Vesicular stomatitis virus glycoprotein mutations that affect membrane fusion activity and abolish virus infectivity

A Cell Membrane Fusion Assay for the Fish Pathogen Spring Viremia of Carp Virus (SVCV)

Fusion of the infected cell membranes is a characteristic effect of a wide number of viral infections. Spring viremia of carp virus (SVCV), a rhabdovirus causing disease in farmed carp, can induce membrane fusion of the infected cells by shifting the pH of the culture medium to slightly acidic. Membrane fusion leads to the formation of clusters of cell nuclei enclosed in a cell membrane, the so-called syncytia, that can be easily visualized by cell staining and light microscope inspection. In the present work, we report a protocol to induce syncytia formation in EPC cells infected with SVCV, where membrane fusion is triggered by a low-pH incubation step. Appearance of syncytia can be observed at 18 hours post infection. The syncytia formation assay described here may serve as an experimental platform to quantitate SVCV, to determine virus infectivity, and a useful tool to study virus entry into the cell as well as to test candidate antiviral compounds that could block the entry of SVCV into cells by inhibiting membrane fusion.

The interaction of viral fusion peptides with lipid membranes

In this paper, we studied fusogenic peptides of class I-III fusion proteins, which are relevant to membrane fusion for certain enveloped viruses, in contact with model lipid membranes. We resolved the vertical structure and examined the adsorption or penetration behavior of the fusogenic peptides at phospholipid Langmuir monolayers with different initial surface pressures with x-ray reflectometry. We show that the fusion loops of tick-borne encephalitis virus (TBEV) glycoprotein E and vesicular stomatitis virus (VSV) G-protein are not able to insert deeply into model lipid membranes, as they adsorbed mainly underneath the headgroups with only limited penetration depths into the lipid films. In contrast, we observed that the hemagglutinin 2 fusion peptide (HA2-FP) and the VSV-transmembrane domain (VSV-TMD) can penetrate deeply into the membranes. However, in the case of VSV-TMD, the penetration was suppressed already at low surface pressures, whereas HA2-FP was able to insert even into highly compressed films. Membrane fusion is accompanied by drastic changes of the membrane curvature. To investigate how the peptides affect the curvature of model lipid membranes, we examined the effect of the fusogenic peptides on the equilibration of cubic monoolein structures after a phase transition from a lamellar state induced by an abrupt hydrostatic pressure reduction. We monitored this process in presence and absence of the peptides with small-angle x-ray scattering and found that HA2-FP and VSV-TMD drastically accelerate the equilibration, while the fusion loops of TBEV and VSV stabilize the swollen state of the lipid structures. In this work, we show that the class I fusion peptide of HA2 penetrates deeply into the hydrophobic region of membranes and is able to promote and accelerate the formation of negative curvature. In contrast, we found that the class II and III fusion loops of TBEV and VSV tend to counteract negative membrane curvature.

Emerging Biomedical Applications of the Vesicular Stomatitis Virus Glycoprotein

Nanoparticles (NPs) made of metals, polymers, micelles, and liposomes are increasingly being used in various biomedical applications. However, most of these NPs are hazardous for long- and short-term use and hence have restricted biomedical applications. Therefore, naturally derived, biocompatible, and biodegradable nanoconstructs are being explored for such applications. Inspired by the biology of viruses, researchers are exploring the viral proteins that hold considerable promise in biomedical applications. The viral proteins are highly stable and further amenable to suit specific biological applications. Among various viral proteins, vesicular stomatitis virus glycoprotein (VSV-G) has emerged as one of the most versatile platforms for biomedical applications. Starting with their first major use in lentivirus/retrovirus packaging systems, the VSV-G-based reagents have been tested for diverse biomedical use, many of which are at various stages of clinical trials. This manuscript discusses the recent advancements in the use of the VSV-G-based reagents in medical, biological research, and clinical applications particularly highlighting emerging applications in biomedical imaging.

Engineering of Extracellular Vesicles for Small Molecule-Regulated Cargo Loading and Cytoplasmic Delivery of Bioactive Proteins

Cytoplasmic delivery of functional proteins into target cells remains challenging for many biological agents to exert their therapeutic effects. Extracellular vesicles (EVs) are expected to be a promising platform for protein delivery; however, efficient loading of proteins of interest (POIs) into EVs remains elusive. In this study, we utilized small compound-induced heterodimerization between FK506 binding protein (FKBP) and FKBP12-rapamycin-binding (FRB) domain to sort bioactive proteins into EVs using the FRB-FKBP system. When CD81, a typical EV marker protein, and POI were fused with FKBP and FRB, respectively, rapamycin induced the binding of these proteins through the FKBP-FRB interaction and recruited the POIs into EVs. The released EVs, displaying the virus-derived membrane fusion protein, delivered the POI cargo into recipient cells and their functionality in the recipient cells was confirmed. Furthermore, we demonstrated that CD81 could be replaced with other EV-enriched proteins, such as CD63 or HIV Gag. Thus, the FRB-FKBP system enables the delivery of functional proteins and paves the way for EV-based protein delivery platforms.

Reporter gene assay for membrane fusion of extracellular vesicles

Extracellular vesicles (EVs) secreted by living cells are expected to deliver biological cargo molecules, including RNA and proteins, to the cytoplasm of recipient cells. There is an increasing need to understand the mechanism of intercellular cargo delivery by EVs. However, the lack of a feasible bioassay has hampered our understanding of the biological processes of EV uptake, membrane fusion, and cargo delivery to recipient cells. Here, we describe a reporter gene assay that can measure the membrane fusion efficiency of EVs during cargo delivery to recipient cells. When EVs containing tetracycline transactivator (tTA)-fused tetraspanins are internalized by recipient cells and fuse with cell membranes, the tTA domain is exposed to the cytoplasm and cleaved by tobacco etch virus protease to induce tetracycline responsive element (TRE)-mediated reporter gene expression in recipient cells. This assay (designated as EV-mediated tetraspanin-tTA delivery assay, ETTD assay), enabled us to assess the cytoplasmic cargo delivery efficiency of EVs in recipient cells. With the help of a vesicular stomatitis virus-derived membrane fusion protein, the ETTD assay could detect significant enhancement of cargo delivery efficiency of EVs. Furthermore, the ETTD assay could evaluate the effect of potential cargo delivery enhancers/inhibitors. Thus, the ETTD assay may contribute to a better understanding of the underlying mechanism of the cytoplasmic cargo delivery by EVs.

Spike protein fusion loop controls SARS-CoV-2 fusogenicity and infectivity

The high SARS-CoV-2 reproductive number driving the COVID-19 pandemic has been a mystery. Our recent in vitro, and in vivo coronaviral pathogenesis studies involving Mouse Hepatitis Virus (MHV-A59) suggest a crucial role for a small host membrane-virus contact initiator region of the Spike protein, called the fusion peptide that enhances the virus fusogenicity and infectivity. Here I study the Spike from five human β-coronaviruses (HCoV) including the SARS-CoV-2, and MHV-A59 for comparison. The structural and dynamics analyses of the Spike show that its fusion loop spatially organizes three fusion peptides contiguous to each other to synergistically trigger the virus-host membrane fusion process. I propose a Contact Initiation Model based on the architecture of the Spike quaternary structure that explains the obligatory participation of the fusion loop in the initiation of the host membrane contact for the virus fusion process. Among all the HCoV Spikes in this study, SARS-CoV-2 has the most hydrophobic surface and the extent of hydrophobicity correlates with the reproductive number and infectivity of the other HCoV. Comparison between results from standard and replica exchange molecular dynamics reveal the unique physicochemical properties of the SARS-CoV-2 fusion peptides, accrued in part from the presence of consecutive prolines that impart backbone rigidity which aids the virus fusogenicity. The priming of the Spike by its cleavage and subsequent fusogenic conformational transition steered by the fusion loop may be critical for the SARS-CoV-2 spread. The importance of the fusion loop makes it an apt target for anti-virals and vaccine candidates.

Real-Time Luminescence Assay for Cytoplasmic Cargo Delivery of Extracellular Vesicles

Extracellular vesicles (EVs) have been considered to deliver biological cargos between cells and mediate intercellular communication and potential drug delivery carriers. However, the mechanisms that underlie the biological process of EV uptake and cytoplasmic cargo release in recipient cells are largely unknown. Quantitative and real-time assays for the assessment of cargo delivery efficiency inside recipient cells have not been feasible. In this study, we developed an EV cargo delivery (EVCD) assay using a split luciferase called a NanoBiT system. Recipient cells expressing LgBiT, a large subunit of luciferase, emit luminescence when EV cargo proteins fused with a small luminescence tag (HiBiT tag) that can complement LgBiT are delivered to the cytoplasm of recipient cells. Using the EVCD assay, the cargo delivery efficiency of EVs could be quantitatively measured in real time. This assay was highly sensitive in detecting a single event of cargo delivery per cell. We found that modification of EVs with a virus-derived fusogenic protein significantly enhanced the cytoplasmic cargo delivery; however, in the absence of a fusogenic protein, the cargo delivery efficiency of EVs was below the threshold of the assay. The EVCD assay could assess the effect of entry inhibitors on EV cargo delivery. Furthermore, using a luminescence microscope, the cytoplasmic cargo delivery of EVs was directly visualized in living cells. This assay could reveal the biological mechanism of the cargo delivery processes of EVs.

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral 'quasispecies'. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

Spike protein fusion loop controls SARS-CoV-2 fusogenicity and infectivity

The high SARS-CoV-2 reproductive number driving the COVID-19 pandemic has been a mystery. Our recent in vitro, and in vivo coronaviral pathogenesis studies involving Mouse Hepatitis Virus (MHV-A59) suggest a crucial role for a small host membrane-virus contact initiator region of the Spike protein, called the fusion peptide that enhances the virus fusogenicity and infectivity. Here I study the Spike from five human β-coronaviruses (HCoV) including the SARS-CoV-2, and MHV-A59 for comparison. The structural and dynamics analyses of the Spike show that its fusion loop spatially organizes three fusion peptides contiguous to each other to synergistically trigger the virus-host membrane fusion process. I propose a Contact Initiation Model based on the architecture of the Spike quaternary structure that explains the obligatory participation of the fusion loop in the initiation of the host membrane contact for the virus fusion process. Among all the HCoV Spikes in this study, SARS-CoV-2 has the most hydrophobic surface and the extent of hydrophobicity correlates with the reproductive number and infectivity of the other HCoV. Comparison between results from standard and replica exchange molecular dynamics reveal the unique physicochemical properties of the SARS-CoV-2 fusion peptides, accrued in part from the presence of consecutive prolines that impart backbone rigidity which aids the virus fusogenicity. The priming of the Spike by its cleavage and subsequent fusogenic conformational transition steered by the fusion loop may be critical for the SARS-CoV-2 spread. The importance of the fusion loop makes it an apt target for anti-virals and vaccine candidates.

A tool with many applications: vesicular stomatitis virus in research and medicine

INTRODUCTION

Vesicular stomatitis virus (VSV) has long been a useful research tool in virology and recently become an essential part of medicinal products. Vesiculovirus research is growing quickly following its adaptation to clinical gene and cell therapy and oncolytic virotherapy.

AREAS COVERED

This article reviews the versatility of VSV as a research tool and biological reagent, its use as a viral and vaccine vector delivering therapeutic and immunogenic transgenes and an oncolytic virus aiding cancer treatment. Challenges such as the immune response against such advanced therapeutic medicinal products and manufacturing constraints are also discussed.

EXPERT OPINION

The field of in vivo gene and cell therapy is advancing rapidly with VSV used in many ways. Comparison of VSV's use as a versatile therapeutic reagent unveils further prospects and problems for each application. Overcoming immunological challenges to aid repeated administration of viral vectors and minimizing harmful host-vector interactions remains one of the major challenges. In the future, exploitation of reverse genetic tools may assist the creation of recombinant viral variants that have improved onco-selectivity and more efficient vaccine vector activity. This will add to the preferential features of VSV as an excellent advanced therapy medicinal product (ATMP) platform.

Single amino acid change at position 255 in rabies virus glycoprotein decreases viral pathogenicity

Previous studies have indicated that the amino acid at position 333 in the glycoprotein (G) is closely related to rabies virus (RABV) pathogenicity. However, whether there are other amino acid residues in G that relate to pathogenicity remain unclear. The aim of this study is to find new amino acid residues in G that could strongly reduce RABV pathogenicity. The present study found that the pathogenicity of a virulent strain was strongly attenuated when the amino acid glycine (Gly) replaced the aspartic acid (Asp) at position 255 in G (D255G) as intracranial (i.c.) infection with this D255G mutant virus did not cause death in adult mice. The indexes of neurotropism of the D255G mutant strain and the parent GD-SH-01 are 0.72 and 10.0, respectively, which indicate that the D255G mutation decreased the neurotropism of RABV. In addition, the D255G mutation significantly decreased RABV replication in the mouse brain. Furthermore, the D255G mutation enhanced the immune response in mice, which contributed to the clearance of RABV after infection. The Asp255 → Gly255 mutation was genetically stable in vitro and in vivo. In this study, we describe a new referenced amino acid site in G that relates to the pathogenicity of RABV.

A proline insertion-deletion in the spike glycoprotein fusion peptide of mouse hepatitis virus strongly alters neuropathology

Fusion peptides (FPs) in spike proteins are key players mediating early events in cell-to-cell fusion, vital for intercellular viral spread. A proline residue located at the central FP region has often been suggested to have a distinctive role in this fusion event. The spike glycoprotein from strain RSA59 (PP) of mouse hepatitis virus (MHV) contains two central, consecutive prolines in the FP. Here, we report that deletion of one of these proline residues, resulting in RSA59 (P), significantly affected neural cell syncytia formation and viral titers postinfection in vitro Transcranial inoculation of C57Bl/6 mice with RSA59 (PP) or RSA59 (P) yielded similar degrees of necrotizing hepatitis and meningitis, but only RSA59 (PP) produced widespread encephalitis that extended deeply into the brain parenchyma. By day 6 postinfection, both virus variants were mostly cleared from the brain. Interestingly, inoculation with the RSA59 (P)-carrying MHV significantly reduced demyelination at the chronic stage. We also found that the presence of two consecutive prolines in FP promotes a more ordered, compact, and rigid structure in the spike protein. These effects on FP structure were due to proline's unique stereochemical properties intrinsic to its secondary amino acid structure, revealed by molecular dynamics and NMR experiments. We therefore propose that the differences in the severity of encephalitis and demyelination between RSA59 (PP) and RSA59 (P) arise from the presence or absence, respectively, of the two consecutive prolines in FP. Our studies define a structural determinant of MHV entry in the brain parenchyma important for altered neuropathogenesis.

Competitive Luminex immunoassays for detection of antibodies to foot-and-mouth disease and vesicular stomatitis viruses in multiple susceptible hosts

Foot-and-mouth disease (FMD) and vesicular stomatitis (VS) cause such similar clinical signs and lesions that laboratory tests are required to distinguish between infections caused by each virus. Using mouse anti-foot-and-mouth disease virus (FMDV) 3B monoclonal or polyclonal anti-vesicular stomatitis virus-New Jersey (VSV-NJ) antibodies and recombinant FMDV 3ABC or VSV-NJ glycoprotein (G) antigens coated to MagPlex beads, competitive Luminex immunoassays (cLIAs) were developed for FMDV and VSV-NJ, respectively. The cLIAs successfully detected antibodies to FMDV 3ABC and VSV-NJ G in sera from infected animals. The diagnostic sensitivity and specificity were 93% and 98%, respectively for FMDV and 93% and 95.4%, respectively for VSV-NJ. These cLIAs are potential alternatives for competitive enzyme-linked immunosorbent assays (cELISAs) and provide the opportunity for multiplexing to reduce time and the amount of serum required for testing.

Environmental Stress Causes Lethal Neuro-Trauma during Asymptomatic Viral Infections

Asymptomatic infections often proceed undetected, yet can still prime the host to be sensitive to secondary environmental stress. While the mechanisms underlying disease caused by asymptomatic infections are unknown, it is believed that productive pathogen replication is required. We report that the environmental stress of carbon dioxide (CO2) anesthesia converts an asymptomatic rhabdovirus infection in Drosophila to one that is lethal. This lethality results from a pool of infectious virus in glial cells and is regulated by the antiviral RNAi pathway of the host. CO2 sensitivity is caused by the fusogenic activity of the viral glycoprotein, which results in fusion of neurons and glia. Expression of the viral glycoprotein, but not a fusion defective mutant, is sufficient to cause CO2 sensitivity, which can occur even in the absence of productive viral replication. These findings highlight how viral proteins, independent of pathogen replication, may predispose hosts to life-threatening environmental stress.

Structural intermediates in the fusion-associated transition of vesiculovirus glycoprotein

Vesiculoviruses enter cells by membrane fusion, driven by a large, low-pH-induced, conformational change in the fusion glycoprotein G that involves transition from a trimeric pre-fusion toward a trimeric post-fusion state via monomeric intermediates. Here, we present the structure of the G fusion protein at intermediate pH for two vesiculoviruses, vesicular stomatitis virus (VSV) and Chandipura virus (CHAV), which is responsible for deadly encephalopathies. First, a CHAV G crystal structure shows two intermediate conformations forming a flat dimer of heterodimers. On virions, electron microscopy (EM) and tomography reveal monomeric spikes similar to one of the crystal conformations. In solution, mass spectrometry shows dimers of G. Finally, mutations at a dimer interface, involving fusion domains associated in an antiparallel manner to form an intermolecular β-sheet, affect G fusion properties. The location of the compensatory mutations restoring fusion activity strongly suggests that this interface is functionally relevant. This work reveals the range of G structural changes and suggests that G monomers can re-associate, through antiparallel interactions between fusion domains, into dimers that play a role at some early stage of the fusion process.

Characterization of chicken interferon-inducible transmembrane protein-10

Interferon-inducible transmembrane protein (IFITM) family proteins are antivirus factors. In the present study, we examined the expression pattern of chicken IFITM10 using quantitative reverse transcription-polymerase chain reaction. In adult chickens, IFITM10 levels were markedly lower than those of IFITM3, which exhibits antivirus activity. On the other hand, IFITM10 was expressed in levels similar to those of IFITM3 in embryonic organs. Primordial germ cells in 2.5-d embryos expressed high levels of IFITM10, which gradually decreased with time. The interferon-α stimulation of embryonic fibroblast cells did not enhance the expression of IFITM10. The forced expression of IFITM10 slightly inhibited the infectivity of the VSV-G-pseudotyped lentiviral vector. Furthermore, cell fusion was inhibited by IFITM10 when HeLa cells transfected with the VSV-G expression vector were treated with low pH buffer. Although it remains unclear whether IFITM10 inhibits viral infections under physiological conditions, these results suggest that chicken IFITM10 exhibits antivirus activity.

Designed proteins induce the formation of nanocage-containing extracellular vesicles

Complex biological processes are often performed by self-organizing nanostructures comprising multiple classes of macromolecules, such as ribosomes (proteins and RNA) or enveloped viruses (proteins, nucleic acids and lipids). Approaches have been developed for designing self-assembling structures consisting of either nucleic acids or proteins, but strategies for engineering hybrid biological materials are only beginning to emerge. Here we describe the design of self-assembling protein nanocages that direct their own release from human cells inside small vesicles in a manner that resembles some viruses. We refer to these hybrid biomaterials as 'enveloped protein nanocages' (EPNs). Robust EPN biogenesis requires protein sequence elements that encode three distinct functions: membrane binding, self-assembly, and recruitment of the endosomal sorting complexes required for transport (ESCRT) machinery. A variety of synthetic proteins with these functional elements induce EPN biogenesis, highlighting the modularity and generality of the design strategy. Biochemical analyses and cryo-electron microscopy reveal that one design, EPN-01, comprises small (~100 nm) vesicles containing multiple protein nanocages that closely match the structure of the designed 60-subunit self-assembling scaffold. EPNs that incorporate the vesicular stomatitis viral glycoprotein can fuse with target cells and deliver their contents, thereby transferring cargoes from one cell to another. These results show how proteins can be programmed to direct the formation of hybrid biological materials that perform complex tasks, and establish EPNs as a class of designed, modular, genetically-encoded nanomaterials that can transfer molecules between cells.

Single-Vector, Single-Injection Recombinant Vesicular Stomatitis Virus Vaccines Against High-Containment Viruses

There are many avenues for making an effective vaccine against viruses. Depending on the virus these can include one of the following: inactivation of whole virions; attenuation of viruses; recombinant viral proteins; non-replication-competent virus particles; or surrogate virus vector systems such as vesicular stomatitis virus (VSV). VSV is a prototypic enveloped animal virus that has been used for over four decades to study virus replication, entry, and assembly due to its ability to replicate to high titers in a wide variety of mammalian and insect cells. The use of reverse genetics to recover infectious and single-cycle replicating VSV from plasmid DNA transfected in cell culture began a revolution in the study of recombinant VSV (rVSV). This platform can be manipulated to study the viral genetic sequences and proteins important in the virus life cycle. Additionally, foreign genes can be inserted between naturally occurring or generated start/stop signals and polyadenylation sites within the VSV genome. VSV has a tolerance for foreign gene expression which has led to numerous rVSVs reported in the literature. Of particular interest are the very effective single-dose rVSV vaccine vectors against high-containment viruses such as filoviruses, henipaviruses, and arenaviruses. Herein we describe the methods for selecting foreign antigenic genes, selecting the location within the VSV genome for insertion, generation of rVSV using reverse genetics, and proper vaccine study designs.

Directed Fusion of Mesenchymal Stem Cells with Cardiomyocytes via VSV-G Facilitates Stem Cell Programming

Mesenchymal stem cells (MSCs) spontaneously fuse with somatic cells in vivo, albeit rarely, and the fusion products are capable of tissue-specific function (mature trait) or proliferation (immature trait), depending on the microenvironment. That stem cells can be programmed, or somatic cells reprogrammed, in this fashion suggests that stem cell fusion holds promise as a therapeutic approach for the repair of damaged tissues, especially tissues not readily capable of functional regeneration, such as the myocardium. In an attempt to increase the frequency of stem cell fusion and, in so doing, increase the potential for cardiac tissue repair, we expressed the fusogen of the vesicular stomatitis virus (VSV-G) in human MSCs. We found VSV-G expressing MSCs (vMSCs) fused with cardiomyocytes (CMs) and these fusion products adopted a CM-like phenotype and morphology in vitro. In vivo, vMSCs delivered to damaged mouse myocardium via a collagen patch were able to home to the myocardium and fuse to cells within the infarct and peri-infarct region of the myocardium. This study provides a basis for the investigation of the biological impact of fusion of stem cells with CMs in vivo and illustrates how viral fusion proteins might better enable such studies.

Molecular and cellular aspects of rhabdovirus entry

Rhabdoviruses enter the cell via the endocytic pathway and subsequently fuse with a cellular membrane within the acidic environment of the endosome. Both receptor recognition and membrane fusion are mediated by a single transmembrane viral glycoprotein (G). Fusion is triggered via a low-pH induced structural rearrangement. G is an atypical fusion protein as there is a pH-dependent equilibrium between its pre- and post-fusion conformations. The elucidation of the atomic structures of these two conformations for the vesicular stomatitis virus (VSV) G has revealed that it is different from the previously characterized class I and class II fusion proteins. In this review, the pre- and post-fusion VSV G structures are presented in detail demonstrating that G combines the features of the class I and class II fusion proteins. In addition to these similarities, these G structures also reveal some particularities that expand our understanding of the working of fusion machineries. Combined with data from recent studies that revealed the cellular aspects of the initial stages of rhabdovirus infection, all these data give an integrated view of the entry pathway of rhabdoviruses into their host cell.

A recombinant vesicular stomatitis virus bearing a lethal mutation in the glycoprotein gene uncovers a second site suppressor that restores fusion

Vesicular stomatitis virus (VSV), a prototype of the Rhabdoviridae family, contains a single surface glycoprotein (G) that is responsible for attachment to cells and mediates membrane fusion. Working with the Indiana serotype of VSV, we employed a reverse genetic approach to produce fully authentic recombinant viral particles bearing lethal mutations in the G gene. By altering the hydrophobicity of the two fusion loops within G, we produced a panel of mutants, W72A, Y73A, Y116A, and A117F, that were nonfusogenic. Propagation of viruses bearing those lethal mutations in G completely depended on complementation by expression of the glycoprotein from the heterologous New Jersey serotype of VSV. The nonfusogenic G proteins oligomerize and are transported normally to the cell surface but fail to mediate acid pH-triggered membrane fusion. The nonfusogenic G proteins also interfered with the ability of wild-type G to mediate fusion, either by formation of mixed trimers or by inhibition of trimer function during fusion. Passage of one recombinant virus, A117F, identified a second site suppressor of the fusion block, E76K. When analyzed in the absence of the A117F substitution, E76K rendered G more sensitive to acid pH-triggered fusion, suggesting that this compensatory mutation is destabilizing. Our work provides a set of authentic recombinant VSV particles bearing lethal mutations in G, confirms that the hydrophobic fusion loops of VSV G protein are critical for membrane fusion, and underscores the importance of the sequence elements surrounding the hydrophobic tips of the fusion loops in driving fusion. This study has implications for understanding dominant targets for inhibition of G-mediated fusion. Moreover, the recombinant viral particles generated here will likely be useful in dissecting the mechanism of G-catalyzed fusion as well as study steps of viral assembly.

HIV-protease inhibitors block the replication of both vesicular stomatitis and influenza viruses at an early post-entry replication step

The inhibitors of HIV-1 protease (PIs) have been designed to block the activity of the viral aspartyl-protease. However, it is now accepted that this family of inhibitors can also affect the activity of cell proteases. Since the replication of many virus species requires the activity of host cell proteases, investigating the effects of PIs on the life cycle of viruses other than HIV would be of interest. Here, the potent inhibition induced by saquinavir and nelfinavir on the replication of both vesicular stomatitis and influenza viruses is described. These are unrelated enveloped RNA viruses infecting target cells upon endocytosis and intracellular fusion. The PI-induced inhibition was apparently a consequence of a block at the level of the fusion between viral envelope and endosomal membranes. These findings would open the way towards the therapeutic use of PIs against enveloped RNA viruses other than HIV.

Immunogenicity and safety of recombinant rabies viruses used for oral vaccination of stray dogs and wildlife

Rabies is a zoonotic disease and stray dogs, wild carnivores and bats are the natural reservoirs of rabies. Oral immunization with live vaccines is the only practical approach to eradicate rabies in free ranging terrestrial animals. We have developed the double glycoprotein (G) rabies virus (RV) variant SPBNGAS-GAS that has great promise to be used as a live-attenuated vaccine. Oral immunization of rodents and several target animal species with this double G RV variant resulted in the induction of protective immunity, superior to that induced by a single RV G variant (SPBNGAS). The high oral efficacy of SPBNGAS-GAS is likely because of its increased ability to infect monocytes or immature dendritic cells (DCs), thereby inducing their conversion into mature DCs. Furthermore, infection of DCs with the double G variant resulted in a strong up-regulation of the expression of genes related to the NFjB signalling pathway including IFN-α and IFN-β, which might underlie the protection conferred by this live RV vaccine. A potential problem associated with the use of live RVs for oral vaccination could rest in the possibility of reversion to the pathogenic phenotype because of the high mutation rate characteristic for all RNA viruses. In this respect, the presence of a second non-pathogenic G gene decreases considerably the risk of reversion to the pathogenic phenotype because a nonpathogenic G is dominant over a pathogenic G in determining the pathogenicity of the double G RV variant. Because of its excellent efficacy and safety, the SPBNGAS-GAS vaccine may provide a distinct advantage over other live RV vaccine in its ability to vaccinate a broad range of mammalian species.

Strong CD8+ T cell antigenicity and immunogenicity of large foreign proteins incorporated in HIV-1 VLPs able to induce a Nef-dependent activation/maturation of dendritic cells

Virus-like particles (VLPs) are excellent tools for vaccines against pathogens and tumors. They can accommodate foreign polypeptides whose incorporation efficiency and immunogenicity however decrease strongly with the increase of their size. We recently described the CD8(+) T cell immune response against a small foreign antigen (i.e., the 98 amino acid long human papilloma virus E7 protein) incorporated in human immunodeficiency virus (HIV)-1 based VLPs as product of fusion with an HIV-1 Nef mutant (Nef(mut)). Here, we extended our previous investigations by testing the antigenic/immunogenic properties of Nef(mut)-based VLPs incorporating much larger heterologous products, i.e., human hepatitis C virus (HCV) NS3 and influenza virus NP proteins, which are composed of 630 and 498 amino acids, respectively. We observed a remarkable cross-presentation of HCV NS3 in dendritic cells challenged with Nef(mut)-NS3 VLPs, as detected using a NS3 specific CD8(+) T cell clone as well as PBMCs from HCV infected patients. On the other hand, when injected in mice, Nef(mut)-NP VLPs elicited strong anti-NP CD8(+) T cell and CTL immune responses. In addition, we revealed the ability of Nef(mut) incorporated in VLPs to activate and mature primary human immature dendritic cells (iDCs). This phenomenon correlated with the activation of Src tyrosine kinase-related intracellular signaling, and can be transmitted from VLP-challenged to bystander iDCs. Overall, these results prove that Nef(mut)-based VLPs represent a rather flexible platform for the design of innovative CD8(+) T cell vaccines.

Structure and working of viral fusion machinery

This chapter discusses the structure and working of viral fusion machinery. The entry of enveloped viruses into cells requires the fusion of viral and cellular membranes, driven by conformational changes in viral glycoproteins. Structural studies have defined three classes of viral membrane fusion proteins. Despite their different structural organizations, all seem to have a common mechanism of action that generates the same lipid organizations during the fusion pathway. The entry of enveloped viruses into host cells requires binding of the virus to one or more receptors present at the cell surface, followed by fusion of the viral envelope with a cellular membrane. These steps are mediated by virally encoded glycoproteins that promote both receptor recognition and membrane fusion. The first crystal structure of a viral fusion protein ectodomain that has been determined is that of influenza virus hemagglutinin (HA) in its prefusion conformation. The structures of viral fusion glycoproteins, of which the conformational change is triggered at low pH, has allowed the identification of amino acid residues that play the role of pH-sensitive molecular switches.

Class III viral membrane fusion proteins

Members of class III of viral fusion proteins share common structural features and molecular architecture, although they belong to evolutionary distant viruses and carry no sequence homology. Based of the experimentally determined three-dimensional structures of their ectodomains, glycoprotein B (gB) of herpesviruses, G protein of rhabdoviruses and glycoprotein 64 (gp64) of baculoviruses have been identified as class III fusion proteins. The structures are proposed to represent post-fusion conformations, and they reveal trimeric, elongated, rod-like molecules, with each protomer being composed of five domains. Sequences which interact with target membranes and form the fusion peptides are located in two loops found at one end of the molecule. Class III fusion proteins are embedded in viral envelope with the principal function of catalyzing fusion of viral and cellular membranes, an event that is essential for infection to occur. In addition, they have been implicated in processes such as attachment to target cells and viral maturation. G protein is the only class III fusion protein for which structures of both pre- and post-fusion states have been determined, shedding light on the mechanism involved in the conformational change and membrane fusion. Whether similar structural organization of class III fusion proteins translates into a common mechanism involved in carrying out membrane fusion remains to be investigated.

Generation of VSV pseudotypes using recombinant ΔG-VSV for studies on virus entry, identification of entry inhibitors, and immune responses to vaccines

Vesicular stomatitis virus (VSV) is a prototypic enveloped animal virus that has been used extensively to study virus entry, replication and assembly due to its broad host range and robust replication properties in a wide variety of mammalian and insect cells. Studies on VSV assembly led to the creation of a recombinant VSV in which the glycoprotein (G) gene was deleted. This recombinant (rVSV-ΔG) has been used to produce VSV pseudotypes containing the envelope glycoproteins of heterologous viruses, including viruses that require high-level biocontainment; however, because the infectivity of rVSV-ΔG pseudotypes is restricted to a single round of replication the analysis can be performed using biosafety level 2 (BSL-2) containment. As such, rVSV-ΔG pseudotypes have facilitated the analysis of virus entry for numerous viral pathogens without the need for specialized containment facilities. The pseudotypes also provide a robust platform to screen libraries for entry inhibitors and to evaluate the neutralizing antibody responses following vaccination. This manuscript describes methods to produce and titer rVSV-ΔG pseudotypes. Procedures to generate rVSV-ΔG stocks and to quantify virus infectivity are also described. These protocols should allow any laboratory knowledgeable in general virological and cell culture techniques to produce successfully replication-restricted rVSV-ΔG pseudotypes for subsequent analysis.

Fusion-active glycoprotein G mediates the cytotoxicity of vesicular stomatitis virus M mutants lacking host shut-off activity

The cytopathogenicity of vesicular stomatitis virus (VSV) has been attributed mainly to the host shut-off activity of the viral matrix (M) protein, which inhibits both nuclear transcription and nucleocytoplasmic RNA transport, thereby effectively suppressing the synthesis of type I interferon (IFN). The M protein from persistently VSV-infected cells was shown to harbour characteristic amino acid substitutions (M51R, V221F and S226R) implicated in IFN induction. This study demonstrates that infection of human fibroblasts with recombinant VSV containing the M51R substitution resulted in IFN induction, whereas neither the V221F nor the S226R substitution effected an IFN-inducing phenotype. Only when V221F was combined with S226R were the host shut-off activity of the M protein abolished and IFN induced, independently of M51R. The M33A substitution, previously implicated in VSV cytotoxicity, did not affect host shut-off activity. M-mutant VSV containing all four amino acid substitutions retained cytotoxic properties in both Vero cells and IFN-competent primary fibroblasts. Infected-cell death was associated with the formation of giant polynucleated cells, suggesting that the fusion activity of the VSV G protein was involved. Accordingly, M-mutant VSV expressing a fusion-defective G protein or with a deletion of the G gene showed significantly reduced cytotoxic properties and caused long-lasting infections in Vero cells and mouse hippocampal slice cultures. In contrast, a G-deleted VSV expressing wild-type M protein remained cytotoxic. These findings indicate that the host shut-off activity of the M protein dominates VSV cytotoxicty, whilst the fusion-active G protein is mainly responsible for the cytotoxicity remaining with M-mutant VSV.

Ngaingan virus, a macropod-associated rhabdovirus, contains a second glycoprotein gene and seven novel open reading frames

Ngaingan virus (NGAV) was isolated from a pool of biting midges that were collected in the tropics of northern Australia. Reported here is the full-length sequence of the NGAV genome, which, at over 15.7 kb, is the largest in any rhabdovirus described to date and contains 13 genes, the highest number of genes observed in any (-) ssRNA virus. Seven of these putative genes show no significant homology to known proteins. Like viruses in the genus Ephemerovirus, NGAV possesses a second glycoprotein gene (G(NS)). Phylogenetic analyses, however, place NGAV within the yet to be classified "Hart Park" group containing Wongabel and Flanders viruses, which do not contain a second glycoprotein gene. Screening of various animal sera from northern Australia has indicated that NGAV is currently circulating in macropods (wallabies, wallaroos and kangaroos), highlighting the need for further studies to determine its potential to cause disease in these species.

Internalization and fusion mechanism of vesicular stomatitis virus and related rhabdoviruses

Members of the Rhabdoviridae infect a wide variety of animals and plants, and are the causative agents of many important diseases. Rhabdoviruses enter host cells following internalization into endosomes, with the glycoprotein (G protein) mediating both receptor binding to host cells and fusion with the cellular membrane. The recently solved crystal structure of vesicular stomatitis virus G has allowed considerable insight into the mechanism of rhabdovirus entry, in particular the low pH-dependent conformational changes that lead to fusion activation. Rhabdovirus entry shows several distinct features compared with other enveloped viruses; first, the entry process appears to consist of two distinct fusion events, initial fusion into vesicles within endosomes followed by back-fusion into the cytosol; second, the conformational changes in the G protein that lead to fusion activation are reversible; and third, the G protein is structurally distinct from other viral fusion proteins and is not proteolytically cleaved. The internalization and fusion mechanisms of rhabdoviruses are discussed in this article, with a focus on viral systems where the G protein has been studied extensively: vesicular stomatitis virus and rabies virus, as well as viral hemorrhagic septicemia virus.

Cell entry of Borna disease virus follows a clathrin-mediated endocytosis pathway that requires Rab5 and microtubules

Borna disease virus (BDV), the prototypic member of the Bornaviridae family within the order Mononegavirales, exhibits high neurotropism and provides an important and unique experimental model system for studying virus-cell interactions within the central nervous system. BDV surface glycoprotein (G) plays a critical role in virus cell entry via receptor-mediated endocytosis, and therefore, G is a critical determinant of virus tissue and cell tropism. However, the specific cell pathways involved in BDV cell entry have not been determined. Here, we provide evidence that BDV uses a clathrin-mediated, caveola-independent cell entry pathway. We also show that BDV G-mediated fusion takes place at an optimal pH of 6.0 to 6.2, corresponding to an early-endosome compartment. Consistent with this finding, BDV cell entry was Rab5 dependent but Rab7 independent and exhibited rapid fusion kinetics. Our results also uncovered a key role for microtubules in BDV cell entry, whereas the integrity and dynamics of actin cytoskeleton were not required for efficient cell entry of BDV.

Class III viral membrane fusion proteins

Accumulating structural studies of viral fusion glycoproteins have revealed unanticipated structural relationships between unrelated virus families and allowed the grouping of these membrane fusogens into three distinct classes. Here we review the newly identified group of class III viral fusion proteins, whose members include fusion proteins from rhabdoviruses, herpesviruses, and baculoviruses. While clearly related in structure, the class III viral fusion proteins exhibit distinct structural features in their architectures as well as in their membrane interacting fusion loops, which are likely related to their virus-specific differences in cellular entry. Further study of the similarities and differences in the class III viral fusion glycoproteins may provide greater insights into protein:membrane interactions that are key to promoting efficient bilayer fusion during virus entry.

Lineage analysis of inner ear cells using genomic tags for clonal identification

To understand the mechanisms of development of the inner ear, it is important to know the lineal relationships among the different cell types and the migrational boundaries of individual clones within the inner ear. This chapter details the basic methods for performing lineage analysis of the inner ear using replication-defective retroviral vectors in chicken embryos. Protocols are provided for generating avian retroviruses pseudotyped with vesicular stomatitis virus (VSV) envelopes to improve infectivity in early embryos. Moreover, we include the pioneering methods of the Cepko laboratory, whereby a library of DNA tags was developed to allow clonal relationships to be confirmed by PCR amplification and sequencing of the tag in dispersed clonal progeny. By varying the site and time of viral delivery, these methods are appropriate to study cell lineages in other tissues of the developing chicken.

The postfusion structure of baculovirus gp64 supports a unified view of viral fusion machines

Viral fusion proteins mediate the merger of host and viral membranes during cell entry for all enveloped viruses. Baculovirus glycoprotein gp64 (gp64) is unusual in promoting entry into both insect and mammalian cells and is distinct from established class I and class II fusion proteins. We report the crystal structure of its postfusion form, which explains a number of gp64's biological properties including its cellular promiscuity, identifies the fusion peptides and shows it to be the third representative of a new class (III) of fusion proteins with unexpected structural homology with vesicular stomatitis virus G and herpes simplex virus type 1 gB proteins. We show that domains of class III proteins have counterparts in both class I and II proteins, suggesting that all these viral fusion machines are structurally more related than previously thought.

Concepts in the pathogenesis of rabies

Rabies is a zoonotic disease that remains an important public health problem worldwide and causes more than 70,000 human deaths each year. The causative agent of rabies is rabies virus (RV), a negative-stranded RNA virus of the rhabdovirus family. Neuroinvasiveness and neurotropism are the main features that define the pathogenesis of rabies. Although RV pathogenicity is a multigenic trait involving several elements of the RV genome, the RV glycoprotein plays a major role in RV pathogenesis by controlling the rate of virus uptake and trans-synaptic virus spread, and by regulating the rate of virus replication. Pathogenic street RV strains differ significantly from tissue culture-adapted RV strains in their neuroinvasiveness. Whereas street RV strains are highly neuroinvasive, most tissue culture-adapted RV strains have either no or only limited ability to invade the CNS from a peripheral site. The high neuroinvasiveness of pathogenic street RVs is, at least in part, due to their ability to evade immune responses and to conserve the structures of neurons. The finding that tissue culture-adapted RV strains replicate very fast and induce strong innate and adaptive immune responses opens new avenues for therapeutic intervention against rabies.

Molecular architecture of the bipartite fusion loops of vesicular stomatitis virus glycoprotein G, a class III viral fusion protein

The glycoprotein of vesicular stomatitis virus (VSV G) mediates fusion of the viral envelope with the host cell, with the conformational changes that mediate VSV G fusion activation occurring in a reversible, low pH-dependent manner. Based on its novel structure, VSV G has been classified as class III viral fusion protein, having a predicted bipartite fusion domain comprising residues Trp-72, Tyr-73, Tyr-116, and Ala-117 that interacts with the host cell membrane to initiate the fusion reaction. Here, we carried out a systematic mutagenesis study of the predicted VSV G fusion loops, to investigate the functional role of the fusion domain. Using assays of low pH-induced cell-cell fusion and infection studies of mutant VSV G incorporated into viral particles, we show a fundamental role for the bipartite fusion domain. We show that Trp-72 is a critical residue for VSV G-mediated membrane fusion. Trp-72 could only tolerate mutation to a phenylalanine residue, which allowed only limited fusion. Tyr-73 and Tyr-116 could be mutated to other aromatic residues without major effect but could not tolerate any other substitution. Ala-117 was a less critical residue, with only charged residues unable to allow fusion activation. These data represent a functional analysis of predicted bipartite fusion loops of VSV G, a founder member of the class III family of viral fusion proteins.

Incorporation of quantum dots on virus in polycationic solution

Developing methods to label viruses with fluorescent moieties has its merits in elucidating viral infection mechanisms and exploring novel antiviral therapeutics. Fluorescent quantum dots (QDs), an emerging probe for biological imaging and medical diagnostics, were employed in this study to tag retrovirus encoding enhanced green fluorescent protein (EGFP) genes. Electrostatic repulsion forces generated from both negatively charged retrovirus and QDs were neutralized by cationic Polybrene, forming colloidal complexes of QDs-virus. By examining the level of EGFP expression in 3T3 fibroblast cells treated with QDs-tagged retroviruses for 24 hours, the infectivity of retrovirus incorporated with QDs was shown to be only slightly decreased. Moreover, the imaging of QDs can be detected in the cellular milieu. In summary, the mild method developed here makes QDs-tagged virus a potential imaging probe for direct tracking the infection process and monitoring distribution of viral particles in infected cells.

Aggregation and hemi-fusion of anionic vesicles induced by the antimicrobial peptide cryptdin-4

We show that cryptdin-4 (Crp4), an antimicrobial peptide found in mice, induces the aggregation and hemi-fusion of charged phospholipid vesicles constructed of the anionic lipid POPG and the zwitterionic lipid POPC. Hemi-fusion is confirmed with positive total lipid-mixing assay results, negative inner monolayer lipid-mixing assay results, and negative results from contents-mixing assays. Aggregation, as quantified by absorbance and dynamic light scattering, is self-limiting, creating finite-sized vesicle assemblies. The rate limiting step in the formation process is the mixing of juxtaposed membrane leaflets, which is regulated by bound peptide concentration as well as vesicle radius (with larger vesicles less prone to hemi-fusion). Bound peptide concentration is readily controlled by total peptide concentration and the fraction of anionic lipid in the vesicles. As little as 1% PEGylated lipid significantly reduces aggregate size by providing a steric barrier for membrane apposition. Finally, as stable hemi-fusion is a rare occurrence, we compare properties of Crp4 to those of many peptides known to induce complete fusion and lend insight into conditions necessary for this unusual type of membrane merger.

Hydrophobic residues that form putative fusion loops of Epstein-Barr virus glycoprotein B are critical for fusion activity

To test the importance of the hydrophobic residues within the putative Epstein-Barr virus (EBV) glycoprotein B (gB) fusion loops in membrane fusion, WY(112-113) and WLIW(193-196) were mutated into alanine, glutamic acid, or the analogous residues from herpes simplex virus type 1 (HSV-1) gB (HR and RVEA). All gB variants exhibited cell surface expression, demonstrating that the substitutions did not perturb gB trafficking. None of six gB variants was, however, capable of mediating fusion with either epithelial or B cells. These data demonstrate that the bulky and hydrophobic EBV loop residues, which differ from the more hydrophilic HSV-1 residues and appear more compatible with membrane insertion, are essential for EBV gB-dependent fusion.

Mutational evidence of internal fusion loops in herpes simplex virus glycoprotein B

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is one of four glycoproteins necessary and sufficient for HSV cellular entry. Recently, the crystal structures of HSV-1 gB and vesicular stomatitis virus glycoprotein G were determined. Surprisingly, the two proteins share remarkable structural homology. Both proteins are homotrimeric and center about a long alpha-helix, features reminiscent of class I fusion proteins, such as influenza virus hemagglutinin or paramyxovirus F. However, these structures revealed that G has internal fusion loops, similar to the fusion loops of the class II fusion proteins, and that these loops are structurally conserved in gB. To examine whether these putative fusion loops are important for gB function, we mutated potential membrane-interacting (hydrophobic) residues to charged amino acids. Of most interest were mutant gB proteins that were expressed on the cell surface and were recognized by monoclonal antibodies against conformational epitopes but lacked the ability to function in cell-cell fusion assays. We find that three of the five hydrophobic amino acids targeted in these loops, tryptophan 174, tyrosine 179, and alanine 261, are integral in the function of gB. Our data suggest that they are part of an important functional domain. We hypothesize that two loops in domain 1 of HSV gB function as fusion loops. Our data are further evidence that gB is a viral fusogen and suggest clues as to how gB may function.

Retro-transduction by virus pseudotyped with glycoprotein of vesicular stomatitis virus

A virus pseudotyped with glycoprotein of vesicular stomatitis virus (VSV-G) can enter various cell types at a relatively high titer. We observed that the amount of viral antigen from VSV-G pseudotyped human immunodeficiency virus type 1 (HIV-1) producing cells was much higher than that from their non-pseudotyped counterparts. This enhanced viral antigen production was not observed when we used HIV-1 pol mutant, viral enzyme inhibitors, HIV Env protein, or VSV-G fusion defective mutants. The transfection experiment using GFP-expressing virus showed time-dependent expansion of GFP-positive cells and viral DNA integration. These results suggested that the increase in viral antigen yield was caused by the release of a progeny virus following retro-transduction by the pseudotyped virus of the cells within the transfected cell culture. The infectivity as well as the amount of VSV-G on virus particles per unit of viral antigen was significantly different before and after the onset of the yield enhancement. This suggests that results of infection assays of the virus pseudotyped with VSV-G may be affected by the occurrence of such enhancement. This means that, while pseudotyping with VSV-G is a simple and effective method, this procedure should be carefully considered when the virus is produced for infectivity assays.

Charged residues are involved in membrane fusion mediated by a hydrophilic peptide located in vesicular stomatitis virus G protein

Membrane fusion is an essential step of the internalization process of the enveloped animal viruses. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion at the acidic environment of the endosomal compartment. In a previous work, we identified a specific sequence in VSV G protein, comprising the residues 145 to 164, directly involved in membrane interaction and fusion. Unlike fusion peptides from other viruses, this sequence is very hydrophilic, containing six charged residues, but it was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Using a carboxyl-modifying agent, dicyclohexylcarbodiimide (DCCD), and several synthetic mutant peptides, we demonstrated that the negative charges of peptide acidic residues, especially Asp153 and Glu158, participate in the formation of a hydrophobic domain at pH 6.0, which is necessary to the peptide-induced membrane fusion. The formation of the hydrophobic region and the membrane fusion itself were dependent on peptide concentration in a higher than linear fashion, suggesting the involvement of peptide oligomerization. His148 was also necessary to hydrophobicity and fusion, suggesting that peptide oligomerization occurs through intermolecular electrostatic interactions between the positively-charged His and a negatively-charged acidic residue of two peptide molecules. Oligomerization of hydrophilic peptides creates a hydrophobic region that is essential for the interaction with the membrane that results in fusion.

Generation and characterization of a stable cell population releasing fluorescent HIV-1-based Virus Like Particles in an inducible way

BACKGROUND

The availability of cell lines releasing fluorescent viral particles can significantly support a variety of investigations, including the study of virus-cell interaction and the screening of antiviral compounds. Regarding HIV-1, the recovery of such biologic reagents represents a very hard challenge due to the intrinsic cytotoxicity of many HIV-1 products. We sought to overcome such a limitation by using a cell line releasing HIV-1 particles in an inducible way, and by exploiting the ability of a HIV-1 Nef mutant to be incorporated in virions at quite high levels.

RESULTS

Here, we report the isolation and characterization of a HIV-1 packaging cell line, termed 18-4s, able to release valuable amounts of fluorescent HIV-1 based Virus-Like Particles (VLPs) in an inducible way. 18-4s cells were recovered by constitutively expressing the HIV-1 NefG3C mutant fused with the enhanced-green fluorescent protein (NefG3C-GFP) in a previously isolated inducible HIV-1 packaging cell line. The G3C mutation creates a palmitoylation site which results in NefG3C-GFP incorporation into virions greatly exceeding that of the wild type counterpart. Upon induction of 18-4s cells with ponasterone A and sodium butyrate, up to 4 mug/ml of VLPs, which had incorporated about 150 molecules of NefG3C-GFP per viral particle, were released into the culture supernatant. Due to their intrinsic strong fluorescence, the 18-4s VLPs were easily detectable by a novel cytofluorometric-based assay developed here. The treatment of target cells with fluorescent 18-4 VLPs pseudotyped with different glycoprotein receptors resulted in these becoming fluorescent as early as two hours post-challenge.

CONCLUSION

We created a stable cell line releasing fluorescent HIV-1 based VLPs upon induction useful for several applications including the study of virus-cell interactions and the screening of antiviral compounds.

Retroviral vector production using suspension-adapted 293GPG cells in a 3L acoustic filter-based perfusion bioreactor

Recombinant retroviruses are now an established tool for gene delivery. Presently they are mainly produced using adherent cells. However, due to the restrictive nature of adherent cell culture, this mode of production is hampered by low cell-specific productivity and small production units. The large-scale production of retroviral vectors could benefit from the adaptation of retrovirus packaging cell lines to suspension culture. Here, we describe the ability of a 293 packaging cell line to produce retroviral vectors in suspension culture at high titer. Adherent 293GPG cells, producing a Moloney Murine Leukemia Virus (MoMLV) retrovirus vector pseudotyped with the vesicular stomatitis virus G (VSVG) envelope protein and expressing a TK-GFP fusion protein, were adapted to suspension culture in calcium-free DMEM. At a cell density similar to adherent cell culture, the suspension culture produced retroviral vector consistently in the range of 1 x 10(7) infectious viral particles/mL (IVP/mL), with a specific productivity threefold higher than adherent culture. Furthermore, at the same medium replacement frequency, the suspension producer cells could be cultured at higher density than their adherent counterparts, which resulted in virus titer of 3-4 x 10(7) IVP/mL at 11.0 x 10(6) cells/mL. This corresponds to a 10-fold increase in viral concentration compared to adherent cells. The capacity to up scale the retroviral vector production was also demonstrated by performing a 2 VVD perfusion culture for 9 days in a 3L Chemap bioreactor. The combination of suspension and perfusion led to a 20-fold increase in maximum virus productivity compared to the adherent culture.

Mutagenesis of the La Crosse Virus glycoprotein supports a role for Gc (1066-1087) as the fusion peptide

The La Crosse Virus (LACV) M segment encodes two glycoproteins (Gn and Gc), and plays a critical role in the neuropathogenesis of LACV infection as the primary determinant of neuroinvasion. A recent study from our group demonstrated that the region comprising the membrane proximal two-thirds of Gc, amino acids 860-1442, is critical in mediating LACV fusion and entry. Furthermore, computational analysis identified structural similarities between a portion of this region, amino acids 970-1350, and the E1 fusion protein of two alphaviruses: Sindbis virus and Semliki Forrest virus (SFV). Within the region 970-1350, a 22-amino-acid hydrophobic segment (1066-1087) is predicted to correlate structurally with the fusion peptides of class II fusion proteins. We performed site-directed mutagenesis of key amino acids in this 22-amino acid segment and determined the functional consequences of these mutations on fusion and entry. Several mutations within this hydrophobic domain affected glycoprotein expression to some extent, but all mutations either shifted the pH threshold of fusion below that of the wild-type protein, reduced fusion efficiency, or abrogated cell-to-cell fusion and pseudotype entry altogether. These results, coupled with the aforementioned computational modeling, suggest that the LACV Gc functions as a class II fusion protein and support a role for the region Gc 1066-1087 as a fusion peptide.

Entry of hepatitis C virus pseudotypes into primary human hepatocytes by clathrin-dependent endocytosis

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. Studies of the early steps of HCV infection have been hampered by the lack of convenient in vitro or in vivo models. Although several cell-surface molecules that mediate the binding of HCV envelope proteins to target cells have been identified, mechanisms of viral entry into human hepatocytes are still poorly understood. Vesicular stomatitis virus/HCV pseudotyped viruses expressing the HCV envelope glycoproteins on the viral envelope were generated and it was found that their entry into human hepatocytes required co-expression of E1 and E2 on the pseudotype surface. Neutralization of pseudotype infection by anti-HCV antibodies suggested that cellular entry was mediated by HCV envelope glycoproteins and by previously characterized cell-surface molecules, including CD81. An entry assay based on the release of a fluorochrome from labelled HCV pseudotypes provided evidence for a pH-dependent fusion of the pseudotype envelope with a cellular compartment. By using a panel of endocytosis inhibitors, it is postulated that penetration of HCV into primary cultures of hepatocytes takes place by clathrin-mediated endocytosis.

Crystal structure of the low-pH form of the vesicular stomatitis virus glycoprotein G

The vesicular stomatitis virus has an atypical membrane fusion glycoprotein (G) exhibiting a pH-dependent equilibrium between two forms at the virus surface. Membrane fusion is triggered during the transition from the high- to low-pH form. The structure of G in its low-pH form shows the classic hairpin conformation observed in all other fusion proteins in their postfusion conformation, in spite of a novel fold combining features of fusion proteins from classes I and II. The structure provides a framework for understanding the reversibility of the G conformational change. Unexpectedly, G is homologous to gB of herpesviruses, which raises important questions on viral evolution.

A single amino acid change in rabies virus glycoprotein increases virus spread and enhances virus pathogenicity

Several rabies virus (RV) vaccine strains containing an aspartic acid (Asp) or glutamic acid (Glu) instead of an arginine (Arg) at position 333 of the RV glycoprotein (G) are apathogenic for immunocompetent mice even after intracranial inoculation. However, we previously showed that the nonpathogenic phenotype of the highly attenuated RV strain SPBNGA, which contains a Glu at position 333 of G, is unstable when this virus is passaged in newborn mice. While the Glu(333) remained unchanged after five mouse passages, an Asn(194)-->Lys(194) mutation occurred in RV G. This mutation was associated with increased pathogenicity for adult mice. Using site-directed mutagenesis to exchange Asn(194) with Lys(194) in the G protein of SPBNGA, resulting in SPBNGA-K, we show here that this mutation is solely responsible for the increase in pathogenicity and that the Asn(194)-->Lys(194) mutation does not arise when Asn(194) is exchanged with Ser(194) (SPBNGA-S). Our data presented indicate that the increased pathogenicity of SPBNGA-K is due to increased viral spread in vivo and in vitro, faster internalization of the pathogenic virus into cells, and a shift in the pH threshold for membrane fusion. These results are consistent with the notion that the RV G protein is a major contributor to RV pathogenesis and that the more pathogenic RVs escape the host responses by a faster spread than that of less pathogenic RVs.

Probing the interaction between vesicular stomatitis virus and phosphatidylserine

The entry of enveloped animal viruses into their host cells always depends on membrane fusion triggered by conformational changes in viral envelope glycoproteins. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion between the viral envelope and the endosomal membrane at the acidic environment of this compartment. In this work, we evaluated VSV interactions with membranes of different phospholipid compositions, at neutral and acidic pH, using atomic force microscopy (AFM) operating in the force spectroscopy mode, isothermal calorimetry (ITC) and molecular dynamics simulation. We found that the binding forces differed dramatically depending on the membrane phospholipid composition, revealing a high specificity of G protein binding to membranes containing phosphatidylserine (PS). In a previous work, we showed that the sequence corresponding amino acid 164 of VSV G protein was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Here, we used this sequence to explore VSV-PS interaction using ITC. We found that peptide binding to membranes was exothermic, suggesting the participation of electrostatic interactions. Peptide-membrane interaction at pH 7.5 was shown to be specific to PS and dependent on the presence of His residues in the fusion peptide. The application of the simplified continuum Gouy-Chapman theory to our system predicted a pH of 5.0 at membrane surface, suggesting that the His residues should be protonated when located close to the membrane. Molecular dynamics simulations suggested that the peptide interacts with the lipid bilayer through its N-terminal residues, especially Val(145) and His(148).