Mechanisms governing inflammasome activation, assembly and pyroptosis induction

Abstract

Inflammasomes are multimeric protein complexes that regulate inflammatory responses and pyroptotic cell death to exert host defense against microbes. Intracellular pattern-recognition receptors such as nucleotide-binding domain and leucine-rich repeat receptors (NLRs) and absent in melanoma 2 like receptors (ALRs) assemble the inflammasome complexes in response to pathogens and danger or altered-self signals in the cell. Inflammasome sensors, in association with an adaptor protein-apoptosis-associated speck-like protein containing a caspase-activation and -recruitment domain (ASC)-activate inflammatory caspase-1 to enable the release of inflammatory cytokines and induce cell death, conferring host defense against pathogens. Beyond infectious diseases, the importance of inflammasomes is implicated in a variety of clinical conditions such as auto-inflammatory diseases, neuro-degeneration and metabolic disorders and the development of cancers. Understanding inflammasome activation and its molecular regulation can unveil therapeutic targets for controlling inflammasome-mediated disorders. In this review, we describe recent advances in inflammasome biology and discuss its activation, structural insights into inflammasome assembly and mechanisms for the execution of pyroptosis.

Keywords: ASC; NLRs; caspase-1; cell death; pathogens.

Fig. 1.

Inflammasome activation mechanisms. (A) Bacillus anthracis toxin containing protective antigen and lethal factor activate the human (contains pyrin domain) NLRP1 inflammasome by inducing cleavage at the N-terminal linker region (red dotted lane). Auto-proteolysis at the FIIND domain (black dotted lane) is also required for NLRP1 activation. The NLRP1 inflammasome activates caspase-1 via ASC-dependent recruitment to the inflammasome complex or by direct association with caspase-1 through CARD–CARD interactions. (B) Various pathogen-derived ligands (PAMPs) and physiological aberrations (DAMPs) activate NLRP3. Assembly of the NLRP3 inflammasome is categorized into canonical (caspase-11 independent) and non-canonical (caspase-11 dependent) inflammasome activation. NEK7 is an upstream activator of NLRP3 inflammasome assembly. (C) Pathogenic bacteria such as Salmonella enterica subspecies Typhimurium operate a T3SS to release effector proteins into the cytosol. These pathogen-associated proteins are recognized by a family of NAIPs and they further recruit NLRC4 to assemble the inflammasome complex. NLRC4 enables ASC-dependent or direct CARD–CARD interaction-dependent casapse-1 activation. (D) The DNA viruses such as cytomegalovirus (CMV) and vaccinia virus and intracellular bacteria such as Francisella release DNA during infection for activating the AIM2 inflammasome. IRF1 induces the expression of GBPs and IRGs to liberate DNA for AIM2 recognition. (E) Pyrin detects the modifications of Rho induced by Rho-inactivating toxins. All these inflammasomes recruit an adaptor protein called ASC, which contains PYD and CARD. ASC further brings caspase-1 to the inflammasome complex by CARD–CARD interactions for its activation. Activated caspase-1 drives the cleavage of pro-inflammatory cytokines pro-IL-1β and pro-IL-18 and also the processing of gasdermin-D protein for executing pyroptosis.

Fig. 2.

Assembly of the NLRC4 inflammasome complex and ASC specks. Bacterial ligands such as flagellin, rod and needle proteins of the T3SS bind to NAIPs, which relieves the auto-inhibitory conformation and enables the accessibility of the oligomerization/interacting surface. The interacting surface of NAIP acts as a scaffold and recruits inactive NLRC4. The binding of NLRC4 monomer to the NAIP leads to the fully activated conformation of the NLRC4. The fully activated NLRC4 in turn exposes its interaction surface to further recruit another NLRC4 protomer to progress the self-oligomerization process. This chain of co-operative recruitment assembles a wheel- or disc-shaped inflammasome complex, which has inner ring of NODs and an outer ring of LRR domains. The available structures for the NLRC4 inflammasome lack CARD domains, which preclude the prediction of its orientation in the inflammasome complex. The activated inflammasome recruits ASC, an adaptor protein. ASC further undergoes self-polymerization to form filamentous structures. These ASC filaments aggregate to form macro-molecular complexes known as ASC specks. The exposed CARD domains of ASC in turn recruit caspase-1 that also forms filamentous structures (not shown). The recruitment of caspase-1 to the ASC filaments leads to caspase-1 activation.

Fig. 3.

Induction of pyroptosis mediated by gasdermin-D. Pathogen-derived PAMPs or physiological aberrations (DAMPs) activate inflammatory caspases. These caspases cleave gasdermin-D to separate its N- and C-terminal domains. The N-terminal domain of gasdermin-D is targeted to the membrane and assembles large, permeable pore complexes in the plasma membrane of the cell to induce pyroptosis.