Hantavirus nucleocapsid protein: a multifunctional molecule with both housekeeping and ambassadorial duties

Cross-reactivity of hantavirus antibodies after immunization with PUUV antigens

Nephropathia epidemica (NE), caused by Puumala (PUUV) orthohantavirus, is endemic in the Republic of Tatarstan (RT). There are limited options for NE prevention in RT. Currently, available vaccines are made using Haantan (HNTV) orthohantavirus antigens. In this study, the efficacy of microvesicles (MVs) loaded with PUUV antigens to induce the humoral immune response in small mammals was analyzed. Additionally, the cross-reactivity of serum from immunized small mammals and NE patients with HNTV, Dobrava, and Andes orthohantaviruses was investigated using nucleocapsid (N) protein peptide libraries. Finally, the selected peptides were analyzed for allergenicity, their ability to induce an autoimmune response, and their interaction with Class II HLA. Several N protein peptides were found to be cross-reactive with serum from MVs immunized small mammals. These cross-reactive epitopes were located in oligomerization perinuclear targeting and Daxx-interacting domains. Most cross-reactive peptides lack allergenic and autoimmune reactivity. Molecular docking revealed two cross-reacting peptides, N6 and N19, to have good binding with three Class II HLA alleles. These peptides could be candidates for developing vaccines and therapeutics for NE.

Negative and ambisense RNA virus ribonucleocapsids: more than protective armor

SUMMARYNegative and ambisense RNA viruses are the causative agents of important human diseases such as influenza, measles, Lassa fever, and Ebola hemorrhagic fever. The viral genome of these RNA viruses consists of one or more single-stranded RNA molecules that are encapsidated by viral nucleocapsid proteins to form a ribonucleoprotein complex (RNP). This RNP acts as protection, as a scaffold for RNA folding, and as the context for viral replication and transcription by a viral RNA polymerase. However, the roles of the viral nucleoproteins extend beyond these functions during the viral infection cycle. Recent advances in structural biology techniques and analysis methods have provided new insights into the formation, function, dynamics, and evolution of negative sense virus nucleocapsid proteins, as well as the role that they play in host innate immune responses against viral infection. In this review, we discuss the various roles of nucleocapsid proteins, both in the context of RNPs and in RNA-free states, as well as the open questions that remain.

Identification and characterization of codon usage pattern and influencing factors in HFRS-causing hantaviruses

Hemorrhagic fever with renal syndrome (HFRS) is an acute viral zoonosis carried and transmitted by infected rodents through urine, droppings, or saliva. The etiology of HFRS is complex due to the involvement of viral factors and host immune and genetic factors which hinder the development of potential therapeutic solutions for HFRS. Hantaan virus (HTNV), Dobrava-Belgrade virus (DOBV), Seoul virus (SEOV), and Puumala virus (PUUV) are predominantly found in hantaviral species that cause HFRS in patients. Despite ongoing prevention and control efforts, HFRS remains a serious economic burden worldwide. Furthermore, recent studies reported that the hantavirus nucleocapsid protein is a multi-functional protein and plays a major role in the replication cycle of the hantavirus. However, the precise mechanism of the nucleoproteins in viral pathogenesis is not completely understood. In the framework of the current study, various in silico approaches were employed to identify the factors influencing the codon usage pattern of hantaviral nucleoproteins. Based on the relative synonymous codon usage (RSCU) values, a comparative analysis was performed between HFRS-causing hantavirus and their hosts, suggesting that HTNV, DOBV, SEOV, and PUUV, were inclined to evolve their codon usage patterns that were comparable to those of their hosts. The results indicated that most of the overrepresented codons had AU-endings, which revealed that mutational pressure is the major force shaping codon usage patterns. However, the influence of natural selection and geographical factors cannot be ignored on viral codon usage bias. Further analysis also demonstrated that HFRS causing hantaviruses adapted host-specific codon usage patterns to sustain successful replication and transmission chains within hosts. To our knowledge, no study to date reported the factors influencing the codon usage pattern within hantaviral nucleoproteins. Thus, the proposed computational scheme can help in understanding the underlying mechanism of codon usage patterns in HFRS-causing hantaviruses which lend a helping hand in designing effective anti-HFRS treatments in future. This study, although comprehensive, relies on in silico methods and thus necessitates experimental validation for more solid outcomes. Beyond the identified factors influencing viral behavior, there could be other yet undiscovered influences. These potential factors should be targets for further research to improve HFRS therapeutic strategies.

Comprehensive computational analysis reveals YXXΦ[I/L/M/F/V] motif and YXXΦ-like tetrapeptides across HFRS causing Hantaviruses and their association with viral pathogenesis and host immune regulation

Hemorrhagic fever with renal syndrome (HFRS) is an acute zoonotic disease transmitted through aerosolized excrement of rodents. The etiology of HFRS is complex due to the involvement of viral factors and host immune and genetic factors. The viral species that dominantly cause HFRS are Puumala virus (PUUV), Seoul virus (SEOV), Dobrava-Belgrade virus (DOBV), and Hantaan virus (HTNV). Despite continuous prevention and control measures, HFRS remains a significant public health problem worldwide. The nucleocapsid protein of PUUV, SEOV, DOBV, and HTNV is a multifunctional viral protein involved in various stages of the viral replication cycle. However, the exact role of nucleoproteins in viral pathogenesis is yet to be discovered. Targeting a universal host protein exploited by most viruses would be a game-changing strategy that offers broad-spectrum solutions and rapid epidemic control. The objective of this study is to understand the replication and pathogenesis of PUUV, SEOV, DOBV, and HTNV by targeting tyrosine-based motif (YXXΦ[I/L/M/F/V]) and YXXΦ-like tetrapeptides. In the light of the current study, in silico analysis uncovered many different YXXΦ[I/L/M/F/V] motifs and YXXΦ-like tetrapeptides within nucleoproteins of PUUV, SEOV, DOBV, and HTNV. Following that, the 3D structures of nucleoproteins were predicted using AlphaFold2 to map the location of YXXΦ[I/L/M/F/V] motif and YXXΦ-like tetrapeptides in a 3D environment. Further, in silico analysis and characterization of Post Translational Modifications (PTMs) revealed multiple PTMs sites within YXXΦ[I/L/M/F/V] motif and YXXΦ-like tetrapeptides, which contribute to virulence and host immune regulation. Our study proposed that the predicted YXXΦ[I/L/M/F/V] motif and YXXΦ-like tetrapeptides may confer specific functions such as virulence, host immune regulation, and pathogenesis to nucleoproteins of PUUV, SEOV, DOBV, and HTNV. However, in vivo and in vitro studies on YXXΦ[I/L/M/F/V] motif and YXXΦ-like tetrapeptides will assign new biological roles to these antiviral targets.

The Role of Nucleoprotein in Immunity to Human Negative-Stranded RNA Viruses—Not Just Another Brick in the Viral Nucleocapsid

Negative-stranded RNA viruses (NSVs) are important human pathogens, including emerging and reemerging viruses that cause respiratory, hemorrhagic and other severe illnesses. Vaccine design traditionally relies on the viral surface glycoproteins. However, surface glycoproteins rarely elicit effective long-term immunity due to high variability. Therefore, an alternative approach is to include conserved structural proteins such as nucleoprotein (NP). NP is engaged in myriad processes in the viral life cycle: coating and protection of viral RNA, regulation of transcription/replication processes and induction of immunosuppression of the host. A broad heterosubtypic T-cellular protection was ascribed very early to this protein. In contrast, the understanding of the humoral immunity to NP is very limited in spite of the high titer of non-neutralizing NP-specific antibodies raised upon natural infection or immunization. In this review, the data with important implications for the understanding of the role of NP in the immune response to human NSVs are revisited. Major implications of the elicited T-cell immune responses to NP are evaluated, and the possible multiple mechanisms of the neglected humoral response to NP are discussed. The intention of this review is to remind that NP is a very promising target for the development of future vaccines.

Global Comprehensive Outlook of Hantavirus Contagion on Humans: A Review

Hantaviruses are rodent viruses that have been identified as etiologic agents of 2 diseases of humans: hemorrhagic fever with renal syndrome (HFRS) and nephropathiaepidemica (NE) in the Old World and Hantavirus pulmonary syndrome (HPS) in the New World. Orthohantavirus is a genus of sin- gle-stranded, enveloped, negative-sense RNA viruses in the family Hantaviridae of the order Bunyavi- rales. The important reservoir of Hantaviruses is rodents. Each virus serotype has its unique rodent host species and is transmitted to human beings with the aid of aerosolized virus, which is shed in urine, fae- ces and saliva and hardly by a bite of the contaminated host. Andes virus is the only Hantavirus identified to be transmitted from human-to-human and its major signs and symptoms include fever, headache, mus- cle aches, lungs filled with fluid etc. In the early 1993, this viral syndrome appeared in the Four Cor- ner location in the south western United States. The only accepted therapeutics for this virus is Ribavirin. Recently, serological examinations to identify Hantavirus antibodies have become most popular for in- vestigation among humans and rodent reservoirs.

Puumala Virus Variants Circulating in Forests of Ardennes, France: Ten Years of Genetic Evolution

In Europe, Puumala virus (PUUV) transmitted by the bank vole (Myodes glareolus) is the causative agent of nephropathia epidemica (NE), a mild form of haemorrhagic fever with renal syndrome. In France, very little is known about the spatial and temporal variability of the virus circulating within bank vole populations. The present study involved monitoring of bank vole population dynamics and PUUV microdiversity over a ten-year period (2000–2009) in two forests of the Ardennes region: Elan and Croix-Scaille. Ardennes region is characterised by different environmental conditions associated with different NE epidemiology. Bank vole density and population parameters were estimated using the capture/marking/recapture method, and blood samples were collected to monitor the overall seroprevalence of PUUV in rodent populations. Phylogenetic analyses of fifty-five sequences were performed to illustrate the genetic diversity of PUUV variants between forests. The pattern of the two forests differed clearly. In the Elan forest, the rodent survival was higher, and this limited turn-over resulted in a lower seroprevalence and diversity of PUUV sequences than in the Croix-Scaille forest. Uncovering the links between host dynamics and virus microevolution is improving our understanding of PUUV distribution in rodents and the NE risk.

Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear membrane-less sub structures that play a critical role in diverse cellular pathways including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chromatin organization, epigenetic regulation, protein turnover, autophagy, intrinsic and innate antiviral immunity. While intrinsic and innate immune functions of PML NBs or PML NB core proteins are well defined in the context of nuclear replicating DNA viruses, several studies also confirm their substantial roles in the context of RNA viruses. In the present review, antiviral activities of PML NBs or its core proteins on diverse RNA viruses that replicate in cytoplasm or the nucleus were discussed. In addition, viral counter mechanisms that reorganize PML NBs, and specifically how viruses usurp PML NB functions in order to create a cellular environment favorable for replication and pathogenesis, are also discussed.

HTNV Sensitizes Host Toward TRAIL-Mediated Apoptosis-A Pivotal Anti-hantaviral Role of TRAIL

Hantaviruses can cause hemorrhagic fever with renal syndrome (HFRS) in Eurasia and have led to public health threat in China. The pathogenesis of HFRS is complex and involves capillary leakage due to the infection of vascular endothelial cells. Accumulating evidence has demonstrated that hantavirus can induce apoptosis in many cells, but the mechanism remains unclear. Our studies showed that Hantaan virus (HTNV) infection could induce TNF-related apoptosis-inducing ligand (TRAIL) expression in primary human umbilical vein endothelial cells (HUVECs) and sensitize host cells toward TRAIL-mediated apoptosis. Furthermore, TRAIL interference could inhibit apoptosis and enhance the production of HTNV as well as reduce IFN-β production, while exogenous TRAIL treatment showed reverse outcome: enhanced apoptosis and IFN-β production as well as a lower level of viral replication. We also observed that nucleocapsid protein (NP) and glycoprotein (GP) of HTNV could promote the transcriptions of TRAIL and its receptors. Thus, TRAIL was upregulated by HTNV infection and then exhibited significant antiviral activities in vitro, and it was further confirmed in the HTNV-infected suckling mice model that TRAIL treatment significantly reduced viral load, alleviated virus-induced tissue lesions, increased apoptotic cells, and decreased the mortality. In conclusion, these results demonstrate that TRAIL-dependent apoptosis and IFN-β production could suppress HTNV replication and TRAIL treatment might be a novel therapeutic target for HTNV infection.

Diverse Morphology and Structural Features of Old and New World Hantaviruses

To further understanding of the structure and morphology of the Orthohantavirus, family Hantaviridae, we have employed cryo-electron microscopy (cryo-EM) for three New World hantaviruses: Andes (ANDV), Sin Nombre (SNV), and Black Creek Canal (BCCV). Building upon our prior cryo-EM and cryo-tomography study of the Old World hantavirus, Hantaan virus (HTNV), we have expanded our studies to examine the entire virion population present in cell culture supernatant. Hence, in contrast to the prior cryo-EM/ET studies in which we used a polyethylene precipitation, a sucrose gradient, and a sucrose cushion, we used two sucrose cushions. We inactivated the material after the first cushion. We tested the method using HTNV which has a known cryo-EM structure and observed equivalent results. Therefore, we used this method to assess the particle distribution of the New World hantaviruses by cryo-EM. Cryo-EM images showed a diverse range of sizes and morphologies for the New World viruses that we classified as round, tubular, and irregular. Strikingly, BCCV virions were mostly tubular. These first cryo-EM images of the New World Orthohantavirus confirm prior EM observations that noted tubular projections of SNV at the plasma membrane during virion morphogenesis but were not confirmed. These findings underscore the need for further investigation of virion morphogenesis of the Orthohantavirus.

Globally emerging hantaviruses: An overview

Hantaviruses are known to cause haemorrhagic fever with renal syndrome in Eurasia and hantavirus cardiopulmonary syndrome in the Americas. They are globally emerging pathogens as newer serotypes are routinely being reported. This review discusses hantavirus biology, clinical features and pathogenesis of hantavirus disease, its diagnostics, distribution and mammalian hosts. Hantavirus research in India is also summarised.

The Andes Virus Nucleocapsid Protein Directs Basal Endothelial Cell Permeability by Activating RhoA

Andes virus (ANDV) predominantly infects microvascular endothelial cells (MECs) and nonlytically causes an acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). In HPS patients, virtually every pulmonary MEC is infected, MECs are enlarged, and infection results in vascular leakage and highly lethal pulmonary edema. We observed that MECs infected with the ANDV hantavirus or expressing the ANDV nucleocapsid (N) protein showed increased size and permeability by activating the Rheb and RhoA GTPases. Expression of ANDV N in MECs increased cell size by preventing tuberous sclerosis complex (TSC) repression of Rheb-mTOR-pS6K. N selectively bound the TSC2 N terminus (1 to 1403) within a complex containing TSC2/TSC1/TBC1D7, and endogenous TSC2 reciprocally coprecipitated N protein from ANDV-infected MECs. TSCs normally restrict RhoA-induced MEC permeability, and we found that ANDV infection or N protein expression constitutively activated RhoA. This suggests that the ANDV N protein alone is sufficient to activate signaling pathways that control MEC size and permeability. Further, RhoA small interfering RNA, dominant-negative RhoA(N19), and the RhoA/Rho kinase inhibitors fasudil and Y27632 dramatically reduced the permeability of ANDV-infected MECs by 80 to 90%. Fasudil also reduced the bradykinin-directed permeability of ANDV and Hantaan virus-infected MECs to control levels. These findings demonstrate that ANDV activation of RhoA causes MEC permeability and reveal a potential edemagenic mechanism for ANDV to constitutively inhibit the basal barrier integrity of infected MECs. The central importance of RhoA activation in MEC permeability further suggests therapeutically targeting RhoA, TSCs, and Rac1 as potential means of resolving capillary leakage during hantavirus infections.

HPS is hallmarked by acute pulmonary edema, hypoxia, respiratory distress, and the ubiquitous infection of pulmonary MECs that occurs without disrupting the endothelium. Mechanisms of MEC permeability and targets for resolving lethal pulmonary edema during HPS remain enigmatic. Our findings suggest a novel underlying mechanism of MEC dysfunction resulting from ANDV activation of the Rheb and RhoA GTPases that, respectively, control MEC size and permeability. Our studies show that inhibition of RhoA blocks ANDV-directed permeability and implicate RhoA as a potential therapeutic target for restoring capillary barrier function to the ANDV-infected endothelium. Since RhoA activation forms a downstream nexus for factors that cause capillary leakage, blocking RhoA activation is liable to restore basal capillary integrity and prevent edema amplified by tissue hypoxia and respiratory distress. Targeting the endothelium has the potential to resolve disease during symptomatic stages, when replication inhibitors lack efficacy, and to be broadly applicable to other hemorrhagic and edematous viral diseases.

What Do We Know about How Hantaviruses Interact with Their Different Hosts?

Hantaviruses, like other members of the Bunyaviridae family, are emerging viruses that are able to cause hemorrhagic fevers. Occasional transmission to humans is due to inhalation of contaminated aerosolized excreta from infected rodents. Hantaviruses are asymptomatic in their rodent or insectivore natural hosts with which they have co-evolved for millions of years. In contrast, hantaviruses cause different pathologies in humans with varying mortality rates, depending on the hantavirus species and its geographic origin. Cases of hemorrhagic fever with renal syndrome (HFRS) have been reported in Europe and Asia, while hantavirus cardiopulmonary syndromes (HCPS) are observed in the Americas. In some cases, diseases caused by Old World hantaviruses exhibit HCPS-like symptoms. Although the etiologic agents of HFRS were identified in the early 1980s, the way hantaviruses interact with their different hosts still remains elusive. What are the entry receptors? How do hantaviruses propagate in the organism and how do they cope with the immune system? This review summarizes recent data documenting interactions established by pathogenic and nonpathogenic hantaviruses with their natural or human hosts that could highlight their different outcomes.

An in silico approach predicted potential therapeutics that can confer protection from maximum pathogenic Hantaviruses

Aim: In silico approach is used to identify most potent epitope and drug against pathogenic Hantavirus against which no approved therapeutics exist. Methods: Nucleocapsid protein sequences were retrieved, aligned and conserved regions were analyzed for the presence of B- and T-cell epitopes, and pockets for potential drugs. Results: T-cell epitope SYLRRTQSM and B-cell epitopes SYLRRTQ and YLRRTQSM appeared to be highly conserved, antigenic, nonallergenic. The T-cell epitope bound to maximum alleles. Thus, SYLRRTQSM would likely elicit both T- and B-cell immunity. High-throughput screening of Traditional Chinese Medicine database by docking technique revealed a potential drug, compound 46547 (1R,11S,15S,18S,20S,21R,22S)-12-oxa-8,17-diazaheptacyclo[15.5.2.0^{1,18}.0^{2,7}.0^{8,22}.0^{11,21}.0^{15,20}]tetracosa-2,4,6-trien-9-one. Conclusion: Our results predict potential therapeutics against multiple strains of pathogenic Hantavirus, but requires validation by in vivo experimentation.

Sin Nombre hantavirus nucleocapsid protein exhibits a metal-dependent DNA-specific endonucleolytic activity

We demonstrate that the nucleocapsid protein of Sin Nombre hantavirus (SNV-N) has a DNA-specific endonuclease activity. Upon incubation of SNV-N with DNA in the presence of magnesium or manganese, we observed DNA digestion in sequence-unspecific manner. In contrast, RNA was not affected under the same conditions. Moreover, pre-treatment of SNV-N with RNase before DNA cleavage increased the endonucleolytic activity. Structure-based protein fold prediction using known structures from the PDB database revealed that Asp residues in positions 88 and 103 of SNV-N show sequence similarity with the active site of the restriction endonuclease HindIII. Crystal structure of HindIII predicts that residues Asp93 and Asp108 are essential for coordination of the metal ions required for HindIII DNA cleavage. Therefore, we hypothesized that homologous residues in SNV-N, Asp88 and Asp103, may have a similar function. Replacing Asp88 and Asp103 by alanine led to an SNV-N protein almost completely abrogated for endonuclease activity.

Novel Insights on Hantavirus Evolution: The Dichotomy in Evolutionary Pressures Acting on Different Hantavirus Segments

Background

Hantaviruses are important emerging zoonotic pathogens. The current understanding of hantavirus evolution is complicated by the lack of consensus on co-divergence of hantaviruses with their animal hosts. In addition, hantaviruses have long-term associations with their reservoir hosts. Analyzing the relative abundance of dinucleotides may shed new light on hantavirus evolution. We studied the relative abundance of dinucleotides and the evolutionary pressures shaping different hantavirus segments.

Methods

A total of 118 sequences were analyzed; this includes 51 sequences of the S segment, 43 sequences of the M segment and 23 sequences of the L segment. The relative abundance of dinucleotides, effective codon number (ENC), codon usage biases were analyzed. Standard methods were used to investigate the relative roles of mutational pressure and translational selection on the three hantavirus segments.

Results

All three segments of hantaviruses are CpG depleted. Mutational pressure is the predominant evolutionary force leading to CpG depletion among hantaviruses. Interestingly, the S segment of hantaviruses is GpU depleted and in contrast to CpG depletion, the depletion of GpU dinucleotides from the S segment is driven by translational selection. Our findings also suggest that mutational pressure is the primary evolutionary pressure acting on the S and the M segments of hantaviruses. While translational selection plays a key role in shaping the evolution of the L segment. Our findings highlight how different evolutionary pressures may contribute disproportionally to the evolution of the three hantavirus segments. These findings provide new insights on the current understanding of hantavirus evolution.

Conclusions

There is a dichotomy among evolutionary pressures shaping a) the relative abundance of different dinucleotides in hantavirus genomes b) the evolution of the three hantavirus segments.

Co-circulation in a single biome of the Juquitiba and Araraquara hantavirus detected in human sera in a sub-tropical region of Brazil

Hantaviruses is an emerging infectious disease. Although HCPS has been reported in several regions of Brazil, more cases of HCPS have recently been reported in Minas Gerais than in any other state. In 2009, we analyzed 27 samples presenting antibodies against hantaviruses. These samples originated from 688 symptomatic patients, as determined based on the Hemorrhagic Fever Protocol. A subsequent SYBR Green-based real-time RT-PCR demonstrated the presence of the virus in 22 of the samples. Among the RT-PCR-positive samples, 17 were analyzed using DNA sequencing; these sequences were compared with others deposited in GenBank and showed similarity with the Araraquara and Juquitiba virus clusters. This work describe the detection of Juquitiba virus, including three fatal cases, in Minas Gerais state, furthermore, showed that it is feasible to characterize the circulating strains using a small fragment of S segment. Finally, the results suggest the co-circulation of Araraquara and Juquitiba virus in a single biome in Minas Gerais state.

Nucleocapsid protein from fig mosaic virus forms cytoplasmic agglomerates that are hauled by endoplasmic reticulum streaming

Although many studies have demonstrated intracellular movement of viral proteins or viral replication complexes, little is known about the mechanisms of their motility. In this study, we analyzed the localization and motility of the nucleocapsid protein (NP) of Fig mosaic virus (FMV), a negative-strand RNA virus belonging to the recently established genus Emaravirus. Electron microscopy of FMV-infected cells using immunogold labeling showed that NPs formed cytoplasmic agglomerates that were predominantly enveloped by the endoplasmic reticulum (ER) membrane, while nonenveloped NP agglomerates also localized along the ER. Likewise, transiently expressed NPs formed agglomerates, designated NP bodies (NBs), in close proximity to the ER, as was the case in FMV-infected cells. Subcellular fractionation and electron microscopic analyses of NP-expressing cells revealed that NBs localized in the cytoplasm. Furthermore, we found that NBs moved rapidly with the streaming of the ER in an actomyosin-dependent manner. Brefeldin A treatment at a high concentration to disturb the ER network configuration induced aberrant accumulation of NBs in the perinuclear region, indicating that the ER network configuration is related to NB localization. Dominant negative inhibition of the class XI myosins, XI-1, XI-2, and XI-K, affected both ER streaming and NB movement in a similar pattern. Taken together, these results showed that NBs localize in the cytoplasm but in close proximity to the ER membrane to form enveloped particles and that this causes passive movements of cytoplasmic NBs by ER streaming.

IMPORTANCE

Intracellular trafficking is a primary and essential step for the cell-to-cell movement of viruses. To date, many studies have demonstrated the rapid intracellular movement of viral factors but have failed to provide evidence for the mechanism or biological significance of this motility. Here, we observed that agglomerates of nucleocapsid protein (NP) moved rapidly throughout the cell, and we performed live imaging and ultrastructural analysis to identify the mechanism of motility. We provide evidence that cytoplasmic protein agglomerates were passively dragged by actomyosin-mediated streaming of the endoplasmic reticulum (ER) in plant cells. In virus-infected cells, NP agglomerates were surrounded by the ER membranes, indicating that NP agglomerates form the basis of enveloped virus particles in close proximity to the ER. Our work provides a sophisticated model of macromolecular trafficking in plant cells and improves our understanding of the formation of enveloped particles of negative-strand RNA viruses.

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.

Genetic analysis of Dobrava-Belgrade virus from western Serbia--a newly detected focus in the Balkan Peninsula

Dobrava-Belgrade virus (DOBV) is a hantavirus species that causes the most severe form of haemorrhagic fever with renal syndrome (HFRS) in Europe. DOBV has been detected in three Apodemus rodents: A. flavicollis, A. agrarius and A. ponticus. These emerging viruses appear throughout the Balkan Peninsula including Serbia as its central part. In this study, we examined the seroprevalence, molecular epidemiology and phylogenetics of DOBV from A. flavicollis captured at six Serbian localities. Furthermore, we applied microsatellite typing of host animal genome to analyse the role of host kinship in DOBV animal transmission. The overall IgG seropositivity rate over 3 years (2008-2010) was 11.9% (22/185). All seropositive samples were subjected to RT-PCR and DNA sequencing for S and L genome segments (pos. 291-1079 nt and 2999-3316 nt, respectively). DOBV was genetically detected in three samples from mountain Tara in western Serbia, a newly detected DOBV focus in the Balkans. No sequence data from human cases from Serbia are available for the studied period. However, collected DOBV isolates in this work phylogenetically clustered together with isolates from Serbian human cases dating from 2002, with 1.9% nucleotide divergence. We determined the level of kinship between seropositive and seronegative animal groups and found no significant difference, suggesting that horizontal virus transmission in the studied population was the same within and among the hatches. Our findings are the first genetic detection of DOBV in rodents in Serbia. We confirm wide and continuous hantavirus presence in the examined parts of the Balkans, underlying the necessity of continual monitoring of hantavirus circulation in A. flavicollis.

Changes in diversification patterns and signatures of selection during the evolution of murinae-associated hantaviruses

In the last 50 years, hantaviruses have significantly affected public health worldwide, but the exact extent of the distribution of hantavirus diseases, species and lineages and the risk of their emergence into new geographic areas are still poorly known. In particular, the determinants of molecular evolution of hantaviruses circulating in different geographical areas or different host species are poorly documented. Yet, this understanding is essential for the establishment of more accurate scenarios of hantavirus emergence under different climatic and environmental constraints. In this study, we focused on Murinae-associated hantaviruses (mainly Seoul Dobrava and Hantaan virus) using sequences available in GenBank and conducted several complementary phylogenetic inferences. We sought for signatures of selection and changes in patterns and rates of diversification in order to characterize hantaviruses’ molecular evolution at different geographical scales (global and local). We then investigated whether these events were localized in particular geographic areas. Our phylogenetic analyses supported the assumption that RNA virus molecular variations were under strong evolutionary constraints and revealed changes in patterns of diversification during the evolutionary history of hantaviruses. These analyses provide new knowledge on the molecular evolution of hantaviruses at different scales of time and space.

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.

Virion factors that target Daxx to overcome intrinsic immunity

PML nuclear bodies and their associated functions are part of an intrinsic cellular mechanism aimed at maintaining transcriptional control over viral gene expression and preventing replication of invading viruses. To overcome these barriers, many viruses express early nonstructural, multifunctional proteins to support the viral replication cycle or modulate host immune responses. Virion proteins constituting the invading particle are traditionally investigated for their role in transport during entry or egress and in the assembly of new virions. The additional functions of virion proteins have largely been ignored, in contrast to those of their nonstructural counterparts. A number of recent reports suggest that several virion proteins may also play vital roles in gene activation processes, in particular by counteracting intrinsic immune mechanisms mediated by the PML nuclear body-associated cellular factors Daxx, ATRX, and Sp100. These virion proteins share several features with their more potent nonstructural counterparts, and they may serve to bridge the gap in the early phase of an infection until immediate early viral gene expression is established. In this review, we discuss how virion proteins are an integral part of gene regulation among several viral families and to what extent structural proteins of incoming virions may contribute to species barrier, latency, and oncogenesis.

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.

Bunyavirus: structure and replication

The Bunyaviridae family is comprised of a large number of negative-sense, single-stranded RNA viruses that infect animals, insects, and plants. The tripartite genome of bunyaviruses, encapsidated in the form of individual ribonucleoprotein complexes, encodes four structural proteins, the glycoproteins Gc and Gn, the nucleoprotein N, and the viral polymerase L. Some bunyaviruses also use an ambi-sense strategy to encode the nonstructural proteins NSs and NSm. While some bunyaviruses have a T = 12 icosahedral symmetry, others only have locally ordered capsids, or capsids with no detectable symmetry. Bunyaviruses enter cells through clathrin-mediated endocytosis or phagocytosis. In endosome, viral glycoproteins facilitate membrane fusion at acidic pH, thus allowing bunyaviruses to uncoat and deliver their genomic RNA into host cytoplasm. Bunyaviruses replicate in cytoplasm where the viral polymerase L catalyzes both transcription and replication of the viral genome. While transcription requires a cap primer for initiation and ends at specific termination signals before the 3' end of the template is reached, replication copies the entire template and does not depend on any primer for initiation. This review will discuss some of the most interesting aspects of bunyavirus replication, including L protein/N protein-mediated cap snatching, prime-and-realign for transcription and replication initiation, translation-coupled transcription, sequence/secondary structure-dependent transcription termination, ribonucleoprotein encapsidation, and N protein-mediated initiation of viral protein translation. Recent developments on the structure and functional characterization of the bunyavirus capsid and the RNA synthesis machineries (including both protein L and N) will also be discussed.

Arbovirus vaccines; opportunities for the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is a well-established technology for the production of heterologous viral (glyco)proteins in cultured cells, applicable for basic scientific research as well as for the development and production of vaccines and diagnostics. Arboviruses form an emerging group of medically important viral pathogens that are transmitted to humans and animals via arthropod vectors, mostly mosquitoes, ticks or midges. Few arboviral vaccines are currently available, but there is a growing need for safe and effective vaccines against some highly pathogenic arboviruses such as Chikungunya, dengue, West Nile, Rift Valley fever and Bluetongue viruses. This comprehensive review discusses the biology and current state of the art in vaccine development for arboviruses belonging to the families Togaviridae, Flaviviridae, Bunyaviridae and Reoviridae and the potential of the baculovirus-insect cell expression system for vaccine antigen production The members of three of these four arbovirus families have enveloped virions and display immunodominant glycoproteins with a complex structure at their surface. Baculovirus expression of viral antigens often leads to correctly folded and processed (glyco)proteins able to induce protective immunity in animal models and humans. As arboviruses occupy a unique position in the virosphere in that they also actively replicate in arthropod cells, the baculovirus-insect cell expression system is well suited to produce arboviral proteins with correct folding and post-translational processing. The opportunities for recombinant baculoviruses to aid in the development of safe and effective subunit and virus-like particle vaccines against arboviral diseases are discussed.

Investigations on the RNA binding and phosphorylation of groundnut bud necrosis virus nucleocapsid protein

Groundnut bud necrosis virus belongs to the genus Tospovirus, infects a wide range of crop plants and causes severe losses. To understand the role of the nucleocapsid protein in the viral life cycle, the protein was overexpressed in E. coli and purified by Ni-NTA chromatography. The purified N protein was well folded and was predominantly alpha-helical. Deletion analysis revealed that the C-terminal unfolded region of the N protein was involved in RNA binding. Furthermore, the N protein could be phosphorylated in vitro by Nicotiana benthamiana plant sap and by purified recombinant kinases such as protein kinase CK2 and calcium-dependent protein kinase. This is the first report of phoshphorylation of a nucleocapsid protein in the family Bunyaviridae. The possible implications of the present findings for the viral life cycle are discussed.

The cytoplasmic tail of hantavirus Gn glycoprotein interacts with RNA

We recently characterized the interaction between the intraviral domains of envelope glycoproteins (Gn and Gc) and ribonucleoprotein (RNP) of Puumala and Tula hantaviruses (genus Hantavirus, family Bunyaviridae). Herein we report a direct interaction between spike-forming glycoprotein and nucleic acid. We show that the envelope glycoprotein Gn of hantaviruses binds genomic RNA through its cytoplasmic tail (CT). The nucleic acid binding of Gn-CT is unspecific, as demonstrated by interactions with unrelated RNA and with single-stranded DNA. Peptide scan and protein deletions of Gn-CT mapped the nucleic acid binding to regions that overlap with the previously characterized N protein binding sites and demonstrated the carboxyl-terminal part of Gn-CT to be the most potent nucleic acid-binding site. We conclude that recognition of the RNP complex by the Gn-CT could be mediated by interactions with both genomic RNA and the N protein. This would provide the required selectivity for the genome packaging of hantaviruses.

Human pathogenic hantaviruses and prevention of infection

Hantaviruses are emerging viruses which are hosted by small mammals. When transmitted to humans, they can cause two clinical syndromes, hemorrhagic fever with renal syndrome or hantavirus cardiopulmonary syndrome. The review compiles the current list of hantaviruses which are thought to be pathogenic in humans on the basis of molecular or at least serological evidence. Whereas induction of a neutralizing humoral immune response is considered to be protective against infection, the dual role of cellular immunity (protection versus immunopathogenicity) is discussed. For active immunisation, inactivated virus vaccines are licensed in certain Asian countries. Moreover, several classical and molecular vaccine approaches are in pre-clinical stages of development. The development of hantavirus vaccines is hampered by the lack of adequate animal models of hantavirus-associated disease. In addition to active immunization strategies, the review summarizes other ways of infection prevention, as passive immunization, chemoprophylaxis, and exposition prophylaxis.

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.

Geo-spatial hotspots of hemorrhagic fever with renal syndrome and genetic characterization of Seoul variants in Beijing, China

BACKGROUND

Hemorrhagic fever with renal syndrome (HFRS) is highly endemic in mainland China, and has extended from rural areas to cities recently. Beijing metropolis is a novel affected region, where the HFRS incidence seems to be diverse from place to place.

METHODOLOGY/PRINCIPAL FINDINGS

The spatial scan analysis based on geographical information system (GIS) identified three geo-spatial "hotspots" of HFRS in Beijing when the passive surveillance data from 2004 to 2006 were used. The Relative Risk (RR) of the three "hotspots" was 5.45, 3.57 and 3.30, respectively. The Phylogenetic analysis based on entire coding region sequence of S segment and partial L segment sequence of Seoul virus (SEOV) revealed that the SEOV strains circulating in Beijing could be classified into at least three lineages regardless of their host origins. Two potential recombination events that happened in lineage #1 were detected and supported by comparative phylogenetic analysis. The SEOV strains in different lineages and strains with distinct special amino acid substitutions for N protein were partially associated with different spatial clustered areas of HFRS.

CONCLUSION/SIGNIFICANCE

Hotspots of HFRS were found in Beijing, a novel endemic region, where intervention should be enhanced. Our data suggested that the genetic variation and recombination of SEOV strains was related to the high risk areas of HFRS, which merited further investigation.

Hantaviruses in the americas and their role as emerging pathogens

The continued emergence and re-emergence of pathogens represent an ongoing, sometimes major, threat to populations. Hantaviruses (family Bunyaviridae) and their associated human diseases were considered to be confined to Eurasia, but the occurrence of an outbreak in 1993–94 in the southwestern United States led to a great increase in their study among virologists worldwide. Well over 40 hantaviral genotypes have been described, the large majority since 1993, and nearly half of them pathogenic for humans. Hantaviruses cause persistent infections in their reservoir hosts, and in the Americas, human disease is manifest as a cardiopulmonary compromise, hantavirus cardiopulmonary syndrome (HCPS), with case-fatality ratios, for the most common viral serotypes, between 30% and 40%. Habitat disturbance and larger-scale ecological disturbances, perhaps including climate change, are among the factors that may have increased the human caseload of HCPS between 1993 and the present. We consider here the features that influence the structure of host population dynamics that may lead to viral outbreaks, as well as the macromolecular determinants of hantaviruses that have been regarded as having potential contribution to pathogenicity.

Characterization of two substrains of Puumala virus that show phenotypes that are different from each other and from the original strain

Hantaviruses, the causative agents of two emerging diseases, are negative-stranded RNA viruses with a tripartite genome. We isolated two substrains from a parental strain of Puumala hantavirus (PUUV-Pa), PUUV-small (PUUV-Sm) and PUUV-large (PUUV-La), named after their focus size when titrated. The two isolates were sequenced; this revealed differences at two positions in the nucleocapsid protein and two positions in the RNA-dependent RNA polymerase, but the glycoproteins were identical. We also detected a 43-nucleotide deletion in the PUUV-La S-segment 5' noncoding region covering a predicted hairpin loop structure that was found to be conserved among all hantaviruses with members of the rodent subfamily Arvicolinae as their hosts. Stocks of PUUV-La showed a lower ratio of viral RNA to infectious particles than stocks of PUUV-Sm and PUUV-Pa, indicating that PUUV-La replicated more efficiently in alpha/beta interferon (IFN-α/β)-defective Vero E6 cells. In Vero E6 cells, PUUV-La replicated to higher titers and PUUV-Sm replicated to lower titers than PUUV-Pa. In contrast, in IFN-competent MRC-5 cells, PUUV-La and PUUV-Sm replicated to similar levels, while PUUV-Pa progeny virus production was strongly inhibited. The different isolates clearly differed in their potential to induce innate immune responses in MRC-5 cells. PUUV-Pa caused stronger induction of IFN-β, ISG56, and MxA than PUUV-La and PUUV-Sm, while PUUV-Sm caused stronger MxA and ISG56 induction than PUUV-La. These data demonstrate that the phenotypes of isolated hantavirus substrains can have substantial differences compared to each other and to the parental strain. Importantly, this implies that the reported differences in phenotypes for hantaviruses might depend more on chance due to spontaneous mutations during passage than inherited true differences between hantaviruses.

Tracking hantavirus nucleocapsid protein using intracellular antibodies

Background

Hantavirus nucleocapsid (N) protein is a multifunctional viral macromolecule involved in multiple stages of the viral replication cycle. The intracellular trafficking of N protein during virus assembly remains unclear.

Methods

We used N protein-specific intracellular expressed antibodies to track the localization and distribution of Hantaan virus and Seoul virus N protein. The N protein-specific antibody single-chain variable antibody fragments (scFvs), which bind an N-terminal linear epitope (L13F3) and C-terminal conformational domain (H34), were intracellularly expressed in the endoplasmic reticulum (ER) by fusion of the SEKDEL retention signal peptide at the carboxyl terminus, and in the cytoplasm (Cyto) by deletion of the ER membrane target signal peptide. Stable Vero-E6 cell lines expressing intracellular scFvs were either infected with hantavirus or transfected with an N protein expression plasmid; virus replication and N protein intracellular localization were determined.

Result

N protein co-localized with scFvs in the ER and cytoplasm with or without viral membrane glycoproteins. Hantavirus replication was inhibited in both the scFvs-ER- and scFvs-Cyto-expressing stable cell lines.

Conclusion

N protein may be expressed in the ER retention signal peptide of KDEL circulating region (ER/cis-Golgi) without the assistance of G protein, and so expression of N protein in both the cytoplasm and within the ER/cis-Golgi plays an important role in virus replication.

Oligomerization of Uukuniemi virus nucleocapsid protein

Background

Uukuniemi virus (UUKV) belongs to the Phlebovirus genus in the family Bunyaviridae. As a non-pathogenic virus for humans UUKV has served as a safe model bunyavirus in a number of studies addressing fundamental questions such as organization and regulation of viral genes, genome replication, structure and assembly. The present study is focused on the oligomerization of the UUKV nucleocapsid (N) protein, which plays an important role in several steps of virus replication. The aim was to locate the domains involved in the N protein oligomerization and study the process in detail.

Results

A set of experiments concentrating on the N- and C-termini of the protein was performed, first by completely or partially deleting putative N-N-interaction domains and then by introducing point mutations of amino acid residues. Mutagenesis strategy was based on the computer modeling of secondary and tertiary structure of the N protein. The N protein mutants were studied in chemical cross-linking, immunofluorescence, mammalian two-hybrid, minigenome, and virus-like particle-forming assays. The data showed that the oligomerization ability of UUKV-N protein depends on the presence of intact α-helices on both termini of the N protein molecule and that a specific structure in the N-terminal region plays a crucial role in the N-N interaction(s). This structure is formed by two α-helices, rich in amino acid residues with aromatic (W7, F10, W19, F27, F31) or long aliphatic (I14, I24) side chains. Furthermore, some of the N-terminal mutations (e.g. I14A, I24A, F31A) affected the N protein functionality both in mammalian two-hybrid and minigenome assays.

Conclusions

UUKV-N protein has ability to form oligomers in chemical cross-linking and mammalian two-hybrid assays. In mutational analysis, some of the introduced single-point mutations abolished the N protein functionality both in mammalian two-hybrid and minigenome assays, suggesting that especially the N-terminal region of the UUKV-N protein is essential for the N-N interaction.

Rubella virus capsid protein: a small protein with big functions

Virus replication occurs in the midst of a life or death struggle between the virus and the infected host cell. To limit virus replication, host cells can activate a number of antiviral pathways, the most drastic of which is programmed cell death. Whereas large DNA viruses have the luxury of encoding accessory proteins whose main function is to interfere with host cell defences, the genomes of RNA viruses are not large enough to encode proteins of this type. Recent studies have revealed that proteins encoded by RNA viruses often play multiple roles in the battles between viruses and host cells. In this article, we discuss the many functions of the rubella virus capsid protein. This protein has well-defined roles in virus assembly, but recent research suggests that it also functions to modulate virus replication and block host cell defences.

Interaction between hantaviral nucleocapsid protein and the cytoplasmic tail of surface glycoprotein Gn

Hantaviral N and Gn proteins were shown to interact, thus providing the long-awaited evidence for one of the crucial steps in the virus replication at which RNPs are directed to the site of the virus assembly. Using pull-down assay and point mutagenesis it was demonstrated that intact, properly folded zinc fingers in the Gn protein cytoplasmic tail as well as the middle domain of the N protein (that includes aa residues 80-248) are essential for the interaction.

Cytoplasmic tails of hantavirus glycoproteins interact with the nucleocapsid protein

Here we characterize the interaction between the glycoproteins (Gn and Gc) and the ribonucleoprotein (RNP) of Puumala virus (PUUV; genus Hantavirus, family Bunyaviridae). The interaction was initially established with native proteins by co-immunoprecipitating PUUV nucleocapsid (N) protein with the glycoprotein complex. Mapping of the interaction sites revealed that the N protein has multiple binding sites in the cytoplasmic tail (CT) of Gn and is also able to bind to the predicted CT of Gc. The importance of Gn- and Gc-CTs to the recognition of RNP was further verified in pull-down assays using soluble peptides with binding capacity to both recombinant N protein and the RNPs of PUUV and Tula virus. Additionally, the N protein of PUUV was demonstrated to interact with peptides of Gn and Gc from a variety of hantavirus species, suggesting a conserved RNP-recognition mechanism within the genus. Based on these and our previous results, we suggest that the complete hetero-oligomeric (Gn-Gc)(4) spike complex of hantaviruses mediates the packaging of RNP into virions.

Computational identification of post-translational modification sites and functional families reveal possible moonlighting role of rotaviral proteins

Rotavirus (RV) diarrhoea causes huge number deaths in children less than 5 years of age. In spite of available vaccines, it has been difficult to combat RV due to large number of antigenically distinct genotypes, high mutation rates, generation of reassortant viruses due to segmented genome. RV is an eukaryotic virus which utilizes host cell machinery for its propagation. Since RV only encodes 12 proteins, post­translational modification (PTM) is important mechanism for modification, which consequently alters their function. A single protein exhibiting different functions in different locations or in different subcellular sites, are known to be ’moonlighting‘. So there is a possibility that viral proteins moonlight in separate location and in different time to exhibit diverse cellular effects. Based on the primary sequence, the putative behaviour of proteins in cellular environment can be predicted, which helps to classify them into different functional families with high reliability score. In this study, sites for phosphorylation, glycosylation and SUMOylation of the six RV structural proteins (VP1, VP2, VP3, VP4, VP6 & VP7) & five non­structural proteins (NSP1, NSP2,NSP3,NSP4 & NSP5) and the functional families were predicted. As NSP6 is a very small protein and not required for virus growth & replication, it was not included in the study. Classification of RV proteins revealed multiple putative functions of each structural protein and varied number of PTM sites, indicating that RV proteins may also moonlight depending on requirements during viral life cycle. Targeting the crucial PTM sites on RV structural proteins may have implications in developing future anti­rotaviral strategies.

Non-structural and nucleocapsid proteins of Punta Toro virus induce apoptosis of hepatocytes through both intrinsic and extrinsic pathways

Punta Toro virus (PTV; family Bunyaviridae, genus Phlebovirus) causes severe hepatic damage through brisk apoptosis of hepatocytes. In the present study, two viral proteins encoded by the S segment of the viral genome, non-structural (NSs) and nucleocapsid protein (N), were examined for their roles in apoptosis. Expression of NSs in HepG2 cells led to apoptosis in 45% of transfected cells, and with N, 28%, on average. These levels represent a four- to an eightfold increase over cells transfected with the mutated protein vectors. Caspase-3, -8 and -9 activities were increased by N protein when compared with the control NC (P < 0.05), and by NSsA and NSsB, as compared to control NSsC (P < 0.01). Treatment of the transfected cells with caspase-8 or -9 inhibitors markedly decreased apoptosis. Neutralization of TNF-alpha or Fas ligand had no effect on apoptosis. These results indicate that both NSs and N are responsible for causing hepatocyte apoptosis by triggering the extrinsic caspase-8 and intrinsic caspase-9 pathways.

Analysis of the immune response to Hantaan virus nucleocapsid protein C-terminal-specific CD8(+) T cells in patients with hemorrhagic fever with renal syndrome

Hantaan virus (HTNV), the prototype member of the Hantavirus genus in the family Bunyaviridae, causes hemorrhagic fever with renal syndrome (HFRS), which is characterized by capillary leakage, hemorrhage, and renal injury, and is an important public health problem in China. Some kinds of immune cells, particularly CD8(+) T cells, are involved in the pathogenesis of Hantavirus infection. The nucleocapsid protein (NP) of the Hantavirus is the most conserved structural protein and the most abundant viral protein produced during infection. It is one of the important target antigens that induce the CD8(+) T-cell response. In this study, we examined the CD8(+) T-cell response to HTNV NP C-terminal polypeptides. We synthesized 23 overlapping C-terminal polypeptides and detected the antigen-specific CD8(+) T cell response in 15 patients with HFRS. The results demonstrated that there were NP-specific T-cell responses in bulk cultures of peripheral blood mononuclear cells (PBMCs) from 9 of 15 patients. The peptide 51 (aa 301-315: SPSSIWVFAGAPDRC), peptide 60 (aa 355-369: LRKKSSFYQSYLRRT), and peptide 70 (aa 415-429: DVKVKEISNQEPLKL) induced strong CD8(+) T-cell responses. Among them, peptide 70 induced CTL responses in donors 7, 9, and 11, and the strongest responses were seen in donor 11. Depletion of CD8(+) T cells from PBMCs completely abrogated the peptide-specific T-cell response, while depletion of CD4(+) T cells did not diminish the number of IFN-gamma spot-forming cells. These data suggest that infection with HTNV results in CTL responses to immunodominant regions on the NP.

Hantaviruses: an emerging public health threat in India? A review

The emerging viral diseases haemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS) are a cause of global concern as they are increasingly reported from newer regions of the world. The hantavirus species causing HFRS include Hantaan virus,Seoul virus, Puumala virus, and Dobrava-Belgrade virus while Sin Nombre virus was responsible for the 1993 outbreak of HCPS in the Four Corners Region of the US. Humans are accidental hosts and get infected by aerosols generated from contaminated urine,feces and saliva of infected rodents. Rodents are the natural hosts of these viruses and develop persistent infection. Human to human infections are rare and the evolution of the virus depends largely on that of the rodent host. The first hantavirus isolate to be cultured, Thottapalayam virus,is the only indigenous isolate from India,isolated from an insectivore in 1964 in Vellore, South India. Research on hantaviruses in India has been slow but steady since 2005. Serological investigation of patients with pyrexic illness revealed presence of anti-hantavirus IgM antibodies in 14.7% of them. The seropositivity of hantavirus infections in the general population is about 4% and people who live and work in close proximity with rodents have a greater risk of acquiring hantavirus infections. Molecular and serological evidence of hantavirus infections in rodents and man has also been documented in this country. The present review on hantaviruses is to increase awareness of these emerging pathogens and the threats they pose to the public health system.

The Hantavirus Glycoprotein G1 Tail Contains Dual CCHC-type Classical Zinc Fingers

Hantaviruses are distributed worldwide and can cause a hemorrhagic fever or a cardiopulmonary syndrome in humans. Mature virions consist of RNA genome, nucleocapsid protein, RNA polymerase, and two transmembrane glycoproteins, G1 and G2. The ectodomain of G1 is surface-exposed; however, it has a 142-residue C-terminal cytoplasmic tail that plays important roles in viral assembly and host-pathogen interaction. Here we show by NMR, circular dichroism spectroscopy, and mutagenesis that a highly conserved cysteine/histidine-rich region in the G1 tail of hantaviruses forms two CCHC-type classical zinc fingers. Unlike classical zinc fingers, however, the two G1 zinc fingers are intimately joined together, forming a compact domain with a unique fold. We discuss the implication of the hantaviral G1 zinc fingers in viral assembly and host-pathogen interaction.

Roles for the recycling endosome, Rab8, and Rab11 in hantavirus release from epithelial cells

Hantavirus structural proteins are believed to localize to intracellular membranes often identified as Golgi membranes, in virus-infected cells. After virus budding into the Golgi luminal space, virus-containing vesicles are transported to the plasma membrane via trafficking pathways that are not well defined. Using the New World hantavirus, Andes virus, we have investigated the role of various Rab proteins in the release of hantavirus particles from infected cells. Rabs 8 and 11 were found to colocalize with Andes virus proteins in virus infected cells and when expressed from cDNA, implicating the recycling endosome as an organelle important for hantavirus infection. Small interfering RNA-mediated downregulation of Rab11a alone or Rab11a and Rab11b together resulted in a decrease in infectious virus particle secretion from infected cells. Downregulation of Rab8a did not alter infectious virus release but reduction of both isoforms did. These data implicate the recycling endosome and the Rab proteins associated with vesicular transport to or from this intracellular organelle as an important pathway for hantavirus trafficking to the plasma membrane.

Oligomerization of hantaviral nucleocapsid protein: charged residues in the N-terminal coiled-coil domain contribute to intermolecular interactions

The nucleocapsid (N) protein of hantaviruses (family Bunyaviridae) is the most abundant component of the virion; it encapsidates genomic RNA segments and participates in viral genome transcription and replication, as well as in virus assembly. During RNA encapsidation, the N protein forms intermediate trimers and then oligomers via 'head-to-head, tail-to-tail' interactions. In previous work, using Tula hantavirus (TULV) N protein as a model, it was demonstrated that an intact coiled-coil structure of the N terminus is crucial for the oligomerization capacity of the N protein and that the hydrophobic 'a' residues from the second alpha-helix are especially important. Here, the importance of charged amino acid residues located within the coiled-coil for trimer formation and oligomerization was analysed. To predict the interacting surfaces of the monomers, the previous in silico model of TULV coiled-coils was first upgraded, taking advantage of the recently published crystal structure of the N-terminal coiled-coil of the Sin Nombre virus N protein. The results obtained using a mammalian two-hybrid assay suggested that conserved, charged amino acid residues within the coiled-coil make a substantial contribution to N protein oligomerization. This contribution probably involves (i) the formation of interacting surfaces of the N monomers (residues D35 and D38, located at the tip of the coiled-coil loop, and R63 appear particularly important) and (ii) stabilization of the coiled-coil via intramolecular ionic bridging (with E55 as a key player). It is hypothesized that the tips of the coiled-coils are the first to come into direct contact and thus to initiate tight packing of the three structures.

NMR structure of the N-terminal coiled coil domain of the Andes hantavirus nucleocapsid protein

The hantaviruses are emerging infectious viruses that in humans can cause a cardiopulmonary syndrome or a hemorrhagic fever with renal syndrome. The nucleocapsid (N) is the most abundant viral protein, and during viral assembly, the N protein forms trimers and packages the viral RNA genome. Here, we report the NMR structure of the N-terminal domain (residues 1-74, called N1-74) of the Andes hantavirus N protein. N1-74 forms two long helices (alpha1 and alpha2) that intertwine into a coiled coil domain. The conserved hydrophobic residues at the helix alpha1-alpha2 interface stabilize the coiled coil; however, there are many conserved surface residues whose function is not known. Site-directed mutagenesis, CD spectroscopy, and immunocytochemistry reveal that a point mutation in the conserved basic surface formed by Arg22 or Lys26 lead to antibody recognition based on the subcellular localization of the N protein. Thus, Arg22 and Lys26 are likely involved in a conformational change or molecular recognition when the N protein is trafficked from the cytoplasm to the Golgi, the site of viral assembly and maturation.

Molecular diversity and phylogeny of Hantaan virus in Guizhou, China: evidence for Guizhou as a radiation center of the present Hantaan virus

To gain further insight into the molecular epidemiology of Hantaan virus (HTNV) in Guizhou, China, rodents were captured in this region in 2004 and 2005. In addition, serum samples were collected from four patients. Ten hantaviruses were isolated successfully in cell culture from four humans, two Apodemus agrarius, three Rattus norvegicus and one Rattus nitidus. The nucleotide sequences for their small (S), medium (M) and partial large (L) segments were determined. Phylogenetic analysis of the S and M segment sequences revealed that all of these isolates belong to the species HTNV, suggesting a spillover of HTNV from A. agrarius to Rattus rats. All available isolates from Guizhou were divided into four distinct groups either in the S segment tree or in the M segment tree. The clustering pattern of these isolates in the S segment tree was not in agreement with that in the M or L segment tree, showing that genetic reassortment between HTNV had occurred naturally. Analysis of the S segment sequences from available HTNV strains indicated that they formed three clades. The first clade, which comprised only viruses from Guizhou, was the outgroup of clades II and III. The viruses in the second clade were found in Guizhou and mainly in the far-east Asian region, including China. However, the viruses in the third clade were found in most areas of China, including Guizhou, in which haemorrhagic fever with renal syndrome (HFRS) is endemic. Our results reveal that the highest genetic diversity of HTNV is in a limited geographical region of Guizhou, and suggest that Guizhou might be a radiation centre of the present form of HTNV.

Quasispecies dynamics and fixation of a synonymous mutation in hantavirus transmission

RNA-dependent RNA polymerases, the key enzymes in replication of RNA viruses, have a low fidelity; thus, these viruses replicate as a swarm of mutants termed viral quasispecies. Constant generation of new mutations allows RNA viruses to adapt swiftly to a novel environment through selection of both pre-existing and de novo-generated genetic variants. Here, quasispecies dynamics were studied in vivo in controlled hantavirus transmission from experimentally infected to naïve rodents through infested cage bedding. An elementary step of virus microevolution was apparent, as one synonymous mutation (A759G) repeatedly became fixed in the viral RNA quasispecies populations in the recipient animals.

Characterization of cross-reactive and serotype-specific epitopes on the nucleocapsid proteins of hantaviruses

The hantavirus nucleocapsid (N) protein fulfills several key roles in virus replication and assembly and is the major antigen in humoral immune responses in humans and mice. Here we report on epitopes involved in serotype-specific and cross-reactive recognition of the N proteins of hantaviruses using monoclonal antibodies (mAbs) against the N proteins of Andes virus (ANDV) and Sin Nombre virus (SNV). The mAbs define at least twelve different epitopic patterns which span eight sequences, including amino acids 17-59, 66-78, 79-91, 157-169, 222-234, 244-263, 274-286 and 326-338 on the SNV and ANDV N proteins. Studies on the cross-reactivity of these mAbs with different hantavirus N proteins indicated that epitopes located within amino acids 244-286 are related to serotype specificity. We analyzed further the location of epitopes with available three-dimensional structure information including the N-terminal coiled-coil and derived exposed and hidden residues of these epitopes. The generated recombinant N proteins and the characterized mAbs are functional tools being now available for hantavirus diagnostics and replication studies.

New and Old World hantaviruses differentially utilize host cytoskeletal components during their life cycles

Recently we reported that the N protein of the Old World hantavirus, Hantaan (HTNV), traffics on microtubules to the ER-Golgi intermediate compartment (ERGIC) prior to its movement to the Golgi and requires an intact ERGIC for viral replication. We have extended these studies to the New World hantaviruses, Andes virus (ANDV) and Black Creek Canal virus (BCCV), and an additional Old World hantavirus, Seoul virus (SEOV). These studies support microtubule-dependent trafficking of the N protein to ERGIC within the perinuclear region for the New and Old World hantaviruses. However, we observed that early entry events were distinct for HTNV and ANDV with respect to the pathway for entry and the dependence on an intact actin (ANDV) versus microtubule (HTNV) cytoskeleton for viral replication. These studies show for the first time that while Old and New World hantaviruses share common features in their pathways, they have evolved differences in their interaction with host cell machinery.