Biochemical Dissection of Innate Immune Signaling Mechanisms Mediated by MAVS and Inflammasomes

Project 1: MAVS Recruits Multiple Ubiquitin E3 Ligases to Activate Antiviral Signaling Cascades The RIG-I antiviral pathway plays a pivotal role in innate immune response against RNA viruses. Upon virus infection, viral RNA in the cytoplasm is detected by the RIG-I family of receptors, which activates an adaptor protein called MAVS (mitochondrial antiviral signaling). MAVS in turn leads to the activation of two transcription factors, NF-κB and IRF3, which coordinately induce type-I interferons and proinflammatory cytokines that are critical in eliminating viral infection. However, the exact mechanism of NF-κB and IRF3 activation by MAVS is still largely unknown. In this study we show that activated MAVS recruits three ubiquitin E3 ligases TRAF6, TRAF2 and TRAF5 to the mitochondria through distinct TRAF-binding motifs upon virus infection. Mutations that disrupt these motifs in MAVS abrogated the recruitment of these E3 ligases, and abolished the ability of MAVS to activate the downstream signaling. Interestingly, virus-induced prion-like aggregation of MAVS is essential for its interaction with these TRAF proteins as well as the downstream activation. Genetic evidence has shown that these E3 ligases function redundantly to activate NF-κB and IRF3, so that they not only amplify the antiviral signal, but also serve as backup systems against specific viruses that can degrade components in the antiviral pathway. Identifying these key players in MAVS pathway will help us explore novel therapeutic targets for infectious diseases caused by RNA viruses. Project 2: Dissecting the Mechanism of NLRP3 Inflammasome Activation Inflammasome is a multi-protein oligomer that serves as a platform for caspase-1-depedent activation of proinflammatory cytokines and the induction of a specific form of cell death termed pyroptosis. A plethora of pathogen-associated-molecular patterns (PAMPs) and damage-associated-molecular-patterns (DAMPs) have been found to activate inflammasome through NLRP3, a Nod-like receptor protein. As a result, the dysfunction of NLRP3 is closely associated with various health problems including autoimmune diseases, neurodegenerative diseases, susceptibility to pathogen infection, and metabolic disorders. Given the chemical and structural diversity of the stimuli, and the lack of evidence that NLRP3 directly interacts with these stimuli, it has been hypothesized that NLRP3 activation is triggered by a common cellular signal, the identity of which is still a mystery. We have reconstituted NLRP3 inflammasome pathway in HEK293T cell and used this system to establish a cell-free assay, in which NLRP3 from stimulated cells was added to cells containing Asc and caspase-1 to activate the downstream signaling. With this assay, we were able to obtain highly purified active NLRP3 for further characterization, and have found that dephosphorylation plays an important role in NLRP3 inflammasome activation. Moreover, both biochemistry and imaging data has suggested that NLRP3 is translocated to specific subcellular structures after stimulation, of which the significance is still under investigation. Our final goal is to dissect the detailed mechanism of NLRP3 activation, which can provide insight into the prevention and treatment of various related human diseases.