{"gene":"SMPD1","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":1992,"finding":"SMPD1 encodes acid sphingomyelinase (ASM), a lysosomal phosphodiesterase that hydrolyzes sphingomyelin to ceramide and phosphocholine. The gene contains six exons and five introns, with exon 2 encoding ~44% of the mature polypeptide. Alternative splicing of exon 3 generates multiple transcript isoforms.","method":"Genomic cloning, nucleotide sequencing, exon/intron junction analysis","journal":"Genomics","confidence":"High","confidence_rationale":"Tier 1 — complete gene structure and biochemical function established by primary sequencing and structural analysis","pmids":["1740330"],"is_preprint":false},{"year":1991,"finding":"The SMPD1 gene was localized to chromosomal region 11p15.1–p15.4 by PCR analysis of somatic cell hybrids and confirmed by in situ hybridization.","method":"Somatic cell hybrid PCR concordance analysis and in situ hybridization","journal":"Genomics","confidence":"High","confidence_rationale":"Tier 1 — two independent orthogonal mapping methods converge on the same chromosomal assignment","pmids":["2004772"],"is_preprint":false},{"year":1998,"finding":"Acidic sphingomyelinase (ASM, encoded by SMPD1) is required for Fas-induced GD3 ganglioside accumulation and efficient apoptosis in lymphoid cells. NPD lymphoblasts lacking ASM activity fail to activate ASM after Fas cross-linking, fail to accumulate GD3, and show impaired apoptosis; mannose receptor-mediated transfer of ASM rescues GD3 accumulation and rapid apoptosis.","method":"Patient-derived NPD lymphoblast cell lines (loss-of-function genetic model), exogenous ceramide rescue, mannose receptor-mediated ASM transfer, GD3 accumulation assay","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — genetic loss-of-function with specific phenotypic readout, rescue by substrate bypass and by protein reconstitution, replicated by orthogonal approaches","pmids":["9500792"],"is_preprint":false},{"year":2006,"finding":"SMPD1 is paternally imprinted; differential methylation of CpG dinucleotides in the SMPD1 promoter results in preferential maternal allele expression, influencing residual ASM activity and disease severity in Niemann-Pick disease.","method":"Genomic sequencing vs. RT-PCR allele discrimination, 5-aza-2'-deoxycytidine demethylation treatment, bisulfite genomic sequencing of SMPD1 promoter","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (allele-specific expression, demethylation rescue, bisulfite sequencing) in a single study","pmids":["16642440"],"is_preprint":false},{"year":2008,"finding":"Common NPD-causing SMPD1 mutations (L302P, H421Y, R496L, ΔR608) abolish ASM catalytic activity without preventing protein expression or lysosomal trafficking; mutant ASM polypeptides are expressed at normal levels and reach lysosomes but are enzymatically inactive. Co-immunoprecipitation with ER chaperone BiP confirmed proper folding attempts.","method":"In vitro and in situ enzyme assays, Western blotting, fluorescent microscopy, co-immunoprecipitation with BiP, 3D structural modeling, reticulocyte lysate expression, transgenic knock-in mice","journal":"Molecular genetics and metabolism","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal biochemical and cell-biological methods in one study","pmids":["18815062"],"is_preprint":false},{"year":2012,"finding":"TRAIL activates ASM in coronary arterial endothelial cells, causing ASM translocation to the plasma membrane, ceramide production, and NADPH oxidase aggregation in membrane raft clusters, thereby generating redox signaling platforms that impair endothelial function. SMPD1-knockout cells show absence of lysosome–membrane fusion and membrane raft clustering after TRAIL stimulation.","method":"Confocal microscopy, FRET imaging of lysosome–membrane raft co-trafficking, NADPH oxidase superoxide measurement, comparison of Smpd1+/+ vs. Smpd1−/− endothelial cells, vasodilation assays","journal":"Journal of molecular medicine","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with clear cellular/vascular phenotype, FRET-based trafficking assay, multiple readouts","pmids":["23108456"],"is_preprint":false},{"year":2016,"finding":"ASM (SMPD1) is a negative regulator of regulatory T cell (Treg) development. ASM-deficient mice have higher numbers of Tregs; in vitro induction with TGF-β and IL-2 produces more Foxp3+ iTregs in ASM-deficient T cells, associated with reduced AKT (Ser473) phosphorylation and lower Rictor levels. Exogenous ceramide (C6) reduces iTreg numbers in both ASM-deficient and WT cells.","method":"Flow cytometry on Smpd1−/− splenocytes and isolated naive T cells, in vitro Treg induction, CFSE proliferation assay, Western blotting, qRT-PCR, ceramide rescue experiment","journal":"Cellular physiology and biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined immunological phenotype and ceramide rescue, single lab","pmids":["27512981"],"is_preprint":false},{"year":2016,"finding":"ASM (SMPD1) controls autophagosome trafficking by regulating lysosomal TRPML1 channel-mediated Ca²⁺ release, which in turn activates dynein motor activity required for autophagosome–lysosome fusion in coronary arterial smooth muscle cells.","method":"Lysosomal Ca²⁺ imaging, dynein activity assay, TRPML1 gene silencing/overexpression, autophagosome trafficking imaging in Smpd1−/− vs. Smpd1+/+ CASMCs","journal":"Frontiers in bioscience (Landmark edition)","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with mechanistic follow-up (TRPML1 rescue), single lab","pmids":["26709800"],"is_preprint":false},{"year":2018,"finding":"ASM (Smpd1) controls autophagy maturation in vascular smooth muscle cells. PDGF-BB stimulation in Smpd1−/− SMCs causes prolonged Akt activation, reduced autophagosome biogenesis, p62/SQSTM1 accumulation, and a myofibroblast-like phenotypic transition; Akt inhibition or p62 silencing rescues this phenotype.","method":"Smpd1−/− primary SMC cultures, Western blotting, immunofluorescence, Akt inhibitor, siRNA silencing of p62, cytokine/adhesion molecule measurement","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with pharmacological and siRNA rescue, defined phenotypic readouts, single lab","pmids":["30451833"],"is_preprint":false},{"year":2018,"finding":"ASM (SMPD1) is a critical regulator of brain endothelial barrier integrity. Increased ASM in aged brain endothelium promotes caveolae-mediated transcytosis and blood-brain barrier (BBB) disruption via protein phosphatase 1-mediated dephosphorylation of ERM (ezrin/radixin/moesin) proteins, reducing caveolae-cytoskeleton interaction; genetic inhibition or endothelial-specific knockdown of ASM reduces BBB breakdown and neurocognitive impairment in aged mice.","method":"Endothelial-specific Smpd1 knockout and overexpression in mice, primary brain endothelial cell culture, caveolae imaging, ERM phosphorylation assays, BBB permeability measurement, cognitive testing","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific conditional KO and OE, mechanistic pathway (PP1/ERM), functional BBB and cognitive readouts","pmids":["30269989"],"is_preprint":false},{"year":2019,"finding":"The SMPD1 p.L302P and p.fsP330 mutations impair trafficking of ASM protein to the lysosome. SMPD1 knockout and knockdown in HeLa and dopaminergic BE(2)-M17 cells result in increased α-synuclein levels, suggesting ASM activity promotes α-synuclein clearance.","method":"CRISPR/Cas9 knockout and siRNA knockdown of SMPD1, lysosomal localization assays (immunofluorescence), α-synuclein immunoblotting, mass spectrometry-based ASM activity assay","journal":"Movement disorders","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO/KD with two cell lines, direct localization experiments for mutant proteins, single lab","pmids":["30788890"],"is_preprint":false},{"year":2019,"finding":"Lysosomal ceramide derived from ASM (Smpd1) drives phenotypic switching of vascular smooth muscle cells (VSCs) and small extracellular vesicle (sEV) secretion, contributing to arterial medial calcification. Smooth muscle-specific Smpd1 overexpression reduces lysosome-MVB interaction and increases sEV release; the ASM inhibitor amitriptyline prevents these effects.","method":"Smpd1 smooth-muscle-specific transgenic mice (Smpd1trg/SMcre), Vitamin D-induced calcification model, immunofluorescence (CD63, AnX2, ALP, LAMP1, VPS16), amitriptyline pharmacological inhibition, calcium deposition assay in CASMCs","journal":"Journal of cellular and molecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — tissue-specific overexpression model with pharmacological rescue, multiple cellular readouts, single lab","pmids":["31743567"],"is_preprint":false},{"year":2020,"finding":"Homozygous Smpd1 deficiency (ASM loss) aggravates brain ischemia/reperfusion injury by increasing ICAM-1 on cerebral microvessels and enhancing polymorphonuclear neutrophil (PMN) infiltration into the brain, while heterozygous deficiency is protective against mild focal ischemia. PMN depletion abrogated the increased injury in Smpd1−/− mice.","method":"Smpd1+/+, Smpd1+/−, Smpd1−/− mice; middle cerebral artery occlusion (MCAO) model; BBB permeability measurement; leukocyte/PMN quantification; anti-Ly6G PMN depletion","journal":"Basic research in cardiology","confidence":"Medium","confidence_rationale":"Tier 2 — three genetic dosage groups with depletion rescue, defined mechanistic pathway via ICAM-1/PMN, single lab","pmids":["33057972"],"is_preprint":false},{"year":2021,"finding":"Ceramide accumulation in preeclamptic trophoblasts drives TFEB nuclear translocation and lysosomal biogenesis; TFEB directly regulates SMPD1 expression, creating a positive feedback loop. Ceramide-induced lysosomal exocytosis translocates SMPD1 to the apical syncytiotrophoblast membrane and releases active SMPD1 via ceramide-enriched exosomes, which impair endothelial tubule formation and cause endothelial activation.","method":"In vitro and in vivo ceramide treatment, TFEB localization imaging, ChIP/reporter assay for TFEB–SMPD1 promoter binding, exosome isolation, endothelial cell functional assays, SMPD1 inhibitor rescue","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (promoter binding, localization, functional assays, pharmacological rescue), single lab","pmids":["34017832"],"is_preprint":false},{"year":2022,"finding":"Caspase-7 cleaves and activates ASM (SMPD1), which generates ceramide to enable membrane repair after gasdermin D pore formation, counteracting pore-driven lysis. This mechanism is required for intestinal epithelial cell extrusion during Salmonella infection (in organoids and mice) and for clearance of bacterial infections after perforin pore attack by NK cells and CTLs in hepatocytes. Caspase-7-deficient cells cannot complete extrusion or apoptosis, and ASM cleavage is the mechanistic link.","method":"Caspase-7-deficient mouse models, intestinal organoids, in vivo infection models (S. Typhimurium, C. violaceum, L. monocytogenes), biochemical cleavage assays, ceramide quantification, membrane repair assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1–2 — genetic KO in vivo and organoids, multiple infection models, direct biochemical cleavage demonstration, published in Nature with rigorous controls","pmids":["35705808"],"is_preprint":false},{"year":2016,"finding":"Resveratrol increases SMPD1 mRNA and ASM enzymatic activity in leukemia and cancer cells through transcriptional upregulation by EGR1 and EGR3 transcription factors, which directly bind the SMPD1 promoter, leading to ceramide accumulation and reduced sphingomyelin.","method":"ASM activity assay, RT-PCR, sphingolipid measurements, transcription factor overexpression with SMPD1 promoter reporter assay, EMSA, ChIP assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — direct promoter binding confirmed by EMSA and ChIP, promoter-reporter functional assay, single lab","pmids":["26809095"],"is_preprint":false},{"year":2021,"finding":"Sortilin facilitates trafficking of ASM from the trans-Golgi network to the lysosome. Sortilin knockdown in mouse prefrontal cortex and hippocampus reduces ASM/ceramide signaling, which decreases RhoA/ROCK2 activation and dendritic spine remodeling, ameliorating depressive-like behaviors.","method":"Sortilin knockdown/overexpression in vivo (CUMS mouse model), ASM inhibitor (SR33557) injection, ceramide level measurement, RhoA/ROCK2 activity assay, dendritic spine imaging","journal":"Acta pharmacologica Sinica","confidence":"Medium","confidence_rationale":"Tier 2 — genetic manipulation in vivo with pharmacological rescue, defined trafficking mechanism, single lab","pmids":["34931016"],"is_preprint":false},{"year":2020,"finding":"Decreased SMPD1 activity in bronchial airway epithelial cells causes pro-oxidative stress, NRF2 activation, increased cytokine production (IL-8, GRO-α, GM-CSF, CCL20), and enhanced neutrophil recruitment, even without infection. Expression of catalytically inactive SMPD1[L225P] but not wild-type ASM activates NRF2, indicating the effect is activity-dependent.","method":"Inducible shRNA knockdown of SMPD1 in BEAS-2B cells, NRF2 reporter assay, cytokine ELISA/qPCR, neutrophil chemotaxis assay, catalytically inactive mutant overexpression","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — activity-specific mutant vs. WT rescue distinguishes enzymatic from structural roles, single lab","pmids":["27865842"],"is_preprint":false},{"year":2021,"finding":"ASM (SMPD1) activity promotes NLRP3 inflammasome activation through a ceramide/CD36/NF-κB/TXNIP signaling axis in macrophages. ASM inhibition by imipramine suppresses LPS/ATP-induced ceramide accumulation and NLRP3/caspase-1/IL-1β/IL-18 expression; exogenous ceramide activates the inflammasome via CD36-dependent NF-κB activation of TXNIP.","method":"ASM activity assay, immunofluorescence, Western blotting, RT-PCR, ELISA, siRNA against TXNIP, CD36 inhibitor (SSO), NF-κB inhibitor (SN50), verapamil (TXNIP inhibitor), imipramine (ASM inhibitor)","journal":"Lipids in health and disease","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological dissection with multiple inhibitors and siRNA defining pathway order, single lab","pmids":["33612104"],"is_preprint":false},{"year":2020,"finding":"SMPD1 knockdown and knockout result in reduced ceramide, increased sphingomyelin, and 5-FU resistance in colorectal cancer cells, with siRNA-SMPD1 treatment of sensitive DLD-1 cells phenocopying the resistant state.","method":"MALDI-MS and LC-MRM-MS lipidomics, siRNA knockdown of SMPD1, drug resistance functional assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — quantitative lipidomics combined with functional gene knockdown, single lab","pmids":["32273521"],"is_preprint":false},{"year":2024,"finding":"Structure-based molecular dynamics simulations of ASM in a lysosomal membrane environment, combined with pathogenicity predictions, identified that SMPD1 missense variants exert pathogenic effects through destabilization of protein structure or through local and long-range effects at catalytic, zinc-binding, and substrate-binding functional sites; predictions were validated against experimental residual activity data for 135 variants.","method":"Molecular dynamics simulation in lysosomal membrane, pathogenicity prediction tools, validation against experimental ASM activity data for 135 variants","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"Low","confidence_rationale":"Tier 4 — primarily computational with experimental validation drawn from published data, not direct experimental mechanistic work","pmids":["38782304"],"is_preprint":false},{"year":2020,"finding":"The SMPD1 p.C133Y mutation in the N-terminal saposin domain completely abolishes ASM catalytic activity despite normal protein expression and proper lysosomal localization, demonstrating that the three-dimensional structure of the saposin domain is essential for catalytic activity.","method":"Patient fibroblast immunoblotting, COS-7 cell transient expression of mutant ASM, subcellular localization by immunofluorescence, enzyme activity assay","journal":"The Tohoku journal of experimental medicine","confidence":"Medium","confidence_rationale":"Tier 2 — structure-function analysis with specific domain mutant, localization confirmed separate from activity loss, single lab","pmids":["31941852"],"is_preprint":false}],"current_model":"SMPD1 encodes lysosomal acid sphingomyelinase (ASM), which hydrolyzes sphingomyelin to ceramide and phosphocholine; it is paternally imprinted, traffics to lysosomes via the trans-Golgi network (facilitated by sortilin), is activated by caspase-7 cleavage to enable membrane repair after gasdermin/perforin pore formation, and generates ceramide that regulates diverse downstream processes including Fas/GD3-mediated apoptosis, NLRP3 inflammasome activation via CD36/NF-κB/TXNIP, brain endothelial barrier integrity through PP1-mediated ERM dephosphorylation and caveolae-cytoskeleton uncoupling, autophagosome trafficking via TRPML1-Ca²⁺/dynein, vascular smooth muscle homeostasis, Treg development, and α-synuclein clearance."},"narrative":{"teleology":[{"year":1991,"claim":"Chromosomal mapping placed SMPD1 at 11p15.1–p15.4, establishing its genomic location and setting the stage for linkage to Niemann-Pick disease loci in this region.","evidence":"Somatic cell hybrid PCR and in situ hybridization","pmids":["2004772"],"confidence":"High","gaps":["Precise regulatory elements at this locus were not defined"]},{"year":1992,"claim":"Cloning of SMPD1 defined its six-exon gene structure and confirmed it encodes acid sphingomyelinase, the enzyme that hydrolyzes sphingomyelin to ceramide and phosphocholine, establishing the molecular identity underlying Niemann-Pick disease.","evidence":"Genomic cloning and nucleotide sequencing with exon/intron junction analysis","pmids":["1740330"],"confidence":"High","gaps":["Catalytic mechanism and domain requirements for activity not yet resolved","Trafficking route to lysosomes unknown"]},{"year":1998,"claim":"Demonstration that ASM is required for Fas-triggered GD3 ganglioside accumulation and apoptosis established ceramide generation as a signaling output, not merely a catabolic step, linking SMPD1 to receptor-mediated cell death.","evidence":"NPD patient lymphoblasts lacking ASM showed impaired Fas-induced apoptosis; mannose receptor-mediated ASM transfer and exogenous ceramide rescued the phenotype","pmids":["9500792"],"confidence":"High","gaps":["Whether ASM translocates to the plasma membrane during Fas signaling was not shown","Generality across non-lymphoid cell types untested"]},{"year":2006,"claim":"Discovery of paternal imprinting of SMPD1 revealed an epigenetic layer of regulation: differential promoter methylation causes preferential maternal allele expression, explaining variable residual ASM activity and disease severity among Niemann-Pick patients with identical genotypes.","evidence":"Allele-specific RT-PCR, bisulfite sequencing, and 5-aza-2'-deoxycytidine demethylation rescue","pmids":["16642440"],"confidence":"High","gaps":["Tissue-specific variation in imprinting not assessed","Whether imprinting status changes with age or disease progression is unknown"]},{"year":2008,"claim":"Structure–function analysis of NPD-causing mutations (L302P, H421Y, R496L, ΔR608) showed that these variants abolish catalytic activity without preventing lysosomal trafficking, separating the protein folding/sorting pathway from enzymatic function.","evidence":"In vitro/in situ enzyme assays, Western blotting, fluorescent microscopy, co-IP with BiP, 3D modeling in transfected cells and knock-in mice","pmids":["18815062"],"confidence":"High","gaps":["Precise structural basis for catalytic inactivation not resolved at atomic level","Whether these mutants exert dominant-negative effects was not tested"]},{"year":2012,"claim":"TRAIL-induced ASM translocation to the plasma membrane and ceramide-dependent clustering of NADPH oxidase in membrane rafts revealed that ASM-generated ceramide reorganizes signaling platforms to produce superoxide in endothelial cells, extending ASM's role to redox signaling.","evidence":"Confocal and FRET imaging, superoxide measurement in Smpd1+/+ vs. Smpd1−/− endothelial cells, vasodilation assays","pmids":["23108456"],"confidence":"High","gaps":["Identity of the signal that triggers ASM translocation in this context not defined","Whether this mechanism operates in non-endothelial vascular cells was untested"]},{"year":2016,"claim":"Two studies expanded ASM's downstream biology: ASM-derived ceramide suppresses Treg induction via AKT/Rictor signaling, and controls autophagosome trafficking through lysosomal TRPML1-Ca²⁺/dynein, connecting SMPD1 to adaptive immunity and autophagy maturation.","evidence":"Smpd1−/− splenocytes with ceramide rescue for Treg; lysosomal Ca²⁺ imaging and TRPML1 manipulation in Smpd1−/− coronary smooth muscle cells for autophagy","pmids":["27512981","26709800"],"confidence":"Medium","gaps":["Treg findings from a single lab; independent replication needed","Whether TRPML1 regulation by ceramide is direct or indirect is unclear","In vivo immune consequences of ASM-Treg axis not demonstrated"]},{"year":2016,"claim":"Identification of EGR1/EGR3 as direct transcriptional activators of the SMPD1 promoter provided the first defined transcription factor–promoter interaction for ASM expression, showing how external stimuli (resveratrol) increase ASM activity.","evidence":"EMSA, ChIP, and promoter-reporter assays in leukemia and cancer cells","pmids":["26809095"],"confidence":"Medium","gaps":["Physiological stimuli that use the EGR–SMPD1 axis remain undefined","Whether TFEB (identified later) and EGR factors cooperate on the SMPD1 promoter is unknown"]},{"year":2018,"claim":"ASM was shown to maintain blood–brain barrier integrity by restraining caveolae-mediated transcytosis: excess ASM in aged brain endothelium activates PP1, dephosphorylates ERM proteins, uncouples caveolae from the cytoskeleton, and increases BBB permeability, establishing a ceramide-driven mechanism of age-related neurovascular decline.","evidence":"Endothelial-specific Smpd1 KO and overexpression in mice, ERM phosphorylation assays, caveolae imaging, BBB permeability and cognitive testing","pmids":["30269989"],"confidence":"High","gaps":["How ASM expression increases with endothelial aging is not defined","Whether pharmacological ASM inhibition can reverse established BBB dysfunction is untested in vivo"]},{"year":2019,"claim":"SMPD1 knockout increases α-synuclein levels, and certain NPD mutations (L302P, fsP330) impair ASM lysosomal trafficking rather than only catalytic activity, linking ASM loss to Parkinson's disease-relevant protein accumulation and refining the genotype–phenotype spectrum.","evidence":"CRISPR/Cas9 KO and siRNA KD in HeLa and dopaminergic BE(2)-M17 cells, immunofluorescence for localization, α-synuclein immunoblotting","pmids":["30788890"],"confidence":"Medium","gaps":["Direct demonstration that α-synuclein accumulation leads to neurodegeneration in this model is lacking","Whether ASM acts on α-synuclein clearance through autophagy or an alternative lysosomal pathway is unresolved"]},{"year":2020,"claim":"The saposin domain was shown to be essential for catalytic function: the C133Y mutation in this domain abolishes activity despite normal expression and lysosomal localization, defining a critical structural requirement beyond the catalytic center.","evidence":"Patient fibroblasts and COS-7 transient expression of C133Y mutant with subcellular localization and enzyme activity assays","pmids":["31941852"],"confidence":"Medium","gaps":["Structural basis for saposin domain contribution to catalysis not resolved at atomic level","Single mutation study; generalizability across saposin domain variants not tested"]},{"year":2021,"claim":"Sortilin was identified as the receptor that mediates ASM trafficking from the trans-Golgi network to lysosomes; sortilin knockdown in brain reduces ASM/ceramide signaling and downstream RhoA/ROCK2-dependent dendritic spine remodeling, linking ASM trafficking to synaptic plasticity.","evidence":"Sortilin KD/OE in mouse prefrontal cortex and hippocampus, ASM inhibitor rescue, RhoA/ROCK2 activity assays, dendritic spine imaging in a CUMS depression model","pmids":["34931016"],"confidence":"Medium","gaps":["Whether sortilin is the sole trafficking receptor for ASM is unknown","Direct physical interaction between sortilin and ASM was not shown in this study"]},{"year":2021,"claim":"A ceramide–TFEB positive feedback loop was delineated in trophoblasts: ceramide drives TFEB nuclear translocation, TFEB directly activates SMPD1 transcription, and lysosomal exocytosis translocates ASM to the cell surface, implicating this circuit in preeclampsia pathogenesis.","evidence":"ChIP and reporter assays for TFEB–SMPD1 promoter binding, exosome isolation, endothelial functional assays, SMPD1 inhibitor rescue","pmids":["34017832"],"confidence":"Medium","gaps":["Whether this TFEB–SMPD1 loop operates in non-placental tissues is unknown","Upstream triggers of initial ceramide accumulation in preeclampsia remain unclear"]},{"year":2022,"claim":"The discovery that caspase-7 cleaves and activates ASM to generate ceramide for membrane repair after gasdermin D and perforin pore formation unified ASM's roles in innate immunity, cytotoxic lymphocyte killing, and epithelial homeostasis, establishing a direct biochemical activation mechanism.","evidence":"Caspase-7-deficient mice, intestinal organoids, multiple in vivo infection models (Salmonella, C. violaceum, L. monocytogenes), biochemical cleavage assays, ceramide quantification, membrane repair assays","pmids":["35705808"],"confidence":"High","gaps":["Whether other caspases can activate ASM in different contexts is unknown","The structural basis for caspase-7 cleavage site selectivity on ASM is not defined"]},{"year":null,"claim":"Key unresolved questions include: the high-resolution structural basis for ASM catalysis and substrate access at the lysosomal membrane, the full spectrum of transcriptional and post-translational inputs that regulate ASM activity in vivo, and whether pharmacological ASM modulation can be therapeutically exploited for neurodegeneration or vascular disease.","evidence":"","pmids":[],"confidence":"Low","gaps":["No co-crystal structure of ASM with substrate in a membrane environment","Systematic in vivo analysis of ASM regulation across tissues is lacking","Therapeutic window for ASM inhibition vs. augmentation not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,2,4,14,21]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,5]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[0,4,10,16,21]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,13,14]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[16]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,2,5,19]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[2,14]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[6,12,18]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[7,8]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,9,18]}],"complexes":[],"partners":["CASP7","SORT1","TFEB","TRPML1","PP1","CD36"],"other_free_text":[]},"mechanistic_narrative":"SMPD1 encodes lysosomal acid sphingomyelinase (ASM), a phosphodiesterase that hydrolyzes sphingomyelin to ceramide and phosphocholine, functioning as a central regulator of sphingolipid metabolism with broad roles in membrane dynamics, cell death signaling, autophagy, immune regulation, and vascular homeostasis. ASM is activated by caspase-7 cleavage to generate ceramide for plasma membrane repair following gasdermin D and perforin pore formation, a mechanism essential for epithelial cell extrusion during infection and for cytotoxic lymphocyte-mediated bacterial clearance [PMID:35705808]. The ceramide produced by ASM organizes membrane raft signaling platforms that mediate Fas-induced apoptosis via GD3 accumulation [PMID:9500792], NLRP3 inflammasome activation through a CD36/NF-κB/TXNIP axis [PMID:33612104], TRAIL-induced NADPH oxidase-dependent redox signaling [PMID:23108456], and blood–brain barrier regulation via PP1-mediated ERM dephosphorylation and caveolae-cytoskeleton uncoupling [PMID:30269989]. Loss-of-function mutations in SMPD1 cause Niemann-Pick disease types A and B, with disease severity modulated by paternal imprinting that results in preferential maternal allele expression [PMID:16642440, PMID:18815062]."},"prefetch_data":{"uniprot":{"accession":"P17405","full_name":"Sphingomyelin phosphodiesterase","aliases":["Acid sphingomyelinase","aSMase"],"length_aa":631,"mass_kda":69.9,"function":"Converts sphingomyelin to ceramide (PubMed:12563314, PubMed:1840600, PubMed:18815062, PubMed:25339683, PubMed:25920558, PubMed:27659707, PubMed:33163980). Exists as two enzymatic forms that arise from alternative trafficking of a single protein precursor, one that is targeted to the endolysosomal compartment, whereas the other is released extracellularly (PubMed:20807762, PubMed:21098024, PubMed:9660788). However, in response to various forms of stress, lysosomal exocytosis may represent a major source of the secretory form (PubMed:12563314, PubMed:20530211, PubMed:20807762, PubMed:22573858, PubMed:9393854) In the lysosomes, converts sphingomyelin to ceramide (PubMed:20807762, PubMed:21098024). Plays an important role in the export of cholesterol from the intraendolysosomal membranes (PubMed:25339683). Also has phospholipase C activities toward 1,2-diacylglycerolphosphocholine and 1,2-diacylglycerolphosphoglycerol (PubMed:25339683). Modulates stress-induced apoptosis through the production of ceramide (PubMed:8706124) When secreted, modulates cell signaling with its ability to reorganize the plasma membrane by converting sphingomyelin to ceramide (PubMed:12563314, PubMed:17303575, PubMed:20807762). Secreted form is increased in response to stress and inflammatory mediators such as IL1B, IFNG or TNF as well as upon infection with bacteria and viruses (PubMed:12563314, PubMed:20807762, PubMed:9393854). Produces the release of ceramide in the outer leaflet of the plasma membrane playing a central role in host defense (PubMed:12563314, PubMed:20807762, PubMed:9393854). Ceramide reorganizes these rafts into larger signaling platforms that are required to internalize P.aeruginosa, induce apoptosis and regulate the cytokine response in infected cells (PubMed:12563314). In wounded cells, the lysosomal form is released extracellularly in the presence of Ca(2+) and promotes endocytosis and plasma membrane repair (PubMed:20530211) This form is generated following cleavage by CASP7 in the extracellular milieu in response to bacterial infection (PubMed:21157428). It shows increased ability to convert sphingomyelin to ceramide and promotes plasma membrane repair (By similarity). Plasma membrane repair by ceramide counteracts the action of gasdermin-D (GSDMD) perforin (PRF1) pores that are formed in response to bacterial infection (By similarity) (Microbial infection) Secretion is activated by bacteria such as P.aeruginosa, N.gonorrhoeae and others, this activation results in the release of ceramide in the outer leaflet of the plasma membrane which facilitates the infection (Microbial infection) Secretion is activated by human coronaviruses SARS-CoV and SARS-CoV-2 as well as Zaire ebolavirus, this activation results in the release of ceramide in the outer leaflet of the plasma membrane which facilitates the infection Lacks residues that bind the cofactor Zn(2+) and has no enzyme activity Lacks residues that bind the cofactor Zn(2+) and has no enzyme activity","subcellular_location":"Secreted, extracellular 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Progressive Supranuclear Palsy-Richardson Syndrome Patients of Chinese Ancestry.","date":"2024","source":"Journal of movement disorders","url":"https://pubmed.ncbi.nlm.nih.gov/38291878","citation_count":6,"is_preprint":false},{"pmid":"27865842","id":"PMC_27865842","title":"Decreasing SMPD1 activity in BEAS-2B bronchial airway epithelial cells results in increased NRF2 activity, cytokine synthesis and neutrophil recruitment.","date":"2016","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/27865842","citation_count":6,"is_preprint":false},{"pmid":"24643943","id":"PMC_24643943","title":"A novel missense SMPD1 gene mutation, T460P, and clinical findings in a patient with Niemann-Pick disease type B presenting to a lipid disorders clinic.","date":"2014","source":"Annals of clinical biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/24643943","citation_count":6,"is_preprint":false},{"pmid":"22264577","id":"PMC_22264577","title":"Reduced 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The gene contains six exons and five introns, with exon 2 encoding ~44% of the mature polypeptide. Alternative splicing of exon 3 generates multiple transcript isoforms.\",\n      \"method\": \"Genomic cloning, nucleotide sequencing, exon/intron junction analysis\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — complete gene structure and biochemical function established by primary sequencing and structural analysis\",\n      \"pmids\": [\"1740330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"The SMPD1 gene was localized to chromosomal region 11p15.1–p15.4 by PCR analysis of somatic cell hybrids and confirmed by in situ hybridization.\",\n      \"method\": \"Somatic cell hybrid PCR concordance analysis and in situ hybridization\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — two independent orthogonal mapping methods converge on the same chromosomal assignment\",\n      \"pmids\": [\"2004772\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Acidic sphingomyelinase (ASM, encoded by SMPD1) is required for Fas-induced GD3 ganglioside accumulation and efficient apoptosis in lymphoid cells. NPD lymphoblasts lacking ASM activity fail to activate ASM after Fas cross-linking, fail to accumulate GD3, and show impaired apoptosis; mannose receptor-mediated transfer of ASM rescues GD3 accumulation and rapid apoptosis.\",\n      \"method\": \"Patient-derived NPD lymphoblast cell lines (loss-of-function genetic model), exogenous ceramide rescue, mannose receptor-mediated ASM transfer, GD3 accumulation assay\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with specific phenotypic readout, rescue by substrate bypass and by protein reconstitution, replicated by orthogonal approaches\",\n      \"pmids\": [\"9500792\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"SMPD1 is paternally imprinted; differential methylation of CpG dinucleotides in the SMPD1 promoter results in preferential maternal allele expression, influencing residual ASM activity and disease severity in Niemann-Pick disease.\",\n      \"method\": \"Genomic sequencing vs. RT-PCR allele discrimination, 5-aza-2'-deoxycytidine demethylation treatment, bisulfite genomic sequencing of SMPD1 promoter\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (allele-specific expression, demethylation rescue, bisulfite sequencing) in a single study\",\n      \"pmids\": [\"16642440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Common NPD-causing SMPD1 mutations (L302P, H421Y, R496L, ΔR608) abolish ASM catalytic activity without preventing protein expression or lysosomal trafficking; mutant ASM polypeptides are expressed at normal levels and reach lysosomes but are enzymatically inactive. Co-immunoprecipitation with ER chaperone BiP confirmed proper folding attempts.\",\n      \"method\": \"In vitro and in situ enzyme assays, Western blotting, fluorescent microscopy, co-immunoprecipitation with BiP, 3D structural modeling, reticulocyte lysate expression, transgenic knock-in mice\",\n      \"journal\": \"Molecular genetics and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal biochemical and cell-biological methods in one study\",\n      \"pmids\": [\"18815062\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"TRAIL activates ASM in coronary arterial endothelial cells, causing ASM translocation to the plasma membrane, ceramide production, and NADPH oxidase aggregation in membrane raft clusters, thereby generating redox signaling platforms that impair endothelial function. SMPD1-knockout cells show absence of lysosome–membrane fusion and membrane raft clustering after TRAIL stimulation.\",\n      \"method\": \"Confocal microscopy, FRET imaging of lysosome–membrane raft co-trafficking, NADPH oxidase superoxide measurement, comparison of Smpd1+/+ vs. Smpd1−/− endothelial cells, vasodilation assays\",\n      \"journal\": \"Journal of molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with clear cellular/vascular phenotype, FRET-based trafficking assay, multiple readouts\",\n      \"pmids\": [\"23108456\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ASM (SMPD1) is a negative regulator of regulatory T cell (Treg) development. ASM-deficient mice have higher numbers of Tregs; in vitro induction with TGF-β and IL-2 produces more Foxp3+ iTregs in ASM-deficient T cells, associated with reduced AKT (Ser473) phosphorylation and lower Rictor levels. Exogenous ceramide (C6) reduces iTreg numbers in both ASM-deficient and WT cells.\",\n      \"method\": \"Flow cytometry on Smpd1−/− splenocytes and isolated naive T cells, in vitro Treg induction, CFSE proliferation assay, Western blotting, qRT-PCR, ceramide rescue experiment\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined immunological phenotype and ceramide rescue, single lab\",\n      \"pmids\": [\"27512981\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ASM (SMPD1) controls autophagosome trafficking by regulating lysosomal TRPML1 channel-mediated Ca²⁺ release, which in turn activates dynein motor activity required for autophagosome–lysosome fusion in coronary arterial smooth muscle cells.\",\n      \"method\": \"Lysosomal Ca²⁺ imaging, dynein activity assay, TRPML1 gene silencing/overexpression, autophagosome trafficking imaging in Smpd1−/− vs. Smpd1+/+ CASMCs\",\n      \"journal\": \"Frontiers in bioscience (Landmark edition)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with mechanistic follow-up (TRPML1 rescue), single lab\",\n      \"pmids\": [\"26709800\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ASM (Smpd1) controls autophagy maturation in vascular smooth muscle cells. PDGF-BB stimulation in Smpd1−/− SMCs causes prolonged Akt activation, reduced autophagosome biogenesis, p62/SQSTM1 accumulation, and a myofibroblast-like phenotypic transition; Akt inhibition or p62 silencing rescues this phenotype.\",\n      \"method\": \"Smpd1−/− primary SMC cultures, Western blotting, immunofluorescence, Akt inhibitor, siRNA silencing of p62, cytokine/adhesion molecule measurement\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with pharmacological and siRNA rescue, defined phenotypic readouts, single lab\",\n      \"pmids\": [\"30451833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ASM (SMPD1) is a critical regulator of brain endothelial barrier integrity. Increased ASM in aged brain endothelium promotes caveolae-mediated transcytosis and blood-brain barrier (BBB) disruption via protein phosphatase 1-mediated dephosphorylation of ERM (ezrin/radixin/moesin) proteins, reducing caveolae-cytoskeleton interaction; genetic inhibition or endothelial-specific knockdown of ASM reduces BBB breakdown and neurocognitive impairment in aged mice.\",\n      \"method\": \"Endothelial-specific Smpd1 knockout and overexpression in mice, primary brain endothelial cell culture, caveolae imaging, ERM phosphorylation assays, BBB permeability measurement, cognitive testing\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific conditional KO and OE, mechanistic pathway (PP1/ERM), functional BBB and cognitive readouts\",\n      \"pmids\": [\"30269989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The SMPD1 p.L302P and p.fsP330 mutations impair trafficking of ASM protein to the lysosome. SMPD1 knockout and knockdown in HeLa and dopaminergic BE(2)-M17 cells result in increased α-synuclein levels, suggesting ASM activity promotes α-synuclein clearance.\",\n      \"method\": \"CRISPR/Cas9 knockout and siRNA knockdown of SMPD1, lysosomal localization assays (immunofluorescence), α-synuclein immunoblotting, mass spectrometry-based ASM activity assay\",\n      \"journal\": \"Movement disorders\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO/KD with two cell lines, direct localization experiments for mutant proteins, single lab\",\n      \"pmids\": [\"30788890\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Lysosomal ceramide derived from ASM (Smpd1) drives phenotypic switching of vascular smooth muscle cells (VSCs) and small extracellular vesicle (sEV) secretion, contributing to arterial medial calcification. Smooth muscle-specific Smpd1 overexpression reduces lysosome-MVB interaction and increases sEV release; the ASM inhibitor amitriptyline prevents these effects.\",\n      \"method\": \"Smpd1 smooth-muscle-specific transgenic mice (Smpd1trg/SMcre), Vitamin D-induced calcification model, immunofluorescence (CD63, AnX2, ALP, LAMP1, VPS16), amitriptyline pharmacological inhibition, calcium deposition assay in CASMCs\",\n      \"journal\": \"Journal of cellular and molecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific overexpression model with pharmacological rescue, multiple cellular readouts, single lab\",\n      \"pmids\": [\"31743567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Homozygous Smpd1 deficiency (ASM loss) aggravates brain ischemia/reperfusion injury by increasing ICAM-1 on cerebral microvessels and enhancing polymorphonuclear neutrophil (PMN) infiltration into the brain, while heterozygous deficiency is protective against mild focal ischemia. PMN depletion abrogated the increased injury in Smpd1−/− mice.\",\n      \"method\": \"Smpd1+/+, Smpd1+/−, Smpd1−/− mice; middle cerebral artery occlusion (MCAO) model; BBB permeability measurement; leukocyte/PMN quantification; anti-Ly6G PMN depletion\",\n      \"journal\": \"Basic research in cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — three genetic dosage groups with depletion rescue, defined mechanistic pathway via ICAM-1/PMN, single lab\",\n      \"pmids\": [\"33057972\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Ceramide accumulation in preeclamptic trophoblasts drives TFEB nuclear translocation and lysosomal biogenesis; TFEB directly regulates SMPD1 expression, creating a positive feedback loop. Ceramide-induced lysosomal exocytosis translocates SMPD1 to the apical syncytiotrophoblast membrane and releases active SMPD1 via ceramide-enriched exosomes, which impair endothelial tubule formation and cause endothelial activation.\",\n      \"method\": \"In vitro and in vivo ceramide treatment, TFEB localization imaging, ChIP/reporter assay for TFEB–SMPD1 promoter binding, exosome isolation, endothelial cell functional assays, SMPD1 inhibitor rescue\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (promoter binding, localization, functional assays, pharmacological rescue), single lab\",\n      \"pmids\": [\"34017832\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Caspase-7 cleaves and activates ASM (SMPD1), which generates ceramide to enable membrane repair after gasdermin D pore formation, counteracting pore-driven lysis. This mechanism is required for intestinal epithelial cell extrusion during Salmonella infection (in organoids and mice) and for clearance of bacterial infections after perforin pore attack by NK cells and CTLs in hepatocytes. Caspase-7-deficient cells cannot complete extrusion or apoptosis, and ASM cleavage is the mechanistic link.\",\n      \"method\": \"Caspase-7-deficient mouse models, intestinal organoids, in vivo infection models (S. Typhimurium, C. violaceum, L. monocytogenes), biochemical cleavage assays, ceramide quantification, membrane repair assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic KO in vivo and organoids, multiple infection models, direct biochemical cleavage demonstration, published in Nature with rigorous controls\",\n      \"pmids\": [\"35705808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Resveratrol increases SMPD1 mRNA and ASM enzymatic activity in leukemia and cancer cells through transcriptional upregulation by EGR1 and EGR3 transcription factors, which directly bind the SMPD1 promoter, leading to ceramide accumulation and reduced sphingomyelin.\",\n      \"method\": \"ASM activity assay, RT-PCR, sphingolipid measurements, transcription factor overexpression with SMPD1 promoter reporter assay, EMSA, ChIP assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct promoter binding confirmed by EMSA and ChIP, promoter-reporter functional assay, single lab\",\n      \"pmids\": [\"26809095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Sortilin facilitates trafficking of ASM from the trans-Golgi network to the lysosome. Sortilin knockdown in mouse prefrontal cortex and hippocampus reduces ASM/ceramide signaling, which decreases RhoA/ROCK2 activation and dendritic spine remodeling, ameliorating depressive-like behaviors.\",\n      \"method\": \"Sortilin knockdown/overexpression in vivo (CUMS mouse model), ASM inhibitor (SR33557) injection, ceramide level measurement, RhoA/ROCK2 activity assay, dendritic spine imaging\",\n      \"journal\": \"Acta pharmacologica Sinica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic manipulation in vivo with pharmacological rescue, defined trafficking mechanism, single lab\",\n      \"pmids\": [\"34931016\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Decreased SMPD1 activity in bronchial airway epithelial cells causes pro-oxidative stress, NRF2 activation, increased cytokine production (IL-8, GRO-α, GM-CSF, CCL20), and enhanced neutrophil recruitment, even without infection. Expression of catalytically inactive SMPD1[L225P] but not wild-type ASM activates NRF2, indicating the effect is activity-dependent.\",\n      \"method\": \"Inducible shRNA knockdown of SMPD1 in BEAS-2B cells, NRF2 reporter assay, cytokine ELISA/qPCR, neutrophil chemotaxis assay, catalytically inactive mutant overexpression\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — activity-specific mutant vs. WT rescue distinguishes enzymatic from structural roles, single lab\",\n      \"pmids\": [\"27865842\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ASM (SMPD1) activity promotes NLRP3 inflammasome activation through a ceramide/CD36/NF-κB/TXNIP signaling axis in macrophages. ASM inhibition by imipramine suppresses LPS/ATP-induced ceramide accumulation and NLRP3/caspase-1/IL-1β/IL-18 expression; exogenous ceramide activates the inflammasome via CD36-dependent NF-κB activation of TXNIP.\",\n      \"method\": \"ASM activity assay, immunofluorescence, Western blotting, RT-PCR, ELISA, siRNA against TXNIP, CD36 inhibitor (SSO), NF-κB inhibitor (SN50), verapamil (TXNIP inhibitor), imipramine (ASM inhibitor)\",\n      \"journal\": \"Lipids in health and disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological dissection with multiple inhibitors and siRNA defining pathway order, single lab\",\n      \"pmids\": [\"33612104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SMPD1 knockdown and knockout result in reduced ceramide, increased sphingomyelin, and 5-FU resistance in colorectal cancer cells, with siRNA-SMPD1 treatment of sensitive DLD-1 cells phenocopying the resistant state.\",\n      \"method\": \"MALDI-MS and LC-MRM-MS lipidomics, siRNA knockdown of SMPD1, drug resistance functional assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — quantitative lipidomics combined with functional gene knockdown, single lab\",\n      \"pmids\": [\"32273521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Structure-based molecular dynamics simulations of ASM in a lysosomal membrane environment, combined with pathogenicity predictions, identified that SMPD1 missense variants exert pathogenic effects through destabilization of protein structure or through local and long-range effects at catalytic, zinc-binding, and substrate-binding functional sites; predictions were validated against experimental residual activity data for 135 variants.\",\n      \"method\": \"Molecular dynamics simulation in lysosomal membrane, pathogenicity prediction tools, validation against experimental ASM activity data for 135 variants\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — primarily computational with experimental validation drawn from published data, not direct experimental mechanistic work\",\n      \"pmids\": [\"38782304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The SMPD1 p.C133Y mutation in the N-terminal saposin domain completely abolishes ASM catalytic activity despite normal protein expression and proper lysosomal localization, demonstrating that the three-dimensional structure of the saposin domain is essential for catalytic activity.\",\n      \"method\": \"Patient fibroblast immunoblotting, COS-7 cell transient expression of mutant ASM, subcellular localization by immunofluorescence, enzyme activity assay\",\n      \"journal\": \"The Tohoku journal of experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — structure-function analysis with specific domain mutant, localization confirmed separate from activity loss, single lab\",\n      \"pmids\": [\"31941852\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SMPD1 encodes lysosomal acid sphingomyelinase (ASM), which hydrolyzes sphingomyelin to ceramide and phosphocholine; it is paternally imprinted, traffics to lysosomes via the trans-Golgi network (facilitated by sortilin), is activated by caspase-7 cleavage to enable membrane repair after gasdermin/perforin pore formation, and generates ceramide that regulates diverse downstream processes including Fas/GD3-mediated apoptosis, NLRP3 inflammasome activation via CD36/NF-κB/TXNIP, brain endothelial barrier integrity through PP1-mediated ERM dephosphorylation and caveolae-cytoskeleton uncoupling, autophagosome trafficking via TRPML1-Ca²⁺/dynein, vascular smooth muscle homeostasis, Treg development, and α-synuclein clearance.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SMPD1 encodes lysosomal acid sphingomyelinase (ASM), a phosphodiesterase that hydrolyzes sphingomyelin to ceramide and phosphocholine, functioning as a central regulator of sphingolipid metabolism with broad roles in membrane dynamics, cell death signaling, autophagy, immune regulation, and vascular homeostasis. ASM is activated by caspase-7 cleavage to generate ceramide for plasma membrane repair following gasdermin D and perforin pore formation, a mechanism essential for epithelial cell extrusion during infection and for cytotoxic lymphocyte-mediated bacterial clearance [PMID:35705808]. The ceramide produced by ASM organizes membrane raft signaling platforms that mediate Fas-induced apoptosis via GD3 accumulation [PMID:9500792], NLRP3 inflammasome activation through a CD36/NF-κB/TXNIP axis [PMID:33612104], TRAIL-induced NADPH oxidase-dependent redox signaling [PMID:23108456], and blood–brain barrier regulation via PP1-mediated ERM dephosphorylation and caveolae-cytoskeleton uncoupling [PMID:30269989]. Loss-of-function mutations in SMPD1 cause Niemann-Pick disease types A and B, with disease severity modulated by paternal imprinting that results in preferential maternal allele expression [PMID:16642440, PMID:18815062].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Chromosomal mapping placed SMPD1 at 11p15.1–p15.4, establishing its genomic location and setting the stage for linkage to Niemann-Pick disease loci in this region.\",\n      \"evidence\": \"Somatic cell hybrid PCR and in situ hybridization\",\n      \"pmids\": [\"2004772\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise regulatory elements at this locus were not defined\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Cloning of SMPD1 defined its six-exon gene structure and confirmed it encodes acid sphingomyelinase, the enzyme that hydrolyzes sphingomyelin to ceramide and phosphocholine, establishing the molecular identity underlying Niemann-Pick disease.\",\n      \"evidence\": \"Genomic cloning and nucleotide sequencing with exon/intron junction analysis\",\n      \"pmids\": [\"1740330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic mechanism and domain requirements for activity not yet resolved\", \"Trafficking route to lysosomes unknown\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Demonstration that ASM is required for Fas-triggered GD3 ganglioside accumulation and apoptosis established ceramide generation as a signaling output, not merely a catabolic step, linking SMPD1 to receptor-mediated cell death.\",\n      \"evidence\": \"NPD patient lymphoblasts lacking ASM showed impaired Fas-induced apoptosis; mannose receptor-mediated ASM transfer and exogenous ceramide rescued the phenotype\",\n      \"pmids\": [\"9500792\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ASM translocates to the plasma membrane during Fas signaling was not shown\", \"Generality across non-lymphoid cell types untested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Discovery of paternal imprinting of SMPD1 revealed an epigenetic layer of regulation: differential promoter methylation causes preferential maternal allele expression, explaining variable residual ASM activity and disease severity among Niemann-Pick patients with identical genotypes.\",\n      \"evidence\": \"Allele-specific RT-PCR, bisulfite sequencing, and 5-aza-2'-deoxycytidine demethylation rescue\",\n      \"pmids\": [\"16642440\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific variation in imprinting not assessed\", \"Whether imprinting status changes with age or disease progression is unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Structure–function analysis of NPD-causing mutations (L302P, H421Y, R496L, ΔR608) showed that these variants abolish catalytic activity without preventing lysosomal trafficking, separating the protein folding/sorting pathway from enzymatic function.\",\n      \"evidence\": \"In vitro/in situ enzyme assays, Western blotting, fluorescent microscopy, co-IP with BiP, 3D modeling in transfected cells and knock-in mice\",\n      \"pmids\": [\"18815062\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise structural basis for catalytic inactivation not resolved at atomic level\", \"Whether these mutants exert dominant-negative effects was not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"TRAIL-induced ASM translocation to the plasma membrane and ceramide-dependent clustering of NADPH oxidase in membrane rafts revealed that ASM-generated ceramide reorganizes signaling platforms to produce superoxide in endothelial cells, extending ASM's role to redox signaling.\",\n      \"evidence\": \"Confocal and FRET imaging, superoxide measurement in Smpd1+/+ vs. Smpd1−/− endothelial cells, vasodilation assays\",\n      \"pmids\": [\"23108456\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the signal that triggers ASM translocation in this context not defined\", \"Whether this mechanism operates in non-endothelial vascular cells was untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Two studies expanded ASM's downstream biology: ASM-derived ceramide suppresses Treg induction via AKT/Rictor signaling, and controls autophagosome trafficking through lysosomal TRPML1-Ca²⁺/dynein, connecting SMPD1 to adaptive immunity and autophagy maturation.\",\n      \"evidence\": \"Smpd1−/− splenocytes with ceramide rescue for Treg; lysosomal Ca²⁺ imaging and TRPML1 manipulation in Smpd1−/− coronary smooth muscle cells for autophagy\",\n      \"pmids\": [\"27512981\", \"26709800\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Treg findings from a single lab; independent replication needed\", \"Whether TRPML1 regulation by ceramide is direct or indirect is unclear\", \"In vivo immune consequences of ASM-Treg axis not demonstrated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identification of EGR1/EGR3 as direct transcriptional activators of the SMPD1 promoter provided the first defined transcription factor–promoter interaction for ASM expression, showing how external stimuli (resveratrol) increase ASM activity.\",\n      \"evidence\": \"EMSA, ChIP, and promoter-reporter assays in leukemia and cancer cells\",\n      \"pmids\": [\"26809095\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological stimuli that use the EGR–SMPD1 axis remain undefined\", \"Whether TFEB (identified later) and EGR factors cooperate on the SMPD1 promoter is unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"ASM was shown to maintain blood–brain barrier integrity by restraining caveolae-mediated transcytosis: excess ASM in aged brain endothelium activates PP1, dephosphorylates ERM proteins, uncouples caveolae from the cytoskeleton, and increases BBB permeability, establishing a ceramide-driven mechanism of age-related neurovascular decline.\",\n      \"evidence\": \"Endothelial-specific Smpd1 KO and overexpression in mice, ERM phosphorylation assays, caveolae imaging, BBB permeability and cognitive testing\",\n      \"pmids\": [\"30269989\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How ASM expression increases with endothelial aging is not defined\", \"Whether pharmacological ASM inhibition can reverse established BBB dysfunction is untested in vivo\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"SMPD1 knockout increases α-synuclein levels, and certain NPD mutations (L302P, fsP330) impair ASM lysosomal trafficking rather than only catalytic activity, linking ASM loss to Parkinson's disease-relevant protein accumulation and refining the genotype–phenotype spectrum.\",\n      \"evidence\": \"CRISPR/Cas9 KO and siRNA KD in HeLa and dopaminergic BE(2)-M17 cells, immunofluorescence for localization, α-synuclein immunoblotting\",\n      \"pmids\": [\"30788890\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct demonstration that α-synuclein accumulation leads to neurodegeneration in this model is lacking\", \"Whether ASM acts on α-synuclein clearance through autophagy or an alternative lysosomal pathway is unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The saposin domain was shown to be essential for catalytic function: the C133Y mutation in this domain abolishes activity despite normal expression and lysosomal localization, defining a critical structural requirement beyond the catalytic center.\",\n      \"evidence\": \"Patient fibroblasts and COS-7 transient expression of C133Y mutant with subcellular localization and enzyme activity assays\",\n      \"pmids\": [\"31941852\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis for saposin domain contribution to catalysis not resolved at atomic level\", \"Single mutation study; generalizability across saposin domain variants not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Sortilin was identified as the receptor that mediates ASM trafficking from the trans-Golgi network to lysosomes; sortilin knockdown in brain reduces ASM/ceramide signaling and downstream RhoA/ROCK2-dependent dendritic spine remodeling, linking ASM trafficking to synaptic plasticity.\",\n      \"evidence\": \"Sortilin KD/OE in mouse prefrontal cortex and hippocampus, ASM inhibitor rescue, RhoA/ROCK2 activity assays, dendritic spine imaging in a CUMS depression model\",\n      \"pmids\": [\"34931016\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether sortilin is the sole trafficking receptor for ASM is unknown\", \"Direct physical interaction between sortilin and ASM was not shown in this study\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"A ceramide–TFEB positive feedback loop was delineated in trophoblasts: ceramide drives TFEB nuclear translocation, TFEB directly activates SMPD1 transcription, and lysosomal exocytosis translocates ASM to the cell surface, implicating this circuit in preeclampsia pathogenesis.\",\n      \"evidence\": \"ChIP and reporter assays for TFEB–SMPD1 promoter binding, exosome isolation, endothelial functional assays, SMPD1 inhibitor rescue\",\n      \"pmids\": [\"34017832\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether this TFEB–SMPD1 loop operates in non-placental tissues is unknown\", \"Upstream triggers of initial ceramide accumulation in preeclampsia remain unclear\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"The discovery that caspase-7 cleaves and activates ASM to generate ceramide for membrane repair after gasdermin D and perforin pore formation unified ASM's roles in innate immunity, cytotoxic lymphocyte killing, and epithelial homeostasis, establishing a direct biochemical activation mechanism.\",\n      \"evidence\": \"Caspase-7-deficient mice, intestinal organoids, multiple in vivo infection models (Salmonella, C. violaceum, L. monocytogenes), biochemical cleavage assays, ceramide quantification, membrane repair assays\",\n      \"pmids\": [\"35705808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other caspases can activate ASM in different contexts is unknown\", \"The structural basis for caspase-7 cleavage site selectivity on ASM is not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the high-resolution structural basis for ASM catalysis and substrate access at the lysosomal membrane, the full spectrum of transcriptional and post-translational inputs that regulate ASM activity in vivo, and whether pharmacological ASM modulation can be therapeutically exploited for neurodegeneration or vascular disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No co-crystal structure of ASM with substrate in a membrane environment\", \"Systematic in vivo analysis of ASM regulation across tissues is lacking\", \"Therapeutic window for ASM inhibition vs. augmentation not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 2, 4, 14, 21]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [0, 4, 10, 16, 21]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 13, 14]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 2, 5, 19]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [2, 14]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [6, 12, 18]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 9, 18]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"CASP7\",\n      \"SORT1\",\n      \"TFEB\",\n      \"TRPML1\",\n      \"PP1\",\n      \"CD36\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}