in viruses with altered replication characteristics in vitro and in vivo. Consequently, a compound blocking the HS-containing structures on cell surfaces was employed to monitor conversion from HS-independency to HS-dependency. It was shown that the porcine PEDSV.15 cell line permitted propagation of CSFV within a limited number of passages without adaptation to HS-binding. The selection of HS-dependent CSFV mutants was also prevented by propagation of the virus in the presence of DSTP 27. The importance of these findings can be seen from the altered ratio of cell-associated to secreted virus upon acquisition of enhanced HS-binding affinity, a phenotype proposed previously to be related to virulence in the natural host.

The synthetic bacterial lipopeptide Pam3CSK4 modulates respiratory syncytial virus infection independent of TLR activation

Respiratory syncytial virus (RSV) is an important cause of acute respiratory disease in infants, immunocompromised subjects and the elderly. However, it is unclear why most primary RSV infections are associated with relatively mild symptoms, whereas some result in severe lower respiratory tract infections and bronchiolitis. Since RSV hospitalization has been associated with respiratory bacterial co-infections, we have tested if bacterial Toll-like receptor (TLR) agonists influence RSV-A2-GFP infection in human primary cells or cell lines. The synthetic bacterial lipopeptide Pam3-Cys-Ser-Lys4 (Pam3CSK4), the prototype ligand for the heterodimeric TLR1/TLR2 complex, enhanced RSV infection in primary epithelial, myeloid and lymphoid cells. Surprisingly, enhancement was optimal when lipopeptides and virus were added simultaneously, whereas addition of Pam3CSK4 immediately after infection had no effect. We have identified two structurally related lipopeptides without TLR-signaling capacity that also modulate RSV infection, whereas Pam3CSK4-reminiscent TLR1/2 agonists did not, and conclude that modulation of infection is independent of TLR activation. A similar TLR-independent enhancement of infection could also be demonstrated for wild-type RSV strains, and for HIV-1, measles virus and human metapneumovirus. We show that the effect of Pam3CSK4 is primarily mediated by enhanced binding of RSV to its target cells. The N-palmitoylated cysteine and the cationic lysines were identified as pivotal for enhanced virus binding. Surprisingly, we observed inhibition of RSV infection in immortalized epithelial cell lines, which was shown to be related to interactions between Pam3CSK4 and negatively charged glycosaminoglycans on these cells, which are known targets for binding of laboratory-adapted but not wild-type RSV. These data suggest a potential role for bacterial lipopeptides in enhanced binding of RSV and other viruses to their target cells, thus affecting viral entry or spread independent of TLR signaling. Moreover, our results also suggest a potential application for these synthetic lipopeptides as adjuvants for live-attenuated viral vaccines.

Respiratory syncytial virus (RSV) infections are an important cause of hospitalization of infants during the winter season. However, RSV is often not the only detectable pathogen, but co-infections with respiratory bacteria are common. It has been hypothesized that this results from epithelial damage caused by the virus, facilitating colonization by pathogenic bacteria such as Streptococcus pneumoniae. However, an inverse order of events is not impossible: bacterial infections may activate respiratory epithelial cells through TLR signaling, resulting in increased susceptibility to virus infections. We tested this hypothesis by screening bacterial TLR agonists for their capacity to modulate RSV infection in different cell types, and identified the lipopeptide and prototype TLR1/2 agonist Pam3CSK4 as an enhancer of RSV infections. However, to our surprise this proved independent of TLR activation, but was mediated by enhancement of binding between virus and target cell. Two structurally related lipopeptides unable to stimulate TLR responses were identified that enhanced infections with RSV, but also with other enveloped viruses including HIV-1, human metapneumovirus, and measles virus. We speculate that bacterial infections may influence the pathogenesis of virus infections by facilitating binding to target cells.

Role of cellular glycosaminoglycans and charged regions of viral G protein in human metapneumovirus infection

Human metapneumovirus (hMPV) is an important cause of lower respiratory tract disease, particularly in infants and young children. hMPV has two major glycoproteins, G and F, which are responsible for virus attachment and membrane fusion, respectively. We investigated the role of cellular glycosaminoglycans (GAGs) and G protein in hMPV infection. The pretreatment of hMPV with soluble heparin markedly inhibited the infection of HEp-2 cells. Recombinant G protein, comprising the extracellular domain of G, bound to heparin-agarose columns and also to HEp-2 cells. hMPV infection and G protein binding to HEp-2 cells was inhibited by other soluble GAGs, including chondroitin sulfates, by the enzymatic removal of cell surface GAGs with GAG lyases or by the pretreatment of cells with basic fibroblast growth factor. The role of cellular GAGs was confirmed by the binding of G protein to wild-type CHO cells but not to GAG-deficient CHO-pgsA745 cells. An analysis of the G protein sequence revealed two adjacent clusters of positively charged amino acids ((149)EKKKTRA(155) and (159)QRRGKGKE(166)). Truncated G fragments were expressed, and only the fragment containing these putative heparin binding domains retained heparin binding. The alanine mutagenesis of charged residues in either of these regions resulted in the loss of binding to heparin and to HEp-2 cells, suggesting that both sites are likely to be required for hMPV attachment. These results, taken together with the inhibition of hMPV infection by soluble G protein, indicate an important role for G protein and cellular GAGs in hMPV infection.

Isolation of a native osteoblast matrix with a specific affinity for BMP2

During their commitment and differentiation toward the osteoblast lineage, mesenchymal stem cells secrete a unique extracellular matrix (ECM) that contains large quantities of glycosaminoglycans (GAGs). Proteoglycans (PGs) are major structural and functional components of the ECM and are composed of a core protein to which one or more glycosaminoglycan sugar chains (GAGs) attach. The association of BMP2, a member of the TGF-beta super-family of growth factors, and a known heparin-binding protein, with GAGs has been implicated as playing a significant role in modulating the growth factor's in vitro bioactivity. Here we have characterised an osteoblast-derived matrix (MX) obtained from decellularised MC3T3-E1 cell monolayers for its structural attributes, using SEM and histology, and for its functional ability to maintain cell growth and viability. Using a combination of histology and anion exchange chromatography, we first confirmed the retention of GAGs within MX following the decellularisation process. Then the binding specificity of the retained GAG species within the MX for BMP2 was examined using a BMP2-HBP/EGFP (BMP2 Heparin-Binding Peptide/Enhanced Green Fluorescent Protein) fusion protein. The results of this study provide further evidence for a central role of the ECM in the regulation of BMP2 bioactivity, hence on mesenchymal stem cell commitment to the osteoblast lineage.