The Furin-S2′ Site in Avian Coronavirus Plays a Key Role in Central Nervous System Damage Progression

The furin cleavage site plays an important role in virus pathogenicity. The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) harbors a furin cleavage site insertion, in contrast to SARS-CoV, which may be related to its stronger communicability. An avian coronavirus with an extra furin cleavage site upstream of the fusion peptide (furin-S2′ site) infected monocytes and neuron cells, leading to viremia and encephalitis, respectively. Immunohistochemistry and real-time quantitative PCR were used to follow disease progression and demonstrated differences between the parent avian coronavirus and mutated avian coronavirus with a furin-S2′ site. Magnetic resonance imaging and biological dye to evaluate the blood-brain barrier permeability showed that avian coronavirus with a furin-S2′ site had increased permeability compared with that of the parent avian coronavirus. Immunohistochemistry of brains after intracerebral injection of avian coronavirus and immunofluorescence staining of primary neuron cells demonstrated that the furin-S2′ site expanded the cell tropism of the mutant avian coronavirus to neuron cells. Tumor necrosis factor alpha (TNF-α), which has a key role in blood-brain barrier permeability, was highly induced by avian coronavirus with a furin-S2′ site in comparison with the parent avian coronavirus. We demonstrated the process involved in mutant avian coronavirus-induced disease and that the addition of a furin-S2′ site changed the virus cell tropism. IMPORTANCE Coronaviruses have broken out three times in 2 decades. Spike (S) protein plays a key role in the process of infection. To clarify the importance of the furin cleavage site in spike protein for coronavirus, we investigated the pathogenesis of neurotropic avian coronavirus, whose spike protein contains an extra furin cleavage site (furin-S2′ site). By combining real-time quantitative PCR and immunohistochemistry, we demonstrated that infectious bronchitis virus (IBV) infects brain instead of trachea when its S protein contains the furin-S2′ site. Moreover, the virus was shown to increase the permeability of the blood-brain barrier, infect neuron cells, and induce high expression of TNF-α. Based on these results, we further showed that the furin cleavage site in S protein plays an important role in coronavirus pathogenicity and cell tropism. Our study extends previous publications on the function of S protein of coronavirus, increasing researchers’ understanding of coronavirus.