Molecular mechanisms regulating NLRP3 inflammasome activation

Abstract

Inflammasomes are multi-protein signaling complexes that trigger the activation of inflammatory caspases and the maturation of interleukin-1β. Among various inflammasome complexes, the NLRP3 inflammasome is best characterized and has been linked with various human autoinflammatory and autoimmune diseases. Thus, the NLRP3 inflammasome may be a promising target for anti-inflammatory therapies. In this review, we summarize the current understanding of the mechanisms by which the NLRP3 inflammasome is activated in the cytosol. We also describe the binding partners of NLRP3 inflammasome complexes activating or inhibiting the inflammasome assembly. Our knowledge of the mechanisms regulating NLRP3 inflammasome signaling and how these influence inflammatory responses offers further insight into potential therapeutic strategies to treat inflammatory diseases associated with dysregulation of the NLRP3 inflammasome.

Figure 1

Both signal 1 and signal 2 are required for NLRP3 inflammasome activation. Activation of the NLRP3 inflammasome requires at least two signals: signal 1, also known as the priming signal, is mediated by microbial ligands recognized by TLRs or cytokines such as TNF-α. Signal 1 activates the NF-κB pathway, leading to upregulation of pro-IL-1β and NLRP3 protein levels. The signal 2 is mediated by numerous PAMP or DAMP stimulation, and promotes the assembly of ASC and pro-caspase-1, leading to activation of the NLRP3 inflammasome complex. Under noninfectious conditions, extracellular ATP and K⁺ efflux leads to the activation of NLRP3 inflammasome via the P2X7 receptor and pannexin-1. Various endogenous and exogenous particulates, including MSU crystals, CPPD crystals, cholesterol crystals, amyloid β, silica crystals, asbestos, and alum, promote lysosomal damage and release cathepsin B into the cytosol, leading to the NLRP3 inflammasome activation. Particulate matters (uric acid, silica, and alum) are also able to trigger inflammasome assembly through multiple purinergic receptor signaling. Additionally, calcium influx through TRPM2 activates NLRP3 inflammasome through mitochondrial ROS. Dissociated TXNIP, which is triggered by intracellular ROS, also activates the NLRP3 inflammasome. ADP, adenosine diphosphate; ATP, adenosine triphosphate; K⁺, potassium; ASC, apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain; CPPD, calcium pyrophosphate dehydrate; DAMPs, damage-associated molecular patterns; NLRP3, NACHT, LRR, and PYD domains-containing protein 3; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; P2X7, P2X purinoceptor 7; P2R, purinergic receptor; PAMPs, pathogen-associated molecular patterns; ROS, reactive oxygen species; TLRs, toll-like receptors; TNF-α, tumor necrosis factor-α TXNIP, thioredoxin (TRX)-interacting protein.

Figure 2

Schematic models of the identified interacting partners for the assembly of NLRP3 inflammasome complex. In response to soluble agents or Salmonella infection, GBP5 is essentially involved in the triggering of the NLRP3 inflammasome activation through binding to the pyrin domain of NLRP3. During RNA viral infection, the GTPase DRP1 and its translocation to mitochondria, resulting in NLRP3 inflammasome activation. Another NLRP3 interacting protein DHX33 can bind to dsRNA as a cytosolic RNA sensor, leading to activation of the NLRP3 inflammasome. dsRNA-dependent protein kinase (PKR) is an interacting partner of NLRP3 and activates inflammasome complex. MAVS, a well-known mitochondrial protein and an interacting partner to NLRP3, also mediates NLRP3 mitochondrial localization and inflammasome activation. Cardiolipin binding to NLRP3 is also critical for ROS-dependent and -independent activation of NLRP3 inflammasome complex. In addition, LUBAC is involved in the activation of NLRP3 inflammasome complex through linear ubiquitination of ASC. Calcium-sensing receptors are also important for the promotion of NLRP3 inflammasome assembly through an increase of calcium influx. The interaction of NLRP3 with mitofusin 2 is required for NLRP3 inflammasome activation during RNA virus infection. NLRP3, NACHT, LRR, and PYD domains-containing protein 3; ASC, apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain; DAMPs, damage-associated molecular patterns; DHX33, DEAH box polypeptide 33; DRP1, dynamin-1-like protein; GBP5, guanylate binding protein 5; K⁺, potassium; LUBAC, linear ubiquitination assembly complex; MAM, mitochondria-associated membrane; MAVS, mitochondrial antiviral signaling protein; PAMPs, pathogen-associated molecular patterns; PKR, protein kinase R; RIP, receptor-interacting serine/threonine-protein kinase; ROS, reactive oxygen species.

Figure 3

Schematic models for the identified negative regulators in NLRP3 inflammasome activation. A20, a well-known inhibitor for NF-κB signaling, acts as a negative regulator of NLRP3 activation and caspase-1 processing. AhR binds to the xenobiotic response element in the NLRP3 promoter and inhibits NLRP3 transcription. Several molecules (e.g., NO, MNS, GPSM3, CARD8, IKKα) play a critical role in the modulation of NLRP3 inflammasome complex assembly. NO and MNS inhibit formation of the ASC pyroptosome and speck formation by targeting the NLRP3 complex. GPSM3 and CARD8 directly bind to NLRP3 and act as negative regulators of the NLRP3 inflammasome. IKKα negatively controls the NLRP3 inflammasome through interaction with the ASC adaptor molecule. LRRFIP2 interacts with Flightless-1, a pseudosubstrate of caspase-1, and inhibits caspase-1 activation. The orphan nuclear receptor SHP interacts with NLRP3, and mediates translocation of NLRP3 into mitochondria, thus regulating NLRP3 inflammasome activation. CO plays a general inhibitory role in mitochondrial ROS generation and translocation of mitochondrial DNA into cytosol. TRIM30 also inhibits the NLRP3 inflammasome activation through modulation of ROS generation, although there is no evidence that it can bind to any partner in NLRP3 inflammasome assembly. ASC, apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain; A20, tumor necrosis factor alpha-induced protein 3; AhR, aryl hydrocarbon receptor; CARD8, caspase recruitment domain; CO, carbon monoxide; IKKα, IκB kinase α GPSM3, G protein signaling modulator-3; LRRFIP2, leucine-rich repeat Fli-I-interacting protein 2; MNS, 3,4-methylenedioxy-β-nitrostyrene; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; NLRP3, NACHT, LRR, and PYD domains-containing protein 3; NO, nitric oxide; SHP, small heterodimer partner; TRIM30, tripartite-motif protein 30.