The NLRP1 and CARD8 inflammasomes

Abstract

Inflammasomes are multiprotein complexes that activate inflammatory cytokines and induce pyroptosis in response to intracellular danger-associated signals. NLRP1 and CARD8 are related germline-encoded pattern recognition receptors that form inflammasomes, but their activation mechanisms and biological purposes have not yet been fully established. Both NLRP1 and CARD8 undergo post-translational autoproteolysis to generate two non-covalently associated polypeptide chains. NLRP1 and CARD8 activators induce the proteasome-mediated destruction of the N-terminal fragment, liberating the C-terminal fragment to form an inflammasome. Here, we review the danger-associated stimuli that have been reported to activate NLRP1 and/or CARD8, including anthrax lethal toxin, Toxoplasma gondii, Shigella flexneri and the small molecule DPP8/9 inhibitor Val-boroPro, focusing on recent mechanistic insights and highlighting unresolved questions. In addition, we discuss the recently identified disease-associated mutations in NLRP1 and CARD8, the potential role that DPP9's protein structure plays in inflammasome regulation, and the emerging link between NLRP1 and metabolism. Finally, we summarize all of this latest research and consider the possible biological purposes of these enigmatic inflammasomes.

Keywords: CARD8; DPP8/9; NLRP1; Val-boroPro; anthrax lethal toxin; inflammasome; proteasome; pyroptosis.

FIGURE 1

Domain architecture of the NLRP1 inflammasome proteins. NLRP1 and CARD8 protein have FIIND and CARD domains and undergo autoproteolysis between the ZU5 and UPA subdomains that comprise the FIIND. NLRP1 proteins have NACHT and LRR domains preceding the FIIND, and human NLRP1 also has an N‐terminal PYD. Some rodent NLRP1 proteins are cleaved by lethal factor (LF) near their N‐termini. ASC contains a PYD and a CARD, and pro‐CASP1 contains a CARD preceding its catalytic p20 and p10 subunits

FIGURE 2

LF activation of the NLRP1B inflammasome. NLRP1B undergoes post‐translational autoproteolysis after its ZU5 subdomain to generate N‐ and C‐terminal fragments that remain non‐covalently associated. LF cleaves between residues K44 and L45 in the N‐terminal fragment, generating an destabilized N‐terminal residue. The N‐end rule E3 ligase UBR2 recognizes and ubiquitinates this neo‐N‐terminus, inducing its proteasome‐mediated degradation. The non‐covalently bound C‐terminal fragment is then freed to recruit and activate pro‐CASP1

FIGURE 3

The molecular decoy hypothesis. In principle, pathogen‐derived activities may target host NLR proteins (top) for destruction in order to enhance pathogen replication. NLRP1 (bottom) may act as a decoy for this host protein (or proteins), sensing the destruction of its N‐terminal fragment to induce an immune response

FIGURE 4

An indirect mechanism to sense pathogen‐associated activities. DPP8/9 inhibitors, T gondii infection and metabolic inhibitors may induce the same perturbation within cells, which in turn activates an E3 ligase to ubiquitinate and degrade the NLRP1 N‐terminus. In this model, unlike the “decoy” model, NLRP1 indirectly senses pathogen‐associated activities

FIGURE 5

Contribution of DPP9 binding to hNLRP1 and CARD8 activation. Inhibition of DPP8/9’s catalytic activity induces the degradation of the hNLRP1 and CARD8 N‐termini, triggering inflammasome assembly. In addition, DPP9 binds to the FIINDs of both hNLRP1 and CARD8, potentially in order to help stabilize the proteins. VbP disrupts the hNLRP1‐DPP9 (A) but not CARD8‐DPP9 (B), interaction. Direct displacement may contribute to hNLRP1 inflammasome activation by destabilizing hNLRP1 and releasing its C‐terminus from autoinhibition

FIGURE 6

Mutations in hNLRP1 that cause autoinflammatory disease. A, The indicated mutations in the N‐terminal fragment of hNLRP1 potentially destabilize this fragment or interfere with its ability to inhibit the C‐terminal fragment. Transparency is used to indicate the possible increased proteasome‐mediated degradation of this fragment. B, The P1214R mutation, which is located immediately after the autoproteolysis site, disrupts the DPP9 binding interaction and causes spontaneous inflammasome activation