LAUSR

Infection of the CNS is a serious complication of human cases of AIV infection. The viral and host factors associated with neurovirulence of AIV infection are not well understood. ABSTRACT Avian influenza viruses (AIVs) are zoonotic viruses that exhibit a range of infectivity and severity in the human host. Severe human cases of AIV infection are often accompanied by neurological symptoms; however, the factors involved in the infection of the central nervous system (CNS) are not well known. In this study, we discovered that the avian-like sialic acid (SA)–α2,3-galactose (α2,3-Gal) receptor is highly presented in mammalian (human and mouse) brains. In the generation of a mouse-adapted neurotropic H9N2 AIV (SD16-MA virus) in BALB/c mice, we identified key adaptive mutations in its hemagglutinin (HA) and polymerase basic protein 2 (PB2) genes that conferred viral replication ability in the mouse brain. The SD16-MA virus showed binding affinity for the avian-like SA–α2,3-Gal receptor, enhanced viral RNP polymerase activity, and increased viral protein production and transport that culminated in elevated progeny virus production and severe pathogenicity. We further established that host fragile X mental retardation protein (FMRP), a highly expressed protein in the brain that is physically associated with viral nucleocapsid protein (NP) to facilitate RNP assembly and export, was an essential host factor for the neuronal replication of neurotropic AIVs (H9N2, H5N1, and H10N7 viruses). Our study identified a mechanistic process for AIVs to acquire neurovirulence in mice. IMPORTANCE Infection of the CNS is a serious complication of human cases of AIV infections. The viral and host factors associated with the neurovirulence of AIV infections are not well understood. We identified and functionally characterized specific changes in the viral HA and PB2 genes of a mouse-adapted neurotropic avian H9N2 virus responsible for enhanced virus replication in neuronal cells and pathogenicity in mice. Importantly, we showed that host FMRP was a crucial host factor that was necessary for neurotropic AIVs (H9N2, H5N1, and H10N7 viruses) to replicate in neuronal cells. Our findings provide insights into the pathogenesis of the neurovirulence of AIV infections.