The Andes hantavirus NSs protein is expressed from the viral small mRNA by a leaky scanning mechanism

Heterogeneous nuclear ribonucleoprotein K promotes cap-independent translation initiation of retroviral mRNAs

Translation initiation of the human immunodeficiency virus-type 1 (HIV-1) genomic mRNA (vRNA) is cap-dependent or mediated by an internal ribosome entry site (IRES). The HIV-1 IRES requires IRES-transacting factors (ITAFs) for function. In this study, we evaluated the role of the heterogeneous nuclear ribonucleoprotein K (hnRNPK) as a potential ITAF for the HIV-1 IRES. In HIV-1-expressing cells, the depletion of hnRNPK reduced HIV-1 vRNA translation. Furthermore, both the depletion and overexpression of hnRNPK modulated HIV-1 IRES activity. Phosphorylations and protein arginine methyltransferase 1 (PRMT1)-induced asymmetrical dimethylation (aDMA) of hnRNPK strongly impacted the protein's ability to promote the activity of the HIV-1 IRES. We also show that hnRNPK acts as an ITAF for the human T cell lymphotropic virus-type 1 (HTLV-1) IRES, present in the 5'UTR of the viral sense mRNA, but not for the IRES present in the antisense spliced transcript encoding the HTLV-1 basic leucine zipper protein (sHBZ). This study provides evidence for a novel role of the host hnRNPK as an ITAF that stimulates IRES-mediated translation initiation for the retroviruses HIV-1 and HTLV-1.

The double-stranded RNA-binding protein, Staufen1, is an IRES-transacting factor regulating HIV-1 cap-independent translation initiation

Translation initiation of the viral genomic mRNA (vRNA) of human immunodeficiency virus-type 1 (HIV-1) can be mediated by a cap- or an internal ribosome entry site (IRES)-dependent mechanism. A previous report shows that Staufen1, a cellular double-stranded (ds) RNA-binding protein (RBP), binds to the 5'untranslated region (5'UTR) of the HIV-1 vRNA and promotes its cap-dependent translation. In this study, we now evaluate the role of Staufen1 as an HIV-1 IRES-transacting factor (ITAF). We first confirm that Staufen1 associates with both the HIV-1 vRNA and the Gag protein during HIV-1 replication. We found that in HIV-1-expressing cells, siRNA-mediated depletion of Staufen1 reduces HIV-1 vRNA translation. Using dual-luciferase bicistronic mRNAs, we show that the siRNA-mediated depletion and cDNA-mediated overexpression of Staufen1 acutely regulates HIV-1 IRES activity. Furthermore, we show that Staufen1-vRNA interaction is required for the enhancement of HIV-1 IRES activity. Interestingly, we find that only Staufen1 harboring an intact dsRNA-binding domain 3 (dsRBD3) rescues HIV-1 IRES activity in Staufen1 CRISPR-Cas9 gene edited cells. Finally, we show that the expression of Staufen1-dsRBD3 alone enhances HIV-1 IRES activity. This study provides evidence of a novel role for Staufen1 as an ITAF promoting HIV-1 vRNA IRES activity.

The viral nucleocapsid protein and the human RNA-binding protein Mex3A promote translation of the Andes orthohantavirus small mRNA

The capped Small segment mRNA (SmRNA) of the Andes orthohantavirus (ANDV) lacks a poly(A) tail. In this study, we characterize the mechanism driving ANDV-SmRNA translation. Results show that the ANDV-nucleocapsid protein (ANDV-N) promotes in vitro translation from capped mRNAs without replacing eukaryotic initiation factor (eIF) 4G. Using an RNA affinity chromatography approach followed by mass spectrometry, we identify the human RNA chaperone Mex3A (hMex3A) as a SmRNA-3’UTR binding protein. Results show that hMex3A enhances SmRNA translation in a 3’UTR dependent manner, either alone or when co-expressed with the ANDV-N. The ANDV-N and hMex3A proteins do not interact in cells, but both proteins interact with eIF4G. The hMex3A–eIF4G interaction showed to be independent of ANDV-infection or ANDV-N expression. Together, our observations suggest that translation of the ANDV SmRNA is enhanced by a 5’-3’ end interaction, mediated by both viral and cellular proteins.

Andes orthohantavirus (ANDV) is endemic in Argentina and Chile and is the primary etiological agent of hantavirus cardiopulmonary syndrome (HCPS) in South America. ANDV is unique among other members of the Hantaviridae family of viruses because of its ability to spread from person to person. The molecular mechanisms driving ANDV protein synthesis remain poorly understood. A previous report showed that translation of the Small segment mRNA (SmRNA) of ANDV relied on both the 5’cap and the 3’untranslated region (UTR) of the SmRNA. In this new study, we further characterize the mechanism by which the 5’ and 3’end of the SmRNA interact to assure viral protein synthesis. We establish that the viral nucleocapsid protein N and the cellular protein hMex3A participate in the process. These observations indicated that both viral and cellular proteins regulate viral gene expression during ANDV infection by enabling the viral mRNA to establish a non-covalent 5’-3’end interaction.

Inhibition of interferon I induction by non-structural protein NSs of Puumala virus and other vole-associated orthohantaviruses: phenotypic plasticity of the protein and potential functional domains

The orthohantavirus Puumala virus (PUUV), which is transmitted by bank voles (Clethrionomys glareolus), and other vole-borne hantaviruses contain in their small (S) genome segment two overlapping open reading frames, coding for the nucleocapsid protein and the non-structural protein NSs, a putative type I interferon (IFN-I) antagonist. To investigate the role of NSs of PUUV and other orthohantaviruses, the expression pattern of recombinant NSs constructs and their ability to inhibit human IFN-I promoter activity were investigated. The NSs proteins of PUUV and related cricetid-borne orthohantaviruses showed strong inhibition of IFN-I promoter induction. We identified protein products originating from three and two methionine initiation codons in the NSs ORF of PUUV during transfection and infection, respectively. The three putative start codons are conserved in all PUUV strains analysed. Translation initiation at these start codons influenced the inhibitory activity of the NSs products, with the wild-type (wt) construct expressing two proteins starting at the first and second methionine and showing strong inhibition activity. Analysis of in vitro-generated variants and naturally occurring PUUV NSs proteins indicated that amino acid variation in the NSs protein is well tolerated, suggesting its phenotypic plasticity. The N-terminal 20-amino-acid region of the NSs protein was found to be associated with strong inhibition and to be highly vulnerable to amino acid exchanges and tag fusions. Infection studies using human, bank vole, and Vero E6 cells did not show obvious differences in the replication capacity of PUUV Sotkamo wt and a strain with a truncated NSs protein (NSs21Stop), showing that the lack of a full-length NSs might be compensated by its N-terminal peptide, as seen in transfection experiments. These results contribute to our understanding of virus-host interactions and highlight the importance of future innate immunity studies in reservoir hosts.

Hantavirus Replication Cycle-An Updated Structural Virology Perspective

Hantaviruses infect a wide range of hosts including insectivores and rodents and can also cause zoonotic infections in humans, which can lead to severe disease with possible fatal outcomes. Hantavirus outbreaks are usually linked to the population dynamics of the host animals and their habitats being in close proximity to humans, which is becoming increasingly important in a globalized world. Currently there is neither an approved vaccine nor a specific and effective antiviral treatment available for use in humans. Hantaviruses belong to the order Bunyavirales with a tri-segmented negative-sense RNA genome. They encode only five viral proteins and replicate and transcribe their genome in the cytoplasm of infected cells. However, many details of the viral amplification cycle are still unknown. In recent years, structural biology methods such as cryo-electron tomography, cryo-electron microscopy, and crystallography have contributed essentially to our understanding of virus entry by membrane fusion as well as genome encapsidation by the nucleoprotein. In this review, we provide an update on the hantavirus replication cycle with a special focus on structural virology aspects.

Mother-to-Child Transmission of Andes Virus through Breast Milk, Chile1

Andes virus (ANDV) is the only hantavirus transmitted between humans through close contact. We detected the genome and proteins of ANDV in breast milk cells from an infected mother in Chile who transmitted the virus to her child, suggesting gastrointestinal infection through breast milk as a route of ANDV person-to-person transmission.

Unconventional viral gene expression mechanisms as therapeutic targets

Unlike the human genome that comprises mostly noncoding and regulatory sequences, viruses have evolved under the constraints of maintaining a small genome size while expanding the efficiency of their coding and regulatory sequences. As a result, viruses use strategies of transcription and translation in which one or more of the steps in the conventional gene-protein production line are altered. These alternative strategies of viral gene expression (also known as gene recoding) can be uniquely brought about by dedicated viral enzymes or by co-opting host factors (known as host dependencies). Targeting these unique enzymatic activities and host factors exposes vulnerabilities of a virus and provides a paradigm for the design of novel antiviral therapies. In this Review, we describe the types and mechanisms of unconventional gene and protein expression in viruses, and provide a perspective on how future basic mechanistic work could inform translational efforts that are aimed at viral eradication.

Chloroquine, an Anti-Malaria Drug as Effective Prevention for Hantavirus Infections

We investigated whether chloroquine can prevent hantavirus infection and disease in vitro and in vivo, using the Hantaan virus newborn C57BL/6 mice model and the Syrian hamster model for Andes virus. In vitro antiviral experiments were performed using Vero E6 cells, and Old World and New World hantavirus species. Hantavirus RNA was detected using quantitative RT-PCR. For all hantavirus species tested, results indicate that the IC₅₀ of chloroquine (mean 10.2 ± 1.43 μM) is significantly lower than the CC₅₀ (mean 260 ± 2.52 μM) yielding an overall selectivity index of 25.5. We also investigated the potential of chloroquine to prevent death in newborn mice after Hantaan virus infection and its antiviral effect in the hantavirus Syrian hamster model. For this purpose, C57Bl/6 mother mice were treated subcutaneously with daily doses of chloroquine. Subsequently, 1-day-old suckling mice were inoculated intracerebrally with 5 x 10² Hantaan virus particles. In litters of untreated mothers, none of the pups survived challenge. The highest survival rate (72.7% of pups) was found when mother mice were administered a concentration of 10 mg/kg chloroquine. Survival rates declined in a dose-dependent manner, with 47.6% survival when treated with 5 mg/kg chloroquine, and 4.2% when treated with 1 mg/kg chloroquine. Assessing the antiviral therapeutic and prophylactic effect of chloroquine in the Syrian hamster model was done using two different administration routes (intraperitoneally and subcutaneously using an osmotic pump system). Evaluating the prophylactic effect, a delay in onset of disease was noted and for the osmotic pump, 60% survival was observed. Our results show that chloroquine can be highly effective against Hantaan virus infection in newborn mice and against Andes virus in Syrian hamsters.

A Look into Bunyavirales Genomes: Functions of Non-Structural (NS) Proteins

In 2016, the Bunyavirales order was established by the International Committee on Taxonomy of Viruses (ICTV) to incorporate the increasing number of related viruses across 13 viral families. While diverse, four of the families (Peribunyaviridae, Nairoviridae, Hantaviridae, and Phenuiviridae) contain known human pathogens and share a similar tri-segmented, negative-sense RNA genomic organization. In addition to the nucleoprotein and envelope glycoproteins encoded by the small and medium segments, respectively, many of the viruses in these families also encode for non-structural (NS) NSs and NSm proteins. The NSs of Phenuiviridae is the most extensively studied as a host interferon antagonist, functioning through a variety of mechanisms seen throughout the other three families. In addition, functions impacting cellular apoptosis, chromatin organization, and transcriptional activities, to name a few, are possessed by NSs across the families. Peribunyaviridae, Nairoviridae, and Phenuiviridae also encode an NSm, although less extensively studied than NSs, that has roles in antagonizing immune responses, promoting viral assembly and infectivity, and even maintenance of infection in host mosquito vectors. Overall, the similar and divergent roles of NS proteins of these human pathogenic Bunyavirales are of particular interest in understanding disease progression, viral pathogenesis, and developing strategies for interventions and treatments.

The internal ribosome entry site of the Dengue virus mRNA is active when cap-dependent translation initiation is inhibited

Dengue virus (DENV) is an enveloped, positive-sense, single-stranded RNA virus belonging to the Flaviviridae family. Translation initiation of the DENV mRNA can occur following a cap-dependent or a cap-independent mechanism. Two non-mutually exclusive cap-independent mechanisms of translation initiation have been described for the DENV mRNA. The first corresponds to a 5'end-dependent internal ribosome entry site (IRES)-independent mechanism, while the second relies on IRES-dependent initiation. In this report, we study the recently discovered DENV IRES. Results show that the DENV IRES is functional in the rabbit reticulocyte (RRL) in vitro translation system. In accordance, the activity of DENV IRES was resistant to the cleavage of eIF4G by the Foot-and-mouth disease virus leader protease in RRL. In cells, the DENV IRES exhibited only a marginal activity under standard culture conditions. The DENV IRES showed weak activity in HEK 293T cells; however, the DENV IRES activity was significantly enhanced in HEK 293T cells expressing the Human rhinovirus 2A protease. These findings suggest that the DENV IRES enables viral protein synthesis under conditions that suppress canonical translation initiation.IMPORTANCE Dengue virus (DENV), the etiological agent of Dengue, a febrile and hemorrhagic disease, infects millions of people per year in tropical and subtropical countries. When infecting cells, DENV induces stress conditions known to inhibit canonical protein synthesis. Under these conditions, DENV mRNA thrives using non-canonical modes of translation initiation. In this study, we characterize the mechanism dependent upon an internal ribosome entry site (IRES). Herein, we describe the activity of the DENV IRES in vitro and cells. We show that in cells, DENV IRES enables the viral mRNA to translate under conditions that suppress canonical translation initiation.

Interactions of Viral Proteins from Pathogenic and Low or Non-Pathogenic Orthohantaviruses with Human Type I Interferon Signaling

Rodent-borne orthohantaviruses are asymptomatic in their natural reservoir, but they can cause severe diseases in humans. Although an exacerbated immune response relates to hantaviral pathologies, orthohantaviruses have to antagonize the antiviral interferon (IFN) response to successfully propagate in infected cells. We studied interactions of structural and nonstructural (NSs) proteins of pathogenic Puumala (PUUV), low-pathogenic Tula (TULV), and non-pathogenic Prospect Hill (PHV) viruses, with human type I and III IFN (IFN-I and IFN-III) pathways. The NSs proteins of all three viruses inhibited the RIG-I-activated IFNβ promoter, while only the glycoprotein precursor (GPC) of PUUV, or its cleavage product Gn/Gc, and the nucleocapsid (N) of TULV inhibited it. Moreover, the GPC of both PUUV and TULV antagonized the promoter of IFN-stimulated responsive elements (ISRE). Different viral proteins could thus contribute to inhibition of IFNβ response in a viral context. While PUUV and TULV strains replicated similarly, whether expressing entire or truncated NSs proteins, only PUUV encoding a wild type NSs protein led to late IFN expression and activation of IFN-stimulated genes (ISG). This, together with the identification of particular domains of NSs proteins and different biological processes that are associated with cellular proteins in complex with NSs proteins, suggested that the activation of IFN-I is probably not the only antiviral pathway to be counteracted by orthohantaviruses and that NSs proteins could have multiple inhibitory functions.

Post-translational modifications of hnRNP A1 differentially modulate retroviral IRES-mediated translation initiation

The full-length mRNAs of the human immunodeficiency virus type-1 (HIV-1), the human T-cell lymphotropic virus type-1 (HTLV-1), and the mouse mammary tumor virus (MMTV) harbor IRESs. The activity of the retroviral-IRESs requires IRES-transacting factors (ITAFs), being hnRNP A1, a known ITAF for the HIV-1 IRES. In this study, we show that hnRNP A1 is also an ITAF for the HTLV-1 and MMTV IRESs. The MMTV IRES proved to be more responsive to hnRNP A1 than either the HTLV-1 or the HIV-1 IRESs. The impact of post-translational modifications of hnRNP A1 on HIV-1, HTLV-1 and MMTV IRES activity was also assessed. Results show that the HIV-1 and HTLV-1 IRESs were equally responsive to hnRNP A1 and its phosphorylation mutants S4A/S6A, S4D/S6D and S199A/D. However, the S4D/S6D mutant stimulated the activity from the MMTV-IRES to levels significantly higher than the wild type hnRNP A1. PRMT5-induced symmetrical di-methylation of arginine residues of hnRNP A1 enabled the ITAF to stimulate the HIV-1 and HTLV-1 IRESs while reducing the stimulatory ability of the ITAF over the MMTV IRES. We conclude that retroviral IRES activity is not only dependent on the recruited ITAFs but also relies on how these proteins are modified at the post-translational level.

Spatial and Temporal Evolutionary Patterns in Puumala Orthohantavirus (PUUV) S Segment

The S segment of bank vole (Clethrionomys glareolus)-associated Puumala orthohantavirus (PUUV) contains two overlapping open reading frames coding for the nucleocapsid (N) and a non-structural (NSs) protein. To identify the influence of bank vole population dynamics on PUUV S segment sequence evolution and test for spillover infections in sympatric rodent species, during 2010–2014, 883 bank voles, 357 yellow-necked mice (Apodemus flavicollis), 62 wood mice (A. sylvaticus), 149 common voles (Microtus arvalis) and 8 field voles (M. agrestis) were collected in Baden-Wuerttemberg and North Rhine-Westphalia, Germany. In total, 27.9% and 22.3% of bank voles were positive for PUUV-reactive antibodies and PUUV-specific RNA, respectively. One of eight field voles was PUUV RNA-positive, indicating a spillover infection, but none of the other species showed evidence of PUUV infection. Phylogenetic and isolation-by-distance analyses demonstrated a spatial clustering of PUUV S segment sequences. In the hantavirus outbreak years 2010 and 2012, PUUV RNA prevalence was higher in our study regions compared to non-outbreak years 2011, 2013 and 2014. NSs amino acid and nucleotide sequence types showed temporal and/or local variation, whereas the N protein was highly conserved in the NSs overlapping region and, to a lower rate, in the N alone coding part.

The Andes Orthohantavirus NSs Protein Antagonizes the Type I Interferon Response by Inhibiting MAVS Signaling

The small messenger RNA (SmRNA) of the Andes orthohantavirus (ANDV), a rodent-borne member of the Hantaviridae family of viruses of the Bunyavirales order, encodes a multifunctional nucleocapsid (N) protein and for a nonstructural (NSs) protein of unknown function. We have previously shown the expression of the ANDV-NSs, but only in infected cell cultures. In this study, we extend our early findings by confirming the expression of the ANDV-NSs protein in the lungs of experimentally infected golden Syrian hamsters. Next, we show, using a virus-free system, that the ANDV-NSs protein antagonizes the type I interferon (IFN) induction pathway by suppressing signals downstream of the melanoma differentiation-associated protein 5 (MDA5) and the retinoic acid-inducible gene 1 (RIG-I) and upstream of TBK1. Consistent with this observation, the ANDV-NSs protein antagonized mitochondrial antiviral-signaling protein (MAVS)-induced IFN-β, NF-κB, IFN-regulatory factor 3 (IRF3), and IFN-sensitive response element (ISRE) promoter activity. Results demonstrate that ANDV-NSs binds to MAVS in cells without disrupting the MAVS-TBK-1 interaction. However, in the presence of the ANDV-NSs ubiquitination of MAVS is reduced. In summary, this study provides evidence showing that the ANDV-NSs protein acts as an antagonist of the cellular innate immune system by suppressing MAVS downstream signaling by a yet not fully understand mechanism. Our findings reveal new insights into the molecular regulation of the hosts' innate immune response by the Andes orthohantavirus.IMPORTANCE Andes orthohantavirus (ANDV) is endemic in Argentina and Chile and is the primary etiological agent of hantavirus cardiopulmonary syndrome (HCPS) in South America. ANDV is distinguished from other hantaviruses by its unique ability to spread from person to person. In a previous report, we identified a novel ANDV protein, ANDV-NSs. Until now, ANDV-NSs had no known function. In this new study, we established that ANDV-NSs acts as an antagonist of cellular innate immunity, the first line of defense against invading pathogens, hindering the cellular antiviral response during infection. This study provides novel insights into the mechanisms used by ANDV to establish its infection.

Viral RNA structure-based strategies to manipulate translation

Viruses must co-opt the cellular translation machinery to produce progeny virions. Eukaryotic viruses have evolved a variety of ways to manipulate the cellular translation apparatus, in many cases using elegant RNA-centred strategies. Viral RNAs can alter or control every phase of protein synthesis and have diverse targets, mechanisms and structures. In addition, as cells attempt to limit infection by downregulating translation, some of these viral RNAs enable the virus to overcome this response or even take advantage of it to promote viral translation over cellular translation. In this Review, we present important examples of viral RNA-based strategies to exploit the cellular translation machinery. We describe what is understood of the structures and mechanisms of diverse viral RNA elements that alter or regulate translation, the advantages that are conferred to the virus and some of the major unknowns that provide motivation for further exploration.

Eukaryotic viruses have evolved a variety of ways to manipulate the cellular translation apparatus. In this Review, Jaafar and Kieft present important examples of viral RNA-based strategies to exploit the cellular translation machinery.

Field vole-associated Traemmersee hantavirus from Germany represents a novel hantavirus species

Vole-associated hantaviruses occur in the Old and New World. Tula orthohantavirus (TULV) is widely distributed throughout the European continent in its reservoir, the common vole (Microtus arvalis), but the virus was also frequently detected in field voles (Microtus agrestis) and other vole species. TULV and common voles are absent from Great Britain. However, field voles there harbor Tatenale and Kielder hantaviruses. Here we screened 126 field voles and 13 common voles from Brandenburg, Germany, for hantavirus infections. One common vole and four field voles were anti-TULV antibody and/or TULV RNA positive. In one additional, seropositive field vole a novel hantavirus sequence was detected. The partial S and L segment nucleotide sequences were only 61.1% and 75.6% identical to sympatrically occurring TULV sequences, but showed highest similarity of approximately 80% to British Tatenale and Kielder hantaviruses. Subsequent determination of the entire nucleocapsid (N), glycoprotein (GPC), and RNA-dependent RNA polymerase encoding sequences and determination of the pairwise evolutionary distance (PED) value for the concatenated N and GPC amino acid sequences confirmed a novel orthohantavirus species, tentatively named Traemmersee orthohantavirus. The identification of this novel hantavirus in a field vole from eastern Germany underlines the necessity of a large-scale, broad geographical hantavirus screening of voles to understand evolutionary processes of virus-host associations and host switches.

Hantavirus entry: Perspectives and recent advances

Hantaviruses are important zoonotic pathogens of public health importance that are found on all continents except Antarctica and are associated with hemorrhagic fever with renal syndrome (HFRS) in the Old World and hantavirus pulmonary syndrome (HPS) in the New World. Despite the significant disease burden they cause, no FDA-approved specific therapeutics or vaccines exist against these lethal viruses. The lack of available interventions is largely due to an incomplete understanding of hantavirus pathogenesis and molecular mechanisms of virus replication, including cellular entry. Hantavirus Gn/Gc glycoproteins are the only viral proteins exposed on the surface of virions and are necessary and sufficient to orchestrate virus attachment and entry. In vitro studies have implicated integrins (β_1-3), DAF/CD55, and gC1qR as candidate receptors that mediate viral attachment for both Old World and New World hantaviruses. Recently, protocadherin-1 (PCDH1) was demonstrated as a requirement for cellular attachment and entry of New World hantaviruses in vitro and lethal HPS in vivo, making it the first clade-specific host factor to be identified. Attachment of hantavirus particles to cellular receptors induces their internalization by clathrin-mediated, dynamin-independent, or macropinocytosis-like mechanisms, followed by particle trafficking to an endosomal compartment where the fusion of viral and endosomal membranes can occur. Following membrane fusion, which requires cholesterol and acid pH, viral nucleocapsids escape into the cytoplasm and launch genome replication. In this review, we discuss the current mechanistic understanding of hantavirus entry, highlight gaps in our existing knowledge, and suggest areas for future inquiry.

Cellular cholesterol abundance regulates potassium accumulation within endosomes and is an important determinant in bunyavirus entry

The Bunyavirales order of segmented negative-sense RNA viruses includes more than 500 isolates that infect insects, animals, and plants and are often associated with severe and fatal disease in humans. To multiply and cause disease, bunyaviruses must translocate their genomes from outside the cell into the cytosol, achieved by transit through the endocytic network. We have previously shown that the model bunyaviruses Bunyamwera virus (BUNV) and Hazara virus (HAZV) exploit the changing potassium concentration ([K⁺]) of maturing endosomes to release their genomes at the appropriate endosomal location. K⁺ was identified as a biochemical cue to activate the viral fusion machinery, promoting fusion between viral and cellular membranes, consequently permitting genome release. In this study, we further define the biochemical prerequisites for BUNV and HAZV entry and their K⁺ dependence. Using drug-mediated cholesterol extraction along with viral entry and K⁺ uptake assays, we report three major findings: BUNV and HAZV require cellular cholesterol during endosomal escape; cholesterol depletion from host cells impairs K⁺ accumulation in maturing endosomes, revealing new insights into endosomal K⁺ homeostasis; and "priming" BUNV and HAZV virions with K⁺ before infection alleviates their cholesterol requirement. Taken together, our findings suggest a model in which cholesterol abundance influences endosomal K⁺ levels and, consequently, the efficiency of bunyavirus infection. The ability to inhibit bunyaviruses with existing cholesterol-lowering drugs may offer new options for future antiviral interventions for pathogenic bunyaviruses.

Seroprevalence, cross antigenicity and circulation sphere of bat-borne hantaviruses revealed by serological and antigenic analyses

Bats are newly identified reservoirs of hantaviruses (HVs) among which very divergent HVs have been discovered in recent years. However, their significance for public health remains unclear since their seroprevalence as well as antigenic relationship with human-infecting HVs have not been investigated. In the present study archived tissues of 1,419 bats of 22 species from 6 families collected in 5 south and southwest provinces in China were screened by pan-HV RT-PCR following viral metagenomic analysis. As a result nine HVs have been identified in two bat species in two provinces and phylogenetically classified into two species, Laibin virus (LAIV, ICTV approved species, 1 strain) and Xuan son virus (XSV, proposed species, 8 strains). Additionally, 709 serum samples of these bats were also analyzed by ELISA to investigate the seroprevalence and cross-reactivity between different HVs using expressed recombinant nucleocapsid proteins (rNPs) of LAIV, XSV and Seoul virus (SEOV). The cross-reactivity of some bat sera were further confirmed by western blot (WB) using three rNPs followed by fluorescent antibody virus neutralization test (FAVNT) against live SEOV. Results showed that the total HV seropositive rate of bat sera was 18.5% (131/709) with many cross reacting with two or all three rNPs and several able to neutralize SEOV. WB analysis using the three rNPs and their specific hyperimmune sera demonstrated cross-reactivity between XSV/SEOV and LAIV/XSV, but not LAIV/SEOV, indicating that XSV is antigenically closer to human-infecting HVs. In addition a study of the distribution of the viruses identified an area covering the region between Chinese Guangxi and North Vietnam, in which XSV and LAIV circulate within different bat colonies with a high seroprevalence. A circulation sphere of bat-borne HVs has therefore been proposed.

Some HVs are life-threatening pathogens predominantly carried and transmitted by rodents. In recent years bat-borne HVs have been identified in a broad range of bat species. To understand their significance to public health the present study conducted extensive investigations on genetic diversity, seroprevalence, distribution and cross antigenicity of bat-borne HVs in south and southwest China. The results provide the first profiling of cross-reactivity between bat-borne and human-infecting HVs, demonstrating that some bat sera can neutralize SEOV in cell culture. They also revealed that divergent bat-borne HVs co-exist and are widely distributed in Chinese Guangxi/Yunnan as well as in north Vietnam, resulting in identification of an area between China and Vietnam in which natural circulation of bat-borne HVs is maintained. Given the existence of bat-borne HVs genetically and antigenically close to human-infecting HVs, the need for extensive future studies is emphasized in order to assess the potential risk of these viruses to public health.

Overlapping genes and the proteins they encode differ significantly in their sequence composition from non-overlapping genes

Overlapping genes represent a fascinating evolutionary puzzle, since they encode two functionally unrelated proteins from the same DNA sequence. They originate by a mechanism of overprinting, in which point mutations in an existing frame allow the expression (the "birth") of a completely new protein from a second frame. In viruses, in which overlapping genes are abundant, these new proteins often play a critical role in infection, yet they are frequently overlooked during genome annotation. This results in erroneous interpretation of mutational studies and in a significant waste of resources. Therefore, overlapping genes need to be correctly detected, especially since they are now thought to be abundant also in eukaryotes. Developing better detection methods and conducting systematic evolutionary studies require a large, reliable benchmark dataset of known cases. We thus assembled a high-quality dataset of 80 viral overlapping genes whose expression is experimentally proven. Many of them were not present in databases. We found that overall, overlapping genes differ significantly from non-overlapping genes in their nucleotide and amino acid composition. In particular, the proteins they encode are enriched in high-degeneracy amino acids and depleted in low-degeneracy ones, which may alleviate the evolutionary constraints acting on overlapping genes. Principal component analysis revealed that the vast majority of overlapping genes follow a similar composition bias, despite their heterogeneity in length and function. Six proven mammalian overlapping genes also followed this bias. We propose that this apparently near-universal composition bias may either favour the birth of overlapping genes, or/and result from selection pressure acting on them.

The nucleocapsid protein of hantaviruses: much more than a genome-wrapping protein

The nucleocapsid (N) protein of hantaviruses represents an impressive example of a viral multifunctional protein. It encompasses properties as diverse as genome packaging, RNA chaperoning, intracellular protein transport, DNA degradation, intervention in host translation, and restricting host immune responses. These functions all rely on the capability of N to interact with RNA and other viral and cellular proteins. We have compiled data on the N protein of different hantavirus species together with information of the recently published three-dimensional structural data of the protein. The array of diverse functional activities accommodated in the hantaviral N protein goes far beyond to be a static structural protein and makes it an interesting target in the development of antiviral therapeutics.

Maporal Hantavirus Causes Mild Pathology in Deer Mice (Peromyscus maniculatus)

Rodent-borne hantaviruses can cause two human diseases with many pathological similarities: hantavirus cardiopulmonary syndrome (HCPS) in the western hemisphere and hemorrhagic fever with renal syndrome in the eastern hemisphere. Each virus is hosted by specific reservoir species without conspicuous disease. HCPS-causing hantaviruses require animal biosafety level-4 (ABSL-4) containment, which substantially limits experimental research of interactions between the viruses and their reservoir hosts. Maporal virus (MAPV) is a South American hantavirus not known to cause disease in humans, thus it can be manipulated under ABSL-3 conditions. The aim of this study was to develop an ABSL-3 hantavirus infection model using the deer mouse (Peromyscus maniculatus), the natural reservoir host of Sin Nombre virus (SNV), and a virus that is pathogenic in another animal model to examine immune response of a reservoir host species. Deer mice were inoculated with MAPV, and viral RNA was detected in several organs of all deer mice during the 56 day experiment. Infected animals generated both nucleocapsid-specific and neutralizing antibodies. Histopathological lesions were minimal to mild with the peak of the lesions detected at 7-14 days postinfection, mainly in the lungs, heart, and liver. Low to modest levels of cytokine gene expression were detected in spleens and lungs of infected deer mice, and deer mouse primary pulmonary cells generated with endothelial cell growth factors were susceptible to MAPV with viral RNA accumulating in the cellular fraction compared to infected Vero cells. Most features resembled that of SNV infection of deer mice, suggesting this model may be an ABSL-3 surrogate for studying the host response of a New World hantavirus reservoir.

Early Bunyavirus-Host Cell Interactions

The Bunyaviridae is the largest family of RNA viruses, with over 350 members worldwide. Several of these viruses cause severe diseases in livestock and humans. With an increasing number and frequency of outbreaks, bunyaviruses represent a growing threat to public health and agricultural productivity globally. Yet, the receptors, cellular factors and endocytic pathways used by these emerging pathogens to infect cells remain largely uncharacterized. The focus of this review is on the early steps of bunyavirus infection, from virus binding to penetration from endosomes. We address current knowledge and advances for members from each genus in the Bunyaviridae family regarding virus receptors, uptake, intracellular trafficking and fusion.

Experimental infection of Rio Mamore hantavirus in Sigmodontinae rodents

This study shows an experimental spillover infection of Sigmodontinae rodents with Rio Mamore hantavirus (RIOMV). Necromys lasiurus and Akodon sp were infected with 103 RNA copies of RIOMV by intraperitoneal administration. The viral genome was detected in heart, lung, and kidney tissues 18 days after infection (ai), and viral excretion in urine and faeces began at four and six ai, respectively. These results reveal that urine and faeces of infected rodents contain the virus for at least 18 days. It is possible that inhaled aerosols of these excreta could transmit hantavirus to humans and other animals.

Polypyrimidine tract-binding protein binds to the 5' untranslated region of the mouse mammary tumor virus mRNA and stimulates cap-independent translation initiation

The 5' untranslated region (UTR) of the full-length mRNA of the mouse mammary tumor virus (MMTV) harbors an internal ribosomal entry site (IRES). In this study, we show that the polypyrimidine tract-binding protein (PTB), an RNA-binding protein with four RNA recognition motifs (RRMs), binds to the MMTV 5' UTR stimulating its IRES activity. There are three isoforms of PTB: PTB1, PTB2, and PTB4. Results show that PTB1 and PTB4, but not PTB2, stimulate MMTV-IRES activity. PTB1 promotes MMTV-IRES-mediated initiation more strongly than PTB4. When expressed in combination, PTB1 further enhanced PTB4 stimulation of the MMTV-IRES, while PTB2 fully abrogates PTB4-induced stimulation. PTB1-induced stimulation of MMTV-IRES was not altered in the presence of PTB4 or PTB2. Mutational analysis reveals that stimulation of MMTV-IRES activity is abrogated when PTB1 is mutated either in RRM1/RRM2 or RRM3/RRM4. In contrast, a PTB4 RRM1/RRM2 mutant has reduced effect over MMTV-IRES activity, while stimulation of the MMTV-IRES activity is still observed when the PTB4 RRM3/RMM4 mutant is used. Therefore, PTB1 and PTB4 differentially stimulate the IRES activity. In contrast, PTB2 acts as a negative modulator of PTB4-induced stimulation of MMTV-IRES. We conclude that PTB1 and PTB4 act as IRES trans-acting factors of the MMTV-IRES.

Molecular method for the detection of Andes hantavirus infection: validation for clinical diagnostics

Hantavirus cardiopulmonary syndrome is a severe disease caused by exposure to New World hantaviruses. Early diagnosis is difficult due to the lack of specific initial symptoms. Antihantavirus antibodies are usually negative until late in the febrile prodrome or the beginning of cardiopulmonary phase, while Andes hantavirus (ANDV) RNA genome can be detected before symptoms onset. We analyzed the effectiveness of quantitative reverse transcription polymerase chain reaction (RT-qPCR) as a diagnostic tool detecting ANDV-Sout genome in peripheral blood cells from 78 confirmed hantavirus patients and 166 negative controls. Our results indicate that RT-qPCR had a low detection limit (~10 copies), with a specificity of 100% and a sensitivity of 94.9%. This suggests the potential for establishing RT-qPCR as the assay of choice for early diagnosis, promoting early effective care of patients, and improving other important aspects of ANDV infection management, such as compliance of biosafety recommendations for health personnel in order to avoid nosocomial transmission.

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.

Development of a novel plaque reduction neutralisation test for hantavirus infection

In the Americas, hantaviruses cause severe cardiopulmonary syndrome (HCPS) with a high fatality rate. Hantavirus infection is commonly diagnosed using serologic techniques and reverse transcription-polymerase chain reaction. This paper presents a novel plaque reduction neutralisation test (PRNT) for detecting antibodies to Brazilian hantavirus. Using PRNT, plaque detection was enhanced by adding 0.6% of dimethyl sulfoxide into the overlay culture medium of the infected cells. This procedure facilitated clear visualisation of small plaques under the microscope and provided for easy and accurate plaque counting. The sera from 37 HCPS patients from the city of Ribeirão Preto, Brazil was evaluated for the Rio Mamoré virus (RIOMV) using PRNT. Six samples exhibited neutralising antibodies; these antibodies exhibited a low titre. The low level of seropositive samples may be due to fewer cross-reactions between two different hantavirus species; the patients were likely infected by Araraquara virus (a virus that has not been isolated) and RIOMV was used for the test. This assay offers a new approach to evaluating and measuring neutralising antibodies produced during hantavirus infections and it can be adapted to other hantaviruses, including viruses that will be isolated in the future.

Antigenic properties of N protein of hantavirus

Hantavirus causes two important rodent-borne viral zoonoses, hemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus pulmonary syndrome (HPS) in North and South America. Twenty-four species that represent sero- and genotypes have been registered within the genus Hantavirus by the International Committee on Taxonomy of Viruses (ICTV). Among the viral proteins, nucleocapsid (N) protein possesses an immunodominant antigen. The antigenicitiy of N protein is conserved compared with that of envelope glycoproteins. Therefore, N protein has been used for serological diagnoses and seroepidemiological studies. An understanding of the antigenic properties of N protein is important for the interpretation of results from serological tests using N antigen. N protein consists of about 430 amino acids and possesses various epitopes. The N-terminal quarter of N protein bears linear and immunodominant epitopes. However, a serotype-specific and multimerization-dependent antigenic site was found in the C-terminal half of N protein. In this paper, the structure, function, and antigenicity of N protein are reviewed.

Differential lymphocyte and antibody responses in deer mice infected with Sin Nombre hantavirus or Andes hantavirus

Hantavirus cardiopulmonary syndrome (HCPS) is a rodent-borne disease with a high case-fatality rate that is caused by several New World hantaviruses. Each pathogenic hantavirus is naturally hosted by a principal rodent species without conspicuous disease and infection is persistent, perhaps for life. Deer mice (Peromyscus maniculatus) are the natural reservoirs of Sin Nombre virus (SNV), the etiologic agent of most HCPS cases in North America. Deer mice remain infected despite a helper T cell response that leads to high-titer neutralizing antibodies. Deer mice are also susceptible to Andes hantavirus (ANDV), which causes most HCPS cases in South America; however, deer mice clear ANDV. We infected deer mice with SNV or ANDV to identify differences in host responses that might account for this differential outcome. SNV RNA levels were higher in the lungs but not different in the heart, spleen, or kidneys. Most ANDV-infected deer mice had seroconverted 14 days after inoculation, but none of the SNV-infected deer mice had. Examination of lymph node cell antigen recall responses identified elevated immune gene expression in deer mice infected with ANDV and suggested maturation toward a Th2 or T follicular helper phenotype in some ANDV-infected deer mice, including activation of the interleukin 4 (IL-4) pathway in T cells and B cells. These data suggest that the rate of maturation of the immune response is substantially higher and of greater magnitude during ANDV infection, and these differences may account for clearance of ANDV and persistence of SNV.

IMPORTANCE

Hantaviruses persistently infect their reservoir rodent hosts without pathology. It is unknown how these viruses evade sterilizing immune responses in the reservoirs. We have determined that infection of the deer mouse with its homologous hantavirus, Sin Nombre virus, results in low levels of immune gene expression in antigen-stimulated lymph node cells and a poor antibody response. However, infection of deer mice with a heterologous hantavirus, Andes virus, results in a robust lymph node cell response, signatures of T and B cell maturation, and production of antibodies. These findings suggest that an early and aggressive immune response to hantaviruses may lead to clearance in a reservoir host and suggest that a modest immune response may be a component of hantavirus ecology.

Hantavirus immunology of rodent reservoirs: current status and future directions

Hantaviruses are hosted by rodents, insectivores and bats. Several rodent-borne hantaviruses cause two diseases that share many features in humans, hemorrhagic fever with renal syndrome in Eurasia or hantavirus cardiopulmonary syndrome in the Americas. It is thought that the immune response plays a significant contributory role in these diseases. However, in reservoir hosts that have been closely examined, little or no pathology occurs and infection is persistent despite evidence of adaptive immune responses. Because most hantavirus reservoirs are not model organisms, it is difficult to conduct meaningful experiments that might shed light on how the viruses evade sterilizing immune responses and why immunopathology does not occur. Despite these limitations, recent advances in instrumentation and bioinformatics will have a dramatic impact on understanding reservoir host responses to hantaviruses by employing a systems biology approach to identify important pathways that mediate virus/reservoir relationships.

Recent evidence of hantavirus circulation in the American tropic

Hantaan virus was discovered in Korea during the 1970s while other similar viruses were later reported in Asia and Europe. There was no information about hantavirus human infection in the Americas until 1993 when an outbreak was described in the United States. This event promoted new studies to find hantaviruses in the Americas. At first, many studies were conducted in Brazil, Argentina, Chile, Uruguay and Paraguay, while other Latin American countries began to report the presence of these agents towards the end of the 20th century. More than 30 hantaviruses have been reported in the Western Hemisphere with more frequent cases registered in the southern cone (Argentina, Chile, Uruguay, Paraguay, Bolivia and Brazil). However there was an important outbreak in 2000 in Panama and some rare events have been described in Peru, Venezuela and French Guiana. Since hantaviruses have only recently emerged as a potential threat in the tropical zones of the Americas, this review compiles recent hantavirus reports in Central America, the Caribbean islands and the northern region of South America. These studies have generated the discovery of new hantaviruses and could help to anticipate the presentation of possible future outbreaks in the region.

The 5' untranslated region of the human T-cell lymphotropic virus type 1 mRNA enables cap-independent translation initiation

The human T-cell leukemia virus type 1 (HTLV-1) is a complex human retrovirus that causes adult T cell leukemia and of HTLV-associated myelopathy/tropical spastic paraparesis. The mRNA of some complex retroviruses, including the human and simian immunodeficiency viruses (HIV and SIV), can initiate translation using a canonical cap-dependent mechanism or through an internal ribosome entry site (IRES). In this study, we present strong evidence showing that like HIV-1 and SIV, the 5'-untranslated region (5'UTR) of the HTLV-1 full-length mRNA harbors an IRES. Cap-independent translational activity was evaluated and demonstrated using dual luciferase bicistronic mRNAs in rabbit reticulocyte lysate, in mammalian cell culture, and in Xenopus laevis oocytes. Characterization of the HTLV-1 IRES shows that its activity is dependent on the ribosomal protein S25 (RPS25) and that its function is highly sensitive to the drug edeine. Together, these findings suggest that the 5'UTR of the HTLV-1 full-length mRNA enables internal recruitment of the eukaryotic translation initiation complex. However, the recognition of the initiation codon requires ribosome scanning. These results suggest that, after internal recruitment by the HTLV-1 IRES, a scanning step takes place for the 40S ribosomal subunit to be positioned at the translation initiation codon.

IMPORTANCE

The mechanism by which retroviral mRNAs recruit the 40S ribosomal subunit internally is not understood. This study provides new insights into the mechanism of translation initiation used by the human T-cell lymphotropic virus type 1 (HTLV-1). The results show that the HTLV-1 mRNA can initiate translation via a noncanonical mechanism mediated by an internal ribosome entry site (IRES). This study also provides evidence showing the involvement of cellular proteins in HTLV-1 IRES-mediated translation initiation. Together, the data presented in this report significantly contribute to the understanding of HTLV-1 gene expression.

An innate immunity-regulating virulence determinant is uniquely encoded by the Andes virus nucleocapsid protein

Andes virus (ANDV) is the only hantavirus known to spread from person to person and shown to cause highly lethal hantavirus pulmonary syndrome (HPS) in patients and Syrian hamsters. Hantaviruses replicate in human endothelial cells and accomplish this by restricting the early induction of beta interferon (IFN-β)- and IFN-stimulated genes (ISGs). Our studies reveal that the ANDV nucleocapsid (N) protein uniquely inhibits IFN signaling responses directed by cytoplasmic double-stranded RNA (dsRNA) sensors RIG-I and MDA5. In contrast, N proteins from Sin Nombre, New York-1, and Prospect Hill hantaviruses had no effect on RIG-I/MDA5-directed transcriptional responses from IFN-β-, IFN-stimulated response element (ISRE)-, or κB-containing promoters. Ablating a potential S-segment nonstructural open reading frame (ORF) (NSs) within the ANDV plasmid expressing N protein failed to alter IFN regulation by ANDV N protein. Further analysis demonstrated that expressing the ANDV N protein inhibited downstream IFN pathway activation directed by MAVS, TBK1, and IκB kinase ε (IKKε) but failed to inhibit transcriptional responses directed by constitutive expression of active interferon regulatory factor IRF3-5D or after stimulation by alpha interferon (IFN-α) or tumor necrosis factor alpha (TNF-α). Consistent with IFN pathway-specific regulation, the ANDV N protein inhibited TBK1-directed IRF3 phosphorylation (phosphorylation of serine 396 [pS396]) and TBK1 autophosphorylation (pS172). Collectively, these findings indicate that the ANDV N inhibits IFN signaling responses by interfering with TBK1 activation, upstream of IRF3 phosphorylation and NF-κB activation. Moreover, our findings reveal that ANDV uniquely carries a gene encoding a virulence determinant within its N protein that is capable of restricting ISG and IFN-β induction and provide a rationale for the novel pathogenesis and spread of ANDV.

Andes virus (ANDV) is distinguished from other hantaviruses by its unique ability to spread from person to person and cause lethal hantavirus pulmonary syndrome (HPS)-like disease in Syrian hamsters. However, virulence determinants that distinguish ANDV from other pathogenic hantaviruses have yet to be defined. Here we reveal that ANDV uniquely contains a virulence determinant within its nucleocapsid (N) protein that potently inhibits innate cellular signaling pathways. This novel function of the N protein provides a new mechanism for hantaviruses to regulate interferon (IFN) and IFN-stimulated gene (ISG) induction that is likely to contribute to the enhanced ability of ANDV to replicate, spread, and cause disease. These findings differentiate ANDV from other HPS-causing hantaviruses and provide a potential target for viral attenuation that needs to be considered in vaccine development.

Hantaviruses: past, present and future

ABSTRACT: 

Hantaviruses productively infect endothelial cells in their rodent reservoirs and humans, but the infection only causes disease in humans – hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome. Despite the enormous progress that has been made in understanding the pathogenesis and immune responses of hantavirus infection, there is a large gap in our molecular-based knowledge of hantaviral proteins in their structures, functions and the mechanisms that facilitate their entry, replication and assembly. Importantly, we know little about the specific viral determinants and viral protein–host interactions that drive differences noted in immune responses between the reservoir and humans. This review discusses our current understanding and future work needed for unraveling the biology of these viruses in their reservoirs and in humans.

Phylogeography and evolutionary history of rodent-borne hantaviruses

Hantavirus (Family Bunyaviridae) are mostly associated to rodents and transmitted to man by inhalation of aerosolized infected excreta of these animals. The human infection by hantaviruses can lead to severe diseases such as hemorrhagic fever with renal syndrome (HFRS) in Asia and Europe, and pulmonary syndrome (HPS) in the Americas. To determine the origin, spreading and evolutionary dynamics of rodent-borne hantaviruses, 190 sequences of nucleoprotein (N) of hantaviruses identified in 30 countries, from 1985 to 2010, were retrieved from the GenBank and analyzed using the BEAST program. Our evolutionary analysis indicates that current genetic diversity of N gene of rodent-borne hantaviruses probably was originated around 2000 years ago. Hantavirus harbored by Murinae and Arvicolinae subfamilies, probably, were originated in Asia 500-700 years ago and later spread toward Siberia, Europe, Africa and North America. Hantavirus carried by Neotominae subfamily, probably, emerged 500-600 years ago in Central America and spread toward North America. Finally, hantaviruses associated to Sigmodontinae occurred in Brazil 400 years ago and were, probably, originated from Neotominae-associated virus from northern South America. These data offer subsidies to understand the time-scale and worldwide dissemination dynamics of rodent-borne hantaviruses.

Discovery of a unique novel clade of mosquito-associated bunyaviruses

Bunyaviruses are the largest known family of RNA viruses, infecting vertebrates, insects, and plants. Here we isolated three novel bunyaviruses from mosquitoes sampled in Côte d'Ivoire, Ghana, and Uganda. The viruses define a highly diversified monophyletic sister clade to all members of the genus Orthobunyavirus and are virtually equidistant to orthobunyaviruses and tospoviruses. Maximal amino acid identities between homologous putative proteins of the novel group and orthobunyaviruses ranged between 12 and 25%. The type isolates, tentatively named Herbert virus (HEBV), Taï virus (TAIV), and Kibale virus (KIBV), comprised genomes with L, M, and S segments of about 7.4 kb, 2.7 kb, and 1.1 kb, respectively. HEBV, TAIV, and KIBV encode the shortest bunyavirus M segments known and did not seem to encode NSs and NSm proteins but contained an elongated L segment with an ∼500-nucleotide (nt) insertion that shows no identity to other bunyaviruses. The viruses replicated to high titers in insect cells but did not replicate in vertebrate cells. The enveloped virions were 90 to 110 nm in diameter and budded at cellular membranes with morphological features typical of the Golgi complex. Viral RNA recovered from infected cells showed 5'-terminal nontemplated sequences of 9 to 22 nt, suggestive of cap snatching during mRNA synthesis, as described for other bunyaviruses. Northern blotting identified RNA species of full and reduced lengths, suggested upon analogy with other bunyaviruses to constitute antigenomic-sense cRNA and transcript mRNAs, respectively. Functional studies will be necessary to determine if this group of viruses constitutes a novel genus in the bunyavirus family.

Role of nucleocapsid protein of hantaviruses in intracellular traffic of viral glycoproteins

To understand the role of nucleocapsid protein (NP) of hantaviruses in viral assembly, the effect of NP on intracellular traffic of viral glycoproteins Gn and Gc was investigated. Double staining of viral and host proteins in Hantaan virus (HTNV)-infected Vero E6 cells showed that Gn and Gc were localized to cis-Golgi, in which virus particles are thought to be formed. When HTNV Gn and Gc were expressed by a plasmid encoding glycoprotein precursor (GPC), which is posttranslationally cleaved into Gn and Gc, Gn was localized to cis-Golgi, whereas Gc showed diffuse distribution in the cytoplasm in 32.9% of Gc-positive cells. The ratio of the diffused Gc-positive cells was significantly decreased to 15.0% by co-expression of HTNV NP. Co-expression of HTNV GPC with NPs of other hantaviruses, such as Seoul virus, Puumala virus and Sin Nombre virus, also reduced the ratios of diffused Gc-positive cells to 13.5%, 25.2%, and 11.6%, respectively. Among amino- and carboxyl-terminally truncated HTNV NPs, NP75-429, NP116-429, NP1-333, NP1-233, and NP1-155 possessed activity to reduce the ratio of diffused Gc-positive cells, while NP155-429 and NP1-116 did not. NP30-429 has partial activity. These results indicate that amino acid region 116-155 of NP is important for the activity, although amino acid region 1-30 is partially related. Truncation of the HTNV Gc cytoplasmic tail caused an increase in diffused Gc-positive cells. In addition, the effect of coexpression of HTNV NP was weakened. These results suggest that HTNV NP has a role to promote Golgi localization of Gc through a mechanism possibly mediated by the Gc cytoplasmic tail.

A rapid method for infectivity titration of Andes hantavirus using flow cytometry

The focus assay is currently the most commonly used technique for hantavirus titer determination. This method requires an incubation time of between 5 and 11 days to allow the appearance of foci after several rounds of viral infection. The following work presents a rapid Andes virus (ANDV) titration assay, based on viral nucleocapsid protein (N) detection in infected cells by flow cytometry. To this end, an anti-N monoclonal antibody was used that was developed and characterized previously. ANDV N could be detected as early as 6 h post-infection, while viral release was not observed until 24-48 h post-infection. Given that ANDV detection was performed during its first round of infection, a time reduction for titer determination was possible and provided results in only two days. The viral titer was calculated from the percentage of N positive cells and agreed with focus assay titers. Furthermore, the assay was applied to quantify the inhibition of ANDV cell entry by patient sera and by preventing endosome acidification. This novel hantavirus titration assay is a highly quantitative and sensitive tool that facilitates infectivity titration of virus stocks, rapid screening for antiviral drugs, and may be further used to detect and quantify infectious virus in human samples.

The N terminus of Andes virus L protein suppresses mRNA and protein expression in mammalian cells

Little is known about the structure and function of the 250-kDa L protein of hantaviruses, although it plays a central role in virus genome transcription and replication. When attempting to study Andes virus (ANDV) L protein in mammalian cells, we encountered difficulties. Even in a strong overexpression system, ANDV L protein could not be detected by immunoblotting. Deletion analysis revealed that the 534 N-terminal amino acid residues determine the low-expression phenotype. Inhibition of translation due to RNA secondary structures around the start codon, rapid proteasomal degradation, and reduced half-life time were excluded. However, ANDV L protein expression could be rescued upon mutation of the catalytic PD-E-K motif and further conserved residues of the putative endonuclease at the N terminus of the protein. In addition, wild-type ANDV L rather than expressible L mutants suppressed the level of L mRNA, as well as reporter mRNAs. Wild-type L protein also reduced the synthesis of cellular proteins in the high-molecular-weight range. Using expressible ANDV L mutants as a tool for localization studies, we show that L protein colocalizes with ANDV N and NSs but not Gc protein. A fraction of L protein also colocalized with the cellular processing (P) body component DCP1a. Overall, these data suggest that ANDV L protein possesses a highly active endonuclease at the N terminus suppressing the level of its own as well as heterologous mRNAs upon recombinant expression in mammalian cells.

Development of a minigenome system for Andes virus, a New World hantavirus

The development of reverse genetics systems for negative-stranded RNA viruses is a rapidly evolving field that has greatly advanced the study of the many different aspects of the viral life cycle. Andes virus (ANDV) is a highly pathogenic hantavirus found in South America that causes hantavirus pulmonary syndrome but to date remains poorly characterized due to the lack of a reverse genetics system for genetic manipulation. Here, we describe the first successful minigenome system for a New World hantavirus, as well as many of the obstacles that still exist in the development of such a system.

Diversity and distribution of hantaviruses in South America

Hantaviruses are important contributors to disease burden in the New World, yet many aspects of their distribution and dynamics remain uncharacterized. To examine the patterns and processes that influence the diversity and geographic distribution of hantaviruses in South America, we performed genetic and phylogeographic analyses of all available South American hantavirus sequences. We sequenced multiple novel and previously described viruses (Anajatuba, Laguna Negra-like, two genotypes of Castelo dos Sonhos, and two genotypes of Rio Mamore) from Brazilian Oligoryzomys rodents and hantavirus pulmonary syndrome cases and identified a previously uncharacterized species of Oligoryzomys associated with a new genotype of Rio Mamore virus. Our analysis indicates that the majority of South American hantaviruses fall into three phylogenetic clades, corresponding to Andes and Andes-like viruses, Laguna Negra and Laguna Negra-like viruses, and Rio Mamore and Rio Mamore-like viruses. In addition, the dynamics and distribution of these viruses appear to be shaped by both the geographic proximity and phylogenetic relatedness of their rodent hosts. The current system of nomenclature used in the hantavirus community is a significant impediment to understanding the ecology and evolutionary history of hantaviruses; here, we suggest strict adherence to a modified taxonomic system, with species and strain designations resembling the numerical system of the enterovirus genus.

Non-canonical translation in RNA viruses

Viral protein synthesis is completely dependent upon the translational machinery of the host cell. However, many RNA virus transcripts have marked structural differences from cellular mRNAs that preclude canonical translation initiation, such as the absence of a 5′ cap structure or the presence of highly structured 5′UTRs containing replication and/or packaging signals. Furthermore, whilst the great majority of cellular mRNAs are apparently monocistronic, RNA viruses must often express multiple proteins from their mRNAs. In addition, RNA viruses have very compact genomes and are under intense selective pressure to optimize usage of the available sequence space. Together, these features have driven the evolution of a plethora of non-canonical translational mechanisms in RNA viruses that help them to meet these challenges. Here, we review the mechanisms utilized by RNA viruses of eukaryotes, focusing on internal ribosome entry, leaky scanning, non-AUG initiation, ribosome shunting, reinitiation, ribosomal frameshifting and stop-codon readthrough. The review will highlight recently discovered examples of unusual translational strategies, besides revisiting some classical cases.