PKR Transduces MDA5-Dependent Signals for Type I IFN Induction

Area of Science:

• Immunology
• Virology
• Molecular Biology

Background:

• Cytosolic RIG-like RNA helicases RIG-I and MDA5 are crucial for type I interferon (IFN) induction during viral infections.
• The role of protein kinase R (PKR) in IFN induction remains incompletely understood, despite its known function as a viral RNA sensor.

Purpose of the Study:

• To elucidate the specific role and mechanism of protein kinase R (PKR) in type I interferon induction during viral infections.
• To determine whether PKR acts redundantly or non-redundantly with RIG-I and MDA5 in initiating host defense.

Main Methods:

• Utilized PKR-deficient cells to assess type I IFN induction in response to various viral infections.
• Investigated the requirement for PKR's catalytic activity and its substrate eIF2α phosphorylation.
• Analyzed IRF3 nuclear translocation and protein-protein interactions between PKR, RIG-I, and MDA5.

Main Results:

• PKR is essential for type I IFN induction by vaccinia virus lacking E3L (VVΔE3L), but not by Sendai virus or influenza A virus lacking NS1.
• PKR's catalytic activity is required, but eIF2α phosphorylation and host translation inhibition are not necessary for IFN induction.
• PKR is required for MDA5-mediated, but not RIG-I-mediated, IRF3 activation and subsequent IFN production.
• PKR interacts with both RIG-I and MDA5, with MDA5 enhancing PKR activation during viral infection.

Conclusions:

• PKR plays a critical, non-redundant role in type I IFN induction downstream of MDA5 activation, independent of RIG-I.
• The kinase activity of PKR, acting through MAVS and IRF3, is essential for this MDA5-dependent IFN production.
• PKR functions as a key mediator in the host's innate immune response to specific viral RNA patterns detected by MDA5.