Inhibition Mechanism Of Interferon Production By SARS-CoV-2 NSP7

COVID-19(Corona Virus Disease 2019)is a kind of respiratory disease caused by SARSCOV-2(Severe Acute Respiratory Syndrome Coronavirus 2)that has spread rapidly and threatened public health seriously worldwide.SARS-CoV-2 is a member of Coronaviridae family,a kind of positive single-stranded RNA virus.The genome of SARS-CoV-2 encodes multiple putative open reading frames,including the large replicase genes expressing two replicative polyproteins(pp1a and pp1ab)that are cleaved into NSP1-16 by viral proteases,the structural genes expressing the spike(S),membrane(M),envelope(E),nucleocapsid(N)proteins and accessory genes.Double-stranded RNA(dsRNA),which is produced by many viruses during replication,is a common viral pathogen-associated molecular pattern that is recognized by pattern recognition receptors(PRRs)such as RIG-I/MDA5 and toll-like receptor 3(TLR3).It is reported that coronavirus infection can lead to the release of mitochondrial DNA,so it also activates the cGAS-STING pathway to defend against certain coronaviruses.RIG-I/MDA5-MAVS,TLR3TRIF and cGAS-STING signaling pathways converge at TBK1/IKKε which can catalyzes IRF3 phosphorylation and the subsequent transcription of types Ⅰ and III IFNs which will activate the JAK-STAT pathway to induce the production of interferon-stimulated genes(ISGs)and chemokinesa in order to activate immune cells and clear the infected cells,establish an antiviral state and inhibit the replication and transmission of virus.SARS-CoV-2 infection can activate RIG-I/MDA5-MAVS,TLR3-TRIF and cGAS-STING signaling pathways.Compared with other respiratory viruses such as influenza virus,SARSCoV-2 infection induces a lower interferon response,but its mechanism remains to be studied.Using real-time quantitative PCR(RT-qPCR)and luciferase reporter assays,we found that the non-structural protein NSP7 of SARS-CoV-2 could inhibit the production of type Ⅰ and type Ⅲinterferon induced by poly(I:C)and SeV.Further experiments show that NSP7 can target multiple members of RIG-I/MDA5-MAVS,TLR3-TRIF and cGAS-STING signaling pathways by targeting multiple molecules of these innate antiviral pathways.Co-IP results indicated that SARS-CoV-2 NSP7 associated with RIG-I,MDA-5,STING and TRIF which is consistent with the results from the colocalization studies.Overexpression of NSP7 can inhibit phosphorylation of TBK1 and IRF3 induced by MAVS,TRIF and STING.Confocal microscopy observations and nucleocytoplasmic separation experiments show that NSP7 could prevent IRF3 from entering the nucleus induced by SeV.This study shows that SARS-CoV-2 NSP7 protein targets RIG-I/MDA5-MAVS,TLR3TRIF and cGAS-STING signaling pathways to inhibit the production of type Ⅰ and Ⅲ IFN and the activation of antiviral immunity,it contributes to our understanding of the molecular mechanism through which SARS-CoV-2 impairs antiviral immunity and provides an essential clue to the pathogenesis and a theoretical basis development of drugs in COVID-19 treatment.