The disulphide bonds of an Asian influenza virus neuraminidase

Anti-neuraminidase immunity in the combat against influenza

INTRODUCTION

Anti-neuraminidase (NA) immunity correlates with the protection against influenza virus infection in both human and animal models. The aim of this review is to better understand the mechanism of anti-NA immunity, and also to evaluate the approaches on developing NA-based influenza vaccines or enhancing immune responses against NA for current influenza vaccines.

AREAS COVERED

In this review, the structure of influenza neuraminidase, the contribution of anti-NA immunity to protection, as well as the efforts and challenges of targeting the immune responses to NA were discussed. We also listed some of the newly discovered anti-NA monoclonal antibodies and discussed their contribution in therapeutic as well as the antigen design of a broadly protective NA vaccine.

EXPERT OPINION

Targeting the immune response to both HA and NA may be critical for achieving the optimal protection since there are different mechanisms of HA and NA elicited protective immunity. Monoclonal antibodies (mAbs) that target the conserved protective lateral face or catalytic sites are effective therapeutics. The epitope discovery using monoclonal antibodies may benefit NA-based vaccine elicited broadly reactive antibody responses. Therefore, the potential for a vaccine that elicits cross-reactive antibodies against neuraminidase is a high priority for next-generation influenza vaccines.

Rapid screening of neuraminidase inhibitors with the benzoic acid skeleton from Paeonia suffruticosa Andrews by solid-phase extraction with an enzyme activity switch combined with mass spectrometry analysis

It is meaningful for drug discovery to discover lead compounds with specific skeletons from medicinal herbs. Screening bioactive compounds with specific skeletons by a simple and rapid strategy is still a challenging task. Solid-phase extraction (SPE) is a simple and time-saving technique in the laboratory and is often used in the concentration of natural products. It is attractive to apply the SPE in the screening of bioactive compounds with specific skeletons. To achieve this goal, SPE with an enzyme activity (EA) switch combined with mass spectrometry analysis was first proposed. The screening of benzoic acid-derived neuraminidase (NA) inhibitors from the root cortex of Paeonia suffruticosa Andrews (CPSA) was used as an example. The NA and crude extract of CPSA were incubated to form a sample solution. Subsequently, the sample was separated, detected, and collected by the SPE with an EA switch. When the detected values reduced significantly, the EA switch was triggered, and the collection was stopped. The collected eluents were treated for LC-MS/MS analysis. Finally, combining diagnostic ions and mass spectrometry data, two benzoic acid NA inhibitors were successfully screened from CPSA. In this study, the separation, detection, and collection were performed on one instrument system. Compared with the traditional isolation strategy, this strategy with the simpler operation and higher experimental efficiency could be an effective tool for the rapid screening of lead compounds with specific skeletons.

Introduction of Cysteines in the Stalk Domain of Recombinant Influenza Virus N1 Neuraminidase Enhances Protein Stability and Immunogenicity in Mice

Influenza virus surface glycoproteins represent the main targets of the immune system during infection and vaccination. Current influenza virus vaccines rely mostly on the hemagglutinin, requiring a close match between the vaccine and circulating strains. Recently, the neuraminidase (NA) has become an attractive target; however low immunogenicity and stability in vaccine preparations remain an obstacles. Here, we took advantage of the hypervariable stalk domain of the NA to introduce cysteines at different positions and to produce more stable multimeric forms of the protein. We generated 11 N1 constructs and characterized the proteins by performing sodium dodecyl sulfate polyacrylamide gel electrophoresis and by testing their enzymatic activity and representation of antigenic epitopes. Moreover, we evaluated their potential to induce a protective immune response in vivo and characterized the polyclonal antibody responses of immunized mice. We observed that the introduction of cysteines at certain positions led to the formation of stable N1 dimers, which are capable of inducing a strong antibody response characterized by neuraminidase inhibiting activity and protection of mice from high dose viral challenge. Overall, our results provide evidence for the feasibility of introducing stalk modifications to enhance the stability and immunogenicity of NA-based recombinant antigens.

Structure of an Influenza A virus N9 neuraminidase with a tetrabrachion-domain stalk

The influenza neuraminidase (NA) is a homotetramer with head, stalk, transmembrane and cytoplasmic regions. The structure of the NA head with a stalk has never been determined. The NA head from an N9 subtype influenza A virus, A/tern/Australia/G70C/1975 (H1N9), was expressed with an artificial stalk derived from the tetrabrachion (TB) tetramerization domain from Staphylothermus marinus. The NA was successfully crystallized both with and without the TB stalk, and the structures were determined to 2.6 and 2.3 Å resolution, respectively. Comparisons of the two NAs with the native N9 NA structure from egg-grown virus showed that the artificial TB stalk maintained the native NA head structure, supporting previous biological observations.

Influenza Virus Neuraminidase Structure and Functions

With the constant threat of emergence of a novel influenza virus pandemic, there must be continued evaluation of the molecular mechanisms that contribute to virulence. Although the influenza A virus surface glycoprotein neuraminidase (NA) has been studied mainly in the context of its role in viral release from cells, accumulating evidence suggests it plays an important, multifunctional role in virus infection and fitness. This review investigates the various structural features of NA, linking these with functional outcomes in viral replication. The contribution of evolving NA activity to viral attachment, entry and release of virions from infected cells, and maintenance of functional balance with the viral hemagglutinin are also discussed. Greater insight into the role of this important antiviral drug target is warranted.

Comparison of the antiviral effect of solid-state copper and silver compounds

Antiviral activities of insoluble solid-state and soluble ionic copper and silver compounds were evaluated against influenza A virus (A/PR8/H1N1) possessing a viral envelope and bacteriophage Qβ lacking an envelope. The viral solutions were exposed on glass samples uniformly loaded with copper and silver compounds. Exposure to solid-state cuprous oxide (Cu2O) efficiently inactivated both influenza A virus and bacteriophage Qβ, whereas solid-state cupric oxide (CuO) and silver sulfide (Ag2S) showed little antiviral activity. Copper ions from copper chloride (CuCl2) had little effect on the activity of bacteriophage Qβ in spite of the fact that copper ions strongly inactivate influenza A in previous studies. Silver ions from silver nitrate (AgNO3) and silver(I) oxide (Ag2O) in solution showed strong inactivation of influenza A and weak inactivation of bacteriophage Qβ. We also investigated the influence of the compounds on the function of two influenza viral proteins, hemagglutinin and neuraminidase. Silver ions from AgNO3 and Ag2O remarkably decreased enzymatic activity of neuraminidase through the breakage of disulfide (SS) bonds, corresponding to the selective inactivation of influenza A virus. By contrast, exposure to Cu2O markedly reduced the activity of hemagglutinin rather than neuraminidase. These findings suggest that solid-state Cu2O disrupts host cell recognition by denaturing protein structures on viral surfaces, leading to the inactivation of viruses regardless of the presence of a viral envelope.

Surface modifications of influenza proteins upon virus inactivation by β-propiolactone

Inactivation of intact influenza viruses using formaldehyde or β-propiolactone (BPL) is essential for vaccine production and safety. The extent of chemical modifications of such reagents on viral proteins needs to be extensively investigated to better control the reactions and quality of vaccines. We have evaluated the effect of BPL inactivation on two candidate re-assortant vaccines (NIBRG-121xp and NYMC-X181A) derived from A/California/07/2009 pandemic influenza viruses using high-resolution FT-ICR MS-based proteomic approaches. We report here an ultra performance LC MS/MS method for determining full-length protein sequences of hemagglutinin and neuraminidase through protein delipidation, various enzymatic digestions, and subsequent mass spectrometric analyses of the proteolytic peptides. We also demonstrate the ability to reliably identify hundreds of unique sites modified by propiolactone on the surface of glycoprotein antigens. The location of these modifications correlated with changes to protein folding, conformation, and stability, but demonstrated no effect on protein disulfide linkages. In some cases, these modifications resulted in suppression of protein function, an effect that correlated with the degree of change of the modified amino acids' side chain length and polarity.

Mapping the sequence mutations of the 2009 H1N1 influenza A virus neuraminidase relative to drug and antibody binding sites

In this work, we study the consequences of sequence variations of the "2009 H1N1" (swine or Mexican flu) influenza A virus strain neuraminidase for drug treatment and vaccination. We find that it is phylogenetically more closely related to European H1N1 swine flu and H5N1 avian flu rather than to the H1N1 counterparts in the Americas. Homology-based 3D structure modeling reveals that the novel mutations are preferentially located at the protein surface and do not interfere with the active site. The latter is the binding cavity for 3 currently used neuraminidase inhibitors: oseltamivir (Tamiflu^®), zanamivir (Relenza^®) and peramivir; thus, the drugs should remain effective for treatment. However, the antigenic regions of the neuraminidase relevant for vaccine development, serological typing and passive antibody treatment can differ from those of previous strains and already vary among patients.

This article was reviewed by Sandor Pongor and L. Aravind.

An influenza A vaccine based on tetrameric ectodomain of matrix protein 2

Matrix protein 2 (M2) of influenza A is a tetrameric type III membrane protein that functions as a proton-selective channel. The extracellular domain (M2e) has remained nearly invariable since the first human influenza strain was isolated in 1933. By linking a modified form of the leucine zipper of the yeast transcription factor GCN4 to M2e, we obtained a recombinant tetrameric protein, M2e-tGCN4. This protein mimics the quaternary structure of the ectodomain of the natural M2 protein. M2e-tGCN4 was purified, biochemically characterized, and used to immunize BALB/c mice. High M2e-specific serum IgG antibody titers were obtained following either intraperitoneal or intranasal administration. Immunized mice were protected fully against a potentially lethal influenza A virus challenge. Antibodies raised by M2e-tGCN4 immunization specifically bound to the surface of influenza-infected cells and to an M2-expressing cell line. Using a M2e peptide competition enzyme-linked immunosorbent assay with M2-expressing cells as target, we obtained evidence that M2e-tGCN4 induces antibodies that are specific for the native tetrameric M2 ectodomain. Therefore, fusion of an oligomerization domain to the extracellular part of a transmembrane protein allows it to mimic the natural quaternary structure and can promote the induction of oligomer-specific antibodies.

Recombinant neuraminidase vaccine protects against lethal influenza

The N2 neuraminidase gene of A/Victoria/3/75 influenza virus was engineered to encode a secretable protein (NAs) by replacing the natural N-terminal membrane anchor sequence with the cleavable signal sequence of the corresponding influenza hemagglutinin gene. Soluble NAs was expressed by a baculovirus/insect cell system and accumulated in the medium at levels between 6 and 8 microgram ml-1. A combination of biochemical and standard chromatographic techniques allowed the purification of NAs to homogeneity. Cross-linking analysis indicated that NAs was partly recovered as an authentic tetrameric protein, while the remaining fraction was composed of dimeric molecules and small amounts of monomeric NAs. Purified NAs was supplemented with low-reactogenic adjuvants and used to immunize mice. After a challenge infection with a lethal dose of homologous mouse-adapted X47 influenza virus, vaccinated animals showed resistance against severe disease symptoms and were protected from lethality. Based on the results of a passive immunization experiment, it may be concluded that performed antibody plays a central role in the mechanism by which vaccination with NAs confers viral protection.

The disulphide bridges in a cellobiohydrolase and an endoglucanase from Trichoderma reesei

The positions of the disulphide bridges of the 1,4-beta-glucan cellobiohydrolase (CBH I) of the fungus Trichoderma reesei have been investigated. The results can be summarized as follows. (1) The enzyme contains 12 disulphide bridges and no free cysteine residues. (2) The location of six disulphide bridges have been determined experimentally. (3) The bonding patterns of the two disulphide bridges in the C-terminal region is suggested on the basis of internal homology. (4) The remaining four disulphide bridges are put into two groups, each containing four half-cystine residues where two are adjacent. (5) A repeating bonding pattern is observed along the peptide chain and a non-local disulphide bond with an unusually long separation distance links the N-terminal and the C-terminal region. (6) The disulphide-bonded CNBr peptides of a 1,4-beta-glucan glucanohydrolase (endoglucanase II) from T. reesei have been isolated and a disulphide bonding pattern is suggested on the basis of the sequence homology between the two enzymes.