Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by viral fusogenic proteins that drive membrane rearrangements needed for fusion between viral and target membranes. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens but do not structurally or functionally resemble classical viral fusogens. We asked whether the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver μDystrophin to skeletal muscle of a mouse model of Duchenne muscular dystrophy and alleviate pathology. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by fusogenic proteins that form a complex between membranes to drive rearrangements needed for fusion. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens, but do not structurally or functionally resemble classical viral fusogens. We asked if the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver micro-Dystrophin (μDys) to skeletal muscle of a mouse model of Duchenne muscular dystrophy. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

Fatal Multisystem Inflammatory Syndrome in Adult after SARS-CoV-2 Natural Infection and COVID-19 Vaccination

We describe a fatal case of multisystem inflammatory syndrome in an adult with onset 22 days after a second dose of mRNA coronavirus disease vaccine. Serologic and clinical findings indicated severe acute respiratory syndrome coronavirus 2 infection occurred before vaccination. The immunopathology of this syndrome, regardless of vaccination status, remains poorly understood.

Generation of SARS-CoV-2 Spike Pseudotyped Virus for Viral Entry and Neutralization Assays: A 1-Week Protocol

The COVID-19 pandemic caused by the SARS-CoV-2 coronavirus requires reliable assays for studying viral entry mechanisms which remains poorly understood. This knowledge is important for the development of therapeutic approaches to control SARS-CoV-2 infection by permitting the screening for neutralizing antibodies and other agents that can block infection. This is particularly important for patients who are at high risk for severe outcomes related to COVID-19. The production of pseudotyped viral particles may seem like a daunting task for a non-virology laboratory without experience in the two most commonly used pseudotyping systems, namely retro/lentiviruses and vesicular stomatitis virus (VSV) which lacks the VSV envelope glycoprotein (VSVΔG). By incorporating the most up-to-date knowledge, we have developed a detailed, easy-to-follow novel protocol for producing SARS-CoV-2 spike-bearing pseudovirus using the VSV-ΔG system. We describe the infection assay which uses GFP fluorescence as a measure of infection in a 24-well live imaging system. We present results of our optimization of the system to enhance viral infection levels through the over-expression of human ACE2 receptor and the overexpression of at least one of two proteases - TMPRSS2 or Furin, as well as, supplementation with Poloxamer 407 (P407) and Prostaglandin E2 (PGE2) as adjuvants. We show that the system works efficiently in three unrelated, clinically relevant cell lines: human 293T (renal epithelial) cells, human Calu-3 (lung epithelial) cells, and the non-human primate (African Green Monkey) cell line, Vero-E6 (renal epithelial) cells. In addition, we have used this system to show infection of human induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs). This system is efficient (virus generation, titration, and infection assays can be performed in 1 week), quantitative, inexpensive, and readily scalable for application in drug development and therapeutic screening approaches.

Heterogeneous antibodies against SARS-CoV-2 spike receptor binding domain and nucleocapsid with implications for COVID-19 immunity

Evaluation of potential immunity against the novel severe acute respiratory syndrome (SARS) coronavirus that emerged in 2019 (SARS-CoV-2) is essential for health, as well as social and economic recovery. Generation of antibody response to SARS-CoV-2 (seroconversion) may inform on acquired immunity from prior exposure, and antibodies against the SARS-CoV-2 spike protein receptor binding domain (S-RBD) are speculated to neutralize virus infection. Some serology assays rely solely on SARS-CoV-2 nucleocapsid protein (N-protein) as the antibody detection antigen; however, whether such immune responses correlate with S-RBD response and COVID-19 immunity remains unknown. Here, we generated a quantitative serological ELISA using recombinant S-RBD and N-protein for the detection of circulating antibodies in 138 serial serum samples from 30 reverse transcription PCR–confirmed, SARS-CoV-2–hospitalized patients, as well as 464 healthy and non–COVID-19 serum samples that were collected between June 2017 and June 2020. Quantitative detection of IgG antibodies against the 2 different viral proteins showed a moderate correlation. Antibodies against N-protein were detected at a rate of 3.6% in healthy and non–COVID-19 sera collected during the pandemic in 2020, whereas 1.9% of these sera were positive for S-RBD. Approximately 86% of individuals positive for S-RBD–binding antibodies exhibited neutralizing capacity, but only 74% of N-protein–positive individuals exhibited neutralizing capacity. Collectively, our studies show that detection of N-protein–binding antibodies does not always correlate with presence of S-RBD–neutralizing antibodies and caution against the extensive use of N-protein–based serology testing for determination of potential COVID-19 immunity.

Detection of SARS-CoV-2 nucleocapsid protein binding antibodies does not always correlate with the presence of spike protein receptor binding domain neutralizing antibodies.

Modulation of endoplasmic reticulum stress and the unfolded protein response in cancerous and noncancerous cells

OBJECTIVES

The bio-field array is a device that generates a dielectrophoretic electromagnetic field when placed in a hypotonic saline solution and a direct current of approximately 3 A is applied. It is known that cell physiology is guided by bioelectrical properties, and there is a significant growth inhibition in cancerous (MDA-MB-231) cells that are grown in media that has been reconstituted with the saline that has been exposed to the bio-field array direct current dielectrophoretic electromagnetic field, alternatively there is no growth inhibition noted in noncancerous cells (MCF-10A) when grown in the bio-field array direct current dielectrophoretic electromagnetic field treated versus control media.

METHODS

To examine the basis for selective growth inhibition in human breast carcinoma, we employed cell death assays, cell cycle assays, microarray analysis and reverse transcription-quantitative polymerase chain reaction.

RESULTS

We found a large transcriptional reprogramming in the cell lines and of the genes affected, those involved in endoplasmic reticulum stress and the unfolded protein response pathways showed some of the most dramatic changes. Cancerous cells grown in media that has been reconstituted with a hypotonic saline solution that has been exposed to the bio-field array direct current dielectrophoretic electromagnetic field show a significant and strong upregulation of the apoptotic arms of the unfolded protein response while the noncancerous cells show a decrease in endoplasmic reticulum stress via microarray analyses and reverse transcription-quantitative polymerase chain reaction.

CONCLUSION

The bio-field array shows potential to initiate apoptosis in cancerous cells while relieving cell stress in noncancerous cells in vitro. These studies lay a foundation for nurses to conduct future in vivo models for the possible development of future adjunct treatments in chronic disease.

Inhibition of melanocortin 1 receptor slows melanoma growth, reduces tumor heterogeneity and increases survival

Melanoma risk is increased in patients with mutations of melanocortin 1 receptor (MC1R) yet the basis for the increased risk remains unknown. Here we report in vivo evidence supporting a critical role for MC1R in regulating melanoma tumor growth and determining overall survival time. Inhibition of MC1R by its physiologically relevant competitive inhibitor, agouti signaling protein (ASIP), reduced melanin synthesis and morphological heterogeneity in murine B16-F10 melanoma cells. In the lungs of syngeneic C57BL/6 mice, mCherry-marked, ASIP-secreting lung tumors inhibited MC1R on neighboring tumors lacking ASIP in a dose dependent manner as evidenced by a proportional loss of pigment in tumors from mice injected with 1:1, 3:1 and 4:1 mixtures of parental B16-F10 to ASIP-expressing tumor cells. ASIP-expressing B16-F10 cells formed poorly pigmented tumors in vivo that correlated with a 20% longer median survival than those bearing parental B16-F10 tumors (p=0.0005). Mice injected with 1:1 mixtures also showed survival benefit (p=0.0054), whereas injection of a 4:1 mixture showed no significant difference in survival. The longer survival time of mice bearing ASIP-expressing tumors correlated with a significantly slower growth rate than parental B16-F10 tumors as judged by quantification of numbers of tumors and total tumor load (p=0.0325), as well as a more homogeneous size and morphology of ASIP-expressing lung tumors. We conclude that MC1R plays an important role in regulating melanoma growth and morphology. Persistent inhibition of MC1R provided a significant survival advantage resulting in part from slower tumor growth, establishing MC1R as a compelling new molecular target for metastatic melanoma.

Rab43 regulates the sorting of a subset of membrane protein cargo through the medial Golgi

To evaluate the role of cytoplasmic domains of membrane-spanning proteins in directing trafficking through the secretory pathway, we generated fluorescently tagged VSV G tsO45 with either the native G tail (G) or a cytoplasmic tail derived from the chicken AE1-4 anion exchanger (G(AE)). We previously showed that these two proteins progressed through the Golgi with distinct kinetics. To investigate the basis for the differential sorting of G and G(AE), we analyzed the role of several Golgi-associated small GTP-binding proteins and found that Rab43 differentially regulated their transport through the Golgi. We show that the expression of GFP-Rab43 arrested the anterograde transport of G(AE) in a Rab43-positive medial Golgi compartment. GFP-Rab43 expression also inhibited the acquisition of endoH-resistant sugars and the surface delivery of G(AE), as well as the surface delivery of the AE1-4 anion exchanger. In contrast, GFP-Rab43 expression did not affect the glycosylation or surface delivery of G. Unexpectedly, down-regulation of endogenous Rab43 using small interfering RNA resulted in an increase in the accumulation of G(AE) on the cell surface while having minimal effect on the surface levels of G. Our data demonstrate that Rab43 regulates the sorting of a subset of membrane-spanning cargo as they progress through the medial Golgi.

Single injection recombinant vesicular stomatitis virus vaccines protect ferrets against lethal Nipah virus disease

Background

Nipah virus (NiV) is a highly pathogenic zoonotic agent in the family Paramyxoviridae that is maintained in nature by bats. Outbreaks have occurred in Malaysia, Singapore, India, and Bangladesh and have been associated with 40 to 75% case fatality rates. There are currently no vaccines or postexposure treatments licensed for combating human NiV infection.

Methods and results

Four groups of ferrets received a single vaccination with different recombinant vesicular stomatitis virus vectors expressing: Group 1, control with no glycoprotein; Group 2, the NiV fusion protein (F); Group 3, the NiV attachment protein (G); and Group 4, a combination of the NiV F and G proteins. Animals were challenged intranasally with NiV 28 days after vaccination. Control ferrets in Group 1 showed characteristic clinical signs of NiV disease including respiratory distress, neurological disorders, viral load in blood and tissues, and gross lesions and antigen in target tissues; all animals in this group succumbed to infection by day 8. Importantly, all specifically vaccinated ferrets in Groups 2-4 showed no evidence of clinical illness and survived challenged. All animals in these groups developed anti-NiV F and/or G IgG and neutralizing antibody titers. While NiV RNA was detected in blood at day 6 post challenge in animals from Groups 2-4, the levels were orders of magnitude lower than animals from control Group 1.

Conclusions

These data show protective efficacy against NiV in a relevant model of human infection. Further development of this technology has the potential to yield effective single injection vaccines for NiV infection.

Inhibition of type I interferon-mediated antiviral action in human glioma cells by the IKK inhibitors BMS-345541 and TPCA-1

The nuclear factor-kappa B (NFκB) signal transduction pathway plays an important role in immunity, inflammation, cell growth, and survival. Since dysregulation of this pathway results in high, constitutive NFκB activation in various cancers and immune disorders, the development of specific drugs to target this pathway has become a focus for treating these diseases. NFκB regulates various aspects of the cellular response to interferon (IFN). However, the role of the upstream regulator of the NFκB signaling pathway, the inhibitor of κB kinase (IKK) complex, on IFN function has not been examined. In the present study, we examined the effects of 2 IKK inhibitors, N-(1,8-Dimethylimidazo[1,2-a]quinoxalin-4-yl)-1,2-ethanediamine hydrochloride (BMS-345541) and 2-[(aminocarbonyl)amino]-5-(4-fluorophenyl)-3-thiophenecarboxamide (TPCA-1), on IFN action in several human glioma cell lines. IKK inhibitors inhibit glioma cell proliferation, as well as TNF-induced RelA (p65) nuclear translocation and NFκB-dependent IL8 gene expression. Importantly, BMS-345541 and TPCA-1 differentially inhibit IFN-induced gene expression, completely suppressing MX1 and GBP1 gene expression, while having only a minor effect on ISG15 expression. Furthermore, these IKK inhibitors displayed marked differences in blocking IFN-induced antiviral action against cytopathic effects and replication of vesicular stomatitis virus (VSV) and encephalomyocarditis virus (EMCV). Our results show that the IKK complex plays an important function in IFN-induced gene expression and antiviral activity. Since VSV and EMCV are oncolytic viruses used in cancer therapy, our results indicate the potential synergy in combining IKK inhibitors with oncolytic viruses.

Use of replication restricted recombinant vesicular stomatitis virus vectors for detection of antigen-specific T cells

Detection of antigen-specific T cells at the single-cell level by ELISpot or flow cytometry techniques employing intracellular cytokine staining (ICS) is now an indispensable tool in many areas of immunology. When precisely mapped, optimal MHC-binding peptide epitopes are unknown, these assays use antigen in a variety of forms, including recombinant proteins, overlapping peptide sets representing one or more target protein sequences, microbial lysates, lysates of microbially-infected cells, or gene delivery vectors such as DNA expression plasmids or recombinant vaccinia or adenoviruses expressing a target protein of interest. Here we introduce replication-restricted, recombinant vesicular stomatitis virus (VSV) vectors as a safe, easy to produce, simple to use, and highly effective vector for genetic antigen delivery for the detection of human antigen-specific helper and cytotoxic T cells. To demonstrate the broad applicability of this approach, we have used these vectors to detect human T cell responses to the immunodominant pp65 antigen of human cytomegalovirus, individual segments of the yellow fever virus polyprotein, and to various influenza proteins.

Utilization of fluorescently-labeled tetracysteine-tagged proteins to study virus entry by live cell microscopy

Viruses exploit cellular machinery to gain entry and initiate their replication cycle within host cells. The development of methods to visualize virus entry in live cells has provided new insights to the cellular processes involved in virus entry and the intracellular locations where viral payloads are deposited. The use of fluorescently labeled virus and high-resolution microscopy is currently the method of choice to study virus entry in live cells. While fluorescent protein fusions (e.g. viral proteins fused to GFP) have been used, the labeling of viral proteins that contain a small tetracysteine (tc) tag with biarsenical fluorescent compounds (e.g. FlAsH, ReAsH, Lumio-x) offers several advantages over conventional xFP-fusion constructs. This article describes methods for generating fluorescently labeled viruses encoding tc-tagged proteins that are suitable for the study of virus entry in live cells by fluorescence microscopy. Critical parameters required to quantify fluorescence signals from the labeled, tc-tagged proteins in individual virus particles during the entry process and the subsequent fate of the labeled viral proteins after virus uncoating are also described.

The protease-sensitive loop of the vesicular stomatitis virus matrix protein is involved in virus assembly and protein translation

To study the contribution of the protease-sensitive loop of the VSV M protein in virus assembly we recovered recombinant VSV (rVSV) with mutations in this region and examined virus replication. Mutations in the highly conserved LXD motif (aa 123-125) resulted in reduced virion budding, reduced virus titers and enhanced M protein exchange with M-ribonucleocapsid complexes (M-RNPs), suggesting that the mutant M proteins were less tightly associated with RNP skeletons. In addition, viral protein synthesis began to decrease at 4h post-infection (hpi) and was reduced by ~80% at 8 hpi for the mutant rVSV-D125A. The reduced protein synthesis was not due to decreased VSV replication or transcription; however, translation of a reporter gene with an EMCV IRES was not reduced, suggesting that cap-dependent, but not cap-independent translation initiation was affected in rVSV-D125A infected cells. These results indicate that the LXD motif is involved in both virus assembly and VSV protein translation.

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.

Binding and activation of host plasminogen on the surface of Francisella tularensis

Background

Francisella tularensis (FT) is a gram-negative facultative intracellular coccobacillus and is the causal agent of a life-threatening zoonotic disease known as tularemia. Although FT preferentially infects phagocytic cells of the host, recent evidence suggests that a significant number of bacteria can be found extracellularly in the plasma fraction of the blood during active infection. This observation suggests that the interaction between FT and host plasma components may play an important role in survival and dissemination of the bacterium during the course of infection. Plasminogen (PLG) is a protein zymogen that is found in abundance in the blood of mammalian hosts. A number of both gram-positive and gram-negative bacterial pathogens have the ability to bind to PLG, giving them a survival advantage by increasing their ability to penetrate extracellular matrices and cross tissue barriers.

Results

We show that PLG binds to the surface of FT and that surface-bound PLG can be activated to plasmin in the presence of tissue PLG activator in vitro. In addition, using Far-Western blotting assays coupled with proteomic analyses of FT outer membrane preparations, we have identified several putative PLG-binding proteins of FT.

Conclusions

The ability of FT to acquire surface bound PLG that can be activated on its surface may be an important virulence mechanism that results in an increase in initial infectivity, survival, and/or dissemination of this bacterium in vivo.

Immunization with heat-killed Francisella tularensis LVS elicits protective antibody-mediated immunity

Francisella tularensis (FT) has been classified by the CDC as a category A pathogen because of its high virulence and the high mortality rate associated with infection via the aerosol route. Because there is no licensed vaccine available for FT, development of prophylactic and therapeutic regimens for the prevention/treatment of infection is a high priority. In this report, heat-killed FT live vaccine strain (HKLVS) was employed as a vaccine immunogen, either alone or in combination with an adjuvant, and was found to elicit protective immunity against high-dose FT live vaccine strain (FTLVS) challenge. FT-specific antibodies produced in response to immunization with HKLVS alone were subsequently found to completely protect naive mice against high-dose FT challenge in both infection-interference and passive immunization experiments. Additional passive immunization trials employing serum collected from mice immunized with a heat-killed preparation of an O-antigen-deficient transposon mutant of FTLVS (HKLVS-OAg(neg)) yielded similar results. These findings demonstrated that FT-specific antibodies alone can confer immunity against high-dose FTLVS challenge, and they reveal that antibody-mediated protection is not dependent upon production of LPS-specific antibodies.

A novel role for a YXXPhi motif in directing the caveolin-dependent sorting of membrane-spanning proteins

Previous studies showed that the sequence between amino acids 38 and 63 of the chicken AE1-4 anion exchanger is sufficient to direct basolateral sorting and recycling to the Golgi when fused to a cytoplasmic tailless F(c)RII B2 receptor. Further characterization of the recycling pathway has indicated that the chimera F(c)38-63 colocalizes with caveolin 1 in the basolateral membrane of MDCK cells, and in early endosomes following its internalization from the cell surface. Studies using small interfering RNA (siRNA) and dominant-negative mutants revealed that F(c)38-63 endocytosis is primarily caveolin-dependent and clathrin-independent. The endocytosis of the chimera is also dependent upon cholesterol and dynamin. Co-precipitation studies indicated that caveolin 1 associates with F(c)38-63. Mutation of the tyrosine or leucine residues in the cytoplasmic sequence Y(47)VEL of F(c)38-63 disrupts this interaction and inhibits the endocytosis of the chimera. Additional analyses revealed that AE1-4 also associates with caveolin 1. Mutation of the leucine in the Y(47)VEL sequence of AE1-4 disrupts this interaction, and blocks the recycling of this transporter from the basolateral membrane to the Golgi. The Y(47)VEL tetrapeptide matches the sequence of a YXXPhi motif, and our results indicate a novel role for this motif in directing caveolin-dependent sorting.

Replication-competent recombinant vesicular stomatitis virus encoding hepatitis C virus envelope proteins

Although in vitro replication of the hepatitis C virus (HCV) JFH1 clone of genotype 2a (HCVcc) has been developed, a robust cell culture system for the 1a and 1b genotypes, which are the most prevalent viruses in the world and resistant to interferon therapy, has not yet been established. As a surrogate virus system, pseudotype viruses transiently bearing HCV envelope proteins based on the vesicular stomatitis virus (VSV) and retrovirus have been developed. Here, we have developed a replication-competent recombinant VSV with a genome encoding unmodified HCV E1 and E2 proteins in place of the VSV envelope protein (HCVrv) in human cell lines. HCVrv and a pseudotype VSV bearing the unmodified HCV envelope proteins (HCVpv) generated in 293T or Huh7 cells exhibited high infectivity in Huh7 cells. Generation of infectious HCVrv was limited in some cell lines examined. Furthermore, HCVrv but not HCVpv was able to propagate and form foci in Huh7 cells. The infection of Huh7 cells with HCVpv and HCVrv was neutralized by anti-hCD81 and anti-E2 antibodies and by sera from chronic HCV patients. The infectivity of HCVrv was inhibited by an endoplasmic reticulum alpha-glucosidase inhibitor, N-(n-nonyl) deoxynojirimycin (Nn-DNJ), but not by a Golgi mannosidase inhibitor, deoxymannojirimycin. Focus formation of HCVrv in Huh7 cells was impaired by Nn-DNJ treatment. These results indicate that the HCVrv developed in this study can be used to study HCV envelope proteins with respect to not only the biological functions in the entry process but also their maturation step.

Generation and characterization of a recombinant vesicular stomatitis virus expressing the glycoprotein of Borna disease virus

Borna disease virus (BDV) is an enveloped virus with a nonsegmented negative-strand RNA genome whose organization is characteristic of mononegavirales. However, based on its unique genetics and biological features, BDV is considered to be the prototypic member of a new virus family, Bornaviridae, within the order Mononegavirales. BDV cell entry occurs via receptor-mediated endocytosis, a process initiated by the recognition of an as yet unidentified receptor at the cell surface by the BDV surface glycoprotein (G). The paucity of cell-free virus associated with BDV infection has hindered studies aimed at the elucidation of cellular receptors and detailed mechanisms involved in BDV cell entry. To overcome this problem, we generated and characterized a replication-competent recombinant vesicular stomatitis virus expressing BDV G (rVSVDeltaG*/BDVG). Cells infected with rVSVDeltaG*/BDVG produced high titers (10(7) PFU/ml) of cell-free virus progeny, but this virus exhibited a highly attenuated phenotype both in cell culture and in vivo. Attenuation of rVSVDeltaG*/BDVG was associated with a delayed kinetics of viral RNA replication and altered genome/N mRNA ratios compared to results for rVSVDeltaG*/VSVG. Likewise, incorporation of BDV G into virions appeared to be restricted despite its high levels of expression and efficient processing in rVSVDeltaG*/BDVG-infected cells. Notably, rVSVDeltaG*/BDVG recreated the cell tropism and entry pathway of bona fide BDV. Our results indicate that rVSVDeltaG*/BDVG represents a unique tool for the investigation of BDV G-mediated cell entry, as well as the roles of BDV G in host immune responses and pathogenesis associated with BDV infection.

IL-12-assisted immunization against Listeria monocytogenes using replication-restricted VSV-based vectors

Co-administration of IL-12 with vaccine immunogens has proven to be an effective strategy for eliciting potent Th1-biased immunity. Unfortunately, the use of IL-12 as a vaccine component has been limited because it is unstable at ambient temperatures, expensive to produce, and toxic when administered at excessive dosages. Using reverse genetics, we created a recombinant replication-restricted vesicular stomatitis virus that expresses large quantities of an IL-12 fusion protein (VSVDeltaG-IL12F), but can only establish a single round of infection because the genome does not encode the viral glycoprotein (G protein) that is required for viral entry into host cells. Here, we report that immunization of mice with a poorly immunogenic listerial antigen preparation (LMAg) in combination with VSVDeltaG-IL12F elicits potent T cell- and B cell-mediated responses that confer protective listerial immunity.

RNA interference is an antiviral defence mechanism in Caenorhabditis elegans

RNA interference (RNAi) is an evolutionarily conserved sequence-specific post-transcriptional gene silencing mechanism that is well defined genetically in Caenorhabditis elegans. RNAi has been postulated to function as an adaptive antiviral immune mechanism in the worm, but there is no experimental evidence for this. Part of the limitation is that there are no known natural viral pathogens of C. elegans. Here we describe an infection model in C. elegans using the mammalian pathogen vesicular stomatitis virus (VSV) to study the role of RNAi in antiviral immunity. VSV infection is potentiated in cells derived from RNAi-defective worm mutants (rde-1; rde-4), leading to the production of infectious progeny virus, and is inhibited in mutants with an enhanced RNAi response (rrf-3; eri-1). Because the RNAi response occurs in the absence of exogenously added VSV small interfering RNAs, these results show that RNAi is activated during VSV infection and that RNAi is a genuine antiviral immune defence mechanism in the worm.

Rhabdovirus assembly and budding

Rhabdoviruses are a diverse, widely-distributed group of enveloped viruses that assemble and bud from the plasma membrane of host cells. Recent advances in the identification of domains on both the envelope glycoprotein and the matrix protein of rhabdoviruses that contribute to virus assembly and release have allowed us to refine current models of rhabdovirus budding and to describe in better detail the interplay between both viral and cellular components involved in the budding process. In this review we discuss the steps involved in rhabdovirus assembly beginning with genome encapsidation and the association of nucleocapsid-matrix protein pre-assembly complexes with the inner leaflet of the plasma membrane, how condensation of these complexes may occur, how microdomains containing the envelope glycoprotein facilitate bud site formation, and how multiple forms of the matrix protein may participate in virion extrusion and release.

Kinetics of protective antibodies are determined by the viral surface antigen

Delayed and weak virus neutralizing antibody (nAb) responses represent a hallmark correlating not only with the establishment of persistent infection but also with unsuccessful vaccine development. Using a reverse genetic approach, we evaluated possible underlying mechanisms in 2 widely studied viral infection models. Swapping the glycoproteins (GPs) of lymphocytic choriomeningitis virus (LCMV, naturally persisting, noncytolytic, inefficient nAb inducer) and vesicular stomatitis virus (VSV, nonpersisting, cytolytic, potent nAb inducer) transferred the only target of nAb's from either virus to the other. We analyzed the nAb response to each of the 2 recombinant and parent viruses in infected mice and found that nAb kinetics were solely determined by the viral surface GP and not by the virus backbone. Moreover, the slowly and poorly nAb-triggering LCMV virion was a potent immunogenic matrix for the more antigenic VSV-GP. These findings indicate that the viral GP determines nAb kinetics largely independently of the specific viral infection context. They further suggest that structural features of viral GPs or coevolutionary adaptation of the virus's GP to the host's naive B cell repertoire, or both, may critically limit nAb kinetics and improvement of vaccine efficacy.

Functional analysis of late-budding domain activity associated with the PSAP motif within the vesicular stomatitis virus M protein

A PPPY motif within the M protein of vesicular stomatitis virus (VSV) functions as a late-budding domain (L-domain); however, L-domain activity has yet to be associated with a downstream PSAP motif. VSV recombinants with mutations in the PPPY and/or PSAP motif were recovered by reverse genetics and examined for growth kinetics, plaque size, and budding efficiency by electron microscopy. Results indicate that unlike the PPPY motif, the PSAP motif alone does not possess L-domain activity. Finally, the insertion of the human immunodeficiency virus type 1 p6 L-domain and flanking sequences into the PSAP region of M protein rescued budding of a PPPY mutant of VSV to wild-type levels.

Recombinant vesicular stomatitis virus vectors as oncolytic agents in the treatment of high-grade gliomas in an organotypic brain tissue slice—glioma coculture model

Object. The purpose of this study was to evaluate both replication-competent and replication-restricted recombinant vesicular stomatitis virus (VSV) vectors as therapeutic agents for high-grade gliomas by using an organotypic brain tissue slice—glioma coculture system.

Methods. The coculture system involved growing different brain structures together to allow neurons from these tissues to develop synaptic connections similar to those found in vivo. Rat C6 or human U87 glioma cells were then introduced into the culture to evaluate VSV as an oncolytic therapy. The authors found that recombinant wild-type VSV (rVSV-wt) rapidly eliminated C6 glioma cells from the coculture, but also caused significant damage to neurons, as measured by a loss of microtubule-associated protein 2 immunoreactivity and a failure in electrophysiological responses from neurons in the tissue slice. Nonetheless, pretreatment with interferon beta (IFNβ) virtually eliminated VSV infection in healthy tissues without impeding any oncolytic effects on tumor cells. Despite the protective effects of the IFNβ pretreatment, the tissue slices still showed signs of cytopathology when exposed to rVSV-wt. In contrast, pretreatment with IFNβ and inoculation with a replication-restricted vector with its glycoprotein gene deleted (rVSV-ΔG) effectively destroyed rat C6 and human U87 glioma cells in the coculture, without causing detectable damage to the neuronal integrity and electrophysiological properties of the healthy tissue in the culture.

Conclusions. Data in this study provide in vitro proof-of-principle that rVSV-ΔG is an effective oncolytic agent that has minimal toxic side effects to neurons compared with rVSV-wt and therefore should be considered for development as an adjuvant to surgery in the treatment of glioma.

Role of the Cyclic AMP-dependent Protein Kinase in Homologous Resensitization of the β1-Adrenergic Receptor

A fundamental question in biology is how the various motifs in G protein-coupled receptors participate in the divergent functions orchestrated by these molecules. Here we describe a fundamental role for a serine residue at position 312 in the third intracellular loop of the human beta(1)-adrenergic receptor (beta(1)-AR) in endocytic recycling of the agonist-internalized receptor. In receptor recycling experiments that were monitored by confocal microscopy, the agonist-internalized wild-type (WT) beta(1)-AR recycled with a t(0.5) of 14 +/- 3 min. Mutagenesis of Ser(312) to alanine (Ser(312) --> Ala beta(1)-AR) or to the phosphoserine mimic aspartic acid (Ser(312) --> Asp beta(1)-AR) resulted in beta(1)-AR constructs that were pharmacologically indistinguishable from the WT beta(1)-AR. The internalized Ser(312) --> Asp beta(1)-AR recycled efficiently with a t(0.5) of 11 +/- 3 min, whereas the internalized Ser(312) --> Ala beta(1)-AR was not recycled or functionally resensitized through the endosomal pathway. Because this serine is a putative residue for phosphorylation by the cyclic AMP-dependent protein kinase (PKA), we examined the role of this kinase in recycling of the internalized beta(1)-AR. Inhibition of PKA biochemically or genetically using a dominant negative PKA construct blocked the recycling of the internalized WT beta(1)-AR. Phosphorylation studies revealed that the beta(1)-AR is partially phosphorylated by PKA and that phosphorylation of the beta(1)-AR by the catalytic subunit of PKA occurs exclusively at Ser(312). Our results identify a new signaling paradigm in which homologous activation of a kinase provides a reversible modification that shifts the itinerary of the internalized receptor toward recycling and resensitization. Therefore, PKA-mediated phosphorylation of G protein-coupled receptors might result in motif-dependent desensitization or resensitization.

Recombinant replication-restricted VSV as an expression vector for murine cytokines

Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus that rapidly and efficiently shuts down the production of host cell-encoded proteins and utilizes the cell's protein production machinery to express high levels of virally encoded proteins. In an effort to take advantage of this characteristic of VSV, we have employed a reverse genetics system to create recombinant forms of VSV encoding a variety of murine cytokines. Previous studies have revealed that cells infected with recombinant VSV that lack expression of the surface glycoprotein (G protein), designated deltaG-VSV, more efficiently express and secrete recombinant proteins than do recombinant "wild-type" VSV. Therefore, murine cytokine-expressing recombinants were produced as deltaG viruses. Propagation of these deltaG viruses in cells that transiently express G protein in vitro results in G-complemented virions that can infect cells, shut down host protein synthesis, and express at high levels each virally encoded protein (including the designated cytokine). We assessed the ability of each deltaG-VSV construct to express recombinant cytokine by infecting BHK cells and then monitoring/measuring the production of the desired cytokine. When possible, the bioactivity of the cytokine products was also measured. The results presented here reveal that large quantities of bioactive cytokines can be produced rapidly and inexpensively using deltaG-VSV as a protein expression system.

The membrane-proximal region of vesicular stomatitis virus glycoprotein G ectodomain is critical for fusion and virus infectivity

The glycoprotein (G) of vesicular stomatitis virus (VSV) is responsible for binding of virus to cells and for mediating virus entry following endocytosis by inducing fusion of the viral envelope with the endosomal membrane. The fusion peptide of G is internal (residues 116 to 137) and exhibits characteristics similar to those of other internal fusion peptides, but recent studies have implicated the region adjacent to the transmembrane domain as also being important for G-mediated membrane fusion. Sequence alignment of the membrane-proximal region of G from several different vesiculoviruses revealed that this domain is highly conserved, suggesting that it is important for G function. Mutational analysis was used to show that this region is not essential for G protein oligomerization, transport to the cell surface, or incorporation into virus particles but that it is essential for acid-induced membrane fusion activity and for virus infectivity. Deletion of the 13 membrane-proximal amino acids (N449 to W461) dramatically reduced cell-cell fusion activity and reduced virus infectivity approximately 100-fold, but mutation of conserved aromatic residues (W457, F458, and W461) either singly or together had only modest effects on cell-cell fusion activity; recombinant virus encoding these mutants replicated as efficiently as wild-type (WT) VSV. Insertion of heterologous sequences in the juxtamembrane region completely abolished membrane fusion activity and virus infectivity, as did deletion of residues F440 to N449. The insertion mutants showed some changes in pH-dependent conformational changes and in virus binding, which could partially explain the defects in membrane fusion activity, but all the other mutants were similar to WT G with respect to conformational changes and virus binding. These data support the hypothesis that the membrane-proximal domain contributes to G-mediated membrane fusion activity, yet the conserved aromatic residues are not essential for membrane fusion or virus infectivity.

The membrane-proximal domain of vesicular stomatitis virus G protein functions as a membrane fusion potentiator and can induce hemifusion

Recently we showed that the membrane-proximal stem region of the vesicular stomatitis virus (VSV) G protein ectodomain (G stem [GS]), together with the transmembrane and cytoplasmic domains, was sufficient to mediate efficient VSV budding (C. S. Robison and M. A. Whitt, J. Virol. 74:2239-2246, 2000). Here, we show that GS can also potentiate the membrane fusion activity of heterologous viral fusion proteins when GS is coexpressed with those proteins. For some fusion proteins, there was as much as a 40-fold increase in syncytium formation when GS was coexpressed compared to that seen when the fusion protein was expressed alone. Fusion potentiation by GS was not protein specific, since it occurred with both pH-dependent as well as pH-independent fusion proteins. Using a recombinant vesicular stomatitis virus encoding GS that contained an N-terminal hemagglutinin (HA) tag (GS(HA) virus), we found that the GS(HA) virus bound to cells as well as the wild-type virus did at pH 7.0; however, the GS(HA) virus was noninfectious. Analysis of cells expressing GS(HA) in a three-color membrane fusion assay revealed that GS(HA) could induce lipid mixing but not cytoplasmic mixing, indicating that GS can induce hemifusion. Treatment of GS(HA) virus-bound cells with the membrane-destabilizing drug chlorpromazine rescued the hemifusion block and allowed entry and subsequent replication of GS(HA) virus, demonstrating that GS-mediated hemifusion was a functional intermediate in the membrane fusion pathway. Using a series of truncation mutants, we also determined that only 14 residues of GS, together with the VSV G transmembrane and cytoplasmic tail, were sufficient for fusion potentiation. To our knowledge, this is the first report which shows that a small domain of one viral glycoprotein can promote the fusion activity of other, unrelated viral glycoproteins.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Identification of two additional translation products from the matrix (M) gene that contribute to vesicular stomatitis virus cytopathology

The matrix (M) protein of vesicular stomatitis virus (VSV) is a multifunctional protein that is responsible for condensation of the ribonucleocapsid core during virus assembly and also plays a critical role in virus budding. The M protein is also responsible for most of the cytopathic effects (CPE) observed in infected cells. VSV CPE include inhibition of host gene expression, disablement of nucleocytoplasmic transport, and disruption of the host cytoskeleton, which results in rounding of infected cells. In this report, we show that the VSV M gene codes for two additional polypeptides, which we have named M2 and M3. These proteins are synthesized from downstream methionines in the same open reading frame as the M protein (which we refer to here as M1) and lack the first 32 (M2) or 50 (M3) amino acids of M1. Infection of cells with a recombinant virus that does not express M2 and M3 (M33,51A) resulted in a delay in cell rounding, but virus yield was not affected. Transient expression of M2 and M3 alone caused cell rounding similar to that with the full-length M1 protein, suggesting that the cell-rounding function of the M protein does not require the N-terminal 50 amino acids. To determine if M2 and M3 were sufficient for VSV-mediated CPE, both M2 and M3 were expressed from a separate cistron in a VSV mutant background that readily establishes persistent infections and that normally lacks CPE. Infection of cells with the recombinant virus that expressed M2 and M3 resulted in cell rounding indistinguishable from that with the wild-type recombinant virus. These results suggest that M2 and M3 are important for cell rounding and may play an important role in viral cytopathogenesis. To our knowledge, this is first report of the multiple coding capacities of a rhabdovirus matrix gene.

Adjuvanticity of an IL-12 fusion protein expressed by recombinant deltaG-vesicular stomatitis virus

The remarkable immunomodulatory and adjuvant properties of rIL-12 have been well described. Many early studies documenting the adjuvanticity of IL-12 were performed using the murine model of Listeria monocytogenes infection. In this report, we describe the construction of an attenuated recombinant vesicular stomatitis virus (VSV-deltaG) that encodes a single-chain IL-12 fusion protein (IL-12F), and the use of this virus as an expression vector to produce large quantities of IL-12F. VSV-expressed IL-12F (vIL-12F) was then co-administered to mice along with a poorly immunogenic listerial antigen preparation as a vaccine regimen and the resulting immune responses were monitored. The vIL-12F was found to have adjuvant properties similar to those observed for rIL-12. Co-administration of vIL-12F and listerial antigen elicited powerful cell-mediated immune responses that conferred long-lived protective listerial immunity. These studies demonstrated that VSVdeltaG-IL12F-infected cells secrete bioactive single-chain IL-12, and laid the foundation for studies using VSVdeltaG-IL12F as a vector for delivery of IL-12F in vivo.

Rhabdoviruses and the cellular ubiquitin-proteasome system: a budding interaction

The matrix (M) proteins of vesicular stomatitis virus (VSV) and rabies virus (RV) play a key role in both assembly and budding of progeny virions. A PPPY motif (PY motif or late-budding domain) is conserved in the M proteins of VSV and RV. These PY motifs are important for virus budding and for mediating interactions with specific cellular proteins containing WW domains. The PY motif and flanking sequences of the M protein of VSV were used as bait to screen a mouse embryo cDNA library for cellular interactors. The mouse Nedd4 protein, a membrane-localized ubiquitin ligase containing multiple WW domains, was identified from this screen. Ubiquitin ligase Rsp5, the yeast homolog of Nedd4, was able to interact both physically and functionally with full-length VSV M protein in a PY-dependent manner. Indeed, the VSV M protein was multiubiquitinated by Rsp5 in an in vitro ubiquitination assay. To demonstrate further that ubiquitin may be involved in the budding process of rhabdoviruses, proteasome inhibitors (e.g., MG132) were used to decrease the level of free ubiquitin in VSV- and RV-infected cells. Viral titers measured from MG132-treated cells were reproducibly 10- to 20-fold lower than those measured from untreated control cells, suggesting that free ubiquitin is important for efficient virus budding. Last, release of a VSV PY mutant was not inhibited in the presence of MG132, signifying that the functional L domain of VSV is required for the inhibitory effect exhibited by MG132. These data suggest that the cellular ubiquitin-proteasome machinery is involved in the budding process of VSV and RV.

Interaction of coxsackievirus B3 with the full length coxsackievirus-adenovirus receptor

Group B coxsackieviruses (CVB) utilize the coxsackievirus-adenovirus receptor (CAR) to recognize host cells. CAR is a membrane protein with two Ig-like extracellular domains (D1 and D2), a transmembrane domain and a cytoplasmic domain. The three-dimensional structure of coxsackievirus B3 (CVB3) in complex with full length human CAR and also with the D1D2 fragment of CAR were determined to approximately 22 A resolution using cryo-electron microscopy (cryo-EM). Pairs of transmembrane domains of CAR associate with each other in a detergent cloud that mimics a cellular plasma membrane. This is the first view of a virus-receptor interaction at this resolution that includes the transmembrane and cytoplasmic portion of the receptor. CAR binds with the distal end of domain D1 in the canyon of CVB3, similar to how other receptor molecules bind to entero- and rhinoviruses. The previously described interface of CAR with the adenovirus knob protein utilizes a side surface of D1.

Characterization of pseudotype VSV possessing HCV envelope proteins

The genome of hepatitis C virus (HCV) encodes two envelope glycoproteins (E1 and E2), which are thought to be responsible for receptor binding and membrane fusion resulting in virus penetration. To investigate cell surface determinants important for HCV infection, we used a recombinant vesicular stomatitis virus (VSV) in which the glycoprotein gene was replaced with a reporter gene encoding green fluorescent protein (GFP) and produced HCV-VSV pseudotypes possessing chimeric HCV E1 or E2 glycoproteins, either individually or together. The infectivity of the pseudotypes was determined by quantifying the number of cells expressing the GFP reporter gene. Pseudotypes that contained both of the chimeric E1 and E2 proteins exhibited 10--20 times higher infectivity on HepG2 cells than the viruses possessing either of the glycoproteins individually. These results indicated that both E1 and E2 envelope proteins are required for maximal infection by HCV. The infectivity of the pseudotype virus was not neutralized by anti-VSV polyclonal antibodies. Bovine lactoferrin specifically inhibited the infection of the pseudotype virus. Treatment of HepG2 cells with Pronase, heparinase, and heparitinase but not with phospholipase C and sodium periodate reduced the infectivity. Therefore, cell surface proteins and some glycosaminoglycans play an important role in binding or entry of HCV into susceptible cells. The pseudotype VSV possessing the chimeric HCV glycoproteins might offer an efficient tool for future research on cellular receptors for HCV and for the development of prophylactics and therapeutics for hepatitis C.

N-terminal domain of Borna disease virus G (p56) protein is sufficient for virus receptor recognition and cell entry

Borna disease virus (BDV) surface glycoprotein (GP) (p56) has a predicted molecular mass of 56 kDa. Due to extensive posttranslational glycosylation the protein migrates as a polypeptide of 84 kDa (gp84). The processing of gp84 by the cellular protease furin generates gp43, which corresponds to the C-terminal part of gp84. Both gp84 and gp43 have been implicated in viral entry involving receptor-mediated endocytosis and pH-dependent fusion. We have investigated the domains of BDV p56 involved in virus entry. For this, we used a pseudotype approach based on a recently developed recombinant vesicular stomatitis virus (VSV) in which the gene for green fluorescent protein was substituted for the VSV G protein gene (VSV Delta G*). Complementation of VSV Delta G* with BDV p56 resulted in infectious VSV Delta G* pseudotypes that contained both BDV gp84 and gp43. BDV-VSV chimeric GPs that contained the N-terminal 244 amino acids of BDV p56 and amino acids 421 to 511 of VSV G protein were efficiently incorporated into VSV Delta G* particles, and the resulting pseudotype virions were neutralized by BDV-specific antiserum. These findings indicate that the N-terminal part of BDV p56 is sufficient for receptor recognition and virus entry.

Mutations in the PPPY motif of vesicular stomatitis virus matrix protein reduce virus budding by inhibiting a late step in virion release

The N terminus of the matrix (M) protein of vesicular stomatitis virus (VSV) and of other rhabdoviruses contains a highly conserved PPPY sequence (or PY motif) similar to the late (L) domains in the Gag proteins of some retroviruses. These L domains in retroviral Gag proteins are required for efficient release of virus particles. In this report, we show that mutations in the PPPY sequence of the VSV M protein reduce virus yield by blocking a late stage in virus budding. We also observed a delay in the ability of mutant viruses to cause inhibition of host gene expression compared to wild-type (WT) VSV. The effect of PY mutations on virus budding appears to be due to a block at a stage just prior to virion release, since electron microscopic examination of PPPA mutant-infected cells showed a large number of assembled virions at the plasma membrane trapped in the process of budding. Deletion of the glycoprotein (G) in addition to these mutations further reduced the virus yield to less than 1% of WT levels, and very few particles were assembled at the cell surface. This observation suggested that G protein aids in the initial stage of budding, presumably during the formation of the bud site. Overall, our results confirm that the PPPY sequence of the VSV M protein possesses L domain activity analogous to that of the retroviral Gag proteins.

The membrane-proximal stem region of vesicular stomatitis virus G protein confers efficient virus assembly

In this report, we show that the glycoprotein of vesicular stomatitis virus (VSV G) contains within its extracellular membrane-proximal stem (GS) a domain that is required for efficient VSV budding. To determine a minimal sequence in GS that provides for high-level virus assembly, we have generated a series of recombinant DeltaG-VSVs which express chimeric glycoproteins having truncated stem sequences. The recombinant viruses having chimeras with 12 or more membrane-proximal residues of the G stem, and including the G protein transmembrane-cytoplasmic tail domains, produced near-wild-type levels of particles. In contrast, viruses encoding chimeras with shorter or no G-stem sequences produced approximately 10- to 20-fold less. This budding domain when present in chimeric glycoproteins also promoted their incorporation into the VSV envelope. We suggest that the G-stem budding domain promotes virus release by inducing membrane curvature at sites where virus budding occurs or by recruiting condensed nucleocapsids to sites on the plasma membrane which are competent for efficient virus budding.

Mutational analysis of the putative fusion domain of Ebola virus glycoprotein

Ebola viruses contain a single glycoprotein (GP) spike, which functions as a receptor binding and membrane fusion protein. It contains a highly conserved hydrophobic region (amino acids 524 to 539) located 24 amino acids downstream of the N terminus of the Ebola virus GP2 subunit. Comparison of this region with the structural features of the transmembrane subunit of avian retroviral GPs suggests that the conserved Ebola virus hydrophobic region may, in fact, serve as the fusion peptide. To test this hypothesis directly, we introduced conservative (alanine) and nonconservative (arginine) amino acid substitutions at eight positions in this region of the GP2 molecule. The effects of these mutations were deduced from the ability of the Ebola virus GP to complement the infectivity of a vesicular stomatitis virus (VSV) lacking the receptor-binding G protein. Some mutations, such as Ile-to-Arg substitutions at positions 532 (I532R), F535R, G536A, and P537R, almost completely abolished the ability of the GP to support VSV infectivity without affecting the transport of GP to the cell surface and its incorporation into virions or the production of virus particles. Other mutations, such as G528R, L529A, L529R, I532A, and F535A, reduced the infectivity of the VSV-Ebola virus pseudotypes by at least one-half. These findings, together with previous reports of liposome association with a peptide corresponding to positions 524 to 539 in the GP molecule, offer compelling support for a fusion peptide role for the conserved hydrophobic region in the Ebola virus GP.

Transcript initiation and 5'-end modifications are separable events during vesicular stomatitis virus transcription

In this report we describe a novel, bipartite vesicular stomatitis virus (VSV) replication system which was used to study the effect of mutations in the transcription start sequence on transcript initiation and 5'-mRNA modifications. The bipartite replication system consisted of two genomic RNAs, one of which (VSVDeltaG) was a recombinant VSV genome with the G gene deleted and the other (GFC) contained the G gene and two non-VSV reporter genes (green fluorescent protein [GFP] and chloramphenicol acetyltransferase [CAT]). Coinfection of cells with these two components resulted in high-level virus production and gave titers similar to that from wild-type-VSV-infected cells. Mutations were introduced within the first 3 nucleotides of the transcription start sequence of the third gene (CAT) of GFC. The effects of these changes on the synthesis and accumulation of CAT transcripts during in vivo transcription (e.g., in infected cells), and during in vitro transcription were determined. As we had reported previously (E. A. Stillman and M. A. Whitt, J. Virol. 71:2127-2137, 1997), changing the first and third nucleotides (NT-1 and NT-3) reduced CAT transcript levels in vivo to near undetectable levels. Similarly, changing NT-2 to a purine also resulted in the detection of very small amounts of CAT mRNA from infected cells. In contrast to the results in vivo, the NT-1C mutant and all of the second-position mutants produced near-wild-type amounts of CAT mRNA in the in vitro system, indicating that the mutations did not prevent transcript initiation per se but, rather, generated transcripts that were unstable in vivo. Oligo (dT) selection and Northern blot analysis revealed that the transcripts produced from these mutants did not contain a poly(A)(+) tail and were truncated, ranging in size from 40 to 200 nucleotides. Immunoprecipitation analysis of cDNA-RNA hybrids with an antibody that recognizes trimethylguanosine revealed that the truncated mutant transcripts were not properly modified at the 5' end, indicating the transcripts either were not capped or were not methylated. This is the first demonstration that transcript initiation and capping/methylation are separable events during VSV transcription. A model is proposed in which polymerase processivity is linked to proper 5'-end modification. The model suggests that a proofreading mechanism exists for VSV and possibly other nonsegmented minus-strand RNA viruses, whereby if some transcripts do not become capped during transcription in a normal infection, a signal is transduced such that the polymerase undergoes abortive elongation and the defective transcript is terminated prematurely and subsequently degraded.

The length and sequence composition of vesicular stomatitis virus intergenic regions affect mRNA levels and the site of transcript initiation

In this study, we used a dicistronic vesicular stomatitis virus (VSV) minigenome to investigate the effects of either single or multiple nucleotide insertions placed immediately after the nontranscribed intergenic dinucleotide of the M gene on VSV transcription. Both Northern blot and primer extension analysis showed that the polymerase responded to the inserted nucleotides in a sequence-specific manner such that some insertions had no effect on mRNA synthesis from the downstream G gene, nor on the site of transcript initiation, whereas other insertions resulted in dramatic reductions in transcript accumulation. Some of these transcripts were initiated at the wild-type site, while others initiated within the inserted sequence. We also examined the transcriptional events that occurred when a natural, 21-nucleotide intergenic region located between the G and L genes from the New Jersey (NJ) serotype of VSV was inserted into the minigenome gene junction. In contrast to the normal 25 to 30% attenuation observed for downstream transcription at gene junctions containing the typical dinucleotide (3'-GA-5') intergenic region, the NJ variant showed greater than 75% attenuation at the gene junction. In addition, the polymerase initiated transcription at two major start sites, one of which was located within the intergenic sequence. Collectively, these data suggest that the polymerase "samples" the intergenic sequences following polyadenylation and termination of the upstream transcript by scanning until an appropriate start site is found. One implication of a scanning polymerase is that the polymerase presumably switches states from a processive elongation mode to a stuttering mode for polyadenylation to one in which no transcription occurs, before it reinitiates at the downstream gene. Our data support the hypothesis that sequences surrounding the intergenic region modulate these events such that appropriate amounts of each mRNA are synthesized.

Attenuation of recombinant vesicular stomatitis viruses encoding mutant glycoproteins demonstrate a critical role for maintaining a high pH threshold for membrane fusion in viral fitness

A plasmid-based recovery system was used to generate four unique vesicular stomatitis virus (VSV) mutants that encode glycoproteins (G proteins) with single or double amino acid substitutions in two conserved acidic residues adjacent to the putative G protein fusion domain. Previously we demonstrated that three of the mutant G proteins (D137-L, E139-L, and DE-SS) have slightly reduced pH thresholds for membrane fusion activity. In this report we show that even though the viruses encoding D137-L, E139-L, and DE-SS were recovered with high efficiency, these mutants were attenuated for growth in cell culture. Plaque formation was significantly delayed with these mutants and the plaques were smaller and more diffuse than those produced by wild type VSV. In addition, cells infected with these mutants produced approximately 5- to 10-fold less infectious virus than cells infected with a similarly recovered VSV encoding the wild-type G protein. Using R18-labeled virus we found that the mutant G proteins had approximately 50% of the fusion activity of wild-type G at pH 6.3 and only 75% activity at pH 5.8. We also show that the mutant viruses were more sensitive to chloroquine inhibition of infection than either wild-type VSV or the mutant E139-T, which has a fusion phenotype similar to wild-type G protein. Reduced fusion activity and attenuation of infectivity was not due to differences in the amount of G protein incorporated into virions, nor to differences in the amount of virus binding to cells at physiological pH. Although infectivity was assayed at neutral pH, we observed an increase in virus binding with both mutant and wild-type virions as the pH was lowered, and the increase in binding occurred near the pH threshold for membrane fusion activity. From these data we propose a model in which VSV entry involves an increase in virus binding to the inner leaflet of the endosomal membrane during endosome acidification. Concomitant with this higher affinity binding, G protein becomes primed to initiate fusion of the viral envelope with the endosomal membrane. Viruses with mutations that delay the onset of increased binding and fusion lag behind wild-type VSV in their ability to initiate a productive infection, potentially because the location within the cytoplasm where these viruses ultimately fuse is not optimal for either virus uncoating or replication of the viral genome.

A system for functional analysis of Ebola virus glycoprotein

Ebola virus causes hemorrhagic fever in humans and nonhuman primates, resulting in mortality rates of up to 90%. Studies of this virus have been hampered by its extraordinary pathogenicity, which requires biosafety level 4 containment. To circumvent this problem, we developed a novel complementation system for functional analysis of Ebola virus glycoproteins. It relies on a recombinant vesicular stomatitis virus (VSV) that contains the green fluorescent protein gene instead of the receptor-binding G protein gene (VSVDeltaG*). Herein we show that Ebola Reston virus glycoprotein (ResGP) is efficiently incorporated into VSV particles. This recombinant VSV with integrated ResGP (VSVDeltaG*-ResGP) infected primate cells more efficiently than any of the other mammalian or avian cells examined, in a manner consistent with the host range tropism of Ebola virus, whereas VSVDeltaG* complemented with VSV G protein (VSVDeltaG*-G) efficiently infected the majority of the cells tested. We also tested the utility of this system for investigating the cellular receptors for Ebola virus. Chemical modification of cells to alter their surface proteins markedly reduced their susceptibility to VSVDeltaG*-ResGP but not to VSVDeltaG*-G. These findings suggest that cell surface glycoproteins with N-linked oligosaccharide chains contribute to the entry of Ebola viruses, presumably acting as a specific receptor and/or cofactor for virus entry. Thus, our VSV system should be useful for investigating the functions of glycoproteins from highly pathogenic viruses or those incapable of being cultured in vitro.

Mutational analyses of the intergenic dinucleotide and the transcriptional start sequence of vesicular stomatitis virus (VSV) define sequences required for efficient termination and initiation of VSV transcripts

We have used dicistronic vesicular stomatitis virus (VSV) minigenomes to dissect the functional importance of the nontranscribed intergenic dinucleotide and the conserved transcription start sequence found at the beginning of all VSV genes. The minigenomes were generated entirely from cDNA and contained the G and M protein genes, flanked by the leader and trailer regions from the Indiana serotype of VSV. All mutations were made either within the nontranscribed M-G intergenic dinucleotide or within the transcription start sequence of the downstream G gene. Immunofluorescence microscopy and immunoprecipitation analysis of the mutated minigenomes indicated that the first three nucleotides of the transcriptional start sequence are the most critical for efficient VSV gene expression, whereas the nontranscribed, intergenic dinucleotide and the other conserved nucleotides found at the 5' mRNA start sequence can tolerate significant sequence variability without affecting G protein production. RNA analysis indicated that nucleotide changes in the transcriptional start sequence which resulted in reduced G protein expression correlated with the amount of transcript present. Therefore, this conserved sequence appears to be required for efficient transcript initiation following polyadenylation of the upstream mRNA. While the minimum sequence for efficient transcription (3'-UYGnn-5') is similar to that of other rhabdoviruses, it is not homologous to the start sites for viruses from the Paramyxoviridae or Filoviridae families. Using Northern blot analysis, we also found that some nucleotide changes in the nontranscribed intergenic region resulted in higher levels of read-through transcription. Therefore, the nontranscribed intergenic dinucleotide plays a role in transcript termination.

The minimal conserved transcription stop-start signal promotes stable expression of a foreign gene in vesicular stomatitis virus

A new transcription unit was generated in the 3' noncoding region of the vesicular stomatitis virus (VSV) glycoprotein gene by introducing the smallest conserved sequence found at each VSV gene junction. This sequence was introduced into a DNA copy of the VSV genome from which infectious VSV can be derived. It contained an 11-nucleotide putative transcription stop/polyadenylation signal for the glycoprotein mRNA, an intergenic dinucleotide, and a 10-nucleotide putative transcription start sequence preceding a downstream foreign gene encoding the bacterial enzyme chloramphenicol acetyltransferase. Infectious recombinant VSV was recovered from this construct and was found to express high levels of functional chloramphenicol acetyltransferase mRNA and protein. The recombinant virus grew to wild-type titers of 5 x 10(9)/ml, and expression of the foreign gene was completely stable for at least 15 passages involving 10(6)-fold expansion at each passage. These results define functionally the transcription stop/polyadenylation and start sequences for VSV and also illustrate the utility of VSV as a stable vector that should have wide application in cell biology and vaccine development.

Mutations at two conserved acidic amino acids in the glycoprotein of vesicular stomatitis virus affect pH-dependent conformational changes and reduce the pH threshold for membrane fusion

Recently, we had shown that amino acid substitutions at residues 124, 127, and 133 either abolished or drastically altered the fusion activity of the glycoprotein (G protein) of vesicular stomatitis virus (VSV), indicating that this region is important for membrane fusion and may constitute an internal fusion domain. In this report we show that amino acid substitutions at two conserved acidic residues located at the C-terminal end of the putative fusion domain also affect the fusion activity of G protein. Two substitutions, D127-L and E139-L, slightly reduced the pH threshold at which G protein mediates membrane fusion. However, both D137-L and E139-L had fusion activities equivalent to that of wild-type G protein after exposure to a pH of 5.7 or below. To determine if the fusion activity of G protein required an acidic residue in this region both D137 and E139 were replaced with serine. This double substitution also reduced the pH threshold for fusion activity, but the effect appeared to be less severe than that for either of the single substitutions. The mutated G protein cDNAs were also introduced into a VSV minigenome to examine the effect of the substitutions on virus assembly and infectivity. All three mutant G proteins assembled into particles and these virions were infectious despite the altered fusion activities of the mutant G proteins. Although particles containing the mutant proteins were infectious they appeared to be attenuated, suggesting that these two acidic residues, which are conserved in several distantly related rhabdoviruses, play an important role in maintaining virus fitness.