Modulation of an ectodomain motif in the influenza A virus neuraminidase alters tetherin sensitivity and results in virus attenuation in vivo

We previously demonstrated that ectodomain residue Asp286 in N2 neuraminidase (NA; Asp268 in N1 NA) present in budding-capable NA proteins contributes to productive NA plasma membrane transport partly by mediating escape from tetherin restriction [Yondola MA, Fernandes F, Belicha-Villanueva A, Uccelini M, Gao Q, Carter C, et al. (2011). Budding capability of the influenza virus neuraminidase can be modulated by tetherin. J Virol, 85, 2480-2491]. Budding-incapable NA proteins contain a G at this position and either co-expression of human immunodeficiency virus type 1 vpu or siRNA-mediated depletion of tetherin rescued budding capabilities in these proteins [Yondola MA, Fernandes F, Belicha-Villanueva A, Uccelini M, Gao Q, Carter C, et al. (2011). Budding capability of the influenza virus neuraminidase can be modulated by tetherin. J Virol, 85, 2480-2491]. Furthermore, replacement of D286 with G in budding-capable NA proteins caused loss of function, preventing release of NA virus-like particles (VLPs). Here, we show that mutation of this residue specifically modulates the ability of NA to escape tetherin restriction at the plasma membrane and results in virus attenuation in vivo. Based on immunogold electron microscopy and co-immunoprecipitation assays, both NA _D286-containing and NA_D286G-containing proteins associated with tetherin in the endoplasmic reticulum (ER). However, the NA _D286G loss-of-function mutant also associated with the host factor outside the ER and in plasma-membrane-localized VLPs as visualized using immunogold electron microscopy. We conclude that the presence of aspartate at residue 286 liberates NA from tetherin-dependent restriction upon exit from the ER compartment thus preventing restriction at the plasma membrane. Underscoring the importance of these observations, knockdown of tetherin resulted in a 1-1.5 log increase in influenza virus growth. Additionally, the loss-of-function mutation conferred attenuation in a mouse model of influenza infection as evidenced by a 5-fold increase in LD₅₀ and increases in either percent survival or time to death dependent on the administered dose in vivo.