Flavivirus nonstructural protein NS1: complementary surprises

Evolution of Tembusu Virus in Ducks, Chickens, Geese, Sparrows, and Mosquitoes in Northern China

Tembusu virus (TMUV) is a contagious pathogen from fowl that mainly infects ducks and geese, causing symptoms of high fever, loss of appetite, retarded growth, neurological symptoms, severe duck-drop syndrome, and even death. During an epidemiological investigation of TMUV in Northern China, we isolated 11 TMUV strains from ducks, chickens, geese, sparrows, and mosquitoes (2011–2017). Phylogenetic analysis of the open-reading frames of genes revealed that these strains clustered into Chinese strains II. The nucleotide and amino acid homologies of NS1 of the strains ranged between 85.8–99.8% and 92.5–99.68%, respectively, which were lower than those of E (86.7–99.9% and 96.5–99.9%, respectively), NS3 (87.6–99.9% and 98.2–99.8%, respectively), and NS5 (86.5–99.9% and 97.8–99.9%, respectively). Predictions of the tertiary structure of the viral proteins indicated that NS1 in 4 of 11 strains had a protein structure mutation at ¹⁸⁰TAV¹⁸² that changed a random crimp into an alpha helix. The protein of 6 of 11 strains had a glycosylation site mutation from NTTD to NITD. Furthermore, epidemiological data suggested that TMUV has been circulating in half of China’s provinces (17 of 34). Our findings, for the first time, have identified the NS1 protein as a potential hypervariable region for genetic evolution. Additionally, the territorial scope of the virus has expanded, requiring strict bio-security measures or a multivalent vaccine to control its spread.

Identification of one B-cell epitope from NS1 protein of duck Tembusu virus with monoclonal antibodies

This study describes the identification of one linear B-cell epitope on TMUV NS1 protein with monoclonal antibody (mAb) 3G2 by indirect enzyme-linked immunosorbent assay (ELISA). In this study, NS1 protein was expressed in prokaryotic expression system and purified. One mAb against NS1 protein was generated from Balb/c mice immunized with recombinant protein NS1. A set of 35 partially-overlapping polypeptides covering the entire NS1 protein was expressed with PGEX-6P-1 vector and screened with mAb 3G2. One polypeptide against the mAb was acquired and identified by indirect ELISA and western-blot. To map the epitope accurately, one or two amino acid residues were removed from the carboxy and amino terminal of polypeptide sequentially. A series of truncated oligopeptides were expressed and purified. The minimal determinant of the linear B cell epitope was recognized and identified with mAb 3G2. The accurate linear B-cell epitope was ²⁶⁹DEKEIV²⁷⁴ located in NS1 protein. Furthermore, sequence alignment showed that the epitope was highly conserved and specific among TMUV strains and other flavivirus respectively. The linear B-cell epitope of TMUV NS1 protein could benefit the development of new vaccines and diagnostic assays.

Prevalence and clinical course of dengue infection in elderly patients with acute febrile illness in a tertiary care hospital in Cali, Colombia

INTRODUCTION

Little is known about the prevalence and clinical course of dengue infection in elderly patients living in endemic areas; it is presumed that there is a lower prevalence but higher severity, complications and mortality. 

OBJECTIVE

To describe the prevalence and clinical course of dengue infection in elderly patients who were admitted to a referral care center for infectious diseases in an endemic region. 

MATERIALS AND METHODS

We conducted an observational and descriptive study between 2011 and 2014, using a cohort of elderly patients with serological diagnosis of dengue. 

RESULTS

A total of 235 febrile elderly patients were assessed, of which 43 patients (18.3%) were found to have dengue. The median age was 71 years; 48.7% were female, and 89% of patients had at least one comorbid condition. According to the serological tests, 51.4% of cases were positive for NS1 Ag, 27% for IgM and 54.1% for IgG, while 64.8% were secondary infections. Dengue was diagnosed in 13 patients (35%), dengue with warning signs in 16 cases (43%), and severe dengue in 8 cases (22%). Nearly 56.7% of patients were admitted to hospital and 21.6%, to the intensive care unit. None died. 

CONCLUSION

We found dengue infection to be more frequent than expected in this sample of elderly patients, due to acute febrile syndrome. Elderly patients also required higher rate of hospitalization and had more complications, however there were no deaths due to good management.

In Vitro and in Vivo Evaluation of Mutations in the NS Region of Lineage 2 West Nile Virus Associated with Neuroinvasiveness in a Mammalian Model

West Nile virus (WNV) strains may differ significantly in neuroinvasiveness in vertebrate hosts. In contrast to genetic lineage 1 WNVs, molecular determinants of pathogenic lineage 2 strains have not been experimentally confirmed so far. A full-length infectious clone of a neurovirulent WNV lineage 2 strain (578/10; Central Europe) was generated and amino acid substitutions that have been shown to attenuate lineage 1 WNVs were introduced into the nonstructural proteins (NS1 (P250L), NS2A (A30P), NS3 (P249H) NS4B (P38G, C102S, E249G)). The mouse neuroinvasive phenotype of each mutant virus was examined following intraperitoneal inoculation of C57BL/6 mice. Only the NS1-P250L mutation was associated with a significant attenuation of virulence in mice compared to the wild-type. Multiplication kinetics in cell culture revealed significantly lower infectious virus titres for the NS1 mutant compared to the wild-type, as well as significantly lower amounts of positive and negative stranded RNA.

Last 20 aa of the West Nile virus NS1' protein are responsible for its retention in cells and the formation of unique heat-stable dimers

West Nile virus (WNV), a mosquito-borne flavivirus, is the major cause of arboviral encephalitis in the USA. As with other members of the Japanese encephalitis virus serogroup, WNV produces an additional non-structural protein, NS1', a C-terminal extended product of NS1 generated as the result of a -1 programmed ribosomal frameshift (PRF). We have previously shown that mutations abolishing the PRF, and consequently NS1', resulted in reduced neuroinvasiveness. However, whether this was caused by the PRF event itself or by the lack of a PRF product, NS1', or a combination of both, remains undetermined. Here, we showed that WNV NS1' formed a unique subpopulation of heat- and low-pH-stable dimers. C-terminal truncations and mutational analysis employing an NS1'-expressing plasmid showed that stability of NS1' dimers was linked to the penultimate 10 aa. To examine the role of NS1' heat-stable dimers in virus replication and pathogenicity, a stop codon mutation was introduced into NS1' to create a WNV producing a truncated version of NS1' lacking the last 20 aa, but not affecting the PRF. NS1' protein produced by this mutant virus was secreted more efficiently than WT NS1', indicating that the sequence of the last 20 aa of NS1' was responsible for its cellular retention. Further analysis of this mutant showed growth kinetics in cells and virulence in weanling mice after peripheral infection similar to the WT WNVKUN, suggesting that full-length NS1' was not essential for virus replication in vitro and for virulence in mice.

NS1' protein expression facilitates production of Japanese encephalitis virus in avian cells and embryonated chicken eggs

Japanese encephalitis virus (JEV), which belongs to the genus Flavivirus of the family Flaviviridae, is a leading cause of meningo-encephalitis in Asian countries. The flavivirus non-structural protein 1 (NS1) plays a role in virus replication and in the elicitation of an immune response. The NS1' protein found among the members of the JEV subgroup is an extended form of NS1 and is generated by a -1 ribosomal frameshift. This protein is known to be involved in viral pathogenicity; however, its specific function is still unknown. Here, we describe an investigation of the molecular function of NS1' protein through the production of JEV NS1'-expressing and -non-expressing clones and their infection of avian and mammalian cells. Efficient NS1' protein expression was observed in avian cells and was found to facilitate JEV production in both avian cultured cells and embryonated chicken eggs. NS1' protein was observed to co-localize with NS5 protein and resulted in increased viral RNA levels in avian cells. These findings clearly indicate that NS1' enhances the production of JEV in avian cells and may facilitate the amplification/maintenance role of birds in the virus transmission cycle in nature.

NS1' colocalizes with NS1 and can substitute for NS1 in West Nile virus replication

NS1' is a C-terminally extended form of the NS1 protein produced only by encephalitic flaviviruses from the Japanese encephalitis virus serogroup. Here we show that West Nile virus (WNV) NS1' and NS1 localize to the same cellular compartments when expressed from plasmid DNAs and also colocalize to viral RNA replication sites in infected cells. Using complementation analysis with NS1-deleted WNV cDNA, we demonstrated that NS1' is able to substitute for the crucial function of NS1 in virus replication.

The flavivirus NS1 protein: molecular and structural biology, immunology, role in pathogenesis and application as a diagnostic biomarker

The flavivirus nonstructural glycoprotein NS1 is an enigmatic protein whose structure and mechanistic function have remained somewhat elusive ever since it was first reported in 1970 as a viral antigen circulating in the sera of dengue-infected patients. All flavivirus NS1 genes share a high degree of homology, encoding a 352-amino-acid polypeptide that has a molecular weight of 46-55 kDa, depending on its glycosylation status. NS1 exists in multiple oligomeric forms and is found in different cellular locations: a cell membrane-bound form in association with virus-induced intracellular vesicular compartments, on the cell surface and as a soluble secreted hexameric lipoparticle. Intracellular NS1 co-localizes with dsRNA and other components of the viral replication complex and plays an essential cofactor role in replication. Although this makes NS1 an ideal target for inhibitor design, the precise nature of its cofactor function has yet to be elucidated. A plethora of potential interacting partners have been identified, particularly for the secreted form of NS1, with many being implicated in immune evasion strategies. Secreted and cell-surface-associated NS1 are highly immunogenic and both the proteins themselves and the antibodies they elicit have been implicated in the seemingly contradictory roles of protection and pathogenesis in the infected host. Finally, NS1 is also an important biomarker for early diagnosis of disease. In this article, we provide an overview of these somewhat disparate areas of research, drawing together the wealth of data generated over more than 40 years of study of this fascinating protein.

West Nile virus

West Nile virus (WNV) is responsible for thousands of cases of morbidity and mortality in birds, horses, and humans. Epidemics were localized to Europe, Africa, the Middle East, and parts of Asia, and primarily caused a mild febrile illness in humans. In the late 1990s, the virus became more virulent and spread to North America. In humans, the clinical presentation ranges from asymptomatic, seen frequently, to encephalitis/paralysis and death, seen rarely. There is no FDA (Food and Drug Administration)-licensed vaccine for human use, and the only recommended treatment is supportive care. Often, there is a long recovery period. This article reviews the current literature summarizing the molecular virology, epidemiology, clinical manifestations, pathogenesis, diagnosis, treatment, immunology, and protective measures against WNV and WNV infections in humans.