Influenza Neuraminidase Characteristics and Potential as a Vaccine Target.

Neuraminidase of influenza A and B viruses plays a critical role in the virus life cycle and is an important target of the host immune system. Here, we highlight the current understanding of influenza neuraminidase structure, function, antigenicity, immunogenicity, and immune protective potential. Neuraminidase inhibiting antibodies have been recognized as correlates of protection against disease caused by natural or experimental influenza A virus infection in humans. In the past years, we have witnessed an increasing interest in the use of influenza neuraminidase to improve the protective potential of currently used influenza vaccines. A number of well-characterized influenza neuraminidase-specific monoclonal antibodies have been described recently, most of which can protect in experimental challenge models by inhibiting the neuraminidase activity or by Fc receptor-dependent mechanisms. The relative instability of the neuraminidase poses a challenge for protein-based antigen design. We critically review the different solutions that have been proposed to solve this problem, ranging from the inclusion of stabilizing heterologous tetramerizing zippers to the introduction of inter-protomer stabilizing mutations. Computationally engineered neuraminidase antigens have been generated that offer broad, within subtype protection in animal challenge models. We also provide an overview of modern vaccine technology platforms that are compatible with the induction of robust neuraminidase-specific immune responses. In the near future, we will likely see the implementation of influenza vaccines that confront the influenza virus with a double punch: targeting both the hemagglutinin and the neuraminidase.

Comparative thermostability analysis of zoonotic and human influenza virus A and B neuraminidase.

Neuraminidase (NA) thermostability of influenza A and B viruses isolated from birds, swine and humans was measured to evaluate its variability associated with host body temperature. The highest 50% inactivation temperature (IT50) was observed with H3N8 avian influenza virus (74 °C), and the lowest IT50 was observed with the seasonal human H3N2 virus (45.5 °C). The IT50 values of A(H1N1)pdm09 viruses 56.4-58.5 °C were statistically higher than that of the prepandemic strain A/Solomon Islands/03/06 (52.5 °C). An analysis of Ca2+ binding sites revealed the correspondence of amino acid changes to NA thermostability. This study demonstrates that changes in NA thermostability correspond to differences in host body temperature.

Design, synthesis and biological evaluation of "Multi-Site"-binding influenza virus neuraminidase inhibitors.

Encouraged by our earlier discovery of neuraminidase inhibitors targeting 150-cavity or 430-cavity, herein, to yield more potent inhibitors, we designed, synthesized, and biologically evaluated a series of novel oseltamivir derivatives via modification of C-1 and C5-NH2 of oseltamivir by exploiting 150-cavity and/or 430-cavity. Among the synthesized compounds, compound 15e, the most potent N1-selective inhibitor targeting 150-cavity, showed 1.5 and 1.8 times greater activity than oseltamivir carboxylate (OSC) against N1 (H5N1) and N1 (H5N1-H274Y). In cellular assays, 15e also exhibited greater potency than OSC against H5N1 with EC50 of 0.66 µM. In addition, 15e demonstrated low cytotoxicity in vitro and low acute toxicity in mice. Molecular docking studies provided insights into the high potency of 15e against N1 and N1-H274Y mutant NA. Overall, we envisioned that the significant breakthrough in the discovery of potent group-1-specific neuraminidase inhibitors may lead to further investigation of more potent anti-influenza agents.

The medicinal potential of natural products for the development of anti- influenza agents.

Influenza neuraminidase (NA) is an important target for designing anti-influenza drugs. By now, three inhibitors, zanamivir, oseltamivir and peramivir have been approved. However, in recent years, the potential threat of influenza pandemics and constant emergence of new drug-resistant influenza virus strains have weaken the defensive role of the current anti-influenza drugs. From another point of view, in this review we focused on some novel NA inhibitors which were mainly derived from natural products that had a variety of structural scaffolds, such as flavonoids, xanthones and diarylheptanoids. Besides interfering the function of NA, some of these compounds also can potently inhibit the replication of influenza virus. It is hoped that these compounds could be the source of leads and provide a guide for discovering new potent anti-influenza virus agents.

[Expression of NA of influenza virus and C3d fusion gene in replication-defective recombinant adenovirus and its immune efficacy analysis].

OBJECTIVE: To construct a replication-defective recombinant adenovirus expressing the fusion gene of neuraminidase (NA) gene in influenza virus A/FM/1/47 and C3d and to evaluate the induced immune efficacy. METHODS: NA-C3d was cloned into shutter vector pAdTrack-CMV, which was cotransformated with adenovirus DNA into E. coli BJ5183. The recombinant adenovirus genomic DNA was generated through homological recombination. The recombinant adenovirus was produced by transfecting 293 cell line with the genomic DNA and the induced immune efficacy in mice were analyzed. RESULTS: The integration of NA-C3d in the adenovirus genomic DNA and its expression were confirmed by PCR and Western-Blot assays respectively. After intranasal immunization, the serum IgG was induced at a titer of 1: 1000 and 1:100 000 in BALB/c mice at primary and secondary immunization respectively. The vaccinated mice were completely survived when challenged with wide influenza virus. CONCLUSION: recombinant adenovirus expressing NA-C3d was successfully constructed and it could induce desired immune efficacy.

[Establishment of drug screening assay and pharmacodynamic evaluation method targeting influenza RNA polymerase].

Influenza virus RNA-dependent RNA polymerase (RdRP) is essential for replication and expression of influenza virus genome. Viral genomic sequences encoding RdRP are highly conservative, thus making it a potential anti-influenza drug target. A cell-based influenza RdRP inhibitor screening assay was established by a luciferase reporter system to analyze the activity of RdRP. Specificity study and statistic analysis showed that the screening assay is sensitive and reproducible.

Development of oseltamivir phosphonate congeners as anti-influenza agents.

Oseltamivir phosphonic acid (tamiphosphor, 3a), its monoethyl ester (3c), guanidino-tamiphosphor (4a), and its monoethyl ester (4c) are potent inhibitors of influenza neuraminidases. They inhibit the replication of influenza viruses, including the oseltamivir-resistant H275Y strain, at low nanomolar to picomolar levels, and significantly protect mice from infection with lethal doses of influenza viruses when orally administered with 1 mg/kg or higher doses. These compounds are stable in simulated gastric fluid, liver microsomes, and human blood and are largely free from binding to plasma proteins. Pharmacokinetic properties of these inhibitors are thoroughly studied in dogs, rats, and mice. The absolute oral bioavailability of these compounds was lower than 12%. No conversion of monoester 4c to phosphonic acid 4a was observed in rats after intravenous administration, but partial conversion of 4c was observed with oral administration. Advanced formulation may be investigated to develop these new anti-influenza agents for better therapeutic use.

Evidence for avian and human host cell factors that affect the activity of influenza virus polymerase.

Typical avian influenza A viruses do not replicate efficiently in humans. The molecular basis of host range restriction and adaptation of avian influenza A viruses to a new host species is still not completely understood. Genetic determinants of host range adaptation have been found on the polymerase complex (PB1, PB2, and PA) as well as on the nucleoprotein (NP). These four viral proteins constitute the minimal set for transcription and replication of influenza viral RNA. It is widely documented that in human cells, avian-derived influenza A viral polymerase is poorly active, but despite extensive study, the reason for this blockade is not known. We monitored the activity of influenza A viral polymerases in heterokaryons formed between avian (DF1) and human (293T) cells. We have discovered that a positive factor present in avian cells enhances the activity of the avian influenza virus polymerase. We found no evidence for the existence of an inhibitory factor for avian virus polymerase in human cells, and we suggest, instead, that the restriction of avian influenza virus polymerases in human cells is the consequence of the absence or the low expression of a compatible positive cofactor. Finally, our results strongly suggest that the well-known adaptative mutation E627K on viral protein PB2 facilitates the ability of a human positive factor to enhance replication of influenza virus in human cells.

Synthesis and in vivo influenza virus-inhibitory effect of ester prodrug of 4-guanidino-7-O-methyl-Neu5Ac2en.

A series of ester prodrugs of 7-O-methyl derivative of Zanamivir (compound 3) was synthesized and their efficacy was evaluated in an influenza infected mice model by intranasal administration. Compound 7c (CS-8958), octanoyl ester prodrug of the C-9 alcohol of compound 3, was found to be much longer-acting than Zanamivir. Furthermore, the in vivo efficacies of compounds 12a, 12b, and 12c, the linear alkyl ester prodrug of the carboxylic acid, were comparable to that exerted by compound 7c.

Influenza virus mRNA translation revisited: is the eIF4E cap-binding factor required for viral mRNA translation?

Influenza virus mRNAs bear a short capped oligonucleotide sequence at their 5' ends derived from the host cell pre-mRNAs by a "cap-snatching" mechanism, followed immediately by a common viral sequence. At their 3' ends, they contain a poly(A) tail. Although cellular and viral mRNAs are structurally similar, influenza virus promotes the selective translation of its mRNAs despite the inhibition of host cell protein synthesis. The viral polymerase performs the cap snatching and binds selectively to the 5' common viral sequence. As viral mRNAs are recognized by their own cap-binding complex, we tested whether viral mRNA translation occurs without the contribution of the eIF4E protein, the cellular factor required for cap-dependent translation. Here, we show that influenza virus infection proceeds normally in different situations of functional impairment of the eIF4E factor. In addition, influenza virus polymerase binds to translation preinitiation complexes, and furthermore, under conditions of decreased eIF4GI association to cap structures, an increase in eIF4GI binding to these structures was found upon influenza virus infection. This is the first report providing evidence that influenza virus mRNA translation proceeds independently of a fully active translation initiation factor (eIF4E). The data reported are in agreement with a role of viral polymerase as a substitute for the eIF4E factor for viral mRNA translation.

[Present data on influenza virus isolated from ducks and chickens, and influenza virus C. Anti-influenza drugs]. / Datos actuales sobre virus de la gripe de patos salvajes y pollos, y virus de la gripe tipo C. Agentes antigripales.

Present data on influenza virus isolated from ducks and chickens, and influenza virus C. Anti-influenza drugs. Within the broad field of Glycopathology and Glycotherapeutics, research on influenza virus types A, B and C from humans and several bird species (particularly migratory birds such as ducks, since they are reservoirs for viruses), as well as the search for improved drugs designed for the prevention or treatment of epidemics/pandemics produced by most of those viruses are issues of relevant interest not only from a scientific point of view but also for repercussions on health and the important economical consequences. The research work begun by the author and collaborators at the Department of Biochemistry and Molecular Biology of the University of Salamanca (Spain) in the middle of the 1970's, developed later in close cooperation with the "(Unité d'Ecologie Virale" of the Pasteur Institute of Paris (Prof. Claude Hannoun and collaborators), has been published in about twenty papers that mainly focus on the theoretic-experimental study of: The sialidase (neuraminidase) activity of human influenza viruses types A and B. The acetylesterase activity of type C virus from humans and dogs. The sialidase activity of type A virus from ducks and pigs, in comparison with that of humans. Certain sialidase inhibitors as useful anti-influenza drugs, especially in the case of possible future influenza pandemics of avian origin.

Structural studies of the resistance of influenza virus neuramindase to inhibitors.

Zanamivir and oseltamivir, specific inhibitors of influenza virus neuraminidase, have significantly different characteristics in resistance studies. In both cases resistance is known to arise through mutations in either the hemagglutinin or neuraminidase surface proteins. A new inhibitor under development by Biocryst Pharmaceuticals, BCX-1812, has both a guanidino group, as in zanamivir, and a bulky hydrophobic group, as in oseltamivir. Using influenza A/NWS/Tern/Australia/G70C/75 (H1N9), neuraminidase variants E119G and R292K have previously been selected by different inhibitors. The sensitivity of these variants to BCX-1812 has now been measured and found in both cases to be intermediate between those of zanamivir and oseltamivir. In addition, the X-ray crystal structures of the complexes of BCX-1812 with the wild type and the two mutant neuraminidases were determined. The ligand is bound in an identical manner in each structure, with a rearrangement of the side chain of E276 from its ligand-free position. A structural explanation of the mechanism of resistance of BCX-1812, relative to zanamivir and oseltamivir in particular, is provided.