Evidence that the respiratory syncytial virus polymerase complex associates with lipid rafts in virus-infected cells: a proteomic analysis

Xuanfei Formula inhibited RSV infection by normalizing the SREBP2-mediated cholesterol synthesis process

Background and aims

Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract infections in children and the elderly, often progressing to pneumonia and severe sequelae. However, there are currently no feasible and cost-effective interventions with proven efficacy for children, making medications with anti-RSV activity urgently needed. Traditional Chinese medicine has shown promising therapeutic efficacy in alleviating viral infection symptoms. Therefore, we aimed to develop effective strategies for RSV treatment based on traditional Chinese medicine.

Methods and results

The infection status was assessed in BALB/c mice with or without Xuanfei Formula (XFF) treatment over a one-week period using H&E staining, cytokine assays and RSV titer testing after RSV challenge. Remarkably, on the first day of XFF intervention, both the pro-inflammation cytokine levels in the serum and RSV-N gene copies in the lung of mice were plummeted, compared to the RSV-infected group. This implied that XFF might possess the immune-independent anti-RSV capability. To elucidate the underlying mechanism, we employed transcriptome analysis followed by k-means analysis. The reversal effects of XFF against RSV primarily focused on the processes of innate and adaptive immunity. Additionally, we found that XFF administration corrected the disordered fatty acid and cholesterol metabolism processes during RSV infection. Lipidomics profiling indicated consistent cholesterol abundance with transcriptional changes but not fatty acids. Cholesterol synthesis-related genes mRNA levels and cholesterol synthesis intermediates detection supported XFF's repression upon cholesterol biosynthesis. Aberrantly increased cholesterol production has been reported as necessary for RSV infection. To mimic that, we observed lovastatin treatment inhibited RSV replication and pro-inflammation cytokine expression in vitro. Transcription factor prediction of differentially expressed genes (DEGs) involved in cholesterol synthesis implicated SREBP2. Through network pharmacology, stigmasterol and β-sitosterol were identified as the effective active ingredients within the XFF, with the help of further molecular docking and mass spectrum detection. In vitro experiments demonstrated β-sitosterol and stigmasterol reinforced the bonding between SREBP cleavage-activation protein (SCAP) and insulin-induced gene proteins (INSIGs) to inhibit SREBP2 cleavage maturation and consequent RSV infection.

Conclusion

Xuanfei Formula (XFF) exhibits excellent anti-RSV efficacy by inhibiting SREBP2-mediated cholesterol synthesis to reduce RSV replication and ameliorate inflammation in the lung of infected mice.

Evidence that an interaction between the respiratory syncytial virus F and G proteins at the distal ends of virus filaments mediates efficient multiple cycle infection

Evidence for a stable interaction between the respiratory syncytial virus (RSV) F and G proteins on the surface of virus filaments was provided using antibody immunoprecipitation studies on purified RSV particles, and by the in situ analysis on the surface of RSV-infected cells using the proximity ligation assay. Imaging of the F and G protein distribution on virus filaments suggested that this protein complex was localised at the distal ends of the virus filaments, and suggested that this protein complex played a direct role in mediating efficient localised cell-to-cell virus transmission. G protein expression was required for efficient localised cell-to-cell transmission of RSV in cell monolayers which provided evidence that this protein complex mediates efficient multiple cycle infection. Collectively, these data provide evidence that F and G proteins form a complex on the surface of RSV particles, and that a role for this protein complex in promoting virus transmission is suggested.

Defining the Assembleome of the Respiratory Syncytial Virus

During respiratory syncytial virus (RSV) particle assembly, the mature RSV particles form as filamentous projections on the surface of RSV-infected cells. The RSV assembly process occurs at the / on the cell surface that is modified by a virus infection, involving a combination of several different host cell factors and cellular processes. This induces changes in the lipid composition and properties of these lipid microdomains, and the virus-induced activation of associated Rho GTPase signaling networks drives the remodeling of the underlying filamentous actin (F-actin) cytoskeleton network. The modified sites that form on the surface of the infected cells form the nexus point for RSV assembly, and in this review chapter, they are referred to as the RSV assembleome. This is to distinguish these unique membrane microdomains that are formed during virus infection from the corresponding membrane microdomains that are present at the cell surface prior to infection. In this article, an overview of the current understanding of the processes that drive the formation of the assembleome during RSV particle assembly is given.

Cellular Lipids-Hijacked Victims of Viruses

Over the millions of years-long co-evolution with their hosts, viruses have evolved plenty of mechanisms through which they are able to escape cellular anti-viral defenses and utilize cellular pathways and organelles for replication and production of infectious virions. In recent years, it has become clear that lipids play an important role during viral replication. Viruses use cellular lipids in a variety of ways throughout their life cycle. They not only physically interact with cellular membranes but also alter cellular lipid metabolic pathways and lipid composition to create an optimal replication environment. This review focuses on examples of how different viruses exploit cellular lipids in different cellular compartments during their life cycles.

Imaging Flow Cytometry and Confocal Immunofluorescence Microscopy of Virus-Host Cell Interactions

Viruses are diverse pathogens that use host factors to enter cells and cause disease. Imaging the entry and replication phases of viruses and their interactions with host factors is key to fully understanding viral infections. This review will discuss how confocal microscopy and imaging flow cytometry are used to investigate virus entry and replication mechanisms in fixed and live cells. Quantification of viral images and the use of cryo-electron microscopy to gather structural information of viruses is also explored. Using imaging to understand how viruses replicate and interact with host factors, we gain insight into cellular processes and identify novel targets to develop antiviral therapeutics and vaccines.

Statin-mediated disruption of Rho GTPase prenylation and activity inhibits respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory tract infections in children. To uncover new antiviral therapies, we developed a live cell-based high content screening approach for rapid identification of RSV inhibitors and characterized five drug classes which inhibit the virus. Among the molecular targets for each hit, there was a strong functional enrichment in lipid metabolic pathways. Modulation of lipid metabolites by statins, a key hit from our screen, decreases the production of infectious virus through a combination of cholesterol and isoprenoid-mediated effects. Notably, RSV infection globally upregulates host protein prenylation, including the prenylation of Rho GTPases. Treatment by statins or perillyl alcohol, a geranylgeranyltransferase inhibitor, reduces infection in vitro. Of the Rho GTPases assayed in our study, a loss in Rac1 activity strongly inhibits the virus through a decrease in F protein surface expression. Our findings provide new insight into the importance of host lipid metabolism to RSV infection and highlight geranylgeranyltransferases as an antiviral target for therapeutic development.

Evolution of proteomics technologies for understanding respiratory syncytial virus pathogenesis

Introduction: Respiratory syncytial virus (RSV) is a major human pathogen associated with long term morbidity. RSV replication occurs primarily in the epithelium, producing a complex cellular response associated with acute inflammation and long-lived changes in pulmonary function and allergic disease. Proteomics approaches provide important insights into post-transcriptional regulatory processes including alterations in cellular complexes regulating the coordinated innate response and epigenome.Areas covered: Peer-reviewed proteomics studies of host responses to RSV infections and proteomics techniques were analyzed. Methodologies identified include 1)." bottom-up" discovery proteomics, 2). Organellar proteomics by LC-gel fractionation; 3). Dynamic changes in protein interaction networks by LC-MS; and 4). selective reaction monitoring MS. We introduce recent developments in single-cell proteomics, top-down mass spectrometry, and photo-cleavable surfactant chemistries that will have impact on understanding how RSV induces extracellular matrix (ECM) composition and airway remodeling.Expert opinion: RSV replication induces global changes in the cellular proteome, dynamic shifts in nuclear proteins, and remodeling of epigenetic regulatory complexes linked to the innate response. Pathways discovered by proteomics technologies have led to deeper mechanistic understanding of the roles of heat shock proteins, redox response, transcriptional elongation complex remodeling and ECM secretion remodeling in host responses to RSV infections and pathological sequelae.

Virus-induced activation of the rac1 protein at the site of respiratory syncytial virus assembly is a requirement for virus particle assembly on infected cells

The distributions of the rac1, rhoA and cdc42 proteins in respiratory syncytial virus (RSV) infected cells was examined. All three rhoGTPases were detected within inclusion bodies, and while the rhoA and rac1 proteins were associated with virus filaments, only the rac1 protein was localised throughout the virus filaments. RSV infection led to increased rac1 protein activation, and using the rac1 protein inhibitor NS23766 we provided evidence that the increased rac1 activation occurred at the site of RSV assembly and facilitated F-actin remodeling during virus morphogenesis. A non-infectious virus-like particle (VLP) assay showed that the RSV VLPs formed in lipid-raft microdomains containing the rac1 protein, together with F-actin and filamin-1 (cell proteins associated with virus filaments). This provided evidence that the virus envelope proteins are trafficked to membrane microdomains containing the rac1 protein. Collectively, these data provide evidence that the rac1 protein plays a direct role in the RSV assembly process.

Respiratory syncytial virus

Respiratory syncytial virus (RSV) is the most common cause of infant hospitalization and causes a high burden of disease in the elderly, too. This enveloped negative-stranded RNA virus has been recently reclassified in the Pneumoviridae family. Infections of the respiratory cells happens when the two major surface glycoproteins, G and F, take contact with the cell receptor CX3CR1 and mediate entry by fusion, respectively. Viral mRNA transcription, genomic RNA synthesis and nucleocapsid formation occur in large cytoplasmic inclusion bodies to avoid recognition by the host innate immune response. Most progeny virions remain associated to the infected cell surface; fusion of infected with adjacent cells results in the formation of large multinucleated syncytia that eventually undergo apoptosis. Desquamated epithelial cells form the plugs that with mucus and fibrin may cause lower airway obstructions. Pathogenetic mechanism of severe RSV disease likely involve both the extent of viral replication and the host immune response. Regarding the latter, single nucleotide polymorphism analysis and genome-wide association studies showed that genetic susceptibility to severe RSV infection is likely a complex trait, in which many different host genetic variants contribute. Recent studies pointed to the fact that bronchiolitis severity depends more on the specific infecting RSV genotypes than on the amount of viral loads. A population-based surveillance system to better define RSV burden of disease would be of valuable help for implementing future vaccination programs.

A sustained antiviral host response in respiratory syncytial virus infected human nasal epithelium does not prevent progeny virus production

Respiratory syncytial virus infection was examined using a human nasal epithelial cell model. Maximum levels of shed-virus were produced at between 3 and 5 days post-infection (dpi), and the infectivity of the shed-virus was stable up to 10 dpi. The highest levels of interferon signalling were recorded at 2dpi, and infection induced a widespread antivirus response in the nasal epithelium, involving both infected cells and non-infected cells. Although these cellular responses were associated with reduced levels of progeny virus production and restricted virus spread, they did not inhibit the infectivity virus that is shed early in infection. In the clinical context these data suggest that although the host cell response in the nasal epithelium may restrict the levels of progeny virus particles produced, the stability of the shed-virus in the nasal mucosa may be an important factor in both disease progression and virus transmission.

Unraveling the role of membrane microdomains during microbial infections

Infectious diseases pose major socioeconomic and health-related threats to millions of people across the globe. Strategies to combat infectious diseases derive from our understanding of the complex interactions between the host and specific bacterial, viral, and fungal pathogens. Lipid rafts are membrane microdomains that play important role in life cycle of microbes. Interaction of microbial pathogens with host membrane rafts influences not only their initial colonization but also their spread and the induction of inflammation. Therefore, intervention strategies aimed at modulating the assembly of membrane rafts and/or regulating raft-directed signaling pathways are attractive approaches for the. management of infectious diseases. The current review discusses the latest advances in terms of techniques used to study the role of membrane microdomains in various pathological conditions and provides updated information regarding the role of membrane rafts during bacterial, viral and fungal infections.

Respiratory Syncytial Virus: Infection, Detection, and New Options for Prevention and Treatment

Respiratory syncytial virus (RSV) infection is a significant cause of hospitalization of children in North America and one of the leading causes of death of infants less than 1 year of age worldwide, second only to malaria. Despite its global impact on human health, there are relatively few therapeutic options available to prevent or treat RSV infection. Paradoxically, there is a very large volume of information that is constantly being refined on RSV replication, the mechanisms of RSV-induced pathology, and community transmission. Compounding the burden of acute RSV infections is the exacerbation of preexisting chronic airway diseases and the chronic sequelae of RSV infection. A mechanistic link is even starting to emerge between asthma and those who suffer severe RSV infection early in childhood. In this article, we discuss developments in the understanding of RSV replication, pathogenesis, diagnostics, and therapeutics. We attempt to reconcile the large body of information on RSV and why after many clinical trials there is still no efficacious RSV vaccine and few therapeutics.

Cholesterol is required for stability and infectivity of influenza A and respiratory syncytial viruses

Cholesterol-rich lipid raft microdomains in the plasma membrane are considered to play a major role in the enveloped virus lifecycle. However, the functional role of cholesterol in assembly, infectivity and stability of respiratory RNA viruses is not fully understood. We previously reported that depletion of cellular cholesterol by cholesterol-reducing agents decreased production of human parainfluenza virus type 1 (hPIV1) particles by inhibiting virus assembly. In this study, we analyzed the role of cholesterol on influenza A virus (IAV) and respiratory syncytial virus (RSV) production. Unlike hPIV1, treatment of human airway cells with the agents did not decrease virus particle production. However, the released virions were less homogeneous in density and unstable. Addition of exogenous cholesterol to the released virions restored virus stability and infectivity. Collectively, these data indicate a critical role of cholesterol in maintaining IAV and RSV membrane structure that is essential for sustaining viral stability and infectivity.

Evidence for a biphasic mode of respiratory syncytial virus transmission in permissive HEp2 cell monolayers

BACKGROUND

During respiratory syncytial virus (RSV) infection filamentous virus particles are formed on the cell surface. Although the virus infectivity remains cell-associated, low levels of cell-free virus is detected during advanced infection. It is currently unclear if this cell-free virus infectivity is due to a low-efficiency specific cell-release mechanism, or if it arises due to mechanical breakage following virus-induced cell damage at the advanced stage of infection. Understanding the origin of this cell-free virus is a prerequisite for understanding the mechanism of RSV transmission in permissive cells. In this study we describe a detailed examination of RSV transmission in permissive HEp2 cell monolayers.

METHODS

HEp2 cell monolayers were infected with RSV using a multiplicity of infection of 0.0002, and the course of infection monitored over 5 days. The progression of the virus infection within the cell monolayers was performed using bright-field microscopy to visualise the cell monolayer and immunofluorescence microscopy to detect virus-infected cells. The cell-associated and cell-free virus infectivity were determined by virus plaque assay, and the virus-induced cell cytotoxicity determined by measuring cell membrane permeability and cellular DNA fragmentation.

RESULTS

At 2 days-post infection (dpi), large clusters of virus-infected cells could be detected indicating localised transmission in the cell monolayer, and during this stage we failed to detect either cell-free virus or cell cytotoxicity. At 3 dpi the presence of much larger infected cell clusters correlated with the begining of virus-induced changes in cell permeability. The presence of cell-free virus correlated with continued increase in cell permeability and cytotoxicity at 4 and 5 dpi. At 5 dpi extensive cell damage, syncytial formation, and increased cellular DNA fragmentation was noted. However, even at 5 dpi the cell-free virus constituted less than 1 % of the total virus infectivity.

CONCLUSIONS

Our data supports a model of RSV transmission that initially involves the localised cell-to-cell spread of virus particles within the HEp2 cell monolayer. However, low levels of cell free-virus infectivity was observed at the advanced stages of infection, which correlated with a general loss in cell monolayer integrity due to virus-induced cytotoxicity.

Direct Inhibition of Cellular Fatty Acid Synthase Impairs Replication of Respiratory Syncytial Virus and Other Respiratory Viruses

Fatty acid synthase (FASN) catalyzes the de novo synthesis of palmitate, a fatty acid utilized for synthesis of more complex fatty acids, plasma membrane structure, and post-translational palmitoylation of host and viral proteins. We have developed a potent inhibitor of FASN (TVB-3166) that reduces the production of respiratory syncytial virus (RSV) progeny in vitro from infected human lung epithelial cells (A549) and in vivo from mice challenged intranasally with RSV. Addition of TVB-3166 to the culture medium of RSV-infected A549 cells reduces viral spread without inducing cytopathic effects. The antiviral effect of the FASN inhibitor is a direct consequence of reducing de novo palmitate synthesis; similar doses are required for both antiviral activity and inhibition of palmitate production, and the addition of exogenous palmitate to TVB-3166-treated cells restores RSV production. TVB-3166 has minimal effect on RSV entry but significantly reduces viral RNA replication, protein levels, viral particle formation and infectivity of released viral particles. TVB-3166 substantially impacts viral replication, reducing production of infectious progeny 250-fold. In vivo, oral administration of TVB-3166 to RSV-A (Long)-infected BALB/c mice on normal chow, starting either on the day of infection or one day post-infection, reduces RSV lung titers 21-fold and 9-fold respectively. Further, TVB-3166 also inhibits the production of RSV B, human parainfluenza 3 (PIV3), and human rhinovirus 16 (HRV16) progeny from A549, HEp2 and HeLa cells respectively. Thus, inhibition of FASN and palmitate synthesis by TVB-3166 significantly reduces RSV progeny both in vitro and in vivo and has broad-spectrum activity against other respiratory viruses. FASN inhibition may alter the composition of regions of the host cell membrane where RSV assembly or replication occurs, or change the membrane composition of RSV progeny particles, decreasing their infectivity.

New insights into viral structure and virus–cell interactions through proteomics

Although genomics techniques such as DNA microarrays have been widely used in virology, much more limited use has been made of proteomics. Although difficult, proteomics can greatly contribute to an understanding of virus-cell interactions, including the ternary structure of viral receptors at the cell surface, post-translational modifications and isoforms of critical viral and cellular proteins and even to the structure of viruses. Proteomics techniques also offer the potential for discovering markers for diagnostic and prognostic tests of viral infections in vivo. This review describes the use of several proteomic approaches for the analysis of HIV-cellular receptor interactions, the molecular mechanisms of transport of herpes simplex virus within neurons, and the structure of the tegument of herpes simplex virus.

Increased hydroxymethylglutaryl coenzyme A reductase activity during respiratory syncytial virus infection mediates actin dependent inter-cellular virus transmission

We have examined the role that hydroxymethylglutaryl coenzyme A reductase (HMGCR) plays during respiratory syncytial virus (RSV) maturation. Imaging analysis indicated that virus-induced changes in F-actin structure correlated with the formation of virus filaments, and that these virus filaments played a direct role in virus cell-to-cell transmission. Treatment with cytochalasin D (CYD) prevented virus filament formation and virus transmission, but this could be reversed by removal of CYD. This observation, together with the presence of F-actin within the virus filaments suggested that newly polymerised F-actin was required for virus transmission. The virus-induced change in F-actin was inhibited by the HMGCR inhibitor lovastatin, and this correlated with the inhibition of both virus filament formation and the incorporation of F-actin in these virus structures. Furthermore, this inhibitory effect on virus filament formation correlated with a significant reduction in RSV transmission. Collectively these data suggested that HMGCR-mediated changes in F-actin structure play an important role in the inter-cellular transmission of mature RSV particles. These data also highlighted the interplay between cellular metabolism and RSV transmission, and demonstrate that this interaction can be targeted using anti-virus strategies.

Molecular mechanisms driving respiratory syncytial virus assembly

Respiratory syncytial virus is a single-stranded RNA virus in the Paramyxoviridae family that preferentially assembles and buds from the apical surface of polarized epithelial cells, forming filamentous structures that contain both viral proteins and the genomic RNA. Recent studies have described both viral and host factors that are involved in ribonucleoprotein assembly and trafficking of viral proteins to the cell surface. At the cell surface, viral proteins assemble into filaments that probably require interactions between viral proteins, host proteins and the cell membrane. Finally, a membrane scission event must occur to release the free virion. This article will review the recent literature describing the mechanisms that drive respiratory syncytial virus assembly and budding.

Lovastatin treatment mitigates the pro-inflammatory cytokine response in respiratory syncytial virus infected macrophage cells

Disease severity following respiratory syncytial virus (RSV) infection is associated with inflammation due to enhanced pro-inflammatory cytokine secretion, and lung macrophage cells play a role in this process. In this study we evaluated the hydroxymethylglutaryl coenzyme A reductase inhibitor lovastatin as an anti-inflammatory drug to control RSV-induced cytokine secretion in the murine RAW 264.7 (RAW) macrophage cell line and in primary murine lung macrophages. These cells could be efficiently infected with RSV in vitro, and although no significant level of infectious virus particles were produced, the increased expression of several virus structural proteins could be detected. Virus infection and gene expression correlated with increased pro-inflammatory cytokine secretion by 24 h post infection. Lovastatin treatment did not reduce the cellular cholesterol levels in RSV-infected cells, nor did it inhibit RSV infection. However, we observed a significant reduction in the pro-inflammatory cytokine levels in lovastatin-treated RSV-infected cells. Since enhanced pro-inflammatory cytokine secretion is a major factor in RSV-associated pathology our findings highlighted the potential use of statins to mitigate and control the inflammatory response due to RSV infection. Furthermore, our study suggested that RAW cells maybe a simple and cost-effective model cell system to screen small molecule libraries to identify compounds that are effective in reducing RSV-induced inflammation.

A systems-based approach to analyse the host response in murine lung macrophages challenged with respiratory syncytial virus

Background

Respiratory syncytial virus (RSV) is an important cause of lower respiratory tract infection in young children. The degree of disease severity is determined by the host response to infection. Lung macrophages play an important early role in the host response to infection and we have used a systems-based approach to examine the host response in RSV-infected lung-derived macrophage cells.

Results

Lung macrophage cells could be efficiently infected (>95%) with RSV in vitro, and the expression of several virus structural proteins could be detected. Although we failed to detect significant levels of virus particle production, virus antigen could be detected up until 96 hours post-infection (hpi). Microarray analysis indicated that 20,086 annotated genes were expressed in the macrophage cells, and RSV infection induced an 8.9% and 11.3% change in the global gene transcriptome at 4 hpi and 24 hpi respectively. Genes showing up-regulated expression were more numerous and exhibited higher changes in expression compared to genes showing down-regulated expression. Based on gene ontology, genes with cytokine, antiviral, cell death, and signal transduction functions showed the highest increases in expression, while signalling transduction, RNA binding and protein kinase genes showed the greatest reduction in expression levels. Analysis of the global gene expression profile using pathway enrichment analysis confirmed that up-regulated expression of pathways related to pathogen recognition, interferon signalling and antigen presentation occurred in the lung macrophage cells challenged with RSV.

Conclusion

Our data provided a comprehensive analysis of RSV-induced gene expression changes in lung macrophages. Although virus gene expression was detected, our data was consistent with an abortive infection and this correlated with the activation of several antivirus signalling pathways such as interferon type I signalling and cell death signalling. RSV infection induced a relatively large increase in pro-inflammatory cytokine expression, however the maintenance of this pro-inflammatory response was not dependent on the production of infectious virus particles. The sustained pro-inflammatory response even in the absence of a productive infection suggests that drugs that control the pro-inflammatory response may be useful in the treatment of patients with severe RSV infection.

Respiratory syncytial virus: virology, reverse genetics, and pathogenesis of disease

Human respiratory syncytial virus (RSV) is an enveloped, nonsegmented negative-strand RNA virus of family Paramyxoviridae. RSV is the most complex member of the family in terms of the number of genes and proteins. It is also relatively divergent and distinct from the prototype members of the family. In the past 30 years, we have seen a tremendous increase in our understanding of the molecular biology of RSV based on a succession of advances involving molecular cloning, reverse genetics, and detailed studies of protein function and structure. Much remains to be learned. RSV disease is complex and variable, and the host and viral factors that determine tropism and disease are poorly understood. RSV is notable for a historic vaccine failure in the 1960s involving a formalin-inactivated vaccine that primed for enhanced disease in RSV naïve recipients. Live vaccine candidates have been shown to be free of this complication. However, development of subunit or other protein-based vaccines for pediatric use is hampered by the possibility of enhanced disease and the difficulty of reliably demonstrating its absence in preclinical studies.

A critical phenylalanine residue in the respiratory syncytial virus fusion protein cytoplasmic tail mediates assembly of internal viral proteins into viral filaments and particles

Respiratory syncytial virus (RSV) is a single-stranded RNA virus in the Paramyxoviridae family that assembles into filamentous structures at the apical surface of polarized epithelial cells. These filaments contain viral genomic RNA and structural proteins, including the fusion (F) protein, matrix (M) protein, nucleoprotein (N), and phosphoprotein (P), while excluding F-actin. It is known that the F protein cytoplasmic tail (FCT) is necessary for filament formation, but the mechanism by which the FCT mediates assembly into filaments is not clear. We hypothesized that the FCT is necessary for interactions with other viral proteins in order to form filaments. In order to test this idea, we expressed the F protein with cytoplasmic tail (CT) truncations or specific point mutations and determined the abilities of these variant F proteins to form filaments independent of viral infection when coexpressed with M, N, and P. Deletion of the terminal three FCT residues (amino acids Phe-Ser-Asn) or mutation of the Phe residue resulted in a loss of filament formation but did not affect F-protein expression or trafficking to the cell surface. Filament formation could be restored by addition of residues Phe-Ser-Asn to an FCT deletion mutant and was unaffected by mutations to Ser or Asn residues. Second, deletion of residues Phe-Ser-Asn or mutation of the Phe residue resulted in a loss of M, N, and P incorporation into virus-like particles. These data suggest that a C-terminal Phe residue in the FCT mediates assembly through incorporation of internal virion proteins into virus filaments at the cell surface.

Respiratory syncytial virus (RSV) is a leading cause of bronchiolitis and pneumonia in infants and the elderly worldwide. There is no licensed RSV vaccine and only limited therapeutics for use in infected patients. Many aspects of the RSV life cycle have been studied, but the mechanisms that drive RSV assembly at the cell surface are not well understood. This study provides evidence that a specific residue in the RSV fusion protein cytoplasmic tail coordinates assembly into viral filaments by mediating the incorporation of internal virion proteins. Understanding the mechanisms that drive RSV assembly could lead to targeted development of novel antiviral drugs. Moreover, since RSV exits infected cells in an ESCRT (endosomal sorting complexes required for transport)-independent manner, these studies may contribute new knowledge about a general strategy by which ESCRT-independent viruses mediate outward bud formation using viral protein-mediated mechanisms during assembly and budding.

Function of membrane rafts in viral lifecycles and host cellular response

Membrane rafts are small (10–200 nm) sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Membrane rafts play an important role in viral infection cycles and viral virulence. Viruses are divided into four main classes, enveloped DNA virus, enveloped RNA virus, nonenveloped DNA virus, and nonenveloped RNA virus. General virus infection cycle is also classified into two sections, the early stage (entry process) and the late stage (assembly, budding, and release processes of virus particles). In the viral cycle, membrane rafts act as a scaffold of many cellular signal transductions, which are associated with symptoms caused by viral infections. In this paper, we describe the functions of membrane rafts in viral lifecycles and host cellular response according to each virus classification, each stage of the virus lifecycle, and each virus-induced signal transduction.

Cholesterol-rich lipid rafts are required for release of infectious human respiratory syncytial virus particles

Cholesterol and sphingolipid enriched lipid raft micro-domains in the plasma membrane play an important role in the life-cycle of numerous enveloped viruses. Although human respiratory syncytial virus (RSV) proteins associate with the raft domains of infected cells and rafts are incorporated in RSV virion particles, the functional role of raft during RSV infection was unknown. In the current study we have identified rafts as an essential component of host cell that is required for RSV infection. Treatment of human lung epithelial cells with raft disrupting agent methyl-beta-cyclodextrin (MBCD) led to drastic loss of RSV infectivity due to diminished release of infectious progeny RSV virion particles from raft disrupted cells. RSV infection of raft deficient Niemann-Pick syndrome type C human fibroblasts and normal human embryonic lung fibroblasts revealed that during productive RSV infection, raft is required for release of infectious RSV particles.

Mass spectrometry based proteomic studies on viruses and hosts--a review

In terms of proteomic research in the 21st century, the realm of virology is still regarded as an enormous challenge mainly brought by three aspects, namely, studying on the complex proteome of the virus with unexpected variations, developing more accurate analytical techniques as well as understanding viral pathogenesis and virus-host interaction dynamics. Progresses in these areas will be helpful to vaccine design and antiviral drugs discovery. Mass spectrometry based proteomics have shown exceptional display of capabilities, not only precisely identifying viral and cellular proteins that are functionally, structurally, and dynamically changed upon virus infection, but also enabling us to detect important pathway proteins. In addition, many isolation and purification techniques and quantitative strategies in conjunction with MS can significantly improve the sensitivity of mass spectrometry for detecting low-abundant proteins, replenishing the stock of virus proteome and enlarging the protein-protein interaction maps. Nevertheless, only a small proportion of the infectious viruses in both of animal and plant have been studied using this approach. As more virus and host genomes are being sequenced, MS-based proteomics is becoming an indispensable tool for virology. In this paper, we provide a brief review of the current technologies and their applications in studying selected viruses and hosts.

A role for caveolin 1 in assembly and budding of the paramyxovirus parainfluenza virus 5

Caveolin 1 (Cav-1) is an integral membrane protein that forms the coat structure of plasma membrane caveolae and regulates caveola-dependent functions. Caveolae are enriched in cholesterol and sphingolipids and are related to lipid rafts. Many studies implicate rafts as sites of assembly and budding of enveloped virus. We show that Cav-1 colocalizes with the paramyxovirus parainfluenza virus 5 (PIV-5) nucleocapsid (NP), matrix (M), and hemagglutinin-neuraminidase (HN) proteins. Moreover, electron microscopy shows that Cav-1 is clustered at sites of viral budding. HN, M, and F(1)/F(2) are associated with detergent-resistant membranes, and these proteins float on sucrose gradients with Cav-1-rich fractions. A complex containing Cav-1 with M, NP, and HN from virus-infected cells and a complex containing Cav-1 and M from M-transfected cells were found on coimmunoprecipitation. A role of Cav-1 in the PIV-5 life cycle was investigated by utilizing MCF-7 human breast cancer cells that stably express Cav-1 (MCF-7/Cav-1). PIV-5 entry into MCF-7 and MCF-7/Cav-1 was found to be Cav-1 independent. However, the interaction between HN and M proteins was dramatically reduced in the Cav-1 null MCF-7 cells, and PIV-5 grown in MCF-7 cells had a reduced infectivity. Similarly, when PIV-5 was grown in MDCK cells that stably expressed dominant negative Cav-1 (MDCK/P132LCav-1), the virus showed a reduced infectivity. Virions lacking Cav-1 were defective and contained high levels of host cellular proteins and reduced levels of HN and M. These data suggest that Cav-1 affects assembly and/or budding, and this is supported by the finding that Cav-1 is incorporated into mature viral particles.

Protein analysis of purified respiratory syncytial virus particles reveals an important role for heat shock protein 90 in virus particle assembly

In this study, we used imaging and proteomics to identify the presence of virus-associated cellular proteins that may play a role in respiratory syncytial virus (RSV) maturation. Fluorescence microscopy of virus-infected cells revealed the presence of virus-induced cytoplasmic inclusion bodies and mature virus particles, the latter appearing as virus filaments. In situ electron tomography suggested that the virus filaments were complex structures that were able to package multiple copies of the virus genome. The virus particles were purified, and the protein content was analyzed by one-dimensional nano-LC MS/MS. In addition to all the major virus structural proteins, 25 cellular proteins were also detected, including proteins associated with the cortical actin network, energy pathways, and heat shock proteins (HSP70, HSC70, and HSP90). Representative actin-associated proteins, HSC70, and HSP90 were selected for further biological validation. The presence of beta-actin, filamin-1, cofilin-1, HSC70, and HSP90 in the virus preparation was confirmed by immunoblotting using relevant antibodies. Immunofluorescence microscopy of infected cells stained with antibodies against relevant virus and cellular proteins confirmed the presence of these cellular proteins in the virus filaments and inclusion bodies. The relevance of HSP90 to virus infection was examined using the specific inhibitors 17-N-Allylamino-17-demethoxygeldanamycin. Although virus protein expression was largely unaffected by these drugs, we noted that the formation of virus particles was inhibited, and virus transmission was impaired, suggesting an important role for HSP90 in virus maturation. This study highlights the utility of proteomics in facilitating both our understanding of the role that cellular proteins play during RSV maturation and, by extrapolation, the identification of new potential targets for antiviral therapy.

Lipids and membrane microdomains in HIV-1 replication

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.

Differential regulation of GM1 and asialo-GM1 expression by T cells and natural killer (NK) cells in respiratory syncytial virus infection

We previously reported that respiratory syncytial virus (RSV) infection increases lung CD8(+) T cell GM1 expression. The related lipid asialo-GM1 (ASGM1) is expressed by T cells in viral infection and by natural killer (NK) cells. The in vivo co-expression of GM1 and ASGM1 by immune cells is not defined. Here we analyzed lung lymphocyte GM1 and ASGM1 expression in RSV-infected mice. GM1 and ASGM1 were coordinately upregulated by activated CD8(+) T cells in RSV-infected BALB/c and C57BL/6 mice. In contrast, RSV infection had no effect on constitutively high NK cell GM1 expression, while increasing NK cell ASGM1 expression. GM1 and ASGM1 co-localized in lipid raft structures in NK and CD8(+) T cells sorted from the lungs of RSV-infected mice. Anti-ASGM1 Ab treatment of RSV-infected BALB/c mice depleted GM1/ASGM1-expressing NK cells and GM1/ASGM1-expressing T cells, reduced lung IFN-gamma levels, increased viral load, delayed viral clearance, and reduced illness. STAT1(-/-) mice are more susceptible to RSV replication and disease than wild-type mice. In RSV-infected STAT1(-/-) mice, anti-ASGM1 Ab altered cytokine levels, but in contrast to BALB/c mice, antibody treatment had no effect on viral load or illness. Taken together, GM1 and ASGM1 expression are differentially regulated by T and NK cells in RSV infection. Also, GM1/ASGM1-expressing cells are important for control of RSV in BALB/c mice, whereas STAT1(-/-) mice clear RSV by an alternative pathway.

Human respiratory syncytial virus glycoproteins are not required for apical targeting and release from polarized epithelial cells

Human respiratory syncytial virus (HRSV) is released from the apical membrane of polarized epithelial cells. However, little is known about the processes of assembly and release of HRSV and which viral gene products are involved in the directional maturation of the virus. Based on previous studies showing that the fusion (F) glycoprotein contained an intrinsic apical sorting signal and that N- and O-linked glycans can act as apical targeting signals, we investigated whether the glycoproteins of HRSV were involved in its directional targeting and release. We generated recombinant viruses with each of the three glycoprotein genes deleted individually or in groups. Each deleted gene was replaced with a reporter gene to maintain wild-type levels of gene expression. The effects of deleting the glycoprotein genes on apical maturation and on targeting of individual proteins in polarized epithelial cells were examined by using biological, biochemical, and microscopic assays. The results of these studies showed that the HRSV glycoproteins are not required for apical maturation or release of the virus. Further, deletion of one or more of the glycoprotein genes did not affect the intracellular targeting of the remaining viral glycoproteins or the nucleocapsid protein to the apical membrane.

Intracellular localization of human respiratory syncytial virus L protein

Replication and transcription of the human respiratory syncytial virus (hRSV) genome is carried out by the ribonucleocapsid complex (RNA together with N, P, M2-1 and L proteins), with the L protein being responsible for all enzymatic activities. In the present study, we obtained anti-L polyclonal sera in mice. These antibodies were functional in immunofluorescence and Western blotting assays in hRSV-infected HEp-2 cells. In the immunofluorescence assays, we detected inclusion bodies in the anti-L staining, similar to the ones seen by anti-N or anti-P staining. The results presented here provide the first evidence of the intracellular localization of the hRSV L protein.

Nonstructural proteins of respiratory syncytial virus suppress premature apoptosis by an NF-kappaB-dependent, interferon-independent mechanism and facilitate virus growth

The two nonstructural (NS) proteins NS1 and NS2 of respiratory syncytial virus (RSV) are abundantly expressed in the infected cell but are not packaged in mature progeny virions. We found that both proteins were expressed early in infection, whereas the infected cells underwent apoptosis much later. Coincident with NS protein expression, a number of cellular antiapoptotic factors were expressed or activated at early stages, which included NF-kappaB and phosphorylated forms of protein kinases AKT, phosphoinositide-dependent protein kinase, and glycogen synthase kinase. Using specific short interfering RNAs (siRNAs), we achieved significant knockdown of one or both NS proteins in the infected cell, which resulted in abrogation of the antiapoptotic functions and led to early apoptosis. NS-dependent suppression of apoptosis was observed in Vero cells that are naturally devoid of type I interferons (IFN). The siRNA-based results were confirmed by the use of NS-deleted RSV mutants. Early activation of epidermal growth factor receptor (EGFR) in the RSV-infected cell did not require NS proteins. Premature apoptosis triggered by the loss of NS or by apoptosis-promoting drugs caused a severe reduction of RSV growth. Finally, recombinantly expressed NS1 and NS2, individually and together, reduced apoptosis by tumor necrosis factor alpha, suggesting an intrinsic antiapoptotic property of both. We conclude that the early-expressed nonstructural proteins of RSV boost viral replication by delaying the apoptosis of the infected cell via a novel IFN- and EGFR-independent pathway.

Respiratory syncytial virus F envelope protein associates with lipid rafts without a requirement for other virus proteins

Like many enveloped viruses, human respiratory syncytial virus (RSV) assembles at and buds from lipid rafts. Translocation of the envelope proteins to these membrane subdomains is essential for production of infectious virus, but the targeting mechanism is poorly understood and it is not known if other virus proteins are required. Here we demonstrate that F protein of RSV intrinsically targets to lipid rafts without a requirement for any other virus protein, including the SH and G envelope proteins. Recombinant virus deficient in SH and G but retaining F protein expression was used to demonstrate that F protein still localized in rafts in both A549 and HEp-2 cells. Expression of a recombinant F gene by use of plasmid vectors demonstrated that F contains its own targeting domain and localized to rafts in the absence of other virus proteins. The domain responsible for translocation was then mapped. Unlike most other virus envelope proteins, F is unusual since the target signal is not contained within the cytoplasmic domain nor did it involve fatty acid modified residues. Furthermore, exchange of the transmembrane domain with that of the vesicular stomatitis virus G protein, a nonraft protein, did not alter F protein raft localization. Taken together, these data suggest that domains present in the extracellular portion of the protein are responsible for lipid raft targeting of the RSV F protein.

Virus infection and lipid rafts

Virus entry, assembly, and budding are important processes in the replication cycle of a virus. Viruses are dependent on host living cells for their replication. Viruses use the proliferative mechanism of host cells for replication of viral components. Lipid rafts, specific membrane microdomains play a critical role in virus replication because localizing and concentrating viral components in the microdomains for entry, assembly, and budding of various types of virus. In this review, we describe the involvement of membrane lipid rafts in the virus replication cycle with our current findings for understanding the role of membrane lipid rafts in virus infection.

The cytoplasmic tail of the human respiratory syncytial virus F protein plays critical roles in cellular localization of the F protein and infectious progeny production

The importance of the F protein cytoplasmic tail (CT) for replication of human respiratory syncytial virus (HRSV) was examined by monitoring the behavior of viruses expressing F proteins with a modified COOH terminus. The F protein mutant viruses were recovered and amplified under conditions where F protein function was complemented by expression of a heterologous viral envelope protein. The effect of the F protein modifications was then examined in the context of a viral infection in standard cell types (Vero and HEp-2). The F protein modifications consisted of a deletion of the predicted CT or a replacement of the CT with the CT of the vesicular stomatitis virus (VSV) G protein. In addition, engineered HRSVs that lacked all homologous glycoprotein genes (SH, G, and F) and expressed instead either the authentic VSV G protein or a VSV G containing the HRSV F protein CT were examined. We found that deletion or replacement of the F protein CT seriously impaired the production of infectious progeny. Cells infected with viruses bearing CT modifications displayed increased F protein surface expression and increased syncytium formation. The distribution of F protein in the plasma membrane of infected cells was altered, resulting in an F protein that was evenly distributed rather than localized predominantly to virus-induced surface filaments. CT deletion or exchange also abrogated interaction of F protein with Triton-insoluble lipid rafts. Addition of the F protein CT to the VSV G protein, expressed as the only viral glycoprotein in an HRSV genome, had the opposite effects: the number of infectious progeny was higher, the surface distribution was changed from relatively even to localized, and the proportion of VSV G protein associated with lipid rafts was higher. Together, these results show that the HRSV F protein CT plays a critical role in F protein cellular localization and production of infectious virus and suggest that the function provided by the CT is independent of the F protein ectodomain and transmembrane domain and is mediated by F protein-lipid raft interaction.

Unravelling the complexities of respiratory syncytial virus RNA synthesis

Human respiratory syncytial virus (RSV) is the leading cause of paediatric respiratory disease and is the focus of antiviral- and vaccine-development programmes. These goals have been aided by an understanding of the virus genome architecture and the mechanisms by which it is expressed and replicated. RSV is a member of the order Mononegavirales and, as such, has a genome consisting of a single strand of negative-sense RNA. At first glance, transcription and genome replication appear straightforward, requiring self-contained promoter regions at the 3' ends of the genome and antigenome RNAs, short cis-acting elements flanking each of the genes and one polymerase. However, from these minimal elements, the virus is able to generate an array of capped, methylated and polyadenylated mRNAs and encapsidated antigenome and genome RNAs, all in the appropriate ratios to facilitate virus replication. The apparent simplicity of genome expression and replication is a consequence of considerable complexity in the polymerase structure and its cognate cis-acting sequences; here, our understanding of mechanisms by which the RSV polymerase proteins interact with signals in the RNA template to produce different RNA products is reviewed.

Evidence that maturation of the N-linked glycans of the respiratory syncytial virus (RSV) glycoproteins is required for virus-mediated cell fusion: The effect of alpha-mannosidase inhibitors on RSV infectivity

Glycan heterogeneity of the respiratory syncytial virus (RSV) fusion (F) protein was demonstrated by proteomics. The effect of maturation of the virus glycoproteins-associated glycans on virus infectivity was therefore examined using the alpha-mannosidase inhibitors deoxymannojirimycin (DMJ) and swainsonine (SW). In the presence of SW the N-linked glycans on the F protein appeared in a partially mature form, whereas in the presence of DMJ no maturation of the glycans was observed. Neither inhibitor had a significant effect on G protein processing or on the formation of progeny virus. Although the level of infectious virus and syncytia formation was not significantly affected by SW-treatment, DMJ-treatment correlated with a one hundred-fold reduction in virus infectivity. Our data suggest that glycan maturation of the RSV glycoproteins, in particular those on the F protein, is an important step in virus maturation and is required for virus infectivity.

Mass spectrometric sequencing and acylation character analysis of C-terminal anchoring segment from Influenza A hemagglutinin

Influenza A virus hemagglutinin (HA) is a major envelope glycoprotein mediating viral and cell membrane fusion. HA is anchored in the viral envelope by a light HA(2) chain containing one transmembrane domain and a cytoplasmic tail. Three cysteine residues in the C-terminal region, one in the transmembrane domain and two in the cytoplasmic tail, are highly conserved and potentially palmitoylated in all HA subtypes. The HA(2) C- terminal anchoring segments were extracted to organic phase from the bromelain-digested viruses (subviral particles) of three strains: A/X-31 (H3 subtype), A/Puerto Rico/8/34 (H1 subtype) and A/FPV/Weybridge/34 (H7 subtype). Their primary structures were assessed by matrix-assisted laser desorption/ionization time-of-flight time-of- flight mass spectrometry (MALDI-ToF-ToF MS). Trypsin-type protease-cleaved peptides prevailed over bromelain- cleaved ones in the peptide mixtures. All of them included transmembrane domains. Several distinctive features of the C-terminal HA(2) peptides acylation character were discovered by MALDI-ToF MS: 1) the peptides isolated from the viruses, which were digested by bromelain in the absence of beta-mercaptoethanol, were predominantly triply acylated; 2) the peptides were acylated not only by palmitic, but also by stearic acid residues; 3) the palmitate/stearate ratio was different for the three strains studied; 4) the A/FPV/Weybridge/34 strain has a priority to stearate binding. This fatty acid residue was discovered at the first of three conservative cysteine residues located in the transmembrane domain. It was found that presence of thiol reagent during preparation of subviral particles led to the appearence of the C-terminal HA(2) peptides acylated to different degrees. Triply, doubly, mono- and even unacylated peptides were detected. It was demonstrated that the thioester bond in the isolated acylpeptides was extremely sensitive to thiol reagents.

Evidence for an association between heat shock protein 70 and the respiratory syncytial virus polymerase complex within lipid-raft membranes during virus infection

In this report, the interaction between respiratory syncytial virus (RSV) and heat shock protein 70 (HSP70) was examined. Although no significant increase in total HSP70 protein levels was observed during virus infection, analysis of the HSP70 content in lipid-raft membranes from mock- and virus-infected cells revealed an increase in the levels of raft-associated HSP70 during virus infection. Fluorescence microscopy demonstrated that this transport of HSP70 into lipid-raft membranes correlated with the appearance of HSP70 within virus-induced inclusion bodies. Furthermore, co-localisation of HSP70 with the virus N protein and the raft lipid GM1 was observed within these structures. Immunoprecipitation experiments demonstrated the ability of HSP70 to interact with the virus polymerase complex in lipid-rafts in an ATP-dependent manner. Collectively, these data suggest that RSV may induce cellular changes which allow the recruitment of specific host-cell factors, via lipid-raft membranes, to the polymerase complex.