Molecular pathogenesis of H5 highly pathogenic avian influenza: the role of the haemagglutinin cleavage site motif

The emergence of H5N1 highly pathogenic avian influenza has caused a heavy socio-economic burden through culling of poultry to minimise human and livestock infection. Although human infections with H5N1 have to date been limited, concerns for the pandemic potential of this zoonotic virus have been greatly intensified following experimental evidence of aerosol transmission of H5N1 viruses in a mammalian infection model. In this review, we discuss the dominance of the haemagglutinin cleavage site motif as a pathogenicity determinant, the host-pathogen molecular interactions driving cleavage activation, reverse genetics manipulations and identification of residues key to haemagglutinin cleavage site functionality and the mechanisms of cell and tissue damage during H5N1 infection. We specifically focus on the disease in chickens, as it is in this species that high pathogenicity frequently evolves and from which transmission to the human population occurs. With >75% of emerging infectious diseases being of zoonotic origin, it is necessary to understand pathogenesis in the primary host to explain spillover events into the human population

Letter: Reduction of nitrate and nitrite in water by immobilized enzymes

NITRATE, a common and serious contaminant of ground water, is removed at present either by physicochemical methods that do not degrade it, or via degradation by microorganisms, which is a slow process1. We report here a rapid and efficient process for nitrate removal which involves catalytic reduction by immobilized enzymes. The reduction is driven by an electrical current, and results in complete conversion of nitrate to N2 without residues. Our electro-bioreactor was constructed by co-immobilizing the enzymes (purified NADH: nitrate reductase from Zea mays2 and crude nitrite reductase and N2O reductase from Rhodop-seudomonas3) with electron-carrying dyes in a polymer matrix, which was then attached in thin layers to the surface of the cathode. Nitrate-laden water is pumped past the anode and through the active matrix on the cathode while a low voltage is applied, resulting in two-stage nitrate reduction to N2 via nitrite. The enzyme activity is higher in the co-immobilized state than in free solution. In principle, such electro-bioreactors could be developed for removal of other water contaminants such as pesticides, if appropriate enzymes and cofactors can be identified. © 1992 Nature Publishing Group

Evolution of high pathogenicity of H5 avian influenza virus: haemagglutinin cleavage site selection of reverse-genetics mutants during passage in chickens

Low pathogenicity avian influenza viruses (LPAIVs) are generally asymptomatic in their natural avian hosts. LPAIVs can evolve into highly pathogenic forms, which can affect avian and human populations with devastating consequences. The switch to highly pathogenic avian influenza virus (HPAIV) from LPAIV precursors requires the acquisition of multiple basic amino acids in the haemagglutinin cleavage site (HACS) motif. Through reverse genetics of an H5N1 HPAIV, and experimental infection of chickens, we determined that viruses containing five or more basic amino acids in the HACS motif were preferentially selected over those with three to four basic amino acids, leading to rapid replacement with virus types containing extended HACS motifs. Conversely, viruses harbouring low pathogenicity motifs containing two basic amino acids did not readily evolve to extended forms, suggesting that a single insertion of a basic amino acid into the cleavage site motif of low-pathogenic viruses may lead to escalating selection for extended motifs. Our results may explain why mid-length forms are rarely detected in nature. The stability of the short motif suggests that pathogenicity switching may require specific conditions of intense selection pressure (such as with high host density) to boost selection of the initial mid-length HACS forms