RIG-I-like receptors (retinoic acid-inducible gene-I-like receptors, or RLRs,) are a type of intracellular pattern recognition receptor involved in the recognition of viruses by the innate immune system. There are three RLRs — RIG-I, MDA5, and LGP2 — that act as sensors of viral replication within the cytoplasm of human cells, and each has a DExD/H box helicase structure. The discovery and analysis of RIG-I preceded the discovery of MDA5 and LGP2, meaning that much more is known about RIG-I than the other two RLRs.
Though each of these three receptors is located in tissues throughout the body, RLRs are especially active in the innate immune defense of epithelial cells, myeloid cells, and CNS cells. As pattern recognition receptors, RLRs live in the cytosol of a cell, so that they can detect the presence of viral DNA or RNA. Upon detection of a viral infection, two of the RLRs, RIG-I and MDA5, possess the ability to activate a complex signaling network that leads to the production of pro-inflammatory molecules. RIG-I and MDA5 recognize distinct viruses but both produce the antiviral immune changes that initiate an innate response and regulate the subsequent adaptive response. LGP2, the remaining RLR, lacks the ability to induce signaling alone, but is necessary for effective RIG-I and MDA5-mediated antiviral responses.
Function and structure
RIG-I and MDA5
Each of the three RLRs contain a C-terminal DExD/H box helicase domain, which binds to double-stranded RNA, or dsRNA. RIG-I and MDA5 each contain two N-terminal caspase activation and recruitment domains (CARD), which will initiate the anti-viral signaling pathway.
LGP2 does not have CARD-domains and therefore does not positively induce the same signaling pathway as RIG-I and MDA5. LGP2 also does not participate in viral detection. Instead, LGP2 modulates the other two RLRs through negative inhibition. It is known that LGP2 binds a repressor domain on the RIG-I C-terminal region to suppress RIG-1 signaling and down-regulate the viral response.
Signaling pathway of RIG-I and MDA5
Because RLRs are pattern recognition receptors, there are certain features of viral dsRNA or ssRNA (single-stranded RNA) that RLRs are equipped to detect. These include double-stranded regions, specific nucleotide sequences, and 5' triphosphate modifications. RIG-I, specifically, detects a 5' triphosphate addition on single-stranded RNA, as a means of differentiating from self-RNA.
RIG-I and MDA5 both contain two CARD-domains for induction of an intracellular response, and as such have a very similar signaling pathway. The binding of the RIG-I or MDA5 helicase to a ssRNA or dsRNA induces a conformational change in the proteins, which releases the CARD domain to initiate signaling. These signals for viral inhibition cannot be transmitted without a functional CARD domain.
During viral infection, cleavage of self-RNA creates a ligand that will also bind to RIG-I and MDA5 to further amplify the anti-viral signaling pathway and increase the intracellular cytokine response (specifically type I IFNs).
Upon activation of RIG-I, the conformational change in the protein allows for CARD-CARD binding to MAVS (mitochondrial antiviral signaling protein, also called IPS-1) a downstream adaptor molecule found in the mitochondrial membrane. MAVS furthers the signaling cascade by facilitating recruitment or activation of subsequent proteins in the signaling cascade; TRADD, TRAF3, and RIP1 activate NEMO/IKKs, which activate IKK complexes that stimulate antiviral transcription factors, including IRF-3 and IRF-7, as well as the NF-κB pathway. In turn, the function, abundance or subcellular distribution of many of these proteins are individually potentiated by an extensive series of post-translational modifications, particularly phosphorylation and ubiquitination. Finally, the NF-κB pathway releases cytokines, antimicrobials, and chemokines, which are all part of the pro-inflammatory innate immune response.
MDA5 activation also promotes CARD-CARD binding to MAVS, inducing the same signaling cascade. As such, the pathways for the two receptors are structurally similar, but will ultimately result in production of different cytokines.
RLRs and the immune system
Studies of RIG-I and MDA5 knock-out mice have shown that though both RLRs lead to production of type I IFNs and inflammatory cytokines, each RLR recognizes separate viral patterns; a deficiency of one type of RLR leads to a lack of cytokine production in response to a particular type of virus. RIG-I has viral ligand specificity for paramyxoviruses, vesicular stomatitis virus, Japanese encephalitis virus, and influenza virus. MDA5 is specific to Picornaviruses, including encephalomyocarditis virus, Mengo virus, and Theiler's virus, and is involved in recognition of poly(I:C), an immunostimulant. Viral proteins have also evolved to counteract and avoid RLR signaling and the ensuing inflammatory response, such as paramyxoviruses, which have V proteins that block RLR signaling through interaction with MDA5.
RLRs work alongside another type of pattern recognition receptor, toll-like receptors (TLRs). Both types of receptor are used for context-dependent sensing of viral infection, initiating the innate immune response, and mediating the adaptive immune response.
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The other two families of PRRs, the NOD-like receptors (NLRs) and the RIG-like helicases (RLHs) are soluble receptors present in the cytosol and act as sensors to detect a variety of viral and bacterial products.
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