The mechanism of NLRP3 inflammasome activation and its pharmacological inhibitors

Abstract

NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3) is a cytosolic pattern recognition receptor (PRR) that recognizes multiple pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Once activated, NLRP3 initiates the inflammasome assembly together with the adaptor ASC and the effector caspase-1, leading to caspase-1 activation and subsequent cleavage of IL-1β and IL-18. Aberrant NLRP3 inflammasome activation is linked with the pathogenesis of multiple inflammatory diseases, such as cryopyrin-associated periodic syndromes, type 2 diabetes, non-alcoholic steatohepatitis, gout, and neurodegenerative diseases. Thus, NLRP3 is an important therapeutic target, and researchers are putting a lot of effort into developing its inhibitors. The review summarizes the latest advances in the mechanism of NLRP3 inflammasome activation and its pharmacological inhibitors.

Keywords: NLRP3; inflammasome; inflammatory diseases; pattern recognition receptor; pharmacological inhibitors.

Figure 1

NLRP3 inflammasome. (A) Domain organization of NLRP3, ASC, and caspase-1. NLRP3 contains an amino­terminal PYRIN (PYD) domain, a nucleotide­binding NACHT domain, and a carboxy­terminal leucine­rich repeat (LRR) domain. The PYD domain is important for recruitment of ASC; the NACHT domain contains ATPase activity required for NLRP3 conformational change and oligomerization. The LRR domain is important for the formation of the NLRP3 cage, which is required to disperse the trans-Golgi network at the early stage of NLRP3 inflammasome activation. (B) NLRP3 inflammasome assembly. Upon activation, NLRP3 recruits ASC via PYD-PYD interactions and ASC recruits caspase-1 via CARD-CARD interactions.

Figure 2

Canonical NLRP3 inflammasome. Canonical NLRP3 inflammasome activation requires two signals: The signal 1 (priming) involves PAMPs or cytokines-induced NF-κB activation and upregulates the gene expression of NLRP3 and the proinflammatory cytokines. Signal 2 is provided by various PAMPs and DAMPs; they activate multiple upstream signaling events, such as K⁺ efflux, Ca²⁺ flux, Cl^- efflux, mtROS production, release of oxidized mitochondrial DNA, lysosome damage, and Golgi disassembly (dispersed trans-Golgi network). NLRP3 inflammasome assembly triggers the auto-cleavage of caspase-1, which in turn cleaves pro–IL-1β and pro–IL-18. Active caspase-1 also cleaves GSDMD and releases GSDMD-N from the autoinhibition and free GSDMD-N oligomerizes in membranes to form pores and induce pyroptosis.

Figure 3

Non-canonical and alternative inflammasome. Non-canonical inflammasome activation is triggered by caspase-4/5/11 activation upon recognition of intracellular LPS, followed by cleavage of GSDMD and subsequent pyroptosis, causing the potassium efflux, which then induces canonical NLRP3 inflammasome. The alternative NLRP3 inflammasome activation refers to the signal 1 only inflammasome response. In monocytes, TLR ligands alone trigger IL-1β secretion via TLR4-TRIF-RIPK1-FADD-CASP8 signaling upstream of NLRP3.

Figure 4

Schematic illustration of targets of NLRP3 inflammasome inhibitors. Upon activation, NLRP3 oligomerizes through interactions between NACHT domains, then recruiting ASC via PYD-PYD interactions, assembled ASC recruits caspase-1, forming NLRP3 inflammasome. NKE7 binds to NLRP3; the interaction is necessary for NLRP3 inflammasome assembly. HSP90 is suggested to bind NLRP3 to promote its activation. K⁺ efflux, Ca²⁺ flux, Cl^- efflux, and mtROS production are upstream signaling events of NLRP3 activation. The targets of NLRP3 inflammasome inhibitors are shown.