Characterization of the Golgi retention motif of Rift Valley fever virus G(N) glycoprotein

Advancements in Rift Valley fever vaccines: a historical overview and prospects for next generation candidates

Rift Valley fever (RVF) is a zoonotic viral disease transmitted by mosquitoes and causes abortion storms, fetal malformations, and newborn animal deaths in livestock ruminants. In humans, RVF can manifest as hemorrhagic fever, encephalitis, or retinitis. Outbreaks of RVF have been occurring in Africa since the early 20th century and continue to pose a threat to both humans and animals in various regions such as Africa, Madagascar, the Comoros, Saudi Arabia, and Yemen. The development of RVF vaccines is crucial in preventing mortality and morbidity and reducing the spread of the virus. While several veterinary vaccines have been licensed in endemic countries, there are currently no licensed RVF vaccines for human use. This review provides an overview of the existing RVF vaccines, as well as potential candidates for future studies on RVF vaccine development, including next-generation vaccines that show promise in combating the disease in both humans and animals.

A Novel Quadruple Gene-Deleted BoHV-1-Vectored RVFV Subunit Vaccine Induces Humoral and Cell-Mediated Immune Response against Rift Valley Fever in Calves

Rift Valley fever virus (RVFV) is considered to be a high biodefense priority based on its threat to livestock and its ability to cause human hemorrhagic fever. RVFV-infected livestock are also a significant risk factor for human infection by direct contact with contaminated blood, tissues, and aborted fetal materials. Therefore, livestock vaccination in the affected regions has the direct dual benefit and one-health approach of protecting the lives of millions of animals and eliminating the risk of severe and sometimes lethal human Rift Valley fever (RVF) disease. Recently, we have developed a bovine herpesvirus type 1 (BoHV-1) quadruple gene mutant virus (BoHV-1qmv) vector that lacks virulence and immunosuppressive properties due to the deletion of envelope proteins UL49.5, glycoprotein G (gG), gE cytoplasmic tail, and US9 coding sequences. In the current study, we engineered the BoHV-1qmv further by incorporating a chimeric gene sequence to express a proteolytically cleavable polyprotein: RVFV envelope proteins Gn ectodomain sequence fused with bovine granulocyte-macrophage colony-stimulating factor (GMCSF) and Gc, resulting in a live BoHV-1qmv-vectored subunit vaccine against RVFV for livestock. In vitro, the resulting recombinant virus, BoHV-1qmv Sub-RVFV, was replicated in cell culture with high titers. The chimeric Gn-GMCSF and Gc proteins expressed by the vaccine virus formed the Gn-Gc complex. In calves, the BoHV-1qmv Sub-RVFV vaccination was safe and induced moderate levels of the RVFV vaccine strain, MP12-specific neutralizing antibody titers. Additionally, the peripheral blood mononuclear cells from the vaccinated calves had six-fold increased levels of interferon-gamma transcription compared with that of the BoHV-1qmv (vector)-vaccinated calves when stimulated with heat-inactivated MP12 antigen in vitro. Based on these findings, we believe that a single dose of BoHV-1qmv Sub-RVFV vaccine generated a protective RVFV-MP12-specific humoral and cellular immune response. Therefore, the BoHV-1qmv sub-RVFV can potentially be a protective subunit vaccine for cattle against RVFV.

Mechanisms of bunyavirus morphogenesis and egress

Unlike many segmented negative-sense RNA viruses, most members of the Bunyavirales bud at Golgi membranes, as opposed to the plasma membrane. Central players in this assembly process are the envelope glycoproteins, Gn and Gc, which upon translation undergo proteolytic processing, glycosylation and trafficking to the Golgi, where they interact with ribonucleoprotein genome segments and bud into Golgi-derived compartments. The processes involved in genome packaging during virion assembly can lead to the generation of reassorted viruses, if a cell is co-infected with two different bunyaviruses, due to mismatching of viral genome segment packaging. This can lead to viruses with high pathogenic potential, as demonstrated by the emergence of Schmallenberg virus. This review focuses on the assembly pathways of tri-segmented bunyaviruses, highlighting some areas in need of further research to understand these important pathogens with zoonotic potential.

Pseudotyped Viruses for Phlebovirus

Rift Valley fever virus (RVFV) is a member of the Phlebovirus genus, one of the 20 genera in the Phenuiviridae family. RVFV causes disease in animals and humans and is transmitted by sandflies or ticks. However, research into RVFV is limited by the requirement for biosafety level 3 (BSL-3) containment. Pseudotyped virus overcomes this limitation as it can be handled in a BSL-2 environment. Pseudotyped RVFV possesses an identical envelope protein structure to that of the authentic virus, simulating the same process of receptor binding and membrane fusion to host cells. Pseudotyped phleboviruses are therefore useful tools to study the infection mechanism of these viruses and for the screening of inhibitory drugs and the development of therapeutic monoclonal antibodies.

Rottlerin inhibits La Crosse virus-induced encephalitis in mice and blocks release of replicating virus from the Golgi body in neurons

La Crosse virus (LACV) is a mosquito-borne orthobunyavirus that causes approximately 60 to 80 hospitalized pediatric encephalitis cases in the United States yearly. The primary treatment for most viral encephalitis, including LACV, is palliative care, and specific antiviral therapeutics are needed. We screened the National Center for Advancing Translational Sciences library of 3,833 FDA-approved and bioactive small molecules for the ability to inhibit LACV-induced death in SH-SY5Y neuronal cells. The top three hits from the initial screen were validated by examining their ability to inhibit virus-induced cell death in multiple neuronal cell lines. Rottlerin consistently reduced LACV-induced death by 50% in multiple human and mouse neuronal cell lines with an effective concentration of 0.16-0.69 µg ml-1 depending on cell line. Rottlerin was effective up to 12 hours post-infection in vitro and inhibited virus particle trafficking from the Golgi apparatus to trans-Golgi vesicles. In human inducible pluripotent stem cell-derived cerebral organoids, rottlerin reduced virus production by one log and cell death by 35% compared with dimethyl sulfoxide-treated controls. Administration of rottlerin in mice by intraperitoneal or intracranial routes starting at 3 days post-infection decreased disease development by 30-50%. Furthermore, rottlerin also inhibited virus replication of other pathogenic California serogroup orthobunyaviruses (Jamestown Canyon and Tahyna virus) in neuronal cell lines.

Characterization of the Molecular Interactions That Govern the Packaging of Viral RNA Segments into Rift Valley Fever Phlebovirus Particles

Rift Valley fever phlebovirus (RVFV) has a single-stranded, negative-sense RNA genome, consisting of L, M, and S segments. The virion carries two envelope glycoproteins, Gn and Gc, along with ribonucleoprotein complexes (RNPs), composed of encapsidated genomes carrying N protein and the viral polymerase, L protein. A quantitative analysis of the profile of viral RNA segments packaged into RVFV particles showed that all three genomic RNA segments had similar packaging abilities, whereas among antigenomic RNA segments, the antigenomic S RNA, which serves as the template for the transcription of mRNA expressing the RVFV virulence factor, NSs, displayed a significantly higher packaging ability. To delineate the factor(s) governing the packaging of RVFV RNA segments, we characterized the interactions between Gn and viral RNPs in RVFV-infected cells. Coimmunoprecipitation analysis demonstrated the interaction of Gn with N protein, L protein, and viral RNAs in RVFV-infected cells. Furthermore, UV-cross-linking and immunoprecipitation analysis revealed, for the first time in bunyaviruses, the presence of a direct interaction between Gn and all the viral RNA segments in RVFV-infected cells. Notably, analysis of the ability of Gn to bind to RVFV RNA segments indicated a positive correlation with their respective packaging abilities and highlighted a binding preference of Gn for antigenomic S RNA, among the antigenomic RNA segments, suggesting the presence of a selection mechanism for antigenomic S RNA incorporation into infectious RVFV particles. Collectively, the results of our study illuminate the importance of a direct interaction between Gn and viral RNA segments in determining their efficiency of incorporation into RVFV particles. IMPORTANCE Rift Valley fever phlebovirus, a bunyavirus, is a mosquito-borne, segmented RNA virus that can cause severe disease in humans and ruminants. An essential step in RVFV life cycle is the packaging of viral RNA segments to produce infectious virus particles for dissemination to new hosts. However, there are key gaps in knowledge regarding the mechanisms that regulate viral RNA packaging efficiency in bunyaviruses. Our studies investigating the mechanism of RNA packaging in RVFV revealed the presence of a direct interaction between the viral envelope glycoprotein, Gn, and the viral RNA segments in infected cells, for the first time in bunyaviruses. Furthermore, our data strongly indicate a critical role for the direct interaction between Gn and viral RNAs in determining the efficiency of incorporation of viral RNA segments into RVFV particles. Clarifying the fundamental mechanisms of RNA packaging in RVFV would be valuable for the development of antivirals and live-attenuated vaccines.

Crystal structure of tomato spotted wilt virus GN reveals a dimer complex formation and evolutionary link to animal-infecting viruses

Tospoviridae is a family of enveloped RNA plant viruses that infect many field crops, inflicting a heavy global economic burden. These tripartite, single-stranded, negative-sense RNA viruses are transmitted from plant to plant by thrips as the insect vector. The medium (M) segment of the viral genome encodes two envelope glycoproteins, GN and GC, which together form the envelope spikes. GC is considered the virus fusogen, while the accompanying GN protein serves as an attachment protein that binds to a yet unknown receptor, mediating the virus acquisition by the thrips carrier. Here we present the crystal structure of glycoprotein N (GN) from the tomato spotted wilt virus (TSWV), a representative member of the Tospoviridae family. The structure suggests that GN is organized as dimers on TSWV's outer shell. Our structural data also suggest that this dimerization is required for maintaining GN structural integrity. Although the structure of the TSWV GN is different from other bunyavirus GN proteins, they all share similar domain connectivity that resembles glycoproteins from unrelated animal-infecting viruses, suggesting a common ancestor for these accompanying proteins.

The interplays between Crimean-Congo hemorrhagic fever virus (CCHFV) M segment-encoded accessory proteins and structural proteins promote virus assembly and infectivity

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tick-borne orthonairovirus that has become a serious threat to the public health. CCHFV has a single-stranded, tripartite RNA genome composed of L, M, and S segments. Cleavage of the M polyprotein precursor generates the two envelope glycoproteins (GPs) as well as three secreted nonstructural proteins GP38 and GP85 or GP160, representing GP38 only or GP38 linked to a mucin-like protein (MLD), and a double-membrane-spanning protein called NSm. Here, we examined the relevance of each M-segment non-structural proteins in virus assembly, egress and infectivity using a well-established CCHFV virus-like-particle system (tc-VLP). Deletion of MLD protein had no impact on infectivity although it reduced by 60% incorporation of GPs into particles. Additional deletion of GP38 abolished production of infectious tc-VLPs. The loss of infectivity was associated with impaired Gc maturation and exclusion from the Golgi, showing that Gn is not sufficient to target CCHFV GPs to the site of assembly. Consistent with this, efficient complementation was achieved in cells expressing MLD-GP38 in trans with increased levels of preGc to Gc conversion, co-targeting to the Golgi, resulting in particle incorporation and restored infectivity. Contrastingly, a MLD-GP38 variant retained in the ER allowed preGc cleavage but failed to rescue miss-localization or infectivity. NSm deletion, conversely, did not affect trafficking of Gc but interfered with Gc processing, particle formation and secretion. NSm expression affected N-glycosylation of different viral proteins most likely due to increased speed of trafficking through the secretory pathway. This highlights a potential role of NSm in overcoming Golgi retention and facilitating CCHFV egress. Thus, deletions of GP38 or NSm demonstrate their important role on CCHFV particle production and infectivity. GP85 is an essential viral factor for preGc cleavage, trafficking and Gc incorporation into particles, whereas NSm protein is involved in CCHFV assembly and virion secretion.

Orthonairoviruses, like the lethal Crimean-Congo hemorrhagic fever virus (CCHFV), encode secreted glycoproteins, such as GP38, in addition to virion envelope glycoproteins (Gn and Gc) that are processed by internal cleavage of the viral M segment encoded polyprotein. CCHFV MLD-GP38 proteins (GP160/GP85) also include an N-terminal domain encompassing a mucin-like protein that is released from GP38 by Furin. The protective effect of non-neutralizing monoclonal antibodies targeting GP38 against lethal CCHFV challenge previously highlighted the importance of GP38 in CCHFV replication. CCHFV also encodes a double-membrane-spanning protein (NSm) of unknown function, located between the Gn and Gc on the polyprotein. To investigate the roles of these so-called accessory proteins encoded by the CCHFV M-segment in virus formation and infectivity, we generated several M-segment deletion mutants and tested them in a CCHFV transcription-entry-competent virus-like particle (tc-VLP) system. Here, we demonstrate that GP38 is crucial for Gc biogenesis, interaction with Gn and trafficking to the Golgi, and that its deletion abrogates formation of infectious particles. We also show that NSm increases the rate of protein trafficking through the secretory pathway with altered N-glycosylation profiles that are advantageous for efficient virus release. These data advanced our understanding of GP38 and NSm roles and CCHFV-host interactions.

A DNA Vaccine Delivery Platform Based on Elastin-Like Recombinamer Nanosystems for Rift Valley Fever Virus

This work analyzes the immunogenicity of six genetically engineered constructs based on elastin-like recombinamers (ELRs) fused to the Gn glycoprotein from Rift Valley fever virus (RVFV). Upon transfection, all constructs showed no effect on cell viability. While fusion constructs including ELR blocks containing hydrophobic amino acids (alanine or isoleucine) did not increase the expression of viral Gn in eukaryotic cells, glutamic acid- or valine-rich fusion proteins showed enhanced expression levels compared with the constructs encoding the viral antigen alone. However, in vivo DNA plasmid immunization assays determined that the more hydrophobic constructs reduced viremia levels after RVFV challenge to a higher extent than glutamic- or valine-rich encoding plasmids and were better inducers of cellular immunity as judged by in vitro restimulation experiments. Although the Gn-ELR fusion constructs did not surpass the protective efficacy of a plasmid vaccine expressing nonfused Gn, our results warrant further experiments directed to take advantage of the immunomodulatory potential of ELR biomaterials for improving vaccines against infectious diseases.

Development of Rift Valley fever (RVF) vaccine by genetic joining of the RVF-glycoprotein Gn with the strong adjuvant subunit B of cholera toxin (CTB) and expression in bacterial system

One of the mosquito-borne zoonotic diseases is the Rift Valley fever virus (RVFV). Currently, there is no completely licensed vaccine that can be used to vaccinate animals or humans outside endemic areas. The aim of this work was to use the RVFV glycoprotein (Gn) and the subunit B of cholera toxin (CTB) at gene level and build up fused recombinant vaccine. The gene of CTB was joined to the gene Gn to work as an adjuvant in the resulting fusion protein. The designed merged genes (CTB-Gn) was tested for restriction sites, open reading frames, expected fusion protein tertiary structure and antigenicity using computer software. The insert sequence was submitted to the BioProject (GenBank). The insert was subcloned into the pQE-31 expression plasmid. The target recombinant protein (rCTB-Gn) was expressed in M15 bacteria, purified and identified by protein gel electrophoresis. The insert got the accession No: PRJNA386723. Analysis of the designed rCTB-Gn protein revealed that it had the right 3D structure, immunogenic and at the correct molecular weight. The presence of the CTB in the proposed vaccine will augment its immunogenicity. Doses and protection levels of the vaccine need to be manipulated.

Evaluation of an Indirect Enzyme-Linked Immunosorbent Assay Based on Recombinant Baculovirus-Expressed Rift Valley Fever Virus Nucleoprotein as the Diagnostic Antigen

The increasing risk of Rift Valley fever virus (RVFV) infection as a global veterinary and public health threat demands the development of safe and accurate diagnostic tests. The aim of this study was to assess the suitability of a baculovirus expression system to produce recombinant RVFV nucleoprotein (N) for use as serodiagnostic antigen in an indirect enzyme-linked immunosorbent assay (ELISA). The ability of the recombinant N antigen to detect RVFV antibody responses was evaluated in ELISA format using antisera from sheep and cattle experimentally infected with two genetically distinct wild-type RVFV strains and sera from indigenous sheep and goat populations exposed to natural RVFV field infection in The Gambia. The recombinant N exhibited specific reactivity with the N-specific monoclonal antibody and various hyperimmune serum samples from ruminants. The indirect ELISA detected N-specific antibody responses in animals with 100% sensitivity compared to the plaque reduction neutralization test (6 to 21 days postinfection) and with 97% and 100% specificity in sheep and cattle, respectively. There was a high level of correlation between the indirect N ELISA and the virus neutralization test for sheep sera (R ² = 0.75; 95% confidence interval [CI] = 0.73 to 0.92) and cattle sera (R ² = 0.80; 95% CI = 0.67 to 0.97); in addition, the N-specific ELISA detected RVFV seroprevalence levels of 26.1% and 54.3% in indigenous sheep and goats, respectively, in The Gambia. The high specificity and correlation with the virus neutralization test support the idea of the feasibility of using the recombinant baculovirus-expressed RVFV N-based indirect ELISA to assess RVFV seroprevalence in livestock in areas of endemicity and nonendemicity.

In vitro and in vivo efficacy of a Rift Valley fever virus vaccine based on pseudovirus

Rift Valley fever virus (RVFV), a recognized category A priority pathogen, causes large outbreaks of Rift Valley fever with some fatalities in humans in humans and huge economic losses in livestock. As wild-type RVFV must be handled in BSL-3 or BSL-4 laboratories, we constructed a high-titer vesicular stomatitis virus (VSV) pseudotype bearing RVFV envelope glycoproteins to detect neutralizing antibodies in vitro under BSL-2 conditions. The neutralizing properties of 39 amino acid mutant sites that have occurred naturally over time in the RVFV envelope glycoproteins were analyzed with their corresponding pseudoviral mutants separately. Compared with the results in the primary strain, the variants showed no statistically significant differences. We next established a Balb/c mouse pseudovirus infection model for detecting neutralizing antibodies against pseudovirus. Five immunizations with pseudoviral DNA protected the mice from infection with the pseudovirus. Bioluminescence imaging, which we used to evaluate viral dissemination and distribution in the mice, showed a good relationship between the neutralizing antibodies titers in vitro. These pseudovirus methods will allow for the safe determination of neutralizing antibodies in vivo and in vitro, and will assist with studies on vaccines and drugs against RVFV with the long term objective of Rift Valley fever prevention.

Molecular aspects of Rift Valley fever virus and the emergence of reassortants

Rift Valley fever phlebovirus (RVFV) is a mosquito-transmitted pathogen endemic to sub-Saharan Africa and the Arabian Peninsula. RVFV is a threat to both animal and human health and has costly economic consequences mainly related to livestock production and trade. Competent hosts and vectors for RVFV are widespread, existing outside of endemic countries including the USA. Thus, the possibility of RVFV spreading to the USA or other countries worldwide is of significant concern. RVFV (genus Phlebovirus) is comprised of an enveloped virion containing a three-segmented, negative-stranded RNA genome that is able to undergo genetic reassortment. Reassortment has the potential to produce viruses that are more pathogenic, easily transmissible, and that have wider vector or host range. This is especially concerning because of the wide use of live attenuated vaccine strains throughout endemic countries. This review focuses on the molecular aspects of RVFV, genetic diversity of RVFV strains, and RVFV reassortment.

Targeting of severe fever with thrombocytopenia syndrome virus structural proteins to the ERGIC (endoplasmic reticulum Golgi intermediate compartment) and Golgi complex

Severe fever with thrombocytopenia syndrome phlebovirus (SFTSV) is a newly emerged phlebovirus identified in China, Japan, and South Korea. Phlebovirus glycoproteins (GP) play a key role in targeting viral structural components to the budding compartments in the ER-Golgi intermediate compartment (ERGIC) and Golgi complex. However, the role of SFTSV GP in targeting structural proteins to the ERGIC and Golgi complex remains unresolved. In this study, we show that SFTSV GP plays a significant role in targeting RNA-dependent RNA polymerase (L) and nucleocapsid protein (NP) to the budding sites. Confocal microscopy was used to investigate the subcellular localization of SFTSV structural proteins. In SFTSV-infected cells, GP and L localized to the ER, ERGIC and Golgi complex, whereas NP localized to the ERGIC and Golgi complex. In addition, GP colocalized with L and NP in infected cells. In cells singly transfected with GP, L or NP, GP localized to the same subcellular compartments as in infected cells. However, L or NP alone did not localize to the ER, ERGIC, or Golgi complex. Cotransfection experiments showed that GP altered the localization of L to the ERGIC and Golgi complex but not that of NP. Interestingly, plasmid-expressed NP fused with a hemagglutinin tag localized to the ERGIC and Golgi complex when expressed in SFTSV-infected cells and colocalised with GP, suggesting that GP plays a role in the subcellular localization of L and NP in infected cells. Thus, the SFTSV structural components start to assemble at the ERGIC to Golgi complex. GP is required for transporting L and NP to the ERGIC and Golgi complex. In addition, targeting of NP requires interaction with other factors besides GP.

A glycosylated recombinant subunit candidate vaccine consisting of Ehrlichia ruminantium major antigenic protein1 induces specific humoral and Th1 type cell responses in sheep

Heartwater, or cowdriosis, is a tick-borne disease of domestic and wild ruminants that is endemic in the Caribbean and sub-Saharan Africa. The disease is caused by an intracellular pathogen, Ehrlichia ruminantium and may be fatal within days of the onset of clinical signs with mortality rates of up to 90% in susceptible hosts. Due to the presence of competent tick vectors in North America, there is substantial risk of introduction of heartwater with potentially devastating consequences to the domestic livestock industry. There is currently no reliable or safe vaccine for use globally. To develop a protective DIVA (differentiate infected from vaccinated animals) subunit vaccine for heartwater, we targeted the E. ruminantium immunodominant major antigenic protein1 (MAP1) with the hypothesis that MAP1 is a glycosylated protein and glycans contained in the antigenic protein are important epitope determinants. Using a eukaryotic recombinant baculovirus expression system, we expressed and characterized, for the first time, a glycoform profile of MAP1 of two Caribbean E. ruminantium isolates, Antigua and Gardel. We have shown that the 37–38 kDa protein corresponded to a glycosylated form of the MAP1 protein, whereas the 31–32 kDa molecular weight band represented the non-glycosylated form of the protein frequently reported in scientific literature. Three groups of sheep (n = 3–6) were vaccinated with increasing doses of a bivalent (Antigua and Gardel MAP1) rMAP1 vaccine cocktail formulation with montanide ISA25 as an adjuvant. The glycosylated recombinant subunit vaccine induced E. ruminantium-specific humoral and Th1 type T cell responses, which are critical for controlling intracellular pathogens, including E. ruminantium, in infected hosts. These results provide an important basis for development of a subunit vaccine as a novel strategy to protect susceptible livestock against heartwater in non-endemic and endemic areas.

Current Status of Rift Valley Fever Vaccine Development

Rift Valley Fever (RVF) is a mosquito-borne zoonotic disease that presents a substantial threat to human and public health. It is caused by Rift Valley fever phlebovirus (RVFV), which belongs to the genus Phlebovirus and the family Phenuiviridae within the order Bunyavirales. The wide distribution of competent vectors in non-endemic areas coupled with global climate change poses a significant threat of the transboundary spread of RVFV. In the last decade, an improved understanding of the molecular biology of RVFV has facilitated significant progress in the development of novel vaccines, including DIVA (differentiating infected from vaccinated animals) vaccines. Despite these advances, there is no fully licensed vaccine for veterinary or human use available in non-endemic countries, whereas in endemic countries, there is no clear policy or practice of routine/strategic livestock vaccinations as a preventive or mitigating strategy against potential RVF disease outbreaks. The purpose of this review was to provide an update on the status of RVF vaccine development and provide perspectives on the best strategies for disease control. Herein, we argue that the routine or strategic vaccination of livestock could be the best control approach for preventing the outbreak and spread of future disease.

Entry of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a globe-shaped virus covered by a dense icosahedral array of glycoproteins Gn/Gc that mediate the attachment of the virus to host cells and the fusion of viral and cellular membranes. Several membrane factors are involved in virus entry, including C-type lectins and nonmuscle myosin heavy chain IIA. The post-fusion crystal structure of the Gc protein suggests that it is a class II membrane fusion protein, similar to the E/E1 protein of flaviviruses and alphaviruses. The virus particles are internalized into host cell endosomes through the clathrin-dependent pathway, where the low pH activates the fusion of the virus with the cell membrane. With information from studies on other bunyaviruses, herein we will review our knowledge of the entry process of SFTSV.

Evidence that Processing of the Severe Fever with Thrombocytopenia Syndrome Virus Gn/Gc Polyprotein Is Critical for Viral Infectivity and Requires an Internal Gc Signal Peptide

The severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging, highly pathogenic bunyavirus against which neither antivirals nor vaccines are available. The SFTSV glycoproteins, Gn and Gc, facilitate viral entry into host cells. Gn and Gc are generated from a precursor protein, Gn/Gc, but it is currently unknown how the precursor is converted into the single proteins and whether this process is required for viral infectivity. Employing a rhabdoviral pseudotyping system, we demonstrate that a predicted signal sequence at the N-terminus of Gc is required for Gn/Gc processing and viral infectivity while potential proprotein convertase cleavage sites in Gc are dispensable. Moreover, we show that expression of Gn or Gc alone is not sufficient for host cell entry while particles bearing both proteins are infectious, and we provide evidence that Gn facilitates Golgi transport and virion incorporation of Gc. Collectively, these results suggest that signal peptidase liberates mature Gc from the Gn/Gc precursor and that this process is essential for viral infectivity and thus constitutes a potential target for antiviral intervention.

Single-Molecule FISH Reveals Non-selective Packaging of Rift Valley Fever Virus Genome Segments

The bunyavirus genome comprises a small (S), medium (M), and large (L) RNA segment of negative polarity. Although genome segmentation confers evolutionary advantages by enabling genome reassortment events with related viruses, genome segmentation also complicates genome replication and packaging. Accumulating evidence suggests that genomes of viruses with eight or more genome segments are incorporated into virions by highly selective processes. Remarkably, little is known about the genome packaging process of the tri-segmented bunyaviruses. Here, we evaluated, by single-molecule RNA fluorescence in situ hybridization (FISH), the intracellular spatio-temporal distribution and replication kinetics of the Rift Valley fever virus (RVFV) genome and determined the segment composition of mature virions. The results reveal that the RVFV genome segments start to replicate near the site of infection before spreading and replicating throughout the cytoplasm followed by translocation to the virion assembly site at the Golgi network. Despite the average intracellular S, M and L genome segments approached a 1:1:1 ratio, major differences in genome segment ratios were observed among cells. We also observed a significant amount of cells lacking evidence of M-segment replication. Analysis of two-segmented replicons and four-segmented viruses subsequently confirmed the previous notion that Golgi recruitment is mediated by the Gn glycoprotein. The absence of colocalization of the different segments in the cytoplasm and the successful rescue of a tri-segmented variant with a codon shuffled M-segment suggested that inter-segment interactions are unlikely to drive the copackaging of the different segments into a single virion. The latter was confirmed by direct visualization of RNPs inside mature virions which showed that the majority of virions lack one or more genome segments. Altogether, this study suggests that RVFV genome packaging is a non-selective process.

The bunyavirus family is one of the largest virus families on Earth, of which several members cause severe disease in humans, animals or plants. Little is known about the mechanisms that facilitate the production of infectious bunyavirus virions, which should contain at least one copy of the small (S), medium (M) and large (L) genome segment. In this study, we investigated the genome packaging process of the Rift Valley fever virus (RVFV) by visualizing individual genome segments inside infected cells and virions. Experiments performed with wild-type virus, two- and four-segmented variants, and a variant with a codon-shuffled M segment showed that the production of infectious virions is a non-selective process and is unlikely to involve the formation of a supramolecular viral RNA complex. These observations have broad implications for understanding the bunyavirus replication cycle and may facilitate the development of new vaccines and the identification of novel antiviral targets.

The Role of Phlebovirus Glycoproteins in Viral Entry, Assembly and Release

Bunyaviruses are enveloped viruses with a tripartite RNA genome that can pose a serious threat to animal and human health. Members of the Phlebovirus genus of the family Bunyaviridae are transmitted by mosquitos and ticks to humans and include highly pathogenic agents like Rift Valley fever virus (RVFV) and severe fever with thrombocytopenia syndrome virus (SFTSV) as well as viruses that do not cause disease in humans, like Uukuniemi virus (UUKV). Phleboviruses and other bunyaviruses use their envelope proteins, Gn and Gc, for entry into target cells and for assembly of progeny particles in infected cells. Thus, binding of Gn and Gc to cell surface factors promotes viral attachment and uptake into cells and exposure to endosomal low pH induces Gc-driven fusion of the viral and the vesicle membranes. Moreover, Gn and Gc facilitate virion incorporation of the viral genome via their intracellular domains and Gn and Gc interactions allow the formation of a highly ordered glycoprotein lattice on the virion surface. Studies conducted in the last decade provided important insights into the configuration of phlebovirus Gn and Gc proteins in the viral membrane, the cellular factors used by phleboviruses for entry and the mechanisms employed by phlebovirus Gc proteins for membrane fusion. Here, we will review our knowledge on the glycoprotein biogenesis and the role of Gn and Gc proteins in the phlebovirus replication cycle.

RNA Encapsidation and Packaging in the Phleboviruses

The Bunyaviridae represents the largest family of segmented RNA viruses, which infect a staggering diversity of plants, animals, and insects. Within the family Bunyaviridae, the Phlebovirus genus includes several important human and animal pathogens, including Rift Valley fever virus (RVFV), severe fever with thrombocytopenia syndrome virus (SFTSV), Uukuniemi virus (UUKV), and the sandfly fever viruses. The phleboviruses have small tripartite RNA genomes that encode a repertoire of 5–7 proteins. These few proteins accomplish the daunting task of recognizing and specifically packaging a tri-segment complement of viral genomic RNA in the midst of an abundance of host components. The critical nucleation events that eventually lead to virion production begin early on in the host cytoplasm as the first strands of nascent viral RNA (vRNA) are synthesized. The interaction between the vRNA and the viral nucleocapsid (N) protein effectively protects and masks the RNA from the host, and also forms the ribonucleoprotein (RNP) architecture that mediates downstream interactions and drives virion formation. Although the mechanism by which all three genomic counterparts are selectively co-packaged is not completely understood, we are beginning to understand the hierarchy of interactions that begins with N-RNA packaging and culminates in RNP packaging into new virus particles. In this review we focus on recent progress that highlights the molecular basis of RNA genome packaging in the phleboviruses.

Mutational Analysis of the Rift Valley Fever Virus Glycoprotein Precursor Proteins for Gn Protein Expression

The Rift Valley fever virus (RVFV) M-segment encodes the 78 kD, NSm, Gn, and Gc proteins. The 1st AUG generates the 78 kD-Gc precursor, the 2nd AUG generates the NSm-Gn-Gc precursor, and the 3rd AUG makes the NSm’-Gn-Gc precursor. To understand biological changes due to abolishment of the precursors, we quantitatively measured Gn secretion using a reporter assay, in which a Gaussia luciferase (gLuc) protein is fused to the RVFV M-segment pre-Gn region. Using the reporter assay, the relative expression of Gn/gLuc fusion proteins was analyzed among various AUG mutants. The reporter assay showed efficient secretion of Gn/gLuc protein from the precursor made from the 2nd AUG, while the removal of the untranslated region upstream of the 2nd AUG (AUG2-M) increased the secretion of the Gn/gLuc protein. Subsequently, recombinant MP-12 strains encoding mutations in the pre-Gn region were rescued, and virological phenotypes were characterized. Recombinant MP-12 encoding the AUG2-M mutation replicated slightly less efficiently than the control, indicating that viral replication is further influenced by the biological processes occurring after Gn expression, rather than the Gn abundance. This study showed that, not only the abolishment of AUG, but also the truncation of viral UTR, affects the expression of Gn protein by the RVFV M-segment.

Characterization of Glycoprotein-Mediated Entry of Severe Fever with Thrombocytopenia Syndrome Virus

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging hemorrhagic fever with a high case fatality rate caused by SFTS virus (SFTSV). Effective vaccines and specific therapies for SFTS are urgently sought, and investigation into virus-host cell interactions is expected to contribute to the development of antiviral strategies. In this study, we have developed a pseudotype vesicular stomatitis virus (VSV) bearing the unmodified Gn/Gc glycoproteins (GPs) of SFTSV (SFTSVpv). We have analyzed the host cell entry of this pseudotype virus and native SFTSV. Both SFTSVpv and SFTSV exhibited high infectivity in various mammalian cell lines. The use of lysosomotropic agents indicated that virus entry occurred via pH-dependent endocytosis. SFTSVpv and SFTSV infectivity was neutralized by serial dilutions of convalescent-phase patient sera. Entry of SFTSVpv and growth of SFTSV were increased in Raji cells expressing not only the C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) but also DC-SIGN-related (DC-SIGNR) and liver and lymph node sinusoidal endothelial cell C-type lectin (LSECtin). 25-Hydroxycholesterol (25HC), a soluble oxysterol metabolite, inhibited the cell entry of SFTSVpv and the membrane fusion of SFTSV. These results indicate that pH-dependent endocytosis of SFTSVpv and SFTSV is enhanced by attachment to certain C-type lectins. SFTSVpv is an appropriate model for the investigation of SFTSV-GP-mediated cell entry and virus neutralization at lower biosafety levels. Furthermore, 25HC may represent a potential antiviral agent against SFTS.

IMPORTANCE

SFTSV is a recently discovered bunyavirus associated with SFTS, a viral hemorrhagic fever with a high case fatality rate endemic to China, South Korea, and Japan. Because little is known about the characteristics of the envelope protein and entry mechanisms of SFTSV, further studies will be required for the development of a vaccine or effective therapies. In this study, we investigated the mechanism of SFTSV cell entry using SFTSVpv and the native virus. SFTSV can grow in nonsusceptible cell lines in the presence of certain C-type lectins. Moreover, 25HC, an oxysterol metabolite, may represent a potential therapeutic inhibitor of SFTSV infection.

Recent advances in hantavirus molecular biology and disease

Hantaviruses are emerging zoonotic pathogens that belong to the Bunyaviridae family. They have been classified as category A pathogens by CDC (centers for disease control and prevention). Hantaviruses pose a serious threat to human health because their infection causes two highly fatal diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). These pathogens are transmitted to humans through aerosolized excreta of their infected rodent hosts. Hantaviruses have a tripartite-segmented negative-sense RNA genome. The three genomic RNA segments, S, M, and L, encode a nucleocapsid protein (N), a precursor glycoprotein that is processed into two envelope glycoproteins (Gn and Gc) and the viral RNA-dependent RNA polymerase (RdRp), respectively. N protein is the major structural component of the virus, its main function is to protect and encapsidate the three genomic RNAs forming three viral ribonucleocapsids. Recent studies have proposed that N in conjunction with RdRp plays important roles in the transcription and replication of viral genome. In addition, N preferentially facilitates the translation of viral mRNA in cells. Glycoproteins, Gn and Gc, play major roles in viral attachment and entry to the host cells, virulence, and assembly and packaging of new virions in infected cells. RdRp functions as RNA replicase and transcriptase to replicate and transcribe the viral RNA and is also thought to have endonuclease activity. Currently, no antiviral therapy or vaccine is available for the treatment of hantavirus-associated diseases. Understanding the molecular details of hantavirus life cycle will help in the identification of targets for antiviral therapeutics and in the design of potential antiviral drug for the treatment of HFRS and HCPS. Due to the alarming fatality of hantavirus diseases, development of an effective vaccine against hantaviruses is a necessity.

Expression and purification of the surface proteins from Andes virus

Andes virus is the main causative agent of Hantavirus cardiopulmonary syndrome in South America. There are currently no vaccines or treatments against Andes virus. However, there are several evidences suggesting that antibodies against Andes virus envelope glycoproteins may be enough to confer full protection against Hantavirus cardiopulmonary syndrome. The goal of the present work was to express, purify and characterize the extracellular domains of Andes virus glycoproteins Gn and Gc. We generated two adenoviral vectors encoding the extracellular domains of Andes virus glycoproteins Gn and Gc. Both molecules were expressed by adenoviral transduction in SiHa cells. We found that sGc ectodomain was mainly secreted into the culture medium, whereas sGn was predominantly retained inside the cells. Both molecules were expressed at very low concentrations (below 1 μg/mL). Treatment with the proteasome inhibitor ALLN raised sGc concentration in the cell culture medium, but did not affect expression levels of sGn. Both ectodomains were purified by immobilized metal ion affinity chromatography, and were recognized by sera from persons previously exposed to Andes virus. To our knowledge, this is the first work that addresses the expression and purification of Andes virus glycoproteins Gn and Gc. Our results demonstrate that sGn and sGc maintain epitopes that are exposed on the surface of the viral envelope. However, our work also highlights the need to explore new strategies to achieve high-level expression of these proteins for development of a vaccine candidate against Andes virus.

Temperature-sensitive mutations for live-attenuated Rift Valley fever vaccines: implications from other RNA viruses

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to the African continent. RVF is characterized by high rate of abortions in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by the Rift Valley fever virus (RVFV: genus Phlebovirus, family Bunyaviridae). Vaccination is the only known effective strategy to prevent the disease, but there are no licensed RVF vaccines available for humans. A live-attenuated vaccine candidate derived from the wild-type pathogenic Egyptian ZH548 strain, MP-12, has been conditionally licensed for veterinary use in the U.S. MP-12 displays a temperature-sensitive (ts) phenotype and does not replicate at 41°C. The ts mutation limits viral replication at a specific body temperature and may lead to an attenuation of the virus. Here we will review well-characterized ts mutations for RNA viruses, and further discuss the potential in designing novel live-attenuated vaccines for RVF.

Rift Valley Fever Virus MP-12 Vaccine Is Fully Attenuated by a Combination of Partial Attenuations in the S, M, and L Segments

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and characterized by a high rate of abortion in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus), which has a tripartite negative-stranded RNA genome (consisting of the S, M, and L segments). Further spread of RVF into countries where the disease is not endemic may affect the economy and public health, and vaccination is an effective approach to prevent the spread of RVFV. A live-attenuated MP-12 vaccine is one of the best-characterized RVF vaccines for safety and efficacy and is currently conditionally licensed for use for veterinary purposes in the United States. Meanwhile, as of 2015, no other RVF vaccine has been conditionally or fully licensed for use in the United States. The MP-12 strain is derived from wild-type pathogenic strain ZH548, and its genome encodes 23 mutations in the three genome segments. However, the mechanism of MP-12 attenuation remains unknown. We characterized the attenuation of wild-type pathogenic strain ZH501 carrying a mutation(s) of the MP-12 S, M, or L segment in a mouse model. Our results indicated that MP-12 is attenuated by the mutations in the S, M, and L segments, while the mutations in the M and L segments confer stronger attenuation than those in the S segment. We identified a combination of 3 amino acid changes, Y259H (Gn), R1182G (Gc), and R1029K (L), that was sufficient to attenuate ZH501. However, strain MP-12 with reversion mutations at those 3 sites was still highly attenuated. Our results indicate that MP-12 attenuation is supported by a combination of multiple partial attenuation mutations and a single reversion mutation is less likely to cause a reversion to virulence of the MP-12 vaccine.

IMPORTANCE

Rift Valley fever (RVF) is a mosquito-transmitted viral disease that is endemic to Africa and that has the potential to spread into other countries. Vaccination is considered an effective way to prevent the disease, and the only available veterinary RVF vaccine in the United States is a live-attenuated MP-12 vaccine, which is conditionally licensed. Strain MP-12 is different from its parental pathogenic RVFV strain, strain ZH548, because of the presence of 23 mutations. This study determined the role of individual mutations in the attenuation of the MP-12 strain. We found that full attenuation of MP-12 occurs by a combination of multiple mutations. Our findings indicate that a single reversion mutation will less likely cause a major reversion to virulence of the MP-12 vaccine.

Creation of Rift Valley fever viruses with four-segmented genomes reveals flexibility in bunyavirus genome packaging

Bunyavirus genomes comprise a small (S), a medium (M), and a large (L) RNA segment of negative polarity. Although the untranslated regions have been shown to comprise signals required for transcription, replication, and encapsidation, the mechanisms that drive the packaging of at least one S, M, and L segment into a single virion to generate infectious virus are largely unknown. One of the most important members of the Bunyaviridae family that causes devastating disease in ruminants and occasionally humans is the Rift Valley fever virus (RVFV). We studied the flexibility of RVFV genome packaging by splitting the glycoprotein precursor gene, encoding the (NSm)GnGc polyprotein, into two individual genes encoding either (NSm)Gn or Gc. Using reverse genetics, six viruses with a segmented glycoprotein precursor gene were rescued, varying from a virus comprising two S-type segments in the absence of an M-type segment to a virus consisting of four segments (RVFV-4s), of which three are M-type. Despite that all virus variants were able to grow in mammalian cell lines, they were unable to spread efficiently in cells of mosquito origin. Moreover, in vivo studies demonstrated that RVFV-4s is unable to cause disseminated infection and disease in mice, even in the presence of the main virulence factor NSs, but induced a protective immune response against a lethal challenge with wild-type virus. In summary, splitting bunyavirus glycoprotein precursor genes provides new opportunities to study bunyavirus genome packaging and offers new methods to develop next-generation live-attenuated bunyavirus vaccines.

IMPORTANCE

Rift Valley fever virus (RVFV) causes devastating disease in ruminants and occasionally humans. Virions capable of productive infection comprise at least one copy of the small (S), medium (M), and large (L) RNA genome segments. The M segment encodes a glycoprotein precursor (GPC) protein that is cotranslationally cleaved into Gn and Gc, which are required for virus entry and fusion. We studied the flexibility of RVFV genome packaging and developed experimental live-attenuated vaccines by applying a unique strategy based on the splitting of the GnGc open reading frame. Several RVFV variants, varying from viruses comprising two S-type segments to viruses consisting of four segments (RVFV-4s), of which three are M-type, could be rescued and were shown to induce a rapid protective immune response. Altogether, the segmentation of bunyavirus GPCs provides a new method for studying bunyavirus genome packaging and facilitates the development of novel live-attenuated bunyavirus vaccines.

Rice stripe tenuivirus NSvc2 glycoproteins targeted to the golgi body by the N-terminal transmembrane domain and adjacent cytosolic 24 amino acids via the COP I- and COP II-dependent secretion pathway

The NSvc2 glycoproteins encoded by Rice stripe tenuivirus (RSV) share many characteristics common to the glycoproteins found among Bunyaviridae. Within this viral family, glycoproteins targeting to the Golgi apparatus play a pivotal role in the maturation of the enveloped spherical particles. RSV particles, however, adopt a long filamentous morphology. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. Here, we demonstrate that the amino-terminal NSvc2 (NSvc2-N) targets to the Golgi apparatus in Nicotiana benthamiana cells, whereas the carboxyl-terminal NSvc2 (NSvc2-C) accumulates in the endoplasmic reticulum (ER). Upon coexpression, NSvc2-N redirects NSvc2-C from the ER to the Golgi bodies. The NSvc2 glycoproteins move together with the Golgi stacks along the ER/actin network. The targeting of the NSvc2 glycoproteins to the Golgi bodies was strictly dependent on functional anterograde traffic out of the ER to the Golgi bodies or on a retrograde transport route from the Golgi apparatus. The analysis of truncated and chimeric NSvc2 proteins demonstrates that the Golgi targeting signal comprises amino acids 269 to 315 of NSvc2-N, encompassing the transmembrane domain and 24 adjacent amino acids in the cytosolic tail. Our findings demonstrate for the first time that the glycoproteins from an unenveloped Tenuivirus could target Golgi bodies in plant cells.

IMPORTANCE

NSvc2 glycoprotein encoded by unenveloped Rice stripe tenuivirus (RSV) share many characteristics in common with glycoprotein found among Bunyaviridae in which all members have membrane-enveloped sphere particle. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. In this study, we demonstrated that the RSV glycoproteins could target Golgi bodies in plant cells. The targeting of NSvc2 glycoproteins to the Golgi bodies was dependent on active COP II or COP I. The Golgi targeting signal was mapped to the 23-amino-acid transmembrane domain and the adjacent 24 amino acids of the cytosolic tail of the NSvc2-N. In light of the evidence from viruses in Bunyaviridae that targeting Golgi bodies is important for the viral particle assembly and vector transmission, we propose that targeting of RSV glycoproteins into Golgi bodies in plant cells represents a physiologically relevant mechanism in the maturation of RSV particle complex for insect vector transmission.

Countermeasure development for Rift Valley fever: deletion, modification or targeting of major virulence factor NSs

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease characterized by a high rate of abortion in ruminants, and febrile illness, hemorrhagic fever, retinitis and encephalitis in humans. RVF is caused by the RVF virus (RVFV), belonging to the genus Phlebovirus of the family Bunyaviridae. RVFV encodes a major virulence factor, NSs, which is dispensable for viral replication, yet required for evasion of host innate immune responses. RVFV NSs inhibits host gene upregulation at the transcriptional level, while promoting viral translation in the cytoplasm. In this article, we summarize the virology and pathology of RVF, and countermeasure development for RVF, with emphasis on NSs function and applications.

A single vaccination with an improved nonspreading Rift Valley fever virus vaccine provides sterile immunity in lambs

Rift Valley fever virus (RVFV) is an important pathogen that affects ruminants and humans. Recently we developed a vaccine based on nonspreading RVFV (NSR) and showed that a single vaccination with this vaccine protects lambs from viremia and clinical signs. However, low levels of viral RNA were detected in the blood of vaccinated lambs shortly after challenge infection. These low levels of virus, when present in a pregnant ewe, could potentially infect the highly susceptible fetus. We therefore aimed to further improve the efficacy of the NSR vaccine. Here we report the expression of Gn, the major immunogenic protein of the virus, from the NSR genome. The resulting NSR-Gn vaccine was shown to elicit superior CD8 and CD4-restricted memory responses and improved virus neutralization titers in mice. A dose titration study in lambs revealed that the highest vaccination dose of 10(6.3) TCID50/ml protected all lambs from clinical signs and viremia. The lambs developed neutralizing antibodies within three weeks after vaccination and no anamnestic responses were observed following challenge. The combined results suggest that sterile immunity was achieved by a single vaccination with the NSR-Gn vaccine.

Role of the cytosolic tails of Rift Valley fever virus envelope glycoproteins in viral morphogenesis

The correct folding, heterodimerization and trafficking of Gn/Gc envelope glycoproteins of Rift Valley fever virus, RVFV (Bunyaviridae and Phlebovirus genus) are essential for Golgi assembly and budding of viral particles. The Gn and Gc carboxy-terminus contain a Golgi targeting and an ER-retrieval signal, respectively. We generated RVFV-like particles with mutations in the cytosolic tails of Gn or Gc and identified regions important for release of infectious particles. The role of specific amino-acids in these regions was further investigated by creating recombinant mutant viruses by reverse-genetics. Residues outside the suspected Golgi targeting motif, i.e. the di-lysine K29-K30 motif and the N43, R44 and I46 residues of the Gn cytosolic domain, appeared important for Golgi localization and RNP packaging. Concerning the Gc tail, replacement of K2 or K3 in the di-lysine motif, had a drastic impact on Gn trafficking and induced an important organelle redistribution and cell remodeling, greatly affecting particle formation and release.

The cytosolic nucleoprotein of the plant-infecting bunyavirus tomato spotted wilt recruits endoplasmic reticulum-resident proteins to endoplasmic reticulum export sites

In contrast with animal-infecting viruses, few known plant viruses contain a lipid envelope, and the processes leading to their membrane envelopment remain largely unknown. Plant viruses with lipid envelopes include viruses of the Bunyaviridae, which obtain their envelope from the Golgi complex. The envelopment process is predominantly dictated by two viral glycoproteins (Gn and Gc) and the viral nucleoprotein (N). During maturation of the plant-infecting bunyavirus Tomato spotted wilt, Gc localizes at endoplasmic reticulum (ER) membranes and becomes ER export competent only upon coexpression with Gn. In the presence of cytosolic N, Gc remains arrested in the ER but changes its distribution from reticular into punctate spots. Here, we show that these areas correspond to ER export sites (ERESs), distinct ER domains where glycoprotein cargo concentrates prior to coat protein II vesicle-mediated transport to the Golgi. Gc concentration at ERES is mediated by an interaction between its cytoplasmic tail (CT) and N. Interestingly, an ER-resident calnexin provided with Gc-CT was similarly recruited to ERES when coexpressed with N. Furthermore, disruption of actin filaments caused the appearance of a larger amount of smaller ERES loaded with N-Gc complexes, suggesting that glycoprotein cargo concentration acts as a trigger for de novo synthesis of ERES.

Rift Valley fever virus structural and nonstructural proteins: recombinant protein expression and immunoreactivity against antisera from sheep

The Rift Valley fever virus (RVFV) encodes the structural proteins nucleoprotein (N), aminoterminal glycoprotein (Gn), carboxyterminal glycoprotein (Gc), and L protein, 78-kD, and the nonstructural proteins NSm and NSs. Using the baculovirus system, we expressed the full-length coding sequence of N, NSs, NSm, Gc, and the ectodomain of the coding sequence of the Gn glycoprotein derived from the virulent strain of RVFV ZH548. Western blot analysis using anti-His antibodies and monoclonal antibodies against Gn and N confirmed expression of the recombinant proteins, and in vitro biochemical analysis showed that the two glycoproteins, Gn and Gc, were expressed in glycosylated form. Immunoreactivity profiles of the recombinant proteins in western blot and in indirect enzyme-linked immunosorbent assay against a panel of antisera obtained from vaccinated or wild type (RVFV)-challenged sheep confirmed the results obtained with anti-His antibodies and demonstrated the suitability of the baculo-expressed antigens for diagnostic assays. In addition, these recombinant proteins could be valuable for the development of diagnostic methods that differentiate infected from vaccinated animals (DIVA).

Anchors aweigh: protein localization and transport mediated by transmembrane domains

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TMDs control the intracellular transport of many membrane proteins.

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The length and hydrophobicity of TMDs determine their sorting.

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Some membrane receptors for sorting TMDs have been identified.

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Lipid partitioning may also participate in the sorting of TMDs.

The transmembrane domains (TMDs) of integral membrane proteins have emerged as major determinants of intracellular localization and transport in the secretory and endocytic pathways. Unlike sorting signals in cytosolic domains, TMD sorting determinants are not conserved amino acid sequences but physical properties such as the length and hydrophilicity of the transmembrane span. The underlying sorting machinery is still poorly characterized, but several mechanisms have been proposed, including TMD recognition by transmembrane sorting receptors and partitioning into membrane lipid domains. Here we review the nature of TMD sorting determinants and how they may dictate transmembrane protein localization and transport.

Characterization of the Uukuniemi virus group (Phlebovirus: Bunyaviridae): evidence for seven distinct species

Evolutionary insights into the phleboviruses are limited because of an imprecise classification scheme based on partial nucleotide sequences and scattered antigenic relationships. In this report, the serologic and phylogenetic relationships of the Uukuniemi group viruses and their relationships with other recently characterized tick-borne phleboviruses are described using full-length genome sequences. We propose that the viruses currently included in the Uukuniemi virus group be assigned to five different species as follows: Uukuniemi virus, EgAn 1825-61 virus, Fin V707 virus, Chizé virus, and Zaliv Terpenia virus would be classified into the Uukuniemi species; Murre virus, RML-105-105355 virus, and Sunday Canyon virus would be classified into a Murre virus species; and Grand Arbaud virus, Precarious Point virus, and Manawa virus would each be given individual species status. Although limited sequence similarity was detected between current members of the Uukuniemi group and Severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus, a clear serological reaction was observed between some of them, indicating that SFTSV and Heartland virus should be considered part of the Uukuniemi virus group. Moreover, based on the genomic diversity of the phleboviruses and given the low correlation observed between complement fixation titers and genetic distance, we propose a system for classification of the Bunyaviridae based on genetic as well as serological data. Finally, the recent descriptions of SFTSV and Heartland virus also indicate that the public health importance of the Uukuniemi group viruses must be reevaluated.

Characterization of the Bhanja serogroup viruses (Bunyaviridae): a novel species of the genus Phlebovirus and its relationship with other emerging tick-borne phleboviruses

Bhanja virus (BHAV) and its antigenically close relatives Forecariah virus (FORV), Kismayo virus (KISV), and Palma virus (PALV) are thought to be members of the family Bunyaviridae, but they have not been assigned to a genus or species. Despite their broad geographical distribution and reports that BHAV causes sporadic cases of febrile illness and encephalitis in humans, the public health importance of the Bhanja serogroup viruses remains unclear, due in part to the lack of sequence and biochemical information for the virus proteins. In order to better define the molecular characteristics of this group, we determined the full-length sequences of the L, M, and S genome segments of multiple isolates of BHAV as well as FORV and PALV. The genome structures of these Bhanja viruses are similar to those of viruses belonging to the genus Phlebovirus. Functional domains and amino acid motifs in the viral proteins that are conserved among other known phleboviruses were also identified in proteins of the BHAV group. Phylogenetic and serological analyses revealed that the BHAVs are most closely related to the novel emerging tick-borne phleboviruses severe fever with thrombocytopenia syndrome virus and Heartland virus, which have recently been implicated as causing severe acute febrile illnesses associated with thrombocytopenia in humans in China and the United States. Our results indicate that the Bhanja serogroup viruses constitute a single novel species in the genus Phlebovirus. The results of this study should facilitate epidemiological surveillance for other, similar tick-borne phleboviruses that may represent unrecognized causes of febrile illness in humans.

Characterization of the Salehabad virus species complex of the genus Phlebovirus (Bunyaviridae)

Genomic and antigenic characterization of the Salehabad virus, a species of the genus Phlebovirus, and four other unclassified phleboviruses (Arbia, Adria, Arumowot and Odrenisrou) demonstrate a serological and genetic relation to one another and are distinct from the eight other recognized species within the genus Phlebovirus. We propose to incorporate these four unclassified viruses as part of the Salehabad species complex within the genus. The known geographical distribution for the members of this species group includes southern Europe, Central Asia and Africa.

An assembly model of rift valley Fever virus

Rift Valley fever virus (RVFV) is a bunyavirus endemic to Africa and the Arabian Peninsula that infects humans and livestock. The virus encodes two glycoproteins, Gn and Gc, which represent the major structural antigens and are responsible for host cell receptor binding and fusion. Both glycoproteins are organized on the virus surface as cylindrical hollow spikes that cluster into distinct capsomers with the overall assembly exhibiting an icosahedral symmetry. Currently, no experimental three-dimensional structure for any entire bunyavirus glycoprotein is available. Using fold recognition, we generated molecular models for both RVFV glycoproteins and found significant structural matches between the RVFV Gn protein and the influenza virus hemagglutinin protein and a separate match between RVFV Gc protein and Sindbis virus envelope protein E1. Using these models, the potential interaction and arrangement of both glycoproteins in the RVFV particle was analyzed, by modeling their placement within the cryo-electron microscopy density map of RVFV. We identified four possible arrangements of the glycoproteins in the virion envelope. Each assembly model proposes that the ectodomain of Gn forms the majority of the protruding capsomer and that Gc is involved in formation of the capsomer base. Furthermore, Gc is suggested to facilitate intercapsomer connections. The proposed arrangement of the two glycoproteins on the RVFV surface is similar to that described for the alphavirus E1-E2 proteins. Our models will provide guidance to better understand the assembly process of phleboviruses and such structural studies can also contribute to the design of targeted antivirals.

Molecular biology and genetic diversity of Rift Valley fever virus

Rift Valley fever virus (RVFV), a member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever (RVF), a mosquito-borne disease of ruminant animals and humans. The generation of a large sequence database has facilitated studies of the evolution and spread of the virus. Bayesian analyses indicate that currently circulating strains of RVFV are descended from an ancestral species that emerged from a natural reservoir in Africa when large-scale cattle and sheep farming were introduced during the 19th century. Viruses descended from multiple lineages persist in that region, through infection of reservoir animals and vertical transmission in mosquitoes, emerging in years of heavy rainfall to cause epizootics and epidemics. On a number of occasions, viruses from these lineages have been transported outside the enzootic region through the movement of infected animals or mosquitoes, triggering outbreaks in countries such as Egypt, Saudi Arabia, Mauritania and Madagascar, where RVF had not previously been seen. Such viruses could potentially become established in their new environments through infection of wild and domestic ruminants and other animals and vertical transmission in local mosquito species. Despite their extensive geographic dispersion, all strains of RVFV remain closely related at the nucleotide and amino acid level. The high degree of conservation of genes encoding the virion surface glycoproteins suggests that a single vaccine should protect against all currently circulating RVFV strains. Similarly, preservation of the sequence of the RNA-dependent RNA polymerase across viral lineages implies that antiviral drugs targeting the enzyme should be effective against all strains. Researchers should be encouraged to collect additional RVFV isolates and perform whole-genome sequencing and phylogenetic analysis, so as to enhance our understanding of the continuing evolution of this important virus. This review forms part of a series of invited papers in Antiviral Research on the genetic diversity of emerging viruses.

Hantavirus structure--molecular interactions behind the scene

Viruses of the genus Hantavirus, carried and transmitted by rodents and insectivores, are the exception in the vector-borne virus family Bunyaviridae, since viruses of the other genera are transmitted via arthropods. The single-stranded, negative-sense, RNA genome of hantaviruses is trisegmented into small, medium and large (S, M and L) segments. The segments, respectively, encode three structural proteins: nucleocapsid (N) protein, two glycoproteins Gn and Gc and an RNA-dependent RNA-polymerase. The genome segments, encapsidated by the N protein to form ribonucleoproteins, are enclosed inside a lipid envelope that is decorated by spikes composed of Gn and Gc. The virion displays round or pleomorphic morphology with a diameter of roughly 120-160 nm depending on the detection method. This review focuses on the structural components of hantaviruses, their interactions, the mechanisms behind virion assembly and the interactions that maintain virion integrity. We attempt to summarize recent results on the virion structure and to suggest mechanisms on how the assembly is driven. We also compare hantaviruses to other bunyaviruses with known structure.

Recent advances in the molecular and cellular biology of bunyaviruses

The family Bunyaviridae of segmented, negative-stranded RNA viruses includes over 350 members that infect a bewildering variety of animals and plants. Many of these bunyaviruses are the causative agents of serious disease in their respective hosts, and are classified as emerging viruses because of their increased incidence in new populations and geographical locations throughout the world. Emerging bunyaviruses, such as Crimean-Congo hemorrhagic fever virus, tomato spotted wilt virus and Rift Valley fever virus, are currently attracting great interest due to migration of their arthropod vectors, a situation possibly linked to climate change. These and other examples of continued emergence suggest that bunyaviruses will probably continue to pose a sustained global threat to agricultural productivity, animal welfare and human health. The threat of emergence is particularly acute in light of the lack of effective preventative or therapeutic treatments for any of these viruses, making their study an important priority. This review presents recent advances in the understanding of the bunyavirus life cycle, including aspects of their molecular, cellular and structural biology. Whilst special emphasis is placed upon the emerging bunyaviruses, we also describe the extensive body of work involving model bunyaviruses, which have been the subject of major contributions to our overall understanding of this important group of viruses.

Rift valley fever: recent insights into pathogenesis and prevention

Rift Valley fever virus (RVFV) is a zoonotic pathogen that primarily affects ruminants but can also be lethal in humans. A negative-stranded RNA virus of the family Bunyaviridae, this pathogen is transmitted mainly via mosquito vectors. RVFV has shown the ability to inflict significant damage to livestock and is also a threat to public health. While outbreaks have traditionally occurred in sub-Saharan Africa, recent outbreaks in the Middle East have raised awareness of the potential of this virus to spread to Europe, Asia, and the Americas. Although the virus was initially characterized almost 80 years ago, the only vaccine approved for widespread veterinary use is an attenuated strain that has been associated with significant pathogenic side effects. However, increased understanding of the molecular biology of the virus over the last few years has led to recent advances in vaccine design and has enabled the development of more-potent prophylactic measures to combat infection. In this review, we discuss several aspects of RVFV, with particular emphasis on the molecular components of the virus and their respective roles in pathogenesis and an overview of current vaccine candidates. Progress in understanding the epidemiology of Rift Valley fever has also enabled prediction of potential outbreaks well in advance, thus providing another tool to combat the physical and economic impact of this disease.

Efficient cellular release of Rift Valley fever virus requires genomic RNA

The Rift Valley fever virus is responsible for periodic, explosive epizootics throughout sub-Saharan Africa. The development of therapeutics targeting this virus is difficult due to a limited understanding of the viral replicative cycle. Utilizing a virus-like particle system, we have established roles for each of the viral structural components in assembly, release, and virus infectivity. The envelope glycoprotein, Gn, was discovered to be necessary and sufficient for packaging of the genome, nucleocapsid protein and the RNA-dependent RNA polymerase into virus particles. Additionally, packaging of the genome was found to be necessary for the efficient release of particles, revealing a novel mechanism for the efficient generation of infectious virus. Our results identify possible conserved targets for development of anti-phlebovirus therapies.

Aguacate virus, a new antigenic complex of the genus Phlebovirus (family Bunyaviridae)

Genomic and antigenic characterization of Aguacate virus, a tentative species of the genus Phlebovirus, and three other unclassified viruses, Armero virus, Durania virus and Ixcanal virus, demonstrate a close relationship to one another. They are distinct from the other nine recognized species within the genus Phlebovirus. We propose to designate them as a new (tenth) serogroup or species (Aguacate virus) within the genus. The four viruses were all isolated from phlebotomine sandflies (Lutzomyia sp.) collected in Central and South America. Aguacate virus appears to be a natural reassortant and serves as one more example of the high frequency of reassortment in this genus.

Characterization of the Candiru antigenic complex (Bunyaviridae: Phlebovirus), a highly diverse and reassorting group of viruses affecting humans in tropical America

The genus Phlebovirus of the family Bunyaviridae consists of approximately 70 named viruses, currently assigned to nine serocomplexes (species) based on antigenic similarities. Sixteen other named viruses that show little serologic relationship to the nine recognized groups are also classified as tentative species in the genus. In an effort to develop a more precise classification system for phleboviruses, we are attempting to sequence most of the named viruses in the genus with the goal of clarifying their phylogenetic relationships. In this report, we describe the serologic and phylogenetic relationships of 13 viruses that were found to be members of the Candiru serocomplex; 6 of them cause disease in humans. Analysis of full genome sequences revealed branching inconsistencies that suggest five reassortment events, all involving the M segment, and thus appear to be natural reassortants. This high rate of reassortment illustrates the inaccuracy of a classification system based solely on antigenic relationships.

Rift valley fever virus infection of human cells and insect hosts is promoted by protein kinase C epsilon

As an arthropod-borne human pathogen, Rift Valley fever virus (RVFV) cycles between an insect vector and mammalian hosts. Little is known about the cellular requirements for infection in either host. Here we developed a tissue culture model for RVFV infection of human and insect cells that is amenable to high-throughput screening. Using this approach we screened a library of 1280 small molecules with pharmacologically defined activities and identified 59 drugs that inhibited RVFV infection with 15 inhibiting RVFV replication in both human and insect cells. Amongst the 15 inhibitors that blocked infection in both hosts was a subset that inhibits protein kinase C. Further studies found that infection is dependent upon the novel protein kinase C isozyme epsilon (PKCε) in both human and insect cells as well as in adult flies. Altogether, these data show that inhibition of cellular factors required for early steps in the infection cycle including PKCε can block RVFV infection, and may represent a starting point for the development of anti-RVFV therapeutics.

Rift Valley fever virus(Bunyaviridae: Phlebovirus): an update on pathogenesis, molecular epidemiology, vectors, diagnostics and prevention

Rift Valley fever(RVF) virus is an arbovirus in the Bunyaviridae family that, from phylogenetic analysis, appears to have first emerged in the mid-19th century and was only identified at the beginning of the 1930's in the Rift Valley region of Kenya. Despite being an arbovirus with a relatively simple but temporally and geographically stable genome, this zoonotic virus has already demonstrated a real capacity for emerging in new territories, as exemplified by the outbreaks in Egypt (1977), Western Africa (1988) and the Arabian Peninsula (2000), or for re-emerging after long periods of silence as observed very recently in Kenya and South Africa. The presence of competent vectors in countries previously free of RVF, the high viral titres in viraemic animals and the global changes in climate, travel and trade all contribute to make this virus a threat that must not be neglected as the consequences of RVF are dramatic, both for human and animal health. In this review, we present the latest advances in RVF virus research. In spite of this renewed interest, aspects of the epidemiology of RVF virus are still not fully understood and safe, effective vaccines are still not freely available for protecting humans and livestock against the dramatic consequences of this virus.

Structure of the Rift Valley fever virus nucleocapsid protein reveals another architecture for RNA encapsidation

Rift Valley fever virus (RVFV) is a negative-sense RNA virus (genus Phlebovirus, family Bunyaviridae) that infects livestock and humans and is endemic to sub-Saharan Africa. Like all negative-sense viruses, the segmented RNA genome of RVFV is encapsidated by a nucleocapsid protein (N). The 1.93-A crystal structure of RVFV N and electron micrographs of ribonucleoprotein (RNP) reveal an encapsidated genome of substantially different organization than in other negative-sense RNA virus families. The RNP polymer, viewed in electron micrographs of both virus RNP and RNP reconstituted from purified N with a defined RNA, has an extended structure without helical symmetry. N-RNA species of approximately 100-kDa apparent molecular weight and heterogeneous composition were obtained by exhaustive ribonuclease treatment of virus RNP, by recombinant expression of N, and by reconstitution from purified N and an RNA oligomer. RNA-free N, obtained by denaturation and refolding, has a novel all-helical fold that is compact and well ordered at both the N and C termini. Unlike N of other negative-sense RNA viruses, RVFV N has no positively charged surface cleft for RNA binding and no protruding termini or loops to stabilize a defined N-RNA oligomer or RNP helix. A potential protein interaction site was identified in a conserved hydrophobic pocket. The nonhelical appearance of phlebovirus RNP, the heterogeneous approximately 100-kDa N-RNA multimer, and the N fold differ substantially from the RNP and N of other negative-sense RNA virus families and provide valuable insights into the structure of the encapsidated phlebovirus genome.