Date Published: March 20, 2014
Publisher: Public Library of Science
Author(s): Ji-Seung Yoo, Kiyohiro Takahasi, Chen Seng Ng, Ryota Ouda, Koji Onomoto, Mitsutoshi Yoneyama, Janice Ching Lai, Simon Lattmann, Yoshikuni Nagamine, Tadashi Matsui, Kuniyoshi Iwabuchi, Hiroki Kato, Takashi Fujita, Michaela U. Gack.
RIG-I is a DExD/H-box RNA helicase and functions as a critical cytoplasmic sensor for RNA viruses to initiate antiviral interferon (IFN) responses. Here we demonstrate that another DExD/H-box RNA helicase DHX36 is a key molecule for RIG-I signaling by regulating double-stranded RNA (dsRNA)-dependent protein kinase (PKR) activation, which has been shown to be essential for the formation of antiviral stress granule (avSG). We found that DHX36 and PKR form a complex in a dsRNA-dependent manner. By forming this complex, DHX36 facilitates dsRNA binding and phosphorylation of PKR through its ATPase/helicase activity. Using DHX36 KO-inducible MEF cells, we demonstrated that DHX36 deficient cells showed defect in IFN production and higher susceptibility in RNA virus infection, indicating the physiological importance of this complex in host defense. In summary, we identify a novel function of DHX36 as a critical regulator of PKR-dependent avSG to facilitate viral RNA recognition by RIG-I-like receptor (RLR).
Cellular responses against various stresses are crucial for maintaining homeostasis. Virus infection is one class of cellular stress that induces a number of host responses through the activation of pattern recognition receptors (PRRs), which detect pathogen-associated molecular patterns (PAMPs) with high specificity for protection against microbial invasion. RIG-I, a DExD/H-box RNA helicase family member, functions as a cytosolic viral RNA sensor, recognizes specific structures of non-self RNAs, including double-stranded RNA (dsRNA) and in vitro transcripts containing 5′ppp- and partial double-stranded structure due to copy back (5′ppp-cbRNA) , . Upon sensing non-self RNA, RIG-I promptly induces type I interferon (IFN) and other cytokines to provide a primary defense program against viruses .
The cytoplasmic viral RNA sensor, RLR, plays a major role in detecting viral infection and triggering antiviral responses. In this report, we describe the involvement of DHX36 in sensing viral infection and subsequent activation of RIG-I. Kim et al. reported that DHX36 functions as DNA sensor in plasmacytoid dendritic cells (pDC) . The reported function of DHX36 is apparently distinct from our discovery. First, we used fibroblast and epithelial cell lines, quite distinct cell types from pDC. Second, the stimuli used did not contain or produce DNA. There is another report describing DHX36 as a dsRNA sensor in myeloid dendritic cells (mDC) . DHX36 appears to function upstream of TIR domain-containing adaptor inducing IFN-ß (TRIF also termed TICAM1) to activate NF-κB and IRF-3/7. Again, their observation is distinct from ours because in our system, RIG-I was essentially involved in the signal induced by dsRNA, IAVΔNS1, and NDV in the fibroblast cells . It remains to be shown whether dsRNA-stimulated mDC promotes PKR activation and SG formation through DHX36.