Pattern recognition receptors (PRRs) in the innate immune system serve as the first line of defense against pathogens. They detect pathogen infection by recognizing conserved molecular patterns, such as viral nucleic acids or bacterial cell wall components, and induce antiviral and inflammatory immune responses. Several families of human PRRs have been identified, which include Toll-like receptors (TLRs), Nod-like receptors (NLRs) and RIG-I like helicases (RLHs). Studies involving knock-out mouse models and human genetic polymorphisms suggest that dysfunctions of these receptors often increase susceptibility to pathogen infection, while their misregulation could lead to inflammatory or autoimmune diseases, such as lupus and rheumatoid arthritis. Significant efforts to identify and develop PRR-based immune modulators, however, have resulted in only a few clinically approved drugs. Harnessing their immune modulatory functions for therapeutic applications would require more precise understanding of their structures, signaling and regulatory mechanisms.
One of our current research focuses is on RIG-I like helicases (RIG-I, MDA5 and LGP2). They are cytoplasmic viral RNA recpetors and play important roles in cell-autonomous recognition of viral infection in a wide range of cell types. RIG-I and MDA5 share similar domain architectures and the signaling adaptor, MAVS, but have different virus and RNA specificities. While RIG-I primarily recopgnizes 5' triphosphate ends of viral RNAs, MDA5 recognizes long viral replication intermediates or genomic dsRNAs in a length-dependent manner (Kato et al, JEM, 2006).
Our investigation of RIG-I and MDA5 began with our findings that these receptors assemble into filamentous oligomers along viral dsRNA, albeit through distinct mechanisms and RNA specificities (Peisley et al, PNAS, 2011 & 2012, and Mol Cell, 2013) (Fig. 1). Our crystal structure of the MDA5:dsRNA complex and reconstitution of the signaling system further revealed precisely how MDA5 and RIG-I divergently evolved to partition their function as non-redundant viral RNA receptors (Wu et al, Cell, 2013 and Peisley et al, Mol Cell, 2013). Furthermore, in collaboration with Dr. Yanick Crow (Univ. of Manchester), we have recently discovered several gain-of-function mutations of MDA5 that over-stabilize the filament and cause aberrant interferon signaling in vitro and inflammatory disease in human (Rice et al, Nat. Genetics, 2014), which offered a new mechanistic link between the dysregulated MDA5 function and immune disorders.
More recently, we determined the crystal structures of the functional, oligomeric form of the signaling domain of RIG-I in complex with K63-linked polyubiquitin, a cofactor required for signal activation (Peisley et al, Nature, 2014), or in complex with MAVS (Wu et al, Mol Cell, 2014). These studies elucidated a long debated question of how RIG-I activates the interferon signaling pathway, revealing novel mechanisms for ubiquitin-mediated receptor oligomerization and previously unrecognized type of receptor:adaptor interactions (Fig. 2).
Our research on RIG-I/MDA5 has also led to the discovery of novel effector-like functions of these receptors (Yao et al, Mol Cell, 2015). In this study, we found that RIG-I/MDA5 can displace viral proteins as well as other roadblocks placed on dsRNA in an ATP dependent manner, and that this activity can confer antiviral activities in a manner independent of their canonical antiviral signaling pathways. We are currently working to further define this novel effector-like functions of RIG-I/MDA5 and their potential abilities to remodel a broad range of viral RNPs
We use a combination of crystallography, electron microscopy, biochemistry and cell-based assays to dissect molecular principles of functions of viral RNA receptors in isolation as well as in the context of signaling pathway and regulatory network in the cell. We believe that our studies would allow us to understand the diversity and commonality of viral detection mechanisms, and complex network of host-virus interaction.