Assembly and Regulation of the Inflammasome Governed by a Unified Polymerization Mechanism

Abstract

The innate immune system employs a diverse set of pattern recognition receptors to detect intrinsic and extrinsic danger signals for host protection. Inflammasomes represent an important class of receptors that elicit inflammatory response through the maturation of pro-inflammatory cytokines and the induction of a form of cell death known as pyroptosis. Assembly of a functional inflammasome typically involves an upstream sensor component, an adaptor component, and an effector caspase. The upstream sensor oligomerizes upon detection of pro-inflammatory triggers to recruit the adaptor, which in turn drives caspase activation. The assembled inflammasome proteolytically cleaves interleukin-1β (IL-1β) and interleukin-18, which are the cytokines responsible for downstream signaling events. Previously, our understanding of this assembly process had been limited by the lack of biochemical data and high-resolution structures. In this dissertation, I present key findings that provide novel perspectives on the assembly and regulation of inflammasomes using structural and biochemical approaches. More specifically, a two-step nucleated polymerization mechanism governs the assembly and activation of inflammasomes that require the filamentous scaffold of an adaptor known as Apoptosis-associated Speck-like protein containing a CARD (ASC). Upstream sensors oligomerize to nucleate the formation of ASC filaments, which in turn nucleate the effector caspase-1 to form another set of filaments. These signaling filaments coalesce into a micron-sized perinuclear puncta, which could be observed under light microscopes. Moreover, I also present evidence to support a capping mechanism for inflammasome regulation. Inhibitor of CARD (INCA) terminates caspase-1 filaments by binding to their ends, which effectively blocks proximity-driven activation of pro-caspase-1 and dampens IL-1β maturation. Both activation and regulation of the inflammasome rely on the special properties of homotypic interaction domains that belong to the death domain superfamily. Collectively, these studies support a novel paradigm in which the innate immune system exploits signaling filaments for the desired functional outcomes. By understanding the structural and biophysical properties of these supramolecular complexes, we may begin to identify novel targets for therapeutic intervention. In the last chapter, I will also discuss some remaining questions of the inflammasome field, particularly the ones that could be addressed by structural and biochemical methods.