Recent advances in the mechanisms of NLRP3 inflammasome activation and its inhibitors

Abstract

The NLRP3 inflammasome is a multimeric protein complex that initiates an inflammatory form of cell death and triggers the release of proinflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome has been implicated in a wide range of diseases, including Alzheimer's disease, Prion diseases, type 2 diabetes, and some infectious diseases. It has been found that a variety of stimuli including danger-associated molecular patterns (DAMPs, such as silica and uric acid crystals) and pathogen-associated molecular patterns (PAMPs) can activate NLRP3 inflammasome, but the specific regulatory mechanisms of NLRP3 inflammasome activation remain unclear. Understanding the mechanisms of NLRP3 activation will enable the development of its specific inhibitors to treat NLRP3-related diseases. In this review, we summarize current understanding of the regulatory mechanisms of NLRP3 inflammasome activation as well as inhibitors that specifically and directly target NLRP3.

Fig. 1. Role of ion fluxes in NLRP3 inflammasome activation.

NLRP3 activator-induced K⁺ efflux leads to mitochondrial damage and mtROS production, which can induce enrichment of CLICs in plasma membrane to promote Cl^– efflux. CLIC-mediated Cl^– efflux can promote NEK7–NLRP3 interaction and subsequent NLRP3 inflammasome assembly. Excessive Ca²⁺ released from ER causes mitochondrial Ca²⁺ overload and mitochondrial damage, leading to mtROS production, which triggers NLRP3 inflammasome activation. The newly identified component of NLRP3 inflammasome NEK7, which can directly bind to NLRP3 protein, also requires K⁺ efflux, ROS production, and Cl^– efflux for NLRP3 inflammasome assembly

Fig. 2. Post-translational modifications (PTMs) regulate NLRP3 inflammasome activation.

Schematic structure and PTM sites on different domains of NLRP3 (LRR, NACHT, and PYD). BRCC3 and PTPN22 target the LRR domain of NLRP3 to promote NLRP3 inflammasome activation by deubiquitination and dephosphorylation, respectively. MARCH7 and FBXL2 ubiquitinate NLRP3 LRR domain to inhibit NLRP3 inflammasome activation. Phosphorylation of NLRP3 at S194 and S293 by JNK1 and PKD, respectively, are key events for NLRP3 inflammasome activation. PKA and ARIH2 can phosphorylate and ubiquitinate NLRP3 NACHT domain, respectively, to abrogate NLRP3 inflammasome activation. TRIM31 directly interacts with PYD domain of NLRP3 and promotes its K48-linked ubiquitination, leading to NLRP3 proteasomal degradation and inhibition. Dephosphorylation of NLRP3 PYD domain at S5 by PP2A promotes NLRP3–ASC as well as NLRP3 PYD–PYD interactions

Fig. 3. Noncanonical inflammasome activation.

LPS of Gram-negative bacteria activates TLR4 to induce caspase-11 transcription. On the other hand, outer membrane vesicles (OMVs) secreted by Gram-negative bacteria act as a vehicle that delivers LPS into the cytosol. LPS that accumulates in the cytosol is directly recognized by the CARD domain of caspase-11, leading to its oligomerization, and then the active caspase-11 cleaves the pore-forming protein gasdermin D within the linker between the N-terminal and C-terminal domains. The N-terminal domain of gasdermin D can induce pyroptosis and activate NLRP3 inflammasome through a K⁺ efflux-dependent process

Fig. 4. Schematic illustration of the mechanism of NLRP3 inhibitors tested via in vitro and in vivo experimental models.

Several molecules have shown inhibitory effects on NLRP3 activation, few of which have been validated in animal models. CY-09 and OLT1177 can inhibit the ATPase activity of the NACHT domain of NLRP3, which is critical for NLRP3 oligomerization. MCC950 is a compound that specifically inhibits NLRP3 inflammasome activation, but its molecular mechanism has not been fully elucidated. Tranilast directly binds to the NACHT domain of NLRP3 to inhibit NLRP3–NLRP3 interaction and the subsequent ASC oligomerization. Oridonin binds to cysteine 279 of NACHT via covalent bond formation to prevent NEK7–NLRP3 interaction and the subsequent NLRP3 inflammasome activation