Inflammasome assembly and activation mechanisms
- School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD 4072, Australia.
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA.
- Program in Cellular and Molecular Medicine, Boston Children’s Hospital, Boston, MA 02115, USA.
- Institute for Molecular Bioscience, University of Queensland, Brisbane, QLD 4072, Australia.
Pattern recognition receptors (PRR) of the innate immune system mediate the first line of defence against infections and cellular stress signals. The inflammasome complex initiated by a subset of PRRs forms a platform for activation of procaspases-1 and -8 to mediate inflammation and cell death responses. Inflammasomes initiated by the AIM2 PRR in response to cytosolic DNA are relevant to viral and bacterial infections and autoimmune diseases caused by recognition of self DNA. Inflammasomes initiated by the NLRP3 PRR, activated by many diverse stimuli, contribute to the pathology of many common diseases including diabetes and atherosclerosis. Inflammasome assembly is mediated by death-fold domains, which include pyrin domains (PYDs), caspase recruitment domains (CARDs) and death effector domains (DEDs). Activation of the PRR induces oligomerisation and PYD clustering, which recruits the adaptor protein ASC and nucleates polymerization of ASC PYD into a helical filament. ASC CARDs condense the complex to form a speck that is synonymous with inflammasome activation and also recruit procaspase-1. We showed that ASC recruits procaspase-8 to the inflammasome to mediate apoptosis. The interaction is mediated by interaction between ASC PYD and procaspase-8 DEDs and nucleates formation of procaspase-8 DED filaments. Studies with reconstituted inflammasomes in HEK cells show that procaspase-8 DED filaments extend from the ASC speck while full-length procaspase-8 is condensed within the ASC speck suggesting inter-filament catalytic domain interactions. Procaspase-1 CARD did not form extensive filaments from the ASC speck in HEK cells thus a similar mechanism cannot be presumed. Time course analysis of endogenous inflammasomes to characterise the mechanism of inflammasome activation indicates rapid activation upon stimulation while live cell imaging indicates rapid ASC filament elongation. Our current models of inflammasome assembly and activation based on our data and insights from structural studies of death-fold domain filaments will be presented.