Abstract
BackgroundDouble stranded RNA (dsRNA) is widely accepted as an RNA motif recognized as a danger signal by the cellular sentries. However, the biology of non-segmented negative strand RNA viruses, or Mononegavirales, is hardly compatible with the production of such dsRNA.Methodology and Principal FindingsDuring measles virus infection, the IFN-β gene transcription was found to be paralleled by the virus transcription, but not by the virus replication. Since the expression of every individual viral mRNA failed to activate the IFN-β gene, we postulated the involvement of the leader RNA, which is a small not capped and not polyadenylated RNA firstly transcribed by Mononegavirales. The measles virus leader RNA, synthesized both in vitro and in vivo, was efficient in inducing the IFN-β expression, provided that it was delivered into the cytosol as a 5′-trisphosphate ended RNA. The use of a human cell line expressing a debilitated RIG-I molecule, together with overexpression studies of wild type RIG-I, showed that the IFN-β induction by virus infection or by leader RNA required RIG-I to be functional. RIG-I binds to leader RNA independently from being 5-trisphosphate ended; while a point mutant, Q299A, predicted to establish contacts with the RNA, fails to bind to leader RNA. Since the 5′-triphosphate is required for optimal RIG-I activation but not for leader RNA binding, our data support that RIG-I is activated upon recognition of the 5′-triphosphate RNA end.Conclusions/SignificanceRIG-I is proposed to recognize Mononegavirales transcription, which occurs in the cytosol, while scanning cytosolic RNAs, and to trigger an IFN response when encountering a free 5′-triphosphate RNA resulting from a mislocated transcription activity, which is therefore considered as the hallmark of a foreign invader.
Highlights
The cellular innate defence is initiated with the recognition of peculiar danger molecular motifs called Pathogen Associated Molecular Patterns (PAMP), by the Pattern Recognition Receptors (PRR), which results in the induction of a type-I interferon (IFNa/b) response
Mononegavirales are characterized by having their RNA genome tightly encapsidated by the viral nucleoprotein N, making them resistant to silencing by siRNA [5], nuclease attacks or high salt concentration, as recently shown by the crystal structure of short vesicular stomatitis virus (VSV) and rabies virus nucleocapsids, which both pointed out that the RNA is fully embedded within the nucleoprotein oligomer [6,7]
To identify which of the two activities, transcription or replication, is required for the production of the PAMP, we compared the kinetics of accumulation of IFN-b mRNAs with the rate of measles virus transcription and replication, using reverse transcription (RT)-QPCR assays [10]
Summary
The cellular innate defence is initiated with the recognition of peculiar danger molecular motifs called Pathogen Associated Molecular Patterns (PAMP), by the Pattern Recognition Receptors (PRR), which results in the induction of a type-I interferon (IFNa/b) response. The helicases RIG-I and Mda-5 induce IFN transcription upon recognition of viral dsRNA [1,2] from different viruses, with Mda-5 and RIG-I being required for sensing picornaviruses and members of the Mononegavirales order [1,2,3,4], respectively. The measles virus leader RNA, synthesized both in vitro and in vivo, was efficient in inducing the IFN-b expression, provided that it was delivered into the cytosol as a 59trisphosphate ended RNA. The use of a human cell line expressing a debilitated RIG-I molecule, together with overexpression studies of wild type RIG-I, showed that the IFN-b induction by virus infection or by leader RNA required RIG-I to be functional. RIG-I is proposed to recognize Mononegavirales transcription, which occurs in the cytosol, while scanning cytosolic RNAs, and to trigger an IFN response when encountering a free 59-triphosphate RNA resulting from a mislocated transcription activity, which is considered as the hallmark of a foreign invader
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.