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

Area of Science:

• Virology
• Immunology
• Molecular Biology

Background:

• Coronaviruses pose significant veterinary and medical threats, including zoonotic viruses like SARS-CoV and MERS-CoV.
• These viruses are adept at evading innate immune responses, notably by suppressing type-I interferon (IFN-I) production.
• This immune evasion suggests an evolutionarily conserved viral mechanism to counteract RNA-based sensing in hosts.

Purpose of the Study:

• To investigate the role of coronavirus endonuclease (EndoU) activity in evading host innate immune responses.
• To determine how EndoU prevents the early induction of double-stranded RNA (dsRNA) mediated cellular responses.
• To elucidate the mechanisms by which coronaviruses overcome host antiviral defenses.

Main Methods:

• Replication studies of wild-type and EndoU-deficient coronaviruses in primary cells and in vivo.
• Analysis of IFN-I expression, RNase L activity, and PKR activation in infected cells.
• Assays to assess the impact of host cell factors (IFN-I, RNase L, PKR) on viral replication.

Main Results:

• Replication of EndoU-deficient coronaviruses was significantly attenuated in vivo and restricted in primary cells.
• EndoU deficiency led to immediate IFN-I expression and RNase L-mediated ribosomal RNA breakdown in macrophages.
• Replication of EndoU-deficient viruses was only observed in cells lacking IFN-I expression/sensing or deficient in both RNase L and PKR.

Conclusions:

• Coronavirus EndoU activity is essential for preventing the simultaneous activation of host dsRNA sensors like Mda5, OAS, and PKR.
• The localization of EndoU within the replicase complex suggests a viral RNA decay pathway evolved to evade early innate and intrinsic antiviral responses.
• Understanding EndoU's function provides insights into coronavirus pathogenesis and potential therapeutic targets.