ASC (PYCARD)

ASC (apoptosis-associated speck-like protein containing a CARD; gene name PYCARD) is a ~22 kDa bipartite adaptor protein that bridges upstream pattern recognition receptors (PRRs) — principally nlrp3, aim2, pyrin, and NLRC4 — to caspase-1 within inflammasome platforms. It is the obligate scaffold for most ASC-dependent inflammasomes and is directly relevant to inflammaging through its role concentrating and amplifying innate immune signaling in aged tissues.

Identity

UniProt: Q9ULZ3 (PYCD_HUMAN) — Swiss-Prot reviewed entry
NCBI Gene: 29108
HGNC symbol: PYCARD
Ensembl: ENSG00000103490
Mouse ortholog: Pycard (one-to-one)
Length: 195 amino acids (canonical isoform); the original Masumoto 1999 paper reported ~22 kDa by SDS-PAGE for the HL-60-derived protein ¹
Aliases in literature: ASC (most common), TMS1 (target of methylation-mediated silencing 1), CARD5

Domain structure

ASC has a simple bipartite architecture — two death-fold domains joined by a short linker:

Domain	Residues (approx.)	Homotypic partner
Pyrin domain (PYD)	1–105	NLRP1/2/3/6, AIM2, IFI16, NLRC4 (partial), pyrin (MEFV)
CARD domain	105–195	Pro-caspase-1, NLRC4 (direct)

Both interactions are homotypic (PYD-PYD, CARD-CARD) and operate via charge-complementary surface patches. Structural work by cryo-EM resolved that ASC polymerizes via its PYD into a three-start helical assembly (C3 symmetry); CARD domains project outward from the filament to nucleate CARD filaments of pro-caspase-1, driving its self-activation ².

Function

Inflammasome assembly and ASC speck formation

Upon danger signal detection, sensor PRRs (e.g., NLRP3 activated by potassium efflux, ATP, or MSU crystals) recruit ASC via PYD-PYD interaction. ASC then polymerizes into a single large (~1 µm diameter) perinuclear aggregate termed the ASC speck — one speck per activated cell, visible by immunofluorescence and serving as a proxy for inflammasome activation in imaging assays ³.

The speck concentrates thousands of ASC molecules, creating a platform that recruits and clusters pro-caspase-1 via CARD-CARD interaction. Proximity-induced autoproteolytic activation of caspase-1 follows, generating the p20/p10 active heterodimer that cleaves pro-IL-1beta and pro-IL-18 to their mature secreted forms, and cleaves gasdermin D to trigger pyroptotic cell death.

Extracellular ASC specks and prionoid propagation

A critical aging-relevant property: when a cell undergoes pyroptosis, assembled ASC specks are released extracellularly intact. These extracellular specks retain the ability to activate neighboring macrophages: phagocytosis of extracellular specks causes lysosomal damage, de novo nucleation of soluble ASC in recipient cells, and caspase-1 and IL-1β activation — amplifying the inflammatory signal beyond the initially activated cell ⁴. This downstream signaling in recipient cells is partially independent of NLRP3 (reduced but not abolished in Nlrp3−/− macrophages), implicating direct ASC seeding as the primary mechanism ⁴. This prionoid-like propagation has been proposed as a mechanism by which a localized sterile inflammatory event can escalate into tissue-wide or systemic inflammaging.

needs-human-replication — Extracellular speck propagation demonstrated in mouse models and cell culture; direct evidence in human aged tissues is limited.

Role in aging

Inflammaging and NLRP3 upregulation

Aged tissues accumulate more activated inflammasomes relative to young controls, reflected by elevated ASC speck frequency per cell and elevated circulating IL-1beta and IL-18. The upstream driver is primarily chronic NLRP3 upregulation — see nlrp3-inflammasome and chronic-inflammation for the evidence base. ASC is the obligate scaffold that translates NLRP3 upregulation into caspase-1 activation; Srinivasula 2002 established that full-length ASC (but not isolated PYD or CARD alone) is required for efficient caspase-1 activation, with the PYRIN domain serving as the oligomerization domain and the CARD as the effector domain ⁵. needs-replication — The specific requirement of ASC for NLRP3-mediated caspase-1 activation was subsequently demonstrated by genetic studies not cited on this page; Srinivasula 2002 used LPS/IFN stimulation models rather than NLRP3-specific activators.

TMAO-ASC axis in metabolic aging

A 2025 study demonstrated that adipocyte FMO3 (not solely hepatic FMO3) is a significant source of circulating TMAO in aging, and that TMAO binds directly to ASC, promoting caspase-1 activation and IL-1β production in white adipose tissue. Adipocyte-specific FMO3 knockout in C57BL/6J mice reduced aging-associated WAT inflammation, senescence, and metabolic dysfunction ⁶. The mechanism links gut microbiome-derived metabolite precursors → adipocyte FMO3 → TMAO → direct ASC binding → inflammasome activation → WAT inflammaging. needs-replication — Single study; direct structural resolution of the TMAO-ASC binding interface not yet published; mouse strain C57BL/6J; human adipocyte data limited to correlative expression analyses.

Evidence quality table

Dimension	Status	Notes
Pathway conserved in humans?	yes	PYCARD highly conserved; speck formation confirmed in human macrophages
Phenotype conserved in humans?	partial	Elevated IL-1beta/IL-18 in aged human plasma; direct speck quantification in aged human tissue sparse
Replicated in humans?	no	Genetic/interventional evidence in humans absent; observational only

Genetic associations

Methylation silencing: PYCARD/TMS1 is epigenetically silenced by promoter methylation in some cancers (breast, colon), eliminating apoptotic and inflammasome function in tumor cells — the flip side of the aging-relevant upregulation phenotype.
No GWAS hits for longevity phenotypes are documented for PYCARD as of this writing. needs-canonical-id — No GenAge entry found for PYCARD.

Therapeutic angles

ASC-PYD inhibitory peptides

Peptides derived from the ASC PYD surface have been used preclinically to block PYD-PYD interactions and interrupt speck assembly. These remain research tools only; no clinical candidate identified.

Upstream NLRP3 inhibitors

MCC950 (small-molecule NLRP3 inhibitor) acts upstream of ASC, blocking NLRP3 ATPase activity and preventing ASC recruitment. This is the most advanced pharmacological approach targeting this pathway — see nlrp3-inflammasome for clinical-stage status.

Anti-IL-1 biologics

Anakinra (IL-1Ra) and canakinumab (anti-IL-1beta mAb) bypass ASC/caspase-1 entirely by blocking the downstream cytokine. The CANTOS trial demonstrated proof-of-concept that anti-IL-1beta reduces cardiovascular events in humans with elevated hsCRP, providing indirect human evidence that this axis is therapeutically tractable ⁷. unsourced — CANTOS ASC-specific mechanistic link not cited directly; cite via nlrp3-inflammasome page.

Pathway membership

nlrp3-inflammasome — central scaffold protein in NLRP3, AIM2, pyrin, and IFI16 inflammasome platforms

Key interactors

caspase-1 — recruited via CARD-CARD; ASC speck is required for efficient caspase-1 activation in NLRP3/AIM2 contexts
nlrp3 — sensor PRR; recruits ASC PYD upon activation
il-1b — downstream substrate of ASC-activated caspase-1
gsdmd — downstream substrate; pore-forming executioner of pyroptosis (page pending — R24d)
bax — UniProt annotation notes BAX interaction in apoptotic contexts (distinct from inflammasome context)

Limitations and gaps

needs-human-replication — Speck accumulation in aged human tissues quantified in only a handful of small studies; no large-scale tissue atlas data.
no-mechanism — Mechanism by which chronic sterile stimuli selectively increase basal ASC speck frequency with aging is not fully characterized (epigenetic upregulation of NLRP3? Mitochondrial ROS? Both?).
needs-canonical-id — No GenAge HAGR entry for PYCARD. No druggability tier from Open Targets yet assigned.
dose-response-unclear — Effective dose range for ASC-PYD peptide inhibitors in vivo not established.

Footnotes

doi:10.1074/jbc.274.48.33835 · Masumoto J et al. · J Biol Chem 1999 · n=N/A · in-vitro · model: HL-60 human promyelocytic leukemia cells · original ASC cloning and characterization; 536 citations ↩
doi:10.1016/j.cell.2014.02.008 · Lu A et al. · Cell 2014 · n=N/A · structural (cryo-EM) · model: recombinant human ASC filaments · unified polymerization mechanism for ASC-dependent inflammasomes; 1284 citations ↩
doi:10.1007/978-1-62703-523-1_8 · Stutz A, Horvath GL, Monks BG, Latz E · Methods Mol Biol 2013 · review/methods · ASC speck formation as quantitative readout for inflammasome activation; 329 citations · no-fulltext-access — book chapter, not_oa; full PDF not in archive ↩
doi:10.1038/ni.2913 · Franklin BS et al. · Nat Immunol 2014 · n=N/A · in-vitro + in-vivo (mouse) · model: murine BMDMs + peritonitis model · extracellular ASC specks propagate inflammation in neighboring cells; 737 citations ↩ ↩²
doi:10.1074/jbc.c200179200 · Srinivasula SM et al. · J Biol Chem 2002 · n=N/A · in-vitro · model: cell-free reconstitution + cell lines · establishes ASC as bipartite PYRIN-CARD adaptor for caspase-1; 561 citations ↩
doi:10.1038/s41467-025-63905-1 · Ganapathy T et al. · Nat Commun 2025 · n=N/A · in-vivo (aged mice) + mechanistic · model: C57BL/6J aging (young 8-wk, middle-aged 48–50-wk, old 85–105-wk) + adipocyte-specific FMO3 knockout (Adipo-FMO3 KO) · adipocyte FMO3-derived TMAO directly binds ASC and promotes caspase-1 activation and IL-1β production in aging WAT; proteomics identified TMAO-interacting inflammasome proteins; locally available PDF ↩
doi:10.1056/NEJMoa1707914 · Ridker PM et al. (CANTOS trial) · N Engl J Med 2017 · n=10,061 · RCT · model: human post-MI patients with hsCRP ≥2 mg/L · canakinumab reduces cardiovascular events; indirect evidence for IL-1beta axis in human aging-related inflammation ↩

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