Role of neuraminidase in influenza virus-induced apoptosis

The virulent influenza virus clone 7a produced a greater level of apoptosis in MDCK cells compared with the attenuated strain A/Fiji, In both cases, a poptosis could be partially blocked by treatment with three anti-neuraminid ase compounds [4-amino- (GR121158A) and 4-guanidino- (GG167; Zanamivir) 2,3 -dehydro-N-acetylneuraminic acid and 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA)] when they were given to cells during the virus attachment/ent ry phase, but not subsequent to this phase. In contrast, GG167, which does not enter cells, did not affect the numbers of infected cells and, in addit ion, acted late in the infection cycle to inhibit virus yields. Clone 7a ne uraminidase was more active than A/Fiji neuraminidase when fetuin was used as the substrate. Similar differences in activity between the two viruses w ere seen when alpha-2,6 sialyl lactose was used as a substrate, but not wit h alpha-2,3 sialyl lactose. No sequence differences in the enzyme active si te of the two neuraminidases were observed, indicating that differences in neuraminidase specificity and activity may be dictated by other residues. T hese results suggest that neuraminidase plays some role in the induction of apoptosis and that it acts prior to or during virus entry. However, apopto sis was considerably reduced when UV-irradiated virus, which retains > 75% of its neuraminidase activity, was used. In addition, ammonium chloride, us ed to prevent virus entry, reduced virus-induced apoptosis, Amantadine, whi ch inhibits virus uncoating, also inhibited apoptosis induced by the amanta dine-sensitive strain A/Udorn/307/72 (H3N2), but not the amantadine-resista nt clone 7a, Hence, one or more intracellular processes are also involved i n influenza virus-induced apoptosis.