Structure, Activation and Regulation of NLRP3 and AIM2 Inflammasomes

Abstract

The inflammasome is a three-component (sensor, adaptor, and effector) filamentous signaling platform that shields from multiple pathogenic infections by stimulating the proteolytical maturation of proinflammatory cytokines and pyroptotic cell death. The signaling process initiates with the detection of endogenous and/or external danger signals by specific sensors, followed by the nucleation and polymerization from sensor to downstream adaptor and then to the effector, caspase-1. Aberrant activation of inflammasomes promotes autoinflammatory diseases, cancer, neurodegeneration, and cardiometabolic disorders. Therefore, an equitable level of regulation is required to maintain the equilibrium between inflammasome activation and inhibition. Recent advancement in the structural and mechanistic understanding of inflammasome assembly potentiates the emergence of novel therapeutics against inflammasome-regulated diseases. In this review, we have comprehensively discussed the recent and updated insights into the structure of inflammasome components, their activation, interaction, mechanism of regulation, and finally, the formation of densely packed filamentous inflammasome complex that exists as micron-sized punctum in the cells and mediates the immune responses.

Keywords: AIM2; ASC (apoptosis-associated speck-like protein containing a CARD), NLRP3; NMR; inflammation; protein assembly; protein structure.

Figure 1

Structural details of NLRP3 and comparison with other pyrin domains (PYDs). (A) Schematic representation of NLRP3 domain organization; (B) 3D NMR solution-state structure of NLRP3^PYD (PDB ID: 2NAQ) [112]; superposition of 3D solution structures of different PYDs with NLRP3^PYD (blue, 2NAQ) which include NLRP1 (magenta, 1PN5); NLRP4, (ivory, 4EWI); NLRP7 (cyan, 2KM6); NLRP10 (orange, 2DO9); and NLRP12 (pink, 2L6A). Superimposed images are adapted from Figure 7A of [112]; (C) superposition of the crystallographic dimeric structure of NLRP3^PYD (green, 3QF2 [111]) onto the monomeric NMR structure (blue); the dimeric interface is shown in the right panel. These superposed images are adapted from Figure 7B of [112].

Figure 2

Activation of NLRC4 and NLRP3. (A) Cryo-EM structure of leucine-rich repeats (LRR) and NACHT domains of NLRP3 in association with NEK7 (PDB ID: 6NPY) [118]. The ribbon diagram is generated with PyMOL molecular graphics software; (B) structural organization and activation of NLRC4. This image is adapted from Figure 2C of [137]; (C) modelling of NLRP3 active conformation and NLRP3 dimer formation in association with NEK7. This image is adapted from Figure 5A,B of [118].

Figure 3

Structural details of AIM2. (A) Schematic representation of AIM2 domain organization; (B) crystal structure of AIM2^PYD with Maltose-binding protein (MBP) fusion tag (PDB ID: 3VD8) [161]; (C) crystal structure of AIM2^HIN in complex with dsDNA (PDB ID: 3RN2) [162]. The ribbon diagrams are generated with PyMOL molecular graphics software.

Figure 4

Structural details of ASC. (A) Schematic representation of ASC domain organization; (B) 3D NMR solution-state structure of full-length ASC (PDB ID: 2KN6) [183]. The ribbon diagram is generated with PyMOL molecular graphics software.

Figure 5

Structure comparison of caspase-activation and recruitment domains (CARDs) from different origins. (A) Superposition of Apaf-1-CARD (dark blue) [195] and iceberg (light blue) [199]. Superposition of RAIDD-CARD (pink) [200] and NOD-1-CARD (purple) [201]. Helix 1 (cylinder) in RAIDD-CARD, although not fragmented, is substantially bent and is shown as two cylinders; (B) electrostatic surface representation of CARDs in the same orientation as displayed in A. This figure is adapted from Figure S4 of [183].

Figure 6

Schematic representation of NLRP3 and AIM2 inflammasomes activation and assembly. Pathogen-associated molecular patterns such as LPS, crystalline/particulate ligands, K⁺ efflux, and ROS trigger the activation of NLRP3. TLR initiates the activation and nuclear translocation of NF-κB, which increases the synthesis of NLRP3 and IL-1β and IL-18 cytokines. AIM2 detects viral and bacterial dsDNA in the cytosol. Assembly of NLRP3 or AIM2 with ASC and procaspase-1 leads to the proximity-induced autoproteolytic maturation of caspase-1, functionalization of IL-1β and IL-18, and pyroptosis cell death mediated by the N-terminal fragment of gasdermin D. Inhibitors of NLRP3, ASC, caspase-1, and AIM2 are shown blue. The reference sources corresponding to the negative-staining electron micrographs are shown in square brackets in each image: ASC^FL adapted from Figure 6 of [198], filamentous NLRP3^PYD-NBD-ASC^PYD binary complex adapted from Figure 2 of [167]; filament of AIM2^FL from Figure 6 of [171], AIM2^FL filament with 600 bp dsDNA adapted from Figure 3 (Copyright National Academy of Science) of [174], and His-GFP-caspase-1^CARD/ASC^FL/AIM2^PYD ternary complex adapted from Figure 6 [167].