Caspase-1 Activation by the Inflammasomes

Thomas Jefferson University, Philadelphia PA

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Abstract

DESCRIPTION (provided by applicant): Caspase-1 activation by the inflammasomes is an important innate immune response to tissue damage and infection by microbial or viral pathogens. Caspase-1 is required for production of the active pro-inflammatory cytokines IL-1 and IL-18, and is also important for pyroptosis or inflammatory cell death. The Nlrp3 inflammasome activates caspase-1 in response to diverse signals including infection with intracellular microbial and viral pathogens, TLR agonists combined with potassium-depleting agents or pore forming toxins, and crystalline and aggregated materials. Uncontrolled activation of the Nlrp3 inflammasome can lead to a number of human diseases, including arthritis, atherosclerosis, and type 2 diabetes. Inherited mutations in Nlrp3 can also lead to a number of human autoinflammatory diseases or periodic fever syndromes. So far the mechanism of activation of the Nlrp3 inflammasome by these seemingly unrelated stimuli is poorly understood. Studies in the applicant's laboratory demonstrated that Nlrp3 is rapidly primed by Toll-like receptor (TLR) signaling through a postranslational modification mechanism that depends on the TLR signaling molecules Myd88 and IRAK1. Additional preliminary evidence suggests that brief TLR signaling primes Nlrp3 by a redox- dependent mechanism, whereas prolonged TLR signaling suppresses inflammasome activation. In this renewal application specific aims are proposed to elucidate how the priming signal through TLR signaling contributes together with signaling from purinergic P2X7 receptor, pore forming toxins or crystalline materials towards the assembly of the Nlrp3 inflammasome, ASC oligomerization and caspase-1 activation. These aims will test the hypothesis that mtROS generation through TLR signaling or direct stimulation of mitochondrial respiratory complexes activates Nlrp3 by a redox-switch mechanism. Additional experiments will extend these findings and will also address alternative possibilities of Nlrp3 priming through IRAK1-dependent phosphorylation and whether the oxidation and phosphorylation mechanisms are interdependent. Experiments are also proposed to define the signaling pathways downstream of Toll-like receptors that mediate ROS production. Additional experiments will investigate how Nlrp3 inflammasome activity is negatively regulated in vitro and in vivo by prolonged TLR signaling, and the role of a number of cellular anti-inflammatory pathways, including autophagosomal and heme oxygenase-1 pathways in this negative regulatory mechanism. Results from this research will provide fundamental new insights into the mechanisms that regulate its activation and those that regulate its suppression. Successful completion of this study should have a high impact on the field by providing a unifying paradigm for how Nlrp3 can be regulated by an exceptionally diverse group of activating stimuli. Understanding these mechanisms is of great scientific and health significance as this should better our understanding of the molecular basis of Nlrp3-related diseases and should in the long term help in the development of therapeutics to alleviate these inflammatory diseases.