Expression of 9-O- and 7,9-O-Acetyl Modified Sialic Acid in Cells and Their Effects on Influenza Viruses

Barnard, Karen N.; Wasik, Brian R.; LaClair, Justin R.; Buchholz, David W.; Weichert, Wendy S.; Alford-Lawrence, Brynn K.; Aguilar, Hector C.; Parrish, Colin R.

Effects of different Sia modifications have also been suggested for the binding of pathogens or on the activities of their sialidases (neuraminidases). However, in general these are still not well documented, with the exception of those that use the modified forms as receptors. Influenza A virus (IAV), IBV, ICV, and IDV use Sia as their primary receptors for host recognition and cell entry, but with different effects of Sia modification. IAV and IBV interact with Sia through two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). For both IAV and IBV, HA binds Sia to initiate the endocytic uptake of the virus by the cell, leading to fusion between the viral envelope and the endosomal membrane at low pH (24, 25) . NA is a sialidase which cleaves Sia off glycan chains when it is present in mucus or on the surface of cells, allowing the virus to penetrate to the epithelial cells and also preventing aggregation of newly produced virus after budding (26, 27) . In contrast to IAV and IBV, ICV and IDV have one surface glycoprotein, the hemagglutinin-esterase fusion protein (HEF), which serves similar purposes to both HA and NA. HEF binds specifically to 9-O-acetyl Sia to initiate uptake of the virus into cells, while the esterase domain removes 9-O-acetyl modifications, releasing the virus from mucus and disassembling virus aggregates after budding (28) (29) (30) (31) . While the role of O-acetyl modified Sia for ICV and IDV infections are well documented, how these modifications affect IAV is not well characterized. In addition, although previous preliminary studies have suggested that the presence of 9-O-Ac on cells may be inhibitory for both NA activity and HA binding of IAV (32, 33) , the details are unclear.