The Mechanisms Of MDA5-mediated Innate Immune Response By RAVER1

Vertebrates are constantly threatened by the invasion of a lot of microorganisms and they have evolved the immune system to defend against infection and clear the pathogens. Mammalian immune system is composed of two branches:innate and adaptive immunity. The innate immune system recognizes microbial components, known as pathogen-associated molecular patterns (PAMPs), via a limited number of germline-encoded pattern-recognition receptors (PRRs). Detection of PAMPs by PRRs initiates intracellular signaling cascades, which leads to the production of type I IFNs, proinflammatory cytokines and chemokines.The production of type I IFNs is a key point of innate immune responses against viral infection, which is dependent on recognition of viral nuclear acid by PRRs. So far, it has been reported that Toll-like receptors (TLRs), such as TLR3, TLR7, TLR8and TLR9, retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), such as RIG-I and MDA5, and recent identified DNA receptors, such as DAI and IFI16are viral nuclear acid sensors. Among the PRRs, the RLRs, RIG-I and MDA5, recognize cytoplasmic viral RNA. Upon viral challenge, RIG-I and MDA5recruit the adapt protein VISA (also known as MAVS、IPS-1and Cardif) through homotypic CARD-CARD interaction. VISA interacts with TRAF2and/or TRAF6to activate IKK kinase complex, leading to activation of transcription factor NF-κB.VISA is associated with TRAF3and TBK1to activate IRF3by phosphorylation. The activated transcription factors IRF3and NF-κB enter into the nucleus and collaboratively trigger the expression of type I IFN genes.Although RIG-I and MDA5are structurally related and mediate similar signaling pathways, these two helicases recognize different ligands. While the regulation of RIG-I activity has been extensively investigated, little is known about the regulatory mechanisms of MDA5activity. Using a biochemical purification approach, we identified ribonucleoprotein PTB-binding1(RAVER1) as a specific MDA5-interacting protein, which directly binds to MDA5. Upon viral infection, RAVER1was associated with MDA5. Overexpression of RAVER1at low dosages enhanced MDA5-but not RIG-I-mediated activation of the IFN-b promoter. On the contrary, knockdown of RAVER1inhibited MDA5-but not RIG-I-mediated induction of downstream antiviral genes. Mechanistically, overexpression of RAVER1enhanced the binding of MDA5to its ligand poly(I:C) whereas knockdown of RAVER1had opposite effect. Our findings suggest that RAVER1specifically regulates MDA5activity, revealing a mechanism of differential regulation of MDA5-and RIG-I-mediated innate immune antiviral response.Further research is needed to understand the precise mechanisms of the facilitation of MDA5binding to its ligand poly(I:C) by RAVER1. RAVER1could not directly recognize long poly(I:C), thus it’s possible that the binding of RAVER1modulate the conformation of MDA5, or there are some other molecules involved in this process. However, this hypothesis requires evidence from structural analysis of MDA5and RAVER1complex. In addition, generation and analysis of raverI-/-mice will help us to gain insight into the function of RAVER1in innate antiviral response.