Tula hantavirus NSs protein accumulates in the perinuclear area in infected and transfected cells

Advances in fluorescence microscopy for orthohantavirus research

Orthohantaviruses are important zoonotic pathogens responsible for a considerable disease burden globally. Partly due to our incomplete understanding of orthohantavirus replication, there is currently no effective antiviral treatment available. Recently, novel microscopy techniques and cutting-edge, automated image analysis algorithms have emerged, enabling to study cellular, subcellular and even molecular processes in unprecedented detail and depth. To date, fluorescence light microscopy allows us to visualize viral and cellular components and macromolecular complexes in live cells which in turn enables the study of specific steps of the viral replication cycle such as particle entry or protein trafficking at high temporal and spatial resolution. In this review, we highlight how fluorescence microscopy has provided new insights and improved our understanding of orthohantavirus biology. We discuss technical challenges such as studying live infected cells, give alternatives with recombinant protein expression and highlight future opportunities for example the application of super-resolution microscopy techniques, which has shown great potential in studies of different cellular processes and viral pathogens.

Innate Immunity to Orthohantaviruses: Could Divergent Immune Interactions Explain Host-specific Disease Outcomes?

The genus Orthohantavirus (family Hantaviridae, order Bunyavirales) consists of numerous genetic and pathologically distinct viral species found within rodent and mammalian insectivore populations world-wide. Although reservoir hosts experience persistent asymptomatic infection, numerous rodent-borne orthohantaviruses cause severe disease when transmitted to humans, with case-fatality rates up to 40%. The first isolation of an orthohantavirus occurred in 1976 and, since then, the field has made significant progress in understanding the immune correlates of disease, viral interactions with the human innate immune response, and the immune kinetics of reservoir hosts. Much still remains elusive regarding the molecular mechanisms of orthohantavirus recognition by the innate immune response and viral antagonism within the reservoir host, however. This review provides a summary of the last 45 years of research into orthohantavirus interaction with the host innate immune response. This summary includes discussion of current knowledge involving human, non-reservoir rodent, and reservoir innate immune responses to viruses which cause hemorrhagic fever with renal syndrome and hantavirus cardio-pulmonary syndrome. Review of the literature concludes with a brief proposition for the development of novel tools needed to drive forward investigations into the molecular mechanisms of innate immune activation and consequences for disease outcomes in the various hosts for orthohantaviruses.

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.

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.

Tula orthohantavirus nucleocapsid protein is cleaved in infected cells and may sequester activated caspase-3 during persistent infection to suppress apoptosis

The family Hantaviridae mostly comprises rodent-borne segmented negative-sense RNA viruses, many of which are capable of causing devastating disease in humans. In contrast, hantavirus infection of rodent hosts results in a persistent and inapparent infection through their ability to evade immune detection and inhibit apoptosis. In this study, we used Tula hantavirus (TULV) to investigate the interplay between viral and host apoptotic responses during early, peak and persistent phases of virus infection in cell culture. Examination of early-phase TULV infection revealed that infected cells were refractory to apoptosis, as evidenced by the complete lack of cleaved caspase-3 (casp-3C) staining, whereas in non-infected bystander cells casp-3C was highly abundant. Interestingly, at later time points, casp-3C was abundant in infected cells, but the cells remained viable and able to continue shedding infectious virus, and together these observations were suggestive of a TULV-associated apoptotic block. To investigate this block, we viewed TULV-infected cells using laser scanning confocal and wide-field deconvolution microscopy, which revealed that TULV nucleocapsid protein (NP) colocalized with, and sequestered, casp-3C within cytoplasmic ultrastructures. Consistent with casp-3C colocalization, we showed for the first time that TULV NP was cleaved in cells and that TULV NP and casp-3C could be co-immunoprecipitated, suggesting that this interaction was stable and thus unlikely to be solely confined to NP binding as a substrate to the casp-3C active site. To account for these findings, we propose a novel mechanism by which TULV NP inhibits apoptosis by spatially sequestering casp-3C from its downstream apoptotic targets within the cytosol.

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.

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.

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.

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.

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.

Uncovering the mysteries of hantavirus infections

Hantaviruses are negative-sense single-stranded RNA viruses that infect many species of rodents, shrews, moles and bats. Infection in these reservoir hosts is almost asymptomatic, but some rodent-borne hantaviruses also infect humans, causing either haemorrhagic fever with renal syndrome (HFRS) or hantavirus cardiopulmonary syndrome (HCPS). In this Review, we discuss the basic molecular properties and cell biology of hantaviruses and offer an overview of virus-induced pathology, in particular vascular leakage and immunopathology.

Hantavirus regulation of type I interferon responses

Hantaviruses primarily infect human endothelial cells (ECs) and cause two highly lethal human diseases. Early addition of Type I interferon (IFN) to ECs blocks hantavirus replication and thus for hantaviruses to be pathogenic they need to prevent early interferon induction. PHV replication is blocked in human ECs, but not inhibited in IFN deficient VeroE6 cells and consistent with this, infecting ECs with PHV results in the early induction of IFNβ and an array of interferon stimulated genes (ISGs). In contrast, ANDV, HTNV, NY-1V and TULV hantaviruses, inhibit early ISG induction and successfully replicate within human ECs. Hantavirus inhibition of IFN responses has been attributed to several viral proteins including regulation by the Gn proteins cytoplasmic tail (Gn-T). The Gn-T interferes with the formation of STING-TBK1-TRAF3 complexes required for IRF3 activation and IFN induction, while the PHV Gn-T fails to alter this complex or regulate IFN induction. These findings indicate that interfering with early IFN induction is necessary for hantaviruses to replicate in human ECs, and suggest that additional determinants are required for hantaviruses to be pathogenic. The mechanism by which Gn-Ts disrupt IFN signaling is likely to reveal potential therapeutic interventions and suggest protein targets for attenuating hantaviruses.

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.

Searching for cellular partners of hantaviral nonstructural protein NSs: Y2H screening of mouse cDNA library and analysis of cellular interactome

Hantaviruses (Bunyaviridae) are negative-strand RNA viruses with a tripartite genome. The small (S) segment encodes the nucleocapsid protein and, in some hantaviruses, also the nonstructural protein (NSs). The aim of this study was to find potential cellular partners for the hantaviral NSs protein. Toward this aim, yeast two-hybrid (Y2H) screening of mouse cDNA library was performed followed by a search for potential NSs protein counterparts via analyzing a cellular interactome. The resulting interaction network was shown to form logical, clustered structures. Furthermore, several potential binding partners for the NSs protein, for instance ACBD3, were identified and, to prove the principle, interaction between NSs and ACBD3 proteins was demonstrated biochemically.

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

The small mRNA (SmRNA) of all Bunyaviridae encodes the nucleocapsid (N) protein. In 4 out of 5 genera in the Bunyaviridae, the smRNA encodes an additional nonstructural protein denominated NSs. In this study, we show that Andes hantavirus (ANDV) SmRNA encodes an NSs protein. Data show that the NSs protein is expressed in the context of an ANDV infection. Additionally, our results suggest that translation initiation from the NSs initiation codon is mediated by ribosomal subunits that have bypassed the upstream N protein initiation codon through a leaky scanning mechanism.

Comparison of innate immune responses to pathogenic and putative non-pathogenic hantaviruses in vitro

Hantaviruses are human pathogens that cause hemorrhagic fever with renal syndrome or hantavirus cardiopulmonary syndrome. The mechanisms accounting for the differences in virulence between pathogenic and non-pathogenic hantaviruses are not well known. We have examined the pathogenesis of different hantavirus groups by comparing the innate immune responses induced in the host cell following infection by pathogenic (Sin Nombre, Hantaan, and Seoul virus) and putative non-pathogenic (Prospect Hill, Tula, and Thottapalayam virus) hantaviruses. Pathogenic hantaviruses were found to replicate more efficiently in interferon-competent A549 cells than putative non-pathogenic hantaviruses. The former also suppressed the expression of the interferon-β and myxovirus resistance protein genes, while the transcription level of both genes increased rapidly within 24 h post-infection in the latter. In addition, the induction level of interferon correlated with the activation level of interferon regulatory factor-3. Taken together, these results suggest that the observed differences are correlated with viral pathogenesis and further indicate that pathogenic and putative non-pathogenic hantaviruses differ in terms of early interferon induction via activation of the interferon regulatory factor-3 in infected host cells.

Hantavirus protein interactions regulate cellular functions and signaling responses

Rodent-borne pathogenic hantaviruses cause two severe and often lethal zoonotic diseases: hemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus cardiopulmonary syndrome (HCPS) in the Americas. Currently, no US FDA-approved therapeutics or vaccines are available for HFRS/HCPS. Infections with hantaviruses are not lytic, and it is currently not known exactly why infections in humans cause disease. A better understanding of how hantaviruses interfere with normal cell functions and activation of innate and adaptive immune responses might provide clues to future development of specific treatment and/or vaccines against hantavirus infection. In this article, the current knowledge regarding immune responses observed in patients, hantavirus interference with cellular proteins and signaling pathways, and possible approaches in the development of therapeutics are discussed.

The C-terminal 42 residues of the Tula virus Gn protein regulate interferon induction

Hantaviruses primarily infect the endothelial cell lining of capillaries and cause two vascular permeability-based diseases. The ability of pathogenic hantaviruses to regulate the early induction of interferon determines whether hantaviruses replicate in endothelial cells. Tula virus (TULV) and Prospect Hill virus (PHV) are hantaviruses which infect human endothelial cells but fail to cause human disease. PHV is unable to inhibit early interferon (IFN) responses and fails to replicate within human endothelial cells. However, TULV replicates successfully in human endothelial cells, suggesting that TULV is capable of regulating cellular IFN responses. We observed a >300-fold reduction in the IFN-stimulated genes (ISGs) MxA and ISG56 following TULV versus PHV infection of endothelial cells 1 day postinfection. Similar to results with pathogenic hantaviruses, expressing the TULV Gn protein cytoplasmic tail (Gn-T) blocked RIG-I- and TBK1-directed transcription from IFN-stimulated response elements (ISREs) and IFN-β promoters (>90%) but not transcription directed by constitutively active IFN regulatory factor-3 (IRF3). In contrast, expressing the PHV Gn-T had no effect on TBK1-induced transcriptional responses. Analysis of Gn-T truncations demonstrated that the C-terminal 42 residues of the Gn-T (Gn-T-C42) from TULV, but not PHV, inhibited IFN induction >70%. These findings demonstrate that the TULV Gn-T inhibits IFN- and ISRE-directed responses upstream of IRF3 at the level of the TBK1 complex and further define a 42-residue domain of the TULV Gn-T that inhibits IFN induction. In contrast to pathogenic hantavirus Gn-Ts, the TULV Gn-T lacks a C-terminal degron domain and failed to bind tumor necrosis factor (TNF) receptor-associated factor 3 (TRAF3), a TBK1 complex component required for IRF3 activation. These findings indicate that the nonpathogenic TULV Gn-T regulates IFN induction but accomplishes this via unique interactions with cellular TBK1 complexes. These findings fundamentally distinguish nonpathogenic hantaviruses, PHV and TULV, and demonstrate that IFN regulation alone is insufficient for hantaviruses to cause disease. Yet regulating the early IFN response is necessary for hantaviruses to replicate within human endothelial cells and to be pathogenic. Thus, in addition to IFN regulation, hantaviruses contain discrete virulence determinants which permit them to be human pathogens.