Multigenic Relation to the Attenuation of Rabies Virus

The Amino Acid at Position 95 in the Matrix Protein of Rabies Virus Is Involved in Antiviral Stress Granule Formation in Infected Cells

Stress granules (SGs) are dynamic structures that store cytosolic messenger ribonucleoproteins. SGs have recently been shown to serve as a platform for activating antiviral innate immunity; however, several pathogenic viruses suppress SG formation to evade innate immunity. In this study, we investigated the relationship between rabies virus (RABV) virulence and SG formation, using viral strains with different levels of virulence. We found that the virulent Nishigahara strain did not induce SG formation, but its avirulent offshoot, the Ni-CE strain, strongly induced SG formation. Furthermore, we demonstrated that the amino acid at position 95 in the RABV matrix protein (M95), a pathogenic determinant for the Nishigahara strain, plays a key role in inhibiting SG formation, followed by protein kinase R (PKR)-dependent phosphorylation of the α subunit of eukaryotic initiation factor 2α (eIF2α). M95 was also implicated in the accumulation of RIG-I, a viral RNA sensor protein, in SGs and in the subsequent acceleration of interferon induction. Taken together, our findings strongly suggest that M95-related inhibition of SG formation contributes to the pathogenesis of RABV by allowing the virus to evade the innate immune responses of the host. IMPORTANCE Rabies virus (RABV) is a neglected zoonotic pathogen that causes lethal infections in almost all mammalian hosts, including humans. Recently, RABV has been reported to induce intracellular formation of stress granules (SGs), also known as platforms that activate innate immune responses. However, the relationship between SG formation capacity and pathogenicity of RABV has remained unclear. In this study, by comparing two RABV strains with completely different levels of virulence, we found that the amino acid mutation from valine to alanine at position 95 of matrix protein (M95), which is known to be one of the amino acid mutations that determine the difference in virulence between the strains, plays a major role in SG formation. Importantly, M95 was involved in the accumulation of RIG-I in SGs and in promoting interferon induction. These findings are the first report of the effect of a single amino acid substitution associated with SGs on viral virulence.

Deactivation of the antiviral state by rabies virus through targeting and accumulation of persistently phosphorylated STAT1

Antagonism of the interferon (IFN)-mediated antiviral state is critical to infection by rabies virus (RABV) and other viruses, and involves interference in the IFN induction and signaling pathways in infected cells, as well as deactivation of the antiviral state in cells previously activated by IFN. The latter is required for viral spread in the host, but the precise mechanisms involved and roles in RABV pathogenesis are poorly defined. Here, we examined the capacity of attenuated and pathogenic strains of RABV that differ only in the IFN-antagonist P protein to overcome an established antiviral state. Importantly, P protein selectively targets IFN-activated phosphorylated STAT1 (pY-STAT1), providing a molecular tool to elucidate specific roles of pY-STAT1. We find that the extended antiviral state is dependent on a low level of pY-STAT1 that appears to persist at a steady state through ongoing phosphorylation/dephosphorylation cycles, following an initial IFN-induced peak. P protein of pathogenic RABV binds and progressively accumulates pY-STAT1 in inactive cytoplasmic complexes, enabling recovery of efficient viral replication over time. Thus, P protein-pY-STAT1 interaction contributes to ‘disarming’ of the antiviral state. P protein of the attenuated RABV is defective in this respect, such that replication remains suppressed over extended periods in cells pre-activated by IFN. These data provide new insights into the nature of the antiviral state, indicating key roles for residual pY-STAT1 signaling. They also elucidate mechanisms of viral deactivation of antiviral responses, including specialized functions of P protein in selective targeting and accumulation of pY-STAT1.

Following viral infection, the host activates multiple antiviral defenses. The ability of viruses to overcome these defenses is critical to disease. The earliest antiviral response involves the production of interferon messenger molecules. Interferons act on infected cells to inhibit viral proliferation, as well as on non-infected cells to establish an antiviral state before infection and so limit viral spread through the host organism. Many strategies used by viruses to overcome the former are well understood, but mechanisms important to the latter, and their importance to disease, are less well defined. In this study, we investigated how rabies virus overcomes a pre-established antiviral state in target cells. We found that the capacity to disable the antiviral state correlates with the ability to cause disease, and involves binding of a viral protein to cellular signaling proteins, which our data indicate are responsible for the maintenance of a prolonged antiviral state. This advances our understanding of antiviral responses, and identifies a key step in lethal infection by rabies virus that causes approximately 60,000 human deaths per year. The findings may contribute to new approaches for the development of vaccines or antivirals.

Analyses of cell death mechanisms related to amino acid substitution at position 95 in the rabies virus matrix protein

We previously reported that the avirulent fixed rabies virus strain Ni-CE induces a clear cytopathic effect in mouse neuroblastoma cells, whereas its virulent progenitor, the Nishigahara strain, does not. Infection with Nishigahara and Ni-CE mutants containing a single amino acid substitution in the matrix protein (M) demonstrated that the amino acid at position 95 of M (M95) is a cytopathic determinant. The characteristics of cell death induced by Ni-CE infection resemble those of apoptosis (rounded and shrunken cells, DNA fragmentation), but the intracellular signalling pathway for this process has not been fully investigated. In this study, we aimed to elucidate the mechanism by which M95 affects cell death induced by human neuroblastoma cell infection with the Nishigahara, Ni-CE and M95-mutated strains. We demonstrated that the Ni-CE strain induced DNA fragmentation, cell membrane disruption, exposure of phosphatidylserine (PS), activation of caspase-3/7 and anti-poly (ADP-ribose) polymerase 1 (PARP-1) cleavage, an early apoptosis indicator, whereas the Nishigahara strain did not induce DNA fragmentation, caspase-3/7 activation, cell membrane disruption, or PARP-1 cleavage, but did induce PS exposure. We also demonstrated that these characteristics were associated with M95 using M95-mutated strains. However, we found that Ni-CE induced cell death despite the presence of a caspase inhibitor, Z-VAD-FMK. In conclusion, our data suggest that M95 mutation-related cell death is caused by both the caspase-dependent and -independent pathways.

Genetic and Phenotypic Characterization of a Rabies Virus Strain Isolated from a Dog in Tokyo, Japan in the 1940s

The rabies virus strain Komatsugawa (Koma), which was isolated from a dog in Tokyo in the 1940s before eradication of rabies in Japan in 1957, is known as the only existent Japanese field strain (street strain). Although this strain potentially provides a useful model to study rabies pathogenesis, little is known about its genetic and phenotypic properties. Notably, this strain underwent serial passages in rodents after isolation, indicating the possibility that it may have lost biological characteristics as a street strain. In this study, to evaluate the utility of the Koma strain for studying rabies pathogenesis, we examined the genetic properties and in vitro and in vivo phenotypes. Genome-wide genetic analyses showed that, consistent with previous findings from partial sequence analyses, the Koma strain is closely related to a Russian street strain within the Arctic-related phylogenetic clade. Phenotypic examinations in vitro revealed that the Koma strain and the representative street strains are less neurotropic than the laboratory strains. Examination by using a mouse model demonstrated that the Koma strain and the street strains are more neuroinvasive than the laboratory strains. These findings indicate that the Koma strain retains phenotypes similar to those of street strains, and is therefore useful for studying rabies pathogenesis.

Molecular Function Analysis of Rabies Virus RNA Polymerase L Protein by Using an L Gene-Deficient Virus

While the RNA-dependent RNA polymerase L protein of rabies virus (RABV), a member of the genus Lyssavirus of the family Rhabdoviridae, has potential to be a therapeutic target for rabies, the molecular functions of this protein have remained largely unknown. In this study, to obtain a novel experimental tool for molecular function analysis of the RABV L protein, we established by using a reverse genetics approach an L gene-deficient RABV (Nishi-ΔL/Nluc), which infects, propagates, and correspondingly produces NanoLuc luciferase in cultured neuroblastoma cells transfected to express the L protein. trans-Complementation with wild-type L protein, but not that with a functionally defective L protein mutant, efficiently supported luciferase production by Nishi-ΔL/Nluc, confirming its potential for function analysis of the L protein. Based on the findings obtained from comprehensive genetic analyses of L genes from various RABV and other lyssavirus species, we examined the functional importance of a highly conserved L protein region at positions 1914 to 1933 by a trans-complementation assay with Nishi-ΔL/Nluc and a series of L protein mutants. The results revealed that the amino acid sequence at positions 1929 to 1933 (NPYNE) is functionally important, and this was supported by other findings that this sequence is critical for binding of the L protein with its essential cofactor, P protein, and thus also for L protein's RNA polymerase activity. Our findings provide useful information for the development of an anti-RABV drug targeting the L-P protein interaction.IMPORTANCE To the best of our knowledge, this is the first report on the establishment of an L gene-deficient, reporter gene-expressing virus in all species of the order Mononegavirales, also highlighting its applicability to a trans-complementation assay, which is useful for molecular function analyses of their L proteins. Moreover, this study revealed for the first time that the NPYNE sequence at positions 1929 to 1933 in the RABV L protein is important for L protein's interaction with the P protein, consistent with and extending the results of a previous study showing that the P protein-binding domain in the L protein is located in its C-terminal region, at positions 1562 to 2127. This study indicates that the NPYNE sequence is a promising target for the development of an inhibitor of viral RNA synthesis, which has high potential as a therapeutic drug for rabies.

Generation of a novel live rabies vaccine strain with a high level of safety by introducing attenuating mutations in the nucleoprotein and glycoprotein

The current live rabies vaccine SAG2 is attenuated by only one mutation (Arg-to-Glu) at position 333 in the glycoprotein (G333). This fact generates a potential risk of the emergence of a pathogenic revertant by a back mutation at this position during viral propagation in the body. To circumvent this risk, it is desirable to generate a live vaccine strain highly and stably attenuated by multiple mutations. However, the information on attenuating mutations other than that at G333 is very limited. We previously reported that amino acids at positions 273 and 394 in the nucleoprotein (N273/394) (Leu and His, respectively) of fixed rabies virus Ni-CE are responsible for the attenuated phenotype by enhancing interferon (IFN)/chemokine gene expressions in infected neural cells. In this study, we found that amino acid substitutions at N273/394 (Phe-to-Leu and Tyr-to-His, respectively) attenuated the pathogenicity of the oral live vaccine ERA, which has a virulent-type Arg at G333. Then we generated ERA-N273/394-G333 attenuated by the combination of the above attenuating mutations at G333 and N273/394, and checked its safety. Similar to the ERA-G333, which is attenuated by only the mutation at G333, ERA-N273/394-G333 did not cause any symptoms in adult mice after intracerebral inoculation, indicating a low level of residual pathogenicity of ERA-N273/394-G333. Further examination revealed that infection with ERA-N273/394-G333 induces IFN-β and CXCL10 mRNA expressions more strongly than ERA-G333 infection in a neuroblastoma cell line. Importantly, we found that the ERA-N273/394-G333 stain has a lower risk for emergence of a pathogenic revertant than does the ERA-G333. These results indicate that ERA-N273/394-G333 has a potential to be a promising candidate for a live rabies vaccine strain with a high level of safety.

Rabies Internalizes into Primary Peripheral Neurons via Clathrin Coated Pits and Requires Fusion at the Cell Body

The single glycoprotein (G) of rabies virus (RABV) dictates all viral entry steps from receptor engagement to membrane fusion. To study the uptake of RABV into primary neuronal cells in culture, we generated a recombinant vesicular stomatitis virus in which the G protein was replaced with that of the neurotropic RABV CVS-11 strain (rVSV CVS G). Using microfluidic compartmentalized culture, we examined the uptake of single virions into the termini of primary neurons of the dorsal root ganglion and ventral spinal cord. By pharmacologically disrupting endocytosis at the distal neurites, we demonstrate that rVSV CVS G uptake and infection are dependent on dynamin. Imaging of single virion uptake with fluorescent endocytic markers further identifies endocytosis via clathrin-coated pits as the predominant internalization mechanism. Transmission electron micrographs also reveal the presence of viral particles in vesicular structures consistent with incompletely coated clathrin pits. This work extends our previous findings of clathrin-mediated uptake of RABV into epithelial cells to two neuronal subtypes involved in rabies infection in vivo. Chemical perturbation of endosomal acidification in the neurite or somal compartment further shows that establishment of infection requires pH-dependent fusion of virions at the cell body. These findings correlate infectivity to existing single particle evidence of long-range endosomal transport of RABV and clathrin dependent uptake at the plasma membrane.

Involvement of the rabies virus phosphoprotein gene in neuroinvasiveness

Rabies virus (RABV), which is transmitted via a bite wound caused by a rabid animal, infects peripheral nerves and then spreads to the central nervous system (CNS) before causing severe neurological symptoms and death in the infected individual. Despite the importance of this ability of the virus to spread from a peripheral site to the CNS (neuroinvasiveness) in the pathogenesis of rabies, little is known about the mechanism underlying the neuroinvasiveness of RABV. In this study, to obtain insights into the mechanism, we conducted comparative analysis of two fixed RABV strains, Nishigahara and the derivative strain Ni-CE, which cause lethal and asymptomatic infections, respectively, in mice after intramuscular inoculation. Examination of a series of chimeric viruses harboring the respective genes from Nishigahara in the genetic background of Ni-CE revealed that the Nishigahara phosphoprotein (P) gene plays a major role in the neuroinvasiveness by mediating infection of peripheral nerves. The results obtained from both in vivo and in vitro experiments strongly suggested that the Nishigahara P gene, but not the Ni-CE P gene, is important for stable viral replication in muscle cells. Further investigation based on the previous finding that RABV phosphoprotein counteracts the host interferon (IFN) system demonstrated that the Nishigahara P gene, but not the Ni-CE P gene, functions to suppress expression of the beta interferon (IFN-β) gene (Ifn-β) and IFN-stimulated genes in muscle cells. In conclusion, we provide the first data strongly suggesting that RABV phosphoprotein assists viral replication in muscle cells by counteracting the host IFN system and, consequently, enhances infection of peripheral nerves.

Genetic and evolutionary characterization of RABVs from China using the phosphoprotein gene

BACKGROUND

While the function of the phosphoprotein (P) gene of the rabies virus (RABV) has been well studied in laboratory adapted RABVs, the genetic diversity and evolution characteristics of the P gene of street RABVs remain unclear. The objective of the present study was to investigate the mutation and evolution of P genes in Chinese street RABVs.

RESULTS

The P gene of 77 RABVs from brain samples of dogs and wild animals collected in eight Chinese provinces through 2003 to 2008 were sequenced. The open reading frame (ORF) of the P genes was 894 nucleotides (nt) in length, with 85-99% (80-89%) amino acid (nucleotide) identity compared with the laboratory RABVs and vaccine strains. Phylogenetic analysis based on the P gene revealed that Chinese RABVs strains could be divided into two distinct clades, and several RABV variants were found to co circulating in the same province. Two conserved (CD1, 2) and two variable (VD1, 2) domains were identified by comparing the deduced primary sequences of the encoded P proteins. Two sequence motifs, one believed to confer binding to the cytoplasmic dynein light chain LC8 and a lysine-rich sequence were conserved throughout the Chinese RABVs. In contrast, the isolates exhibited lower conservation of one phosphate acceptor and one internal translation initiation site identified in the P protein of the rabies challenge virus standard (CVS) strain. Bayesian coalescent analysis showed that the P gene in Chinese RABVs have a substitution rate (3.305x10(-4) substitutions per site per year) and evolution history (592 years ago) similar to values for the glycoprotein (G) and nucleoprotein (N) reported previously.

CONCLUSION

Several substitutions were found in the P gene of Chinese RABVs strains compared to the laboratory adapted and vaccine strains, whether these variations could affect the biological characteristics of Chinese RABVs need to be further investigated. The substitution rate and evolution history of P gene is similar to G and N gene, combine the topology of phylogenetic tree based on the P gene is similar to the G and N gene trees, indicate that the P, G and N genes are equally valid for examining the phylogenetics of RABVs.

Unique characteristics of bat rabies viruses in big brown bats (Eptesicus fuscus)

Rabies virus infection has been documented in several North American bat species, including Eptesicus fuscus. The virus-host relationship between bats and rabies virus (RV) is not well understood. The incidence of non-lethal RV exposure, based on the presence of viral neutralizing antibodies, demonstrates that exposure to RV does not always lead to clinical infection in bats. It is unknown how the route of exposure, rabies virus variant, or health of the bat affects the outcome following exposure. This paper describes the pathogenesis of two big brown bat RV variants in homologous host species. Our study demonstrates that RV variants obtained from the same species of bat from similar geographical areas may result in a diverse clinical progression of disease.

Rabies virus as a research tool and viral vaccine vector

Until recently, single-stranded negative sense RNA viruses (ssNSVs) were one of only a few important human viral pathogens, which could not be created from cDNA. The inability to manipulate their genomes hindered their detailed genetic analysis. A key paper from Conzelmann's laboratory in 1994 changed this with the publication of a method to recover rabies virus (RABV) from cDNA. This discovery not only dramatically changed the broader field of ssNSV biology but also opened a whole new avenue for studying RABV pathogenicity, developing novel RABV vaccines as well a new generation of RABV-based vaccine vectors, and creating research tools important in neuroscience such as neuronal tracing.

Gene-specific contributions to mumps virus neurovirulence and neuroattenuation

Mumps virus (MuV) is highly neurotropic and was the leading cause of aseptic meningitis in the Western Hemisphere prior to widespread use of live attenuated MuV vaccines. Due to the absence of markers of virus neuroattenuation and neurovirulence, ensuring mumps vaccine safety has proven problematic, as demonstrated by the occurrence of aseptic meningitis in recipients of certain vaccine strains. Here we examined the genetic basis of MuV neuroattenuation and neurovirulence by generating a series of recombinant viruses consisting of combinations of genes derived from a cDNA clone of the neurovirulent wild-type 88-1961 strain (r88) and from a cDNA clone of the highly attenuated Jeryl Lynn vaccine strain (rJL). Testing of these viruses in rats demonstrated the ability of several individual rJL genes and gene combinations to significantly neuroattenuate r88, with the greatest effect imparted by the rJL nucleoprotein/matrix protein combination. Interestingly, no tested combination of r88 genes, including the nucleoprotein/matrix protein combination, was able to convert rJL into a highly neurovirulent virus, highlighting mechanistic differences between processes involved in neuroattenuation and neurovirulence.

Rabies virus (RV) glycoprotein expression levels are not critical for pathogenicity of RV

Previous comparisons of different rabies virus (RV) strains suggested an inverse relationship between pathogenicity and the amount of glycoprotein produced in infected cells. In order to provide more insight into this relationship, we pursued an experimental approach that allowed us to alter the glycoprotein expression level without altering the glycoprotein sequence, thereby eliminating the contribution of amino acid changes to differences in viral virulence. To this end, we constructed an infectious clone of the highly pathogenic rabies virus strain CVS-N2c and replaced its cognate glycoprotein gene with synthetic versions in which silent mutations were introduced to replace wild-type codons with the most or least frequently used synonymous codons. A recombinant N2c variant containing the fully codon-optimized G gene and three variants carrying a partially codon-deoptimized G gene were recovered on mouse neuroblastoma cells and shown to express 2- to 3-fold more and less glycoprotein, respectively, than wild-type N2c. Pathogenicity studies in mice revealed the WT-N2c virus to be the most pathogenic strain. Variants containing partially codon-deoptimized glycoprotein genes or the codon-optimized gene were less pathogenic than WT-N2c but still caused significant mortality. We conclude that the expression level of the glycoprotein gene does have an impact on pathogenicity but is not a dominant factor that determines pathogenicity. Thus, strategies such as changes in codon usage that aim solely at altering the expression level of the glycoprotein gene do not suffice to render a pathogenic rabies virus apathogenic and are not a viable and safe approach for attenuation of a pathogenic strain.

Amino acids at positions 273 and 394 in rabies virus nucleoprotein are important for both evasion of host RIG-I-mediated antiviral response and pathogenicity

We previously reported that nucleoprotein (N) is related to the different pathogenicities of the virulent rabies virus strain Nishigahara (Ni) and avirulent strain Ni-CE and also that Ni N, but not Ni-CE N, functions to evade retinoic acid-inducible gene I (RIG-I)-mediated innate immunity. There are three amino acid differences between Ni and Ni-CE N (at positions 273, 394 and 395), indicating that one of these mutations or a combination of mutations is important for the pathogenicity and evasion of innate immunity. We generated Ni-CE mutants in which the amino acids in Ni-CE N were replaced with those of Ni in all combinations. Among the mutants, CE(NiN273/394) with mutations at positions 273 and 394 evaded activation of RIG-I-mediated signaling most efficiently and also showed the highest pathogenicity. This correlation reinforces the relation between evasion of host RIG-I-mediated innate immunity and pathogenicity of rabies virus.

Molecular basis of neurovirulence of flury rabies virus vaccine strains: importance of the polymerase and the glycoprotein R333Q mutation

The molecular mechanisms associated with rabies virus (RV) virulence are not fully understood. In this study, the RV Flury low-egg-passage (LEP) and high-egg-passage (HEP) strains were used as models to explore the attenuation mechanism of RV. The results of our studies confirmed that the R333Q mutation in the glycoprotein (G(R333Q)) is crucial for the attenuation of Flury RV in mice. The R333Q mutation is stably maintained in the HEP genome background but not in the LEP genome background during replication in mouse brain tissue or cell culture. Further investigation using chimeric viruses revealed that the polymerase L gene determines the genetic stability of the G(R333Q) mutation during replication. Moreover, a recombinant RV containing the LEP G protein with the R333Q mutation and the HEP L gene showed significant attenuation, genetic stability, enhancement of apoptosis, and immunogenicity. These results indicate that attenuation of the RV Flury strain results from the coevolution of G and L elements and provide important information for the generation of safer and more effective modified live rabies vaccine.

Role of interferon antagonist activity of rabies virus phosphoprotein in viral pathogenicity

The fixed rabies virus (RV) strain Nishigahara kills adult mice after intracerebral inoculation, whereas the chicken embryo fibroblast cell-adapted strain Ni-CE causes nonlethal infection in adult mice. We previously reported that the chimeric CE(NiP) strain, which has the phosphoprotein (P protein) gene from the Nishigahara strain in the genetic background of the Ni-CE strain, causes lethal infection in adult mice, indicating that the P gene is responsible for the different pathogenicities of the Nishigahara and Ni-CE strains. Previous studies demonstrated that RV P protein binds to the interferon (IFN)-activated transcription factor STAT1 and blocks IFN signaling by preventing its translocation to the nucleus. In this study, we examine the molecular mechanism by which RV P protein determines viral pathogenicity by comparing the IFN antagonist activities of the Nishigahara and Ni-CE P proteins. The results, obtained from both RV-infected cells and cells transfected to express P protein only, show that Ni-CE P protein is significantly impaired for its capacity to block IFN-activated STAT1 nuclear translocation and, consequently, inhibits IFN signaling less efficiently than Nishigahara P protein. Further, it was demonstrated that a defect in the nuclear export of Ni-CE P protein correlates with a defect in its ability to cause the mislocalization of STAT1. These data provide the first evidence that the capacity of the RV P protein to inhibit STAT1 nuclear translocation and IFN signaling correlates with the viral pathogenicity.

Rabies virus nucleoprotein functions to evade activation of the RIG-I-mediated antiviral response

The rabies virus Ni-CE strain causes nonlethal infection in adult mice after intracerebral inoculation, whereas the parental Nishigahara (Ni) strain kills mice. We previously reported that the chimeric CE(NiN) strain with the N gene from the Ni strain in the genetic background of the Ni-CE strain kills adult mice, indicating that the N gene is related to the different pathogenicities of Ni and Ni-CE strains. In the present study, to obtain an insight into the mechanism by which the N gene determines viral pathogenicity, we compared the effects of Ni, Ni-CE, and CE(NiN) infections on host gene expressions using a human neuroblastoma cell line. Microarray analysis of these infected cells revealed that the expression levels of particular genes in Ni- and CE(NiN)-infected cells, including beta interferon (IFN-beta) and chemokine genes (i.e., CXCL10 and CCL5) were lower than those in Ni-CE-infected cells. We also demonstrated that Ni-CE infection activated the interferon regulatory factor 3 (IRF-3)-dependent IFN-beta promoter and induced IRF-3 nuclear translocation more efficiently than did Ni or CE(NiN) infection. Furthermore, we showed that Ni-CE infection, but not Ni or CE(NiN) infection, strongly activates the IRF-3 pathway through activation of RIG-I, which is known as a cellular sensor of virus infection. These findings indicate that the N protein of rabies virus (Ni strain) has a function to evade the activation of RIG-I. To our knowledge, this is the first report that the Mononegavirales N protein functions to evade induction of host IFN and chemokines.

Characterization of a rabies virus isolate from a ferret badger (Melogale moschata) with unique molecular differences in glycoprotein antigenic site III

Rabies virus was isolated from the brain of a Chinese ferret badger (Melogale moschata) and identified as having an R333Q substitution within its glycoprotein antigenic site III. Additionally, compared with vaccine strains and other rabies virus isolates from dogs and ferret badgers in China, the isolate had five other amino acid substitutions in its glycoprotein: P(-17)L in the signal peptide, R88H, L225M, and D422E in the ectoplasmic region, and G478E in the cytoplasmic region. This isolate possessed high virulence in suckling, weanling and adult mice. These data indicate that this is a unique rabies virus with a molecular signature that differentiates it from other strains circulating in terrestrial mammals in China. We propose that rabies virus circulates in some ferret badgers in an independent epidemiological cycle unique to China following spillover from domestic dogs or other hosts infected with rabies.

A unique substitution at position 333 on the glycoprotein of rabies virus street strains isolated from non-hematophagous bats in Brazil

The amino acid R or K at position 333 on the glycoprotein of the rabies virus is considered necessary for virulence in adult mice. Although some exceptions exist, substitution at this position causes expression of a phenotype that is either less pathogenic or non-virulent. To date, such substitutions have only been found in fixed strains of rabies virus. In this study, the authors found 333H, 333N, and 333Q substitutions at this position in rabies virus street strains isolated from non-hematophagous bats in Brazil. These strains showed pathogenicity and lethality on passage using adult mice with the intracerebral route and were confirmed rabies-positive by immunofluorescent assay. This suggests that these strains maintain virulence. Our findings indicate that rabies virus street strains with these substitutions exist in the field and may result in infection cycles.

Genetic characterisation of attenuated SAD rabies virus strains used for oral vaccination of wildlife

The elimination of rabies from the red fox (Vulpes vulpes) in Western Europe has been achieved by the oral rabies vaccination (ORV) of wildlife with a range of attenuated rabies virus strains. With the exception of the vaccinia rabies glycoprotein recombinant vaccine (VRG), all strains were originally derived from a common ancestor; the Street Alabama Dufferin (SAD) field strain. However, after more than 30 years of ORV it is still not possible to distinguish these vaccine strains and there is little information on the genetic basis for their attenuation. We therefore sequenced and compared the full-length genome of five commercially available SAD vaccine viruses (SAD B19, SAD P5/88, SAG2, SAD VA1 and SAD Bern) and four other SAD strains (the original SAD Bern, SAD VA1, ERA and SAD 1-3670 Wistar). Nucleotide sequencing allowed identifying each vaccine strain unambiguously. Phylogenetic analysis revealed that the majority of the currently used commercial attenuated rabies virus vaccines appear to be derived from SAD B19 rather than from SAD Bern. One commercially available vaccine virus did not contain the SAD strain mentioned in the product information of the producer. Two SAD vaccine strains appeared to consist of mixed genomic sequences. Furthermore, in-del events targeting A-rich sequences (in positive strand) within the 3' non-coding regions of M and G genes were observed in SAD-derivates developed in Europe. Our data also supports the idea of a possible recombination that had occurred during the derivation of the European branch of SAD viruses. If confirmed, this recombination event would be the first one reported among RABV vaccine strains.

Lyssaviruses: current trends

Various technological developments have revitalized the approaches employed to study the disease of rabies. In particular, reverse genetics has facilitated the generation of novel viruses used to improve our understanding of the fundamental aspects of rabies virus (RABV) biology and pathogenicity and yielded novel constructs potentially useful as vaccines against rabies and other diseases. Other techniques such as high throughput methods to examine the impact of rabies virus infection on host cell gene expression and two hybrid systems to explore detailed protein-protein interactions also contribute substantially to our understanding of virus-host interactions. This review summarizes much of the increased knowledge about rabies that has resulted from such studies but acknowledges that this is still insufficient to allow rational attempts at curing those who present with clinical disease.

[Progression in studies on pathogenesis of rabies virus]

Rabies virus causes lethal neurological symptoms in humans and animals. Rabies epidemics have continued to occur throughout the world, despite the fact that rabies can be effectively prevented by vaccination. The development of inexpensive and safe attenuated live vaccines and the establishment of cures are the keys to control rabies. To achieve these objectives, it is important to elucidate mechanism by which rabies virus causes disease. Here, previous studies on the pathogenesis of rabies virus are reviewed and ways to apply previous findings to rabies control are also discussed.

Control of rabies: Epidemiology of rabies in Asia and development of new-generation vaccines for rabies

Rabies is an enzootic viral disease widespread throughout the world. Although it is a vaccine-preventable disease, the annual number of human deaths caused by rabies is estimated to be 32,000 in Asia. Phylogenetic analysis based on sequence data of the partial N gene of rabies viruses in Asia has shown that the viruses are divided into five genogroups, distributed in Middle East, South Asia, South East Asia, Malay, and Arctic regions. The genetic relationships among these rabies viruses agree basically with the results of previous studies. Meanwhile, new types of vaccines are being developed by applying gene manipulation techniques to rabies virus in order to overcome the disadvantages of current vaccines. This article reviews the molecular epidemiology of rabies in Asia and progress made in the development of new-generation rabies vaccines with the goal of elimination or control of rabies in Asia.

Are all lyssavirus genes equal for phylogenetic analyses?

Individual lyssavirus genes were evaluated for phylogenetic studies from available full genome sequences. The full genome of the ERA rabies virus was sequenced and its accuracy was confirmed through virus recovery by reverse genetics. The full length of the ERA is 11,931 nucleotides (nt), with a leader sequence of 58 nt, the nucleoprotein (N) gene of 1350 nt, phosphoprotein (P) gene of 891 nt, matrix protein (M) gene of 606 nt, glycoprotein (G) gene of 1572 nt, RNA-dependent RNA polymerase (L) gene of 6384 nt, Psi-region (or G-L intergenic region) of 400 nt, and a trailer region of 70 nt. The five mono-cistrons are separated by intergenic regions of 2, 5, 5 and 24 nt, respectively. One obvious difference between the ERA and SAD-B19 rabies virus strains was the putative stop/polyadenylation signal of the G gene, with a poly(A(8)) tract for ERA, and a poly(A(5)) for SAD-B19. The TGpoly(A(8)) sequence tract was identified to be a leaky termination signal in the ERA strain. Through analyses of nt diversity, protein co-variations, structural and functional constraints, and reconstruction of phylogenetic trees from comprehensive datasets, we propose lyssavirus genes probably are of similar value for phylogenetic analyses.

Sensitivity of rabies virus to type I interferon is determined by the phosphoprotein gene

The growth of a virulent strain of fixed rabies virus, Nishigahara, in mouse neuroblastoma NA cells treated with type I interferon (IFN) was compared with that of a derivative avirulent strain, Ni-CE. Nishigahara strain was slightly sensitive to IFN treatment but still grew more efficiently than did Ni-CE strain in IFN-treated NA cells. Furthermore, a virulent chimeric virus with the phosphoprotein gene from Nishigahara strain in the Ni-CE genome was less sensitive to IFN treatment than was Ni-CE strain, indicating that the IFN sensitivity is determined by the phosphoprotein gene of the virus.

Involvement of nucleoprotein, phosphoprotein, and matrix protein genes of rabies virus in virulence for adult mice

Rabies virus Ni-CE strain causes nonlethal infection in adult mice after intracerebral inoculation, whereas the parental Nishigahara strain kills mice. In this study, to identify viral gene(s) related to the difference in pathogenicity between Ni-CE and Nishigahara strains, we generated chimeric viruses with respective genes of the virulent Nishigahara strain in the background of the avirulent Ni-CE genome. Since chimeric viruses, which had the N, P, or M genes of the Nishigahara strain, respectively, killed adult mice after intracerebral inoculation, it became evident that the N, P, and M genes are related to the difference in pathogenicity between Ni-CE and Nishigahara strains. Previously, we showed that the G gene is a major contributor to the difference in pathogenicity between another avirulent strain, RC-HL, and the parental Nishigahara strain. These results imply that the attenuation mechanism of the Ni-CE strain is different from that of the RC-HL strain, thus suggesting that rabies virus can be attenuated by diverse mechanisms. This is the first report of changes in viral genes other than the G gene of rabies virus causing the reversion of pathogenicity of an avirulent strain.