Two surface glycoproteins of influenza virus, haemagglutinin (HA) and 
neuraminidase (NA), play opposite roles in terms of their interaction with host 
sialic acid receptors. HA attaches to sialic acid on host cell surface receptors 
to initiate virus infection while NA removes these sialic acids to facilitate 
release of progeny virions. This functional opposition requires a balance. To 
explore what might happen when NA of an influenza virus was replaced by one from 
another isolate or subtype, in this study, we generated three recombinant 
influenza A viruses in the background of A/PR/8/34 (PR8) (H1N1) and with NA 
genes obtained respectively from the 2009 pandemic H1N1 virus, a highly 
pathogenic avian H5N1 virus, and a lowly pathogenic avian H9N2 virus. These 
recombinant viruses, rPR8-H1N1NA, rPR8-H5N1NA, and rPR8-H9N2NA, were shown to 
have similar growth kinetics in cells and pathogenicity in mice. However, much 
more rPR8-H5N1NA and PR8-wt virions were released from chicken erythrocytes than 
virions of rPR8-H1N1NA and rPR8-H9N2NA after 1 h. In addition, in MDCK cells, 
rPR8-H5N1NA and rPR8-H9N2NA infected a higher percentage of cells, and induced 
cell-cell fusion faster and more extensively than PR8-wt and rPR8-H1N1NA did in 
the early phase of infection. In conclusion, NA replacement in this study did 
not affect virus replication kinetics but had different effects on infection 
initiation, virus release and fusion of infected cells. These phenomena might be 
partially due to NA proteins' different specificity to α2-3/2-6-sialylated 
carbohydrate chains, but the exact mechanism remains to be explored.