Molecular Mechanism of Signal Perception and Integration by the Innate Immune Sensor Retinoic Acid-inducible Gene-I (RIG-I)

Marco Binder,Florian Eberle,Stefan Seitz,Norbert Mücke,Christian M Hüber,Narsis Kiani,Lars Kaderali,Volker Lohmann,Alexander Dalpke,Ralf Bartenschlager

doi:10.1074/jbc.m111.256974

Abstract

RIG-I is a major innate immune sensor for viral infection, triggering an interferon (IFN)-mediated antiviral response upon cytosolic detection of viral RNA. Double-strandedness and 5'-terminal triphosphates were identified as motifs required to elicit optimal immunological signaling. However, very little is known about the response dynamics of the RIG-I pathway, which is crucial for the ability of the cell to react to diverse classes of viral RNA while maintaining self-tolerance. In the present study, we addressed the molecular mechanism of RIG-I signal detection and its translation into pathway activation. By employing highly quantitative methods, we could establish the length of the double-stranded RNA (dsRNA) to be the most critical determinant of response strength. Size exclusion chromatography and direct visualization in scanning force microscopy suggested that this was due to cooperative oligomerization of RIG-I along dsRNA. The initiation efficiency of this oligomerization process critically depended on the presence of high affinity motifs, like a 5'-triphosphate. It is noteworthy that for dsRNA longer than 200 bp, internal initiation could effectively compensate for a lack of terminal triphosphates. In summary, our data demonstrate a very flexible response behavior of the RIG-I pathway, in which sensing and integration of at least two distinct signals, initiation efficiency and double strand length, allow the host cell to mount an antiviral response that is tightly adjusted to the type of the detected signal, such as viral genomes, replication intermediates, or small by-products.

Highlights

Strength of this initial innate immune response is key in setting the overall course of the disease and, in many instances, determines whether the virus will eventually be cleared
Viral 5Ј-Triphosphorylated RNAs Are Differentially Recognized by RIG-I—The human hepatoma cell line Huh7.5 is deficient for RIG-I signaling because of a dominant negative mutation [29]
We have previously shown that Huh7.5 stably transduced with wild-type RIG-I (Huh7.5/RIG-I) exhibit a strong, RIG-I-dependent IFN regulatory factor 3 (IRF-3) activation upon stimulation [22], which was not due to up-regulation of any other factors as evidenced by genome-wide transcriptional profiling

Summary

Introduction

Strength of this initial innate immune response is key in setting the overall course of the disease and, in many instances, determines whether the virus will eventually be cleared. RIG-I,3 a member of the cytosolic RIG-I like receptor family, has received increasing attention over the past years [1, 2] It detects viral RNA products and upon recognition associates with the adapter molecule MAVS (formerly known as Cardif, IPS-1, or VISA) [3,4,5,6] via its two N-terminal caspase activation and recruitment domains. There have been reports on RIG-I signaling in response to dsRNA lacking a 5Ј-ppp, like RNaseL cleavage products [19] or poly(I1⁄7C) (10, 20 –22). It was reported that the stimulatory potential is higher for short dsRNA [21], the underlying mechanism remains enigmatic, and helicase/translocase activity has been shown to increase on longer substrates [28]. The presented model provides an example of a complex adaptive achievement of the immune system to detect viral invaders with maximum sensitivity while maintaining self-tolerance

Methods

Results

Conclusion