Extraction of catalytically active neuraminidase of H5N1 influenza virus using thrombin proteolytic cleavage

Investigating proteins and proteases composing amniotic and allantoic fluids during chicken embryonic development

In amniotes, the amniotic fluid is a significant contributor to fetal development and health. While numerous studies have been conducted in mammalian amniotic fluid, the composition of amniotic and other extraembryonic fluids in avian egg along with their physiological functions remain largely unexplored. In such a context, our objective was to characterize the chicken amniotic fluid (AmF) and allantoic fluid (AlF) properties, protein composition, and some associated functions from day 8 to day 16 of incubation. SDS-PAGE combined to mass spectrometry analysis revealed common and specific proteins to each fluid, suggesting distinct properties and functions. Indeed, major AlF proteins are mostly "egg yolk" proteins involved in lipid, vitamin metabolisms, and metal ion transport, while major AmF proteins resemble those of albumen. Drastic changes in the AmF protein profiles were observed during incubation, when the albumen transfers from day 12 onwards, while few changes were detected for the AlF protein profile. The decreases in osmolality (from 231 to 183 mOsm/kg) and pH (from 8.26 to 7.26) observed in the AlF during incubation are associated with water and electrolytes reallocation for the embryo needs. In contrast, AmF pH value remained stable (≈7.5). Active proteolytic enzymes have been identified in the 2 fluids using gelatin zymography, followed by mass spectrometry analysis for protease identification. A total of 12 proteases was detected in the AlF, compared to 5 in the AmF. We have shown that AlF concentrates proteolytic enzymes assumed to participate in digestive processes: aminopeptidase N, dipeptidyl peptidase-4, meprin A, and 72 kDa type IV collagenase preproprotein. The other proteases identified in both fluids also could have a role in morphogenesis (hepatocyte growth factor activator, suppressor of tumorigenicity 14, astacin-like metalloendopeptidase) and hemostasis (prothrombin and coagulation factor X). Altogether, these data suggest that the roles of chicken AlF and AmF are not merely associated with protection of the embryo and regulation of metabolic disposable wastes, but also they could have more sophisticated roles during embryonic development.

Substrate specificity of avian influenza H5N1 neuraminidase

AIM: To characterise neuraminidase (NA) substrate specificity of avian influenza H5N1 strains from humans and birds comparing to seasonal influenza virus.

METHODS: Avian influenza H5N1 strains from humans and birds were recruited for characterising their NA substrate specificity by using a modified commercial fluorescence Amplex Red assay. This method can identify the preference of α2,6-linked sialic acid or α2,3-linked sialic acid. Moreover, to avoid the bias of input virus, reverse genetic virus using NA gene from human isolated H5N1 were generated and used to compare with the seasonal influenza virus. Lastly, the substrate specificity profile was further confirmed by high-performance liquid chromatography (HPLC) analysis of the enzymatic product.

RESULTS: The H5N1 NA showed higher activity on α2,3-linked sialic acid than α2,6-linked (P < 0.0001). To compare the NA activity between the H5N1 and seasonal influenza viruses, reverse genetic viruses carrying the NA of H5N1 viruses and NA from a seasonal H3N2 virus was generated. In these reverse genetic viruses, the NA activity of the H5N1 showed markedly higher activity against α2,3-linked sialic acid than that of the H3N2 virus, whereas the activities on α2,6-linkage were comparable. Interestingly, NA from an H5N1 human isolate that was previously shown to have heamagglutinin (HA) with dual specificity showed reduced activity on α2,3-linkage. To confirm the substrate specificity profile, HPLC analytic of enzymatic product was performed. Similar to Amplex red assay, H5N1 virus showed abundant preference on α2,3-linked sialic acid.

CONCLUSION: H5N1 virus maintains the avian specific NA and NA changes may be needed to accompany changes in HA receptor preference for the viral adaptation to humans.

Inhibition of influenza A virus replication by influenza B virus nucleoprotein: an insight into interference between influenza A and B viruses

Given that co-infection of cells with equivalent titers of influenza A and B viruses (FluA and FluB) has been shown to result in suppression of FluA growth, it is possible that FluB-specific proteins might hinder FluA polymerase activity and replication. We addressed this possibility by individually determining the effect of each gene of FluB on the FluA polymerase assay and found that the nucleoprotein of FluB (NP(FluB)) inhibits polymerase activity of FluA in a dose-dependent manner. Mutational analyses of NP(FluB) suggest that functional NP(FluB) is necessary for this inhibition. Slower growth of FluA was also observed in MDCK cells stably expressing NP(FluB). Further analysis of NP(FluB) indicated that it does not affect nuclear import of NP(FluA). Taken together, these findings suggest a novel role of NP(FluB) in inhibiting replication of FluA, providing more insights into the mechanism of interference between FluA and FluB and the lack of reassortants between them.

Enhanced expression of secretable influenza virus neuraminidase in suspension mammalian cells by influenza virus nonstructural protein 1

Influenza neuraminidase (NA) is a major target for anti-influenza drugs. With an increasing number of viruses resistant to the anti-NA drug oseltamivir, functionally active recombinant NA is needed for screening novel anti-NA compounds. In this study, the secretable NA (sNA) head domain of influenza A/Vietnam/DT-036/05 (H5N1) virus was expressed successfully in human embryonic kidney (HEK-293T) cells and shown to be enzymatically active. The inclusion of a plasmid encoding nonstructural protein 1 (NS1) of influenza A/Puerto Rico/8/34 virus with the sNA plasmid in the cotransfection demonstrated an increase in H5N1 sNA expression by 7.4 fold. Subsequently, the sNA/NS1 cotransfection protocol in serum-free 293-F suspension cell culture was optimized to develop a rapid transient gene expression (TGE) system for expression of large amounts of H5N1 sNA. Under optimized conditions, NS1 enhanced H5N1 sNA expression by 4.2 fold. The resulting H5N1 sNA displayed comparable molecular weight, glycosylation, K(m) for MUNANA, and K(i) for oseltamivir carboxylate to those of H5N1 NA on the virus surface. Taken together, the NS1-enhancing sNA expression strategy presented in this study could be used for rapid high-level expression of enzymatically active H5N1 sNA in suspension mammalian cells. This strategy may be applied for expression of sNA of other strains of influenza virus as well as the other recombinant proteins.