Inflammasomes: mechanism of action, role in disease, and therapeutics

Abstract

The inflammasomes are innate immune system receptors and sensors that regulate the activation of caspase-1 and induce inflammation in response to infectious microbes and molecules derived from host proteins. They have been implicated in a host of inflammatory disorders. Recent developments have greatly enhanced our understanding of the molecular mechanisms by which different inflammasomes are activated. Additionally, increasing evidence in mouse models, supported by human data, strongly implicates an involvement of the inflammasome in the initiation or progression of diseases with a high impact on public health, such as metabolic disorders and neurodegenerative diseases. Finally, recent developments pointing toward promising therapeutics that target inflammasome activity in inflammatory diseases have been reported. This review will focus on these three areas of inflammasome research.

Figure 1. Mechanisms of NLRP3 inflammasome activation

NLRP3 must be primed before activation. Priming involves two distinct steps. First, an NF-κB–activating stimulus, such as LPS binding to TLR4, induces elevated expression of NLRP3 (as well as IL1B), which leads to increased expression of NLRP3 protein. Additionally, priming immediately licenses NLRP3 by inducing its deubiquitination. The adaptor protein ASC must become linearly ubiquitinated and phosphorylated for inflammasome assembly to occur. After priming, canonical NLRP3 inflammasome activation requires a second, distinct signal to activate NLRP3 and lead to the formation of the NLRP3 inflammasome complex. The most commonly accepted activating stimuli for NLRP3 include relocalization of NLRP3 to the mitochondria, the sensation of mitochondrial factors released into the cytosol (mitochondrial ROS, mitochondrial DNA, or cardiolipin), potassium efflux through ion channels, and cathepsin release following destabilization of lysosomal membranes. Recent studies have determined that activated NLRP3 nucleates ASC into prion-like filaments through PYD-PYD interactions. Pro-caspase-1 filaments subsequently form off of the ASC filaments through CARD-CARD interactions, allowing autoproteolytic activation of pro-caspase-1. Inset shows domain arrangement of the NLRP3 inflammasome components. Pro-caspase-1 and caspase-1 domains are simplified for clarity, the CARD domain is actually removed by cleavage, and two heterodimers form with the p20 and p10 effector domains (p20/10).

Figure 2. Mechanisms of NLRC4, AIM2 and noncanonical NLRP3 inflammasome activation

(a) NLRC4 inflammasome agonists such as the bacterial needle protein bind directly to regions within the NACHT domains of the NAIP subfamily of proteins. hNAIP and mNAIP1 bind needle protein, mNAIP2 binds rod protein, and both mNAIP5 and mNAIP6 bind flagellin. Ligand-bound NAIP proteins then oligomerize with NLRC4 to form a caspase-1–activating inflammasome. Though NLRC4 can directly oligomerize with caspase-1 through CARD-CARD interactions, ASC is required for caspase-1 activation by the NLRC4 inflammasome, possibly through the formation of prion-like filaments (blue) by ASC. However, ASC is dispensable for the induction of pyroptosis. Inset shows domain arrangement of NLRC4 inflammasome components. NAIP proteins have three N-terminal BIR domains. hNAIP, human NAIP; mNAIP, mouse NAIP. (b) The mechanism of AIM2 inflammasome activation is well defined. The HIN domain of AIM2 directly binds cytosolic dsDNA, displacing the PYD and relieving autoinhibition. This allows oligomerization of AIM2 PYD with ASC PYD, converting ASC into its prion form. Prion-like filaments of pro-caspase-1 (violet) are then able to form off of the ASC filaments, inducing caspase-1 activation. Inset shows domain arrangement of AIM2 inflammasome components. (c) Studies have determined that mouse pro-caspase-11 (mPro-caspase-11) and human pro-caspases-4 and -5 (hPro-caspase-4/5) can directly bind intracellular LPS and activate a noncanonical NLRP3 inflammasome. This induces oligomerization of these pro-caspases, leading to their proximity-induced activation. This is sufficient for the induction of pyroptosis but not for the processing of pro-IL-1β. However, active mCaspase-11 and hCaspase-4 can promote full assembly and activation of the NLRP3 inflammasome following a priming signal.

Figure 3. Mechanisms of NLRP3 inflammasome action in Alzheimer’s disease

In Alzheimer’s disease, CD36 mediates the internalization of soluble amyloid-β and its intracellular conversion to fibrillary amyloid-β. This leads to disruption of the phagolysosome and activation of the NLRP3 inflammasome due to cathepsin B release. However, this does not exclude the possibility that phagocytosis of extracellular fibrillary amyloid-β also activates the NLRP3 inflammasome. Cathepsin B inhibition prevents amyloid-β–induced NLRP3 activation.

Figure 4. Mechanism of inflammasome activation in inflammatory disease

(a) In atherosclerosis, free fatty acids (FFA) can prime the NLRP3 inflammasome through TLR2-TLR4 signaling. Additionally, oxidized low-density lipoprotein (oxLDL) primes the NLRP3 inflammasome through a CD36-TLR4-TLR6 signaling complex. CD36 also facilitates the internalization of oxLDL and its intracellular conversion to cholesterol crystals, which disrupt the phagolysosome and activate the NLRP3 inflammasome through cathepsin release. Phagocytosis of extracellular cholesterol crystals may also contribute to inflammasome activation. Cathepsin inhibition prevents the NLRP3 inflammasome activation induced by cholesterol crystals. (b) In type 2 diabetes (T2D), the NLRP3 inflammasome is activated in both islet β-cells and myeloid cells. In β-cells, elevated glucose increases thioredoxin (TRX)-interacting protein (TXNIP). Intracellular ROS also causes a conformational change in TXNIP, leading to its dissociation from TRX. TXNIP then binds NLRP3 and promotes NLRP3 inflammasome activation. In myeloid cells, the endocannabinoid anandamide binds the CB1 receptor to increase the expression of NLRP3, ASC and IL1B. Saturated fatty acid (SFA) inhibits intracellular AMP-activated protein kinase (AMPK). This decreases autophagy, which leads to an increase in mitochondrial ROS (mtROS), a known NLRP3 inflammasome stimulus. CD36 facilitates the internalization of soluble islet amyloid polypeptide (IAPP), which is converted intracellularly to its amyloid form. This disrupts the phagolysosome and activates the NLRP3 inflammasome due to cathepsin release. As the amyloid form of IAPP builds up in the pancreatic islets of individuals with T2D, phagocytosis of extracellular amyloid IAPP may also contribute to NLRP3 inflammasome activation.