Phosphoproteome of SARS-CoV-2 infected Human Nasal Epithelium

The nasal epithelium is the primary initial site of SARS-CoV-2 entry in the human body. Since much of the molecular detail defining coronavirus entry and replication was derived from non-nasal cell lines, it remains unclear how SARS-CoV-2 overcomes the physical nasal mucus and periciliary mucin layers to infect and spread through the nasal epithelium. Using air-liquid interface cultured primary nasal epithelial cells, we observed that SARS-CoV-2 attaches to motile cilia during the initial stage of infection. Depletion of cilia inhibited SARS-CoV-2, as well as respiratory syncytial virus and parainfluenza virus infection, suggesting a widely-used ciliary mechanism for respiratory viral entry. Using electron and immunofluorescence microscopy, we further observed that SARS-CoV-2 progeny virions attached to airway microvilli 24 hours post infection and triggered the formation of apically extended and highly branched microvilli that organize viral egress from the microvillar base back into the mucus layer, supporting a model of virus dispersion throughout airway tissue via mucociliary transport. Chemical perturbation of microvillus formation severely impaired viral egress and subsequent spread. Phosphoproteomic analyses indicate that virally-triggered microvillar branching is linked to the p21-activated kinase 1 and 4 (PAK1/4) signaling pathway and viral infection is impaired by PAK1/4 kinase inhibitors. Our work provides insight into the mechanisms by which SARS-CoV-2 and potentially many respiratory viruses penetrate the physical nasal epithelium barrier, a first line of defense against pathogens, thus revealing a new view of the motile cilia and microvilli as critical host factors required for viral entry and egress.