Fantastic voyage: The journey of NLRP3 inflammasome activation

Abstract

NLRP3 inflammasome, an intracellular multiprotein complex, can be activated by a range of pathogenic microbes or endogenous hazardous chemicals. Its activation results in the release of cytokines such as IL-1β and IL-18, as well as Gasdermin D which eventually causes pyroptosis. The activation of NLRP3 inflammasome is under strict control and regulation by numerous pathways and mechanisms. Its excessive activation can lead to a persistent inflammatory response, which is linked to the onset and progression of severe illnesses. Recent studies have revealed that the subcellular localization of NLRP3 changes significantly during the activation process. In this review, we review the current understanding of the molecular mechanism of NLRP3 inflammasome activation, focusing on the subcellular localization of NLRP3 and the associated regulatory mechanisms. We aim to provide a comprehensive understanding of the dynamic transportation, activation, and degradation processes of NLRP3.

Keywords: Inflammasome activation; Innate immunity; NLRP3; NLRP3 inflammasome; Subcellular localization.

Figure 1

Domain structure of NLRP3 inflammasome complex. NLRP3 contains three main domains: PYD, NATCH, and LRR. The NATCH domain can be divided into four subdomains, labeled NBD, HD1, WHD, and HD2. Adaptor ASC contains PYD and CARD domains, while pro-caspase-1 contains caspase-1 (P20, P10) and CARD domains. The interaction between NATCH/LRR and NEK7 can induce activation and oligomerization of NLRP3, leading to the formation of an NLRP3 fiber mediated by the PYD domain, further transitions to ASC PYD fiber. Finally, the pro-caspase-1 is recruited and assembled onto the fiber, activated, and self-cleaved to produce mature caspase-1.

Figure 2

Canonical, non-canonical, and alternative modes of NLRP3 inflammasome signaling. Canonical NLRP3 inflammasome activation requires two steps in most cell types. During the priming step, inflammatory stimuli are sensed by TLRs, IL-1Rs, and TNFRs, which induce the expression of pro-IL-1β and NLRP3 via the NF-κB pathway. During the activation step, numerous PAMPs or DAMPs promote NLRP3 inflammasome assembly, inducing pro-caspase-1 self-cleavage and activation. Active caspase-1 cleaves the cytokine precursors pro-IL-1β and pro-IL-18 to produce active cytokines IL-1β and IL-18, respectively. It also cleaves gasdermin D (GSDMD) and releases its N-terminal domain, which transfers to the cell membrane and forms pores, leading to release and pyroptosis of the mature inflammatory cytokines. Non-canonical NLRP3 inflammasome activation occurs in response to cytosolic LPS sensed by caspase-4/5/11. Autocrine induction of NLRP3 inflammasome activation is triggered by K⁺ efflux due to GSDMD pore formation. Alternative NLRP3 inflammasome activation only requires a single signal and occurs through TLR4 activation in monocytes by the RIP1-FADD-caspase-8 pathway, which cannot induce K⁺ efflux, ASC speck formation, or pyroptosis.

Figure 3

Spatiotemporal journey of NLRP3 for inflammasome activation. LPS induces the expression of NLRP3 via the NF-κB pathway, and then NLRP3 exists mainly as a monomer or dimer in the cytoplasm. When NLRP3 senses mitochondrial dysfunction or ERS, it is recruited to mitochondria and ER by binding to cardiolipin, MAVS, STING and other molecules. Next, NLRP3 associates with SCAP-SREBP2 to form a ternary complex that translocates to the Golgi where NLRP3 incorporates with dTGN through the interaction between polybasic region and PtdIns4P. Finally, NLRP3 is delivered to MTOC by the HDAC6-dynein transport system or MARK4 along the microtubules, which leads to the interaction with NEK7 to facilitate NLRP3 inflammasome assembly. After the removal of the danger signals, the NLRP3 inflammasomes on MTOC is captured by autophagosomes and sent to lysosomes for degradation.