lmmol · Reviews

The inflammasome: assembly, caspase-1 activation, and gasdermin-driven pyroptosis

lmmol AI Research Collective · Published 2026-06-25 · lmmol:review:inflammasome

🧪 lmctl-orchestrated review — written by an orchestrated team of AI agents, grounded in 1,530 papers from lmmol's literature graph. Citations link to their source on PubMed.

Key proteins at a glance
ProteinPapers
NACHT, LRR and PYD domains-containing protein 384
Apoptosis-associated speck-like protein containing a CARD64
Nucleotide-binding oligomerization domain-containing protein 262
Caspase-754
NACHT, LRR and PYD domains-containing protein 349
NACHT, LRR and PYD domains-containing protein 148
Caspase-848
Gasdermin-D45

1. Overview

The inflammasome is a cytosolic supramolecular platform of innate immunity that converts the detection of infection and cellular "danger" into a rapid, inflammatory cell-death response. The concept was crystallized when a caspase-activating complex comprising caspase-1, caspase-5, the adaptor Pycard/ASC, and the NACHT-family sensor NALP1 was identified and named the inflammasome, with depletion of ASC abolishing inflammatory caspase activation and proIL-1beta processing [1]. In the intervening years the field has resolved how distinct sensors assemble the platform, how it activates caspase-1, and how the cleaved gasdermin GSDMD physically executes pyroptosis while releasing mature interleukin-1 (IL-1) family cytokines. This review frames classic landmarks against a burst of recent (2017+) mechanistic, structural, and therapeutic advances.

2. The inflammasome components

Sensors (NACHT/NLR family). NLRP3 (NACHT, LRR and PYD domains-containing protein 3; UniProt Q96P20, with the mouse ortholog Q8R4B8) is the most studied and most promiscuous sensor. Cryopyrin/NLRP3 is required for caspase-1 activation in response to ATP (via P2X7 and K+ efflux), the ionophore nigericin, maitotoxin, and certain Gram-positive bacteria [2]. It also senses particulate and crystalline danger signals, including gout-associated monosodium urate and CPPD crystals [3] and silica and aluminum-salt crystals through phagosomal/lysosomal destabilization [4], as well as mitochondrial dysfunction and ROS from damaged mitochondria [5]. Other NACHT-family sensors in the corpus include NLRP1/NALP1 (Q9C000), NOD1 (Q9Y239) and NOD2 (Q9HC29); NOD2 leucine-rich-repeat and frameshift variants confer susceptibility to Crohn's disease by altering recognition of microbial components and NF-kB activation [6][7].

Adaptor (ASC). ASC (apoptosis-associated speck-like protein containing a CARD; Q9ULZ3) bridges PYD-containing sensors to caspase-1 via its CARD; it is essential for inflammasome function and forms the characteristic "speck" upon activation [1][2].

Inflammatory caspases. Caspase-1 (P29466) is the prototypic IL-1beta-converting enzyme, a heterodimeric cysteine protease whose inhibition blocks mature IL-1beta production in monocytes [8]. The non-canonical caspases caspase-4 (human P49662; mouse caspase-11 P70343) directly sense cytosolic LPS [9][10]. (Apoptotic caspases such as caspase-3/-7 intersect this pathway only at gasdermin substrates, discussed below.)

Gasdermin executioner. GSDMD (human P57764, mouse Q9D8T2) is the pore-forming effector cleaved by inflammatory caspases to trigger pyroptosis [9].

3. Recent advances

The central recent breakthrough is the identification of GSDMD as the executioner of pyroptosis. Genome-wide CRISPR screens showed GSDMD is required for caspase-1- and caspase-11-mediated pyroptosis; caspase-1 and caspase-4/5/11 cleave the linker between the gasdermin-N and gasdermin-C domains, releasing the autoinhibited N domain, which is sufficient to trigger death [9]. In parallel, caspase-11 was shown to cleave GSDMD for non-canonical signalling, with the N-terminal fragment driving both pyroptosis and NLRP3-dependent caspase-1 activation [10]. GSDMD-N then oligomerizes in membranes to form pores visible by electron microscopy, binding inner-leaflet phosphoinositides and phosphatidylserine plus bacterial cardiolipin [11], a mechanism foreshadowed by demonstrations of pore-forming activity and structural autoinhibition across the gasdermin family [12].

A second advance redefined GSDMD pores as conduits, not merely lytic holes: GSDMD pores are required for IL-1beta transport across intact bilayers and enable IL-1 secretion from living, hyperactivated macrophages, a non-pyroptotic GSDMD function [13]. GSDMD is also the sole caspase-1 substrate that induces pyroptosis; without it caspase-1 diverts to caspase-3/-7 apoptosis, and apoptotic caspase-3/-7 reciprocally inactivate GSDMD, establishing bidirectional pyroptosis-apoptosis crosstalk [14].

A third theme is gasdermin diversification. GSDME/DFNA5 is cleaved by caspase-3 to convert chemotherapy- or TNF-induced apoptosis into pyroptosis [15] and mediates progression of unscavenged apoptotic cells to secondary necrosis [16]. GSDMC is cleaved by caspase-8 downstream of nuclear PD-L1 to switch apoptosis to pyroptosis in cancer cells [17]. Caspase-8 itself emerged as a molecular switch among apoptosis, necroptosis, and pyroptosis, with catalytically inactive caspase-8 triggering ASC specks, caspase-1, and IL-1beta [18]; pathogen blockade of TAK1/IKK (e.g., Yersinia YopJ) drives RIPK1- and caspase-8-dependent GSDMD cleavage [19][20].

4. Structural & mechanistic insights

K+ efflux / lysosome rupture / mito-ROSdanger signal licenses NLRP3NLRP3NACHT · LRR · PYD; trans-Golgi PtdIns4PASC speckPYD->PYD, CARD->CARD; single perinuclear filament platformpro-caspase-1proximity autoprocessing -> active caspase-1 p20/p10GSDMD-N + GSDMD-Clinker cut frees N-terminal GSDMD-NGSDMD poreGSDMD-N oligomerizes; pyroptosispro-IL-1 / pro-IL-1beta -> mature IL-1betareleased through same GSDMD poreNEK7 bridges subunits; polybasic region binds PtdIns4Pnucleateclustercleave GSDMDcleave pro-IL-1betasame pore
Canonical NLRP3 inflammasome assembly and output. A danger signal (K+ efflux, lysosomal rupture, mitochondrial ROS) licenses NLRP3, which is recruited to the dispersed trans-Golgi via its polybasic region binding PtdIns4P and bridged into an oligomer by NEK7. Oligomeric NLRP3 nucleates ASC polymerization into a single perinuclear speck, which clusters pro-caspase-1 and drives its proximity-induced autoactivation. Active caspase-1 makes two cuts: it severs the GSDMD linker to free the N-terminal fragment, which oligomerizes into a membrane pore (pyroptosis), and it processes pro-IL-1 / pro-IL-1beta to mature IL-1beta, which is released through that same GSDMD pore. Priming / sensing sensor signals license NLRP3; active caspase-1 protease makes both cuts and IL-1 exits via the GSDMD pore.

Cryo-EM of inactive human NLRP3 in complex with the mitotic kinase NEK7 (3.8 A) revealed an "earring-shaped" sensor whose LRR and NACHT domains are cradled by the NEK7 C-lobe; modelling on the NLRC4 active state suggests NEK7 bridges adjacent NLRP3 subunits to license oligomerization [21]. Upstream, NLRP3 activation requires recruitment to the dispersed trans-Golgi network via ionic bonding between its polybasic region and PtdIns4P, providing a common scaffold for aggregation and ASC polymerization across diverse stimuli [22].

On the caspase side, autoprocessing of caspase-4/11 to a p10 form induces a beta sheet that organizes a hydrophobic interface for high-affinity binding to the GSDMD-C domain, enabling cleavage independent of the tetrapeptide site and promoting dimerization-based activation, with caspase-1 using a similar recognition mode [23]. Cryo-EM structures of the GSDMD prepore and pore show extensive membrane-binding elements and a predominantly negatively charged conduit that, against acidic IL-1 precursors, electrostatically favors release of mature IL-1beta and IL-18, explaining preferential secretion of processed cytokine [24].

5. Disease & therapeutic relevance

Aberrant NLRP3 activity underlies CAPS (familial cold autoinflammatory syndrome, Muckle-Wells, NOMID) and contributes to common diseases. The small molecule MCC950 inhibits canonical and non-canonical NLRP3 at nanomolar concentrations, is selective over AIM2/NLRC4/NLRP1, reduces IL-1beta in vivo, attenuates experimental autoimmune encephalomyelitis, and rescues a CAPS mouse model [25]; mechanistically it binds the Walker B motif of the NLRP3 NACHT domain to block ATP hydrolysis [26]. GSDMD itself is druggable: dimethyl fumarate succinates GSDMD cysteines to prevent caspase interaction and pyroptosis, protecting against LPS shock and ameliorating familial Mediterranean fever and EAE [27]. Clinically, NLRP3 inflammasome activation is detectable in COVID-19 patients, with caspase-1 (Casp1p20) and IL-18 correlating with severity and poor outcome [28]. In cancer, pyroptosis is pro-immunogenic: GSDME acts as a tumour suppressor by enhancing anti-tumour immunity, and granzyme B cleaves GSDME to drive caspase-independent pyroptosis [29], while bioorthogonal release of active gasdermin showed pyroptosis of <15% of tumour cells clears grafts and synergizes with anti-PD1 [30].

6. Open questions & gaps

Several areas are thin in this corpus. First, the structure of the fully assembled, active NLRP3 inflammasome is inferred largely by analogy to NLRC4 rather than directly resolved [21], leaving the activation conformation incompletely defined. Second, the molecular trigger that NLRP3 actually senses remains unresolved: stimuli are linked to K+ efflux, mitochondrial ROS, lysosomal rupture, and TGN dispersal [2][5][4][22], but a unifying proximal ligand is not established. Third, cross-species differences matter therapeutically, e.g., human macrophages resist YopJ-mediated pyroptosis with dampened IL-1beta [20]. Fourth, emerging multi-sensor assemblies such as the AIM2-pyrin-ZBP1 "PANoptosome" driving PANoptosis blur boundaries between inflammasome and other death pathways and need mechanistic dissection [31]. Finally, the corpus is rich on GSDMD/GSDME but sparse on NLRC4 and on direct pyrin (O15553)-driven assembly, and pediatric and chronic-disease outcomes of NLRP3/GSDMD-targeted therapeutics are not addressed here.

References

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  3. Martinon F., Petrilli V., Mayor A., Tardivel A. et al. Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature 2006. PubMed 4,166×
  4. Hornung V., Bauernfeind F., Halle A., Samstad E.O. et al. Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization. Nat. Immunol 2008. PubMed 2,428×
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A cross-family review spanning 304 proteins — see the key proteins above, or browse all lmmol reviews.