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.