Early endonuclease-mediated evasion of RNA sensing ensures efficient coronavirus replication

Eveline Kindler, et al. PLoS Pathog 13(2): e1006195

Speaker: Ming-Yu Shieh (謝明育)                               Time: 13:00-14:00, Jun. 7, 2017

Commentator: Dr. Shun-Hua Chen (陳舜華老師)        Place: Room 601

Abstract:

       Coronaviruses (CoV) are single-stranded RNA viruses that cause severe human diseases such as SARS and MERS. It had been demonstrated that they evolve multiple strategies to escape from host innate immune responses, including ribose-2’-O methylation of viral RNA and interference with the type 1 interferon (IFN-I) pathway by some viral non-structural proteins (nsp). Nsp15 contains a uridylate-specific endonuclease (EndoU) domain that degrades ssRNA and dsRNA, but the role of the EndoU activity in virus-host interaction remains unknown [1]. In this study, the authors found that EndoU-deficient CoV failed to propagate in mice and its replication in macrophages in vitro was severely impaired. Notably, replication of the EndoU-deficient CoV was highly susceptible to IFN-I pretreatment and can be largely restored in the mice/macrophages with the genetic background of IFNAR^-/- but not Mda5^-/-, TLR7^-/-, or MAVS^-/-, suggesting that the viral EndoU activity majorly deals with the host responses downstream of IFN-I autocrine/paracrine. The authors further found that the activity of RNase L and PKR, two IFN-induced, RNA-sensing antiviral enzymes, was detected in the macrophages infected with EndoU-deficient CoV but not the wild-type virus [2][3]. Dual knockout of RNase L and PKR genes was able to restore replication of the EndoU-deficient CoV to the similar level observed in the IFNAR^-/- background, suggesting that RNase L and PKR are two major effectors to suppress EndoU-deficient CoV. At last, the authors found that, compared to infection with wild-type CoV, the cells infected with EndoU-deficient CoV contained more dsRNA, which may trigger activation of RNase L and PKR and antiviral responses. In conclusion, this study reveals a new and critical mechanism of immune evasion of CoV, probably by EndoU-mediated reduction of cytosolic dsRNA to escape from sensing by RNase L and PKR. Therefore, EndoU is an attractive pharmaceutic target for anti-CoV therapy.

Reference:

1.       Ivanov KA, Hertzig T, Rozanov M, Bayer S, Thiel V, Gorbalenya AE, et al. Major genetic marker of nidoviruses encodes a replicative endoribonuclease. Proc Natl Acad Sci U S A. 2004; 101(34):12694–9.

2.       Zhao L, Jha BK, Wu A, Elliott R, Ziebuhr J, Gorbalenya AE, et al. Antagonism of the interferon-induced OAS-RNase L pathway by murine coronavirus ns2 protein is required for virus replication and liver pathology. Cell host & microbe. 2012; 11(6):607–16.

3.       Zhou A, Paranjape JM, Der SD, Williams BR, Silverman RH. Interferon action in triply deficient mice reveals the existence of alternative antiviral pathways. Virology. 1999; 258(2):435–40.