Studies on the neuraminidase of influenza virus

Discovery and characterization of a novel peptide inhibitor against influenza neuraminidase

Neuraminidase, an abundant glycoprotein on the influenza virus surface, plays crucial roles in virus replication. Targeting neuraminidase could be a splendid way for the prevention of the spread of influenza infections. Herein, we have identified an octapeptide (errKPAQP) from a synthesized peptide library, originating from mimicking the binding pocket of oseltamivir in neuraminidase, as a potent peptide neuraminidase inhibitor. The docking-based virtual studies showed that errKPAQP exhibited a strong binding affinity (a docking score of -20.03) and nanomolar affinity (11 nM) to influenza neuraminidase, and can inhibit neuraminidase activity at a concentration as low as 4.25 μM, leading to effective protection of MDCK cells from influenza virus-induced death and replication. Furthermore, errKPAQP presented low hemolytic activity, minimal cytotoxicity, and good pharmacokinetic characteristics, which are imperative for an anti-influenza drug. Importantly, errKPAQP was capable of reducing influenza virus-induced inflammation, the serious damage to the lung tissues, and mortality rates in infected mice, indicating that it could protect against the lethal challenge of influenza viruses in vivo. Therefore, we have developed a novel neuraminidase peptide inhibitor with advantageous biological properties and high inhibitory activity towards neuraminidase, and it can serve as a promising anti-influenza drug.

Antiviral adhesion molecular mechanisms for influenza: W. G. Laver's lifetime obsession

Infection by the influenza virus depends firstly on cell adhesion via the sialic-acid-binding viral surface protein, haemagglutinin, and secondly on the successful escape of progeny viruses from the host cell to enable the virus to spread to other cells. To achieve the latter, influenza uses another glycoprotein, the enzyme neuraminidase (NA), to cleave the sialic acid receptors from the surface of the original host cell. This paper traces the development of anti-influenza drugs, from the initial suggestion by MacFarlane Burnet in 1948 that an effective 'competitive poison' of the virus' NA might be useful in controlling infection by the virus, through to the determination of the structure of NA by X-ray crystallography and the realization of Burnet's idea with the design of NA inhibitors. A focus is the contribution of the late William Graeme Laver, FRS, to this research.

Investigation of the binding and cleavage characteristics of N1 neuraminidases from avian, seasonal, and pandemic influenza viruses using saturation transfer difference nuclear magnetic resonance

Objectives

The main function of influenza neuraminidase (NA) involves enzymatic cleavage of sialic acid from the surface of host cells resulting in the release of the newly produced virions from infected cells, as well as aiding the movement of virions through sialylated mucus present in the respiratory tract. However, there has previously been little information on the binding affinity of different forms of sialylated glycan with NA. Our objectives were then to investigate both sialic acid binding and cleavage of neuraminidase at an atomic resolution level.

Design

Nuclear magnetic resonance (NMR) spectroscopy was used to investigate pH and temperature effects on binding and cleavage as well as to interrogate the selectivity of human-like or avian-like receptors for influenza neuraminidase N1 derived from a range of different influenza virus strains including human seasonal H1N1, H1N1pdm09 and avian H5N1.

Results

We demonstrated that an acidic pH and physiological temperature are required for efficient NA enzymatic activity; however a change in the pH had a minimum effect on the NA-sialic acid binding affinity. Our data comparing α-2,3- and α-2,6-sialyllactose indicated that the variation in neuraminidase activity on different ligands correlated with a change in binding affinity. Epitope mapping of the sialylglycans interacting with NAs from different viral origin showed different binding profiles suggesting that different binding conformations were adopted.

Conclusions

The data presented in this study demonstrated that physicochemical conditions (pH in particular) could affect the NA enzymatic activity with minor effect on ligand binding. NA cleavage specificity seemed to be associated with a difference in binding affinity to different ligands, suggesting a relationship between the two events. These findings have implications regarding the replication cycle of influenza infection in the host where different sialidase activities would influence penetration through the respiratory mucin barrier and the release of the newly generated virus from the infected cells.

Antiviral drugs for influenza: Tamiflu® past, present and future

Two antiviral drugs that are currently available for the treatment of influenza are effective against all strains of the virus, if used correctly. These are the neuraminidase inhibitors, zanamivir (Relenza®) and oseltamivir (Tamiflu®). These drugs are the result of basic research performed over a 60-year period by many people around the world. They were deliberately synthesized from a knowledge of the x-ray crystal structure of influenza virus neuraminidase. This article provides a brief historical account of some of the scientific events that lead to their creation.

Pandemic influenza: its origin and control

A "new" influenza virus will appear at some time in the future. This virus will arise by natural processes, which we do not fully understand, or it might be created by some bioterrorist. The world's population will have no immunity to the new virus, which will spread like wild-fire, causing much misery, economic disruption and many deaths. Vaccines will take time to develop and the only means of control, at least in the early stages of the epidemic, are anti-viral drugs, of which the neuraminidase inhibitors currently seem the most effective.

Effect of Ca++ on the stability of influenza virus neuraminidase

The neuraminidases of different strains of influenza virus varied in their stability at 37 degrees C. The enzymes of the strains with N1 neuraminidases were found to be unstable during incubation at 37 degrees C whereas the enzymes of the strains with the N2 neuraminidases were stable. Among the strains with N2 neuraminidases, the enzymes of some strains were inactivated during dialysis at 37 degrees C whereas the enzymes of others were stable. This observed loss of enzyme activity during dialysis at 37 degrees C was not restricted to a single substrate as the same loss of enzyme activity was observed irrespective of the size of the substrate used in the assay. The enzymically inactive neuraminidase was found to be non-antigenic and non-immunogenic. The inactivation of the enzyme could be prevented by the addition of Ca++ but not Mg++. Out results suggest that Ca++ is essential for the stability of the enzyme at 37 degrees C. The results would also suggest that the enzymic, antigenic and immunogenic sites are either the same or very closely situated on the surface of the neuraminidase molecule.

The chemical reactions of the haemagglutinins and neuraminidases of different strains of influenza viruses. 3. Effects of proteolytic enzymes

The action of trypsin and pronase on the haemagglutinins and neuraminidases of eight strains of influenza virus has been examined.The haemagglutinins of all the strains were highly susceptible to digestion by pronase but there were great variations in resistance to trypsin.The neuraminidases of the eight strains were of three types. The neuraminidases of the A 1 strains and the DSP strain of virus A were highly susceptible to destruction by both enzymes. The neuraminidases of the PR 8 and Swine strains showed partial resistance especially to trypsin, while the A 2 strains and the LEE strains of virus B possessed neuraminidases that were completely resistant to both trypsin and pronase.Proteolytic enzymes released free neuraminidases from the A 2 and LEE viruses the morphology of which was different from that of neuraminidases released by detergent treatment.

Isolation of hemagglutinin and neuraminidase subunits of hemagglutinating virus of Japan

When purified hemagglutinating virus of Japan (HVJ) was treated with trypsin, two major surface antigens were released from the virus. The "hemagglutinin" subunits obtained by this method were reactive with homologous hemagglutination-inhibition antibody and could be detected by an antibody-blocking test. They adsorbed to but did not agglutinate red cells and thus appeared to be "monovalent." The neuraminidase subunits were obtained in fully active form and did not adsorb to red cells. This finding suggests that these two activities of HVJ are associated with different subunits of the virus particle. The hemagglutinin and neuraminidase subunits could be partially separated by zonal rate centrifugation or gel filtration on Sephadex G-200. The molecular weights estimated for these subunits were approximately 124,000 and 114,000, respectively. After treatment with trypsin, virus-associated hemagglutinin and neuraminidase activities were both reduced significantly. The electron micrographs of such trypsinized virus particles showed complete or partial loss of surface projections. These results suggested that the subunits obtained by this method seemed to be those projections liberated from the virus by the action of trypsin.

Developmental sequence and intracellular sites of synthesis of three structural protein antigens of influenza A2 virus

Specific antisera for hemagglutinin (HA) and neuraminidase antigens of influenza A(2) virus (A(2)E) were produced through the segregation of the two proteins in reciprocal viral recombinants of A(2)E and A(0)e viruses. Gamma globulin fractions of these specific antisera and of antiserum specific for the nucleoprotein (NP) antigen of A(0)e virus were conjugated with fluorescein isothiocyanate and employed to follow the synthesis of the three structural proteins in clone 1-5C-4 human aneuploid cells, with parallel measurement of serological and biological activity of the antigens by other techniques. In this system, NP antigen appeared first (at 3 hr) in the cell nucleus, whereas HA and neuraminidase appeared coincidentally, at 4 hr after infection, in the cytoplasm. The initial detectability of biological or complement-fixing activity of the proteins coincided with their demonstrability as stainable antigens. Late in infection, all three antigens were detected at the cell surface. Antibody specific for HA partially blocked the intracellular staining of neuraminidase and inhibited the enzymatic activity of both extracted and intact extracellular virus. These observations suggest the close intracytoplasmic proximity of the two envelope antigens and perhaps their initial association in a larger protein.

The chemical reactions of the haemagglutinins and neuraminidases of different strains of influenza viruses. II. Effects of reagents modifying the higher order structure of the protein molecule

The results of treatment of influenza virus strains with chemical reagents acting on the higher-order structure of protein molecules shows that both the haemagglutinating and enzymic activities are susceptible to these agents but there are considerable differences between the different strains and the neuraminidase activity is more sensitive than the haemagglutinating activity.

The neuraminidase activity of A and A1strains is destroyed by urea, guanidine, urea+dithiothreitol and mercuric chloride. The haemagglutinin of the PR 8 and SWINE strains is resistant to urea and mercuric chloride but destroyed by guanidine and by urea+dithiothreitol. The haemagglutinin of the DSP strain of virus A and the A1strains is resistant to urea, guanidine and mercuric chloride but is destroyed by urea+dithiothreitol.

The neuraminidase activity of the A2strains is more resistant than that of the A and A1strains. It is resistant to mercuric chloride and partially resistant to urea but is destroyed by guanidine and by urea+dithiothreitol. The A2haemagglutinin is resistant to urea, urea+dithiothreitol, and mercuric chloride but is destroyed by guanidine.

The LEE virus neuraminidase is resistant to urea and partially resistant to guanidine but is destroyed by urea+dithiothreitol and mercuric chloride. The LEE haemagglutinin is resistant to urea, guanidine and mercuric chloride but is destroyed by urea+dithiothreitol.

It is suggested that the surface projections of the virus particle are protein polymers each made up of three or four monomers which are the components of the V antigen complex. Antigenic activity is a function of the primary or secondary structure of the monomers, haemagglutinin activity is a function of the tertiary structure of the monomers, while neuraminidase activity is a function of the quaternary structure of the polymer.

From studies of the chemical reactions of their haemagglutinins and neuraminidases strains of influenza virus A can be classified into groups. These groups are very similar to but not precisely identical with groupings made by serological methods.

Antiviral activity of antiserum specific for an influenza virus neuraminidase

Antiserum specific for influenza A(2) neuraminidase was produced by immunization of rabbits with the purified enzyme which had been isolated by electrophoresis from the proteins of a detergent-disrupted A(0)A(2) influenza virus recombinant [X-7 (F1)]. This recombinant contained hemagglutinin of the A(0) subtype and A(2) neuraminidase. Antiserum to the isolated A(2) neuraminidase did not react in any of four serological tests with A(0) or A(2) subtype viruses that lacked the A(2) enzyme. In contrast, the antiserum inhibited the neuraminidase activity only of wild-type and recombinant viruses containing the A(2) enzyme, regardless of the nature of their hemagglutinin proteins. The antiserum caused hemagglutination-inhibition of some, but not all, viruses bearing the A(2) enzyme, and it reduced the plaque size or plaque number of all viruses tested that contained A(2) neuraminidase. In the chick embryo and in cell culture, low dilutions of antiserum reduced the yield of virus. True neutralization of virus in the chick embryo did not occur. We conclude that an antiserum specific for A(2) neuraminidase influenced the yield and release of virus from influenza virus-infected cells.

Neuraminidases and Influenza Virus Infection in Embryonated Eggs

Highly purified neuraminidase from influenza virus prevents embryonated eggs from infection by Lee-B influenza virus. The degree of prevention afforded is equal to that of neuraminidases from Vibrio cholerae and from Clostridium perfringens when assayed at the same enzyme activity. The preventive effect is much lower against PR-8 virus. The results support the concept that the intact "receptor" containing sialic acid is required for the binding and infection of cells by influenza viruses.