where its LRR domains are thought to bind the heat-shock chaperone protein HSP90 and the co-chaperone SGT1, which may hold NLRP3 in an inactive state (see Fig. 3.19). Several events seem to induce NLRP3 signaling: reduced intracellular potassium, the generation of reactive oxygen species (ROS), or the disruption of lysosomes by particulate or crystalline matter. The loss of intracellular potassium through efflux can occur during infection with, for example, intracellular bacteria such as Staphylococcus aureus that produce pore-forming toxins. Also, death of nearby cells can release ATP into the extracellular space; this ATP would activate the purinergic receptor P2X7, which itself is a potassium channel, and allow K+ ion efflux. In one model, it is the reduction of intracellular K+ concentration that triggers NLRP3 signaling by causing the dissociation of HSP90 and SGT1. A model proposed for ROS-induced NLRP3 activation involves the intermediate oxidization of sensor proteins collectively called thioredoxin (TRX). Normally TRX proteins are bound to thioredoxin-interacting protein (TXNIP), but oxidation of TRX by ROS causes the dissociation of TXNIP from TRX. The free TXNIP may then displace HSP90 and SGT1 from NLRP3, again causing its activation. In both of these cases, NLRP3 activation involves aggregation of multiple monomers via their LLR and NOD domains to induce signaling. Finally, phagocytosis of particulate matter, such as the adjuvant alum, a crystalline salt of aluminum potassium sulfate, may lead to the rupture of lysosomes and release of the active protease cathepsin B, which can activate NLRP3 by an unknown mechanism.
Rather than activating NFκB as in NOD1 and NOD2 signaling, NLRP3 signaling leads to the generation of pro-inflammatory cytokines and to cell death through formation of a multiprotein complex known as the inflammasome (see Fig. 3.19). Activation of the inflammasome proceeds in several stages. The first is the aggregation of LRR domains of several NLRP3 molecules, or other NLRP molecules, by a specific trigger or recognition event. This aggregation induces the pyrin domains of NLRP3 to interact with pyrin domains of another protein named ASC (also called PYCARD). ASC is an adaptor protein composed of an amino-terminal pyrin domain and a carboxy-terminal CARD domain. Pyrin and CARD domains are each able to form polymeric filamentous structures (Fig. 3.20). The interaction of NLRP3 with ASC further drives the formation of a polymeric ASC filament, with the pyrin domains in the center and CARD domains facing outward. These CARD domains then interact with CARD domains of the inactive protease pro-caspase 1, initiating
Fig. 3.20 The inflammasome is composed of several filamentous protein polymers created by aggregated CARD and pyrin domains.
Top panel: an electron micrograph of structures formed by full-length ASC, the pyrin domain of AIM2, and the CARD domain of caspase 1. The central dark region represents anti-ASC staining with a gold-labeled (15 nm) antibody. The long outward filaments represent the polymer composed of the caspase 1 CARD domain. Bottom panel: Schematic interpretation of NLRP3 inflammasome assembly. In this model, CARD regions of ASC and caspase 1 aggregate into a filamentous structure. The adaptor ASC translates aggregation of NLRP3 into aggregation of pro-caspase 1. Electron micrograph courtesy of Hao Wu.