PKR Transduces MDA5-Dependent Signals for Type I IFN Induction

Abstract

Sensing invading pathogens early in infection is critical for establishing host defense. Two cytosolic RIG-like RNA helicases, RIG-I and MDA5, are key to type I interferon (IFN) induction in response to viral infection. Mounting evidence suggests that another viral RNA sensor, protein kinase R (PKR), may also be critical for IFN induction during infection, although its exact contribution and mechanism of action are not completely understood. Using PKR-deficient cells, we found that PKR was required for type I IFN induction in response to infection by vaccinia virus lacking the PKR antagonist E3L (VVΔE3L), but not by Sendai virus or influenza A virus lacking the IFN-antagonist NS1 (FluΔNS1). IFN induction required the catalytic activity of PKR, but not the phosphorylation of its principal substrate, eIF2α, or the resulting inhibition of host translation. In the absence of PKR, IRF3 nuclear translocation was impaired in response to MDA5 activators, VVΔE3L and encephalomyocarditis virus, but not during infection with a RIG-I-activating virus. Interestingly, PKR interacted with both RIG-I and MDA5; however, PKR was only required for MDA5-mediated, but not RIG-I-mediated, IFN production. Using an artificially activated form of PKR, we showed that PKR activity alone was sufficient for IFN induction. This effect required MAVS and correlated with IRF3 activation, but no longer required MDA5. Nonetheless, PKR activation during viral infection was enhanced by MDA5, as virus-stimulated catalytic activity was impaired in MDA5-null cells. Taken together, our data describe a critical and non-redundant role for PKR following MDA5, but not RIG-I, activation to mediate MAVS-dependent induction of type I IFN through a kinase-dependent mechanism.

Disruption of MDA5-Mediated Innate Immune Responses by the 3C Proteins of Coxsackievirus A16, Coxsackievirus A6, and Enterovirus D68

“…The CARDs of RIG-I and MDA5 are responsible for transducing signals to the downstream adaptor MAVS through CARD-CARD interactions (59,60,64,65).To investigate whether CV-A16 3C pro also antagonizes RIG-I-induced MAVS activation in the RLR pathway, we performed an IFN-␤ luciferase assay in HEK293T cells. A RIG-IN expression vector, which includes the caspase recruitment domain, was employed (64,66).…”

Standing on three legs: antiviral activities of RIG-I against influenza viruses

“…All three RLRs are RNA helicases, and RIG-I and MDA5 additionally possess N-terminal caspase recruitment domains (CARDs) to activate the antiviral signaling chain via the CARD-containing adapter MAVS [3]. Whereas RIG-I mainly responds to short 5 0 triphosphorylated (5 0 ppp) blunt-ended double-stranded (ds) RNA with a minimal length of 10 base pairs, MDA5 is triggered by longer double-stranded dsRNA, ideally with a higher-order structure [2,[4][5][6]. For LGP2, the picture emerges that it is a negative regulator of RIG-I, but a positive regulator of MDA5 [7,8].…”

DNAJB1/HSP40 Suppresses Melanoma Differentiation-Associated Gene 5-Mitochondrial Antiviral Signaling Protein Function in Conjunction with HSP70

“…A recent report has shown that the dsRNA sensor PKR works downstream of MDA5 and upstream of MAVS, leading to MAVS-dependent type I IFN induction [38]. Our data suggest that DNAJB1 is involved in the MDA5-MAVS axis, but not RIG-I or TICAM-1.…”

“…Our data suggest that DNAJB1 is involved in the MDA5-MAVS axis, but not RIG-I or TICAM-1. PKR does not interact with RIG-I but is involved in MDA5-MAVS signaling [38]. Furthermore, dsRNA-mediated PKR signaling is dependent on MAVS, but not TICAM-1 [39].…”