{"gene":"NEU1","run_date":"2026-04-29T11:37:56","timeline":{"discoveries":[{"year":2003,"finding":"NEU1 mutations cause sialidosis by disrupting lysosomal sialidase activity, intracellular localization, stability, and supramolecular organization. A structural model of the sialidase enabled localization of mutations and prediction of their impact on tertiary structure and biochemical properties.","method":"Expression of mutant NEU1 proteins with activity assays, localization studies, and structural modeling","journal":"Human mutation","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (activity assays, localization, structural modeling) across many patient mutations; replicated across labs","pmids":["14517945"],"is_preprint":false},{"year":2009,"finding":"NEU1 suppresses colon cancer metastasis by desialylating integrin β4, which decreases integrin phosphorylation and attenuates downstream FAK and ERK1/2 signaling, and also downregulates MMP-7.","method":"NEU1 overexpression/silencing in HT-29 cells; lectin blotting of integrin β4 immunoprecipitates; phosphorylation assays; in vivo liver metastasis model; cell surface biotinylation and immunofluorescence","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (lectin blotting, phosphorylation assays, in vivo model, immunofluorescence) in a single study with rigorous controls","pmids":["19151752"],"is_preprint":false},{"year":2009,"finding":"NEU1 forms a heterodimeric complex with its chaperone protective protein/cathepsin A (PPCA), which prevents NEU1 from self-associating into chain-like oligomers. PPCA binds NEU1 at overlapping sites used for NEU1-NEU1 self-association, and can disassemble pre-formed NEU1 oligomers.","method":"Hydrodynamic analysis, identification of binding sites by mutagenesis, structural modeling of NEU1 oligomer and PPCA-NEU1 heterodimer","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution and structural modeling with multiple orthogonal biophysical methods","pmids":["19666471"],"is_preprint":false},{"year":2013,"finding":"NEU1 deficiency in lysosomes leads to accumulation and amyloidogenic processing of oversialylated amyloid precursor protein (APP) in lysosomes, and extracellular release of Aβ peptides via excessive lysosomal exocytosis, establishing NEU1 as a regulator of APP processing and Aβ secretion.","method":"NEU1-deficient mouse model; biochemical analysis of APP sialylation and processing; lysosomal exocytosis assays; cerebral injection of NEU1 in AD mouse model","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in vivo and in vitro, including rescue experiment with NEU1 injection in AD model","pmids":["24225533"],"is_preprint":false},{"year":2010,"finding":"NEU1 forms a complex with MMP-9 on the cell surface of neurons and TrkA-expressing cells. Nerve growth factor (NGF) binding to TrkA activates GPCR Gαi signaling and MMP-9, which in turn activates NEU1 sialidase; this NEU1 activity is required for Trk receptor activation and downstream neurotrophin signaling (neurite outgrowth).","method":"Co-immunoprecipitation of NEU1/MMP-9/TrkA complex; sialidase activity assays in live cells; Tamiflu inhibition of sialidase with IC50 determination; neurite outgrowth assays in TrkA-PC12 cells and primary neurons","journal":"Cellular signalling","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP, in-cell sialidase assay, pharmacological inhibition with functional readout, multiple cell types","pmids":["20347965"],"is_preprint":false},{"year":2010,"finding":"NEU1 sialidase is constitutively associated in a complex with TLR-2, TLR-3, and TLR-4 receptors on the cell surface, and is activated upon ligand binding. Activated NEU1 hydrolyzes sialyl α-2,3-linked β-galactosyl residues to remove steric hindrance to TLR-4 dimerization, enabling MyD88/TLR4 complex recruitment, NFκB activation, and pro-inflammatory responses.","method":"Sialidase activity assays in live cells; co-immunoprecipitation; pharmacological inhibition studies","journal":"Journal of visualized experiments","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, mechanistic model supported by sialidase assay and co-IP but limited orthogonal validation in this paper alone","pmids":["20864924"],"is_preprint":false},{"year":2013,"finding":"Elastin-derived peptides signal through the elastin receptor complex (containing NEU1/neuraminidase 1) to activate PI3Kγ in monocytes, promoting ROS production, migration, and atherosclerosis development in vivo; absence of the cathepsin A–NEU1 complex in hematopoietic cells abolishes plaque progression.","method":"Mouse models of atherosclerosis with EP injection; PI3Kγ knockout bone marrow transplant; in vitro monocyte migration and ROS assays with neuraminidase inhibition; genetic ablation of cathepsin A–NEU1 complex in hematopoietic lineage","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic epistasis combined with in vitro mechanistic dissection and multiple orthogonal approaches","pmids":["24357053"],"is_preprint":false},{"year":2018,"finding":"NEU1 expression in monocytes/macrophages is induced by IL-1β and LPS, and NEU1 in turn enhances TNF-α and IL-1β expression, forming a positive feedback loop that promotes M1 macrophage polarization. CRISPR/Cas9 knockout of NEU1 attenuates LPS- and IL-1β-induced cytokine expression.","method":"Lentiviral NEU1 overexpression; CRISPR/Cas9 knockout; siRNA knockdown in primary human CD14+ monocytes; cytokine expression assays","journal":"Vascular pharmacology","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic perturbation methods (overexpression, CRISPR KO, siRNA) in both cell lines and primary human cells with consistent functional readouts","pmids":["29371126"],"is_preprint":false},{"year":2015,"finding":"NEU1 desialylates MUC1 ectodomain (ED) in airway epithelia in response to P. aeruginosa flagellin binding, increasing bacterial adhesion; NEU1 also promotes shedding of the desialylated MUC1-ED, which acts as a competitive decoy receptor blocking bacterial adhesion. NEU1 association with MUC1 increases upon flagellin stimulation.","method":"Scatchard binding analysis; co-immunoprecipitation of NEU1 with MUC1; NEU1-driven desialylation assays; adhesion/invasion assays; bronchoalveolar lavage analysis from patients","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (binding assay, co-IP, functional invasion assay, ex vivo patient samples) in single study","pmids":["25963144"],"is_preprint":false},{"year":2016,"finding":"NEU1 overexpression in pulmonary fibroblasts increases collagen types I and III levels and accelerates degradation of MMP-14. NEU1 overexpression in small airway epithelial cells impairs migration, while in endothelial cells it disrupts capillary-like tube formation and increases T cell adhesion. Intratracheal delivery of NEU1 in vivo induces lymphocyte accumulation and collagen deposition consistent with lung fibrosis.","method":"Adenovirus-mediated NEU1 overexpression in primary human cells; in vivo intratracheal instillation; wounding assays; collagen assays; gene expression analysis","journal":"American journal of physiology. Lung cellular and molecular physiology","confidence":"High","confidence_rationale":"Tier 2 — multiple cell types, both in vitro and in vivo, with functional phenotypic readouts","pmids":["26993524"],"is_preprint":false},{"year":2019,"finding":"In Leishmania donovani-infected macrophages, NEU1 associates with TLR4 and its desialylation of TLR4 promotes TLR4-MyD88 association, MAP kinase signaling, NFκB nuclear translocation, and Th1 cytokine production. Impaired translocation of NEU1 from cytosol to membrane in infected cells is attributed to reduced NEU1 tyrosine-phosphorylation and diminished association with cathepsin A.","method":"Co-immunoprecipitation of NEU1 with TLR4 and cathepsin A; NEU1 overexpression/silencing; sialic acid modification analysis; downstream signaling assays; parasite burden assays","journal":"Frontiers in immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (co-IP, overexpression, silencing, signaling assays) with functional outcome (parasite burden)","pmids":["31649671"],"is_preprint":false},{"year":2021,"finding":"NEU1 and siglec-E act in a reciprocal interplay using sialic acids on TLR4 as a common substrate: NEU1 desialylation of TLR4 promotes MyD88-dependent signaling, while siglec-E engagement of hypersialylated TLR4 activates TRIF-dependent signaling via SHP1/SHP2. NEU1 overexpression also prevents TLR4 ubiquitination and degradation by Triad3A.","method":"NEU1 overexpression and siglec-E silencing; co-immunoprecipitation; phosphorylation and ubiquitination assays; cytokine and IFN-β measurements","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic and biochemical methods, but single lab","pmids":["33763070"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM/X-ray structure of murine NEU1 reveals that the enzyme oligomerizes through two self-association interfaces, displays a wide substrate-binding cavity, and has a catalytic loop in an inactive conformation. A mechanism is proposed whereby binding of protective protein/cathepsin A (PPCA) induces a conformational change in the catalytic loop to activate NEU1.","method":"Crystal/cryo-EM structure determination of murine NEU1; structural analysis of oligomerization interfaces and substrate-binding cavity; comparison with PPCA-bound complex","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1 — three-dimensional structure with mechanistic interpretation and structural validation","pmids":["37205763"],"is_preprint":false},{"year":2022,"finding":"Activated microglia release NEU1 (along with cathepsin A) into culture medium via lysosomal exocytosis. Extracellular NEU1 desialylates microglial phagocytic receptors Trem2 and MerTK, increasing galectin-3 binding to Trem2 and enhancing phagocytosis. NEU1 released from activated microglia also desialylates neurons and sensitizes them to glutamate-induced death.","method":"Neu1 knockdown in microglia; lysosomal exocytosis inhibitors; NEU1 overexpression; neuraminidase activity assays; phagocytosis assays; receptor desialylation assays; neuron co-culture experiments","journal":"Frontiers in cellular neuroscience","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches (KD, OE, pharmacological inhibition, functional phagocytosis and neuronal death assays) in single study","pmids":["35693885"],"is_preprint":false},{"year":2020,"finding":"NEU1 suppresses bladder cancer cell proliferation and promotes apoptosis by disrupting fibronectin–integrin α5β1 interaction (assessed by immunoprecipitation and ELISA) and deactivating the Akt signaling pathway.","method":"NEU1 overexpression and siRNA knockdown; immunoprecipitation; ELISA; flow cytometry; western blot; in vivo xenograft in BALB/c-nu mice","journal":"Cell communication and signaling","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (co-IP, ELISA, in vivo xenograft) with consistent functional phenotypes","pmids":["32164705"],"is_preprint":false},{"year":2016,"finding":"miR-125b directly targets NEU1 (validated by luciferase assay), and this targeting is involved in miR-125b-mediated promotion of gastric cancer cell migration and invasion.","method":"Luciferase reporter assay for miR-125b targeting of NEU1 3'UTR; cell migration and invasion assays with NEU1 manipulation","journal":"Tumour biology","confidence":"Medium","confidence_rationale":"Tier 3 — single lab, luciferase assay with functional follow-up but limited mechanistic depth for NEU1 specifically","pmids":["27220320"],"is_preprint":false},{"year":2020,"finding":"Dipsacoside B binds NEU1 protein (molecular docking, binding energy −7.86 kcal/mol) and inhibits NEU1 activity, reducing NEU1-mediated suppression of lysosomal exocytosis, thereby promoting lysosomal autophagy and protecting against acetaminophen-induced liver injury.","method":"NEU1 activity assay using fluorogenic substrate; western blot for NEU1 and autophagy markers; molecular docking; in vivo mouse model; in vitro HepG2 cells","journal":"Annals of translational medicine","confidence":"Medium","confidence_rationale":"Tier 3 — enzymatic activity assay and in vivo model, but molecular docking is computational and mechanistic link for NEU1 in lysosomal exocytosis inferred rather than directly demonstrated","pmids":["32793668"],"is_preprint":false},{"year":2015,"finding":"Novel missense mutation p.R347Q in NEU1 causes sialidosis with loss of lysosomal sialidase enzymatic activity, confirmed in patient-derived cells.","method":"Sequencing of NEU1 coding region in patient cells; enzymatic activity assays in patient-derived cells; structural conservation analysis","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — direct enzymatic activity measurement in patient cells with novel mutation, but single study","pmids":["25600812"],"is_preprint":false}],"current_model":"NEU1 is a lysosomal and cell-surface sialidase that removes sialic acids from glycoproteins and glycolipids; it is chaperoned to lysosomes by protective protein/cathepsin A (PPCA), which prevents premature NEU1 oligomerization and activates NEU1 via a conformational change in its catalytic loop (crystal structure established). On the cell surface, NEU1 forms complexes with multiple receptors (TLRs, Trk receptors, integrins, MUC1) and desialylates them to regulate downstream signaling: NEU1-driven desialylation of TLR4 enables TLR dimerization and MyD88-dependent NFκB activation; desialylation of Trk receptors (via MMP-9 cross-talk) is required for neurotrophin signaling; desialylation of integrin β4 attenuates FAK/ERK signaling to suppress metastasis; and desialylation of MUC1 modulates bacterial adhesion and decoy receptor shedding. NEU1 deficiency causes lysosomal storage disease (sialidosis) and promotes excessive lysosomal exocytosis with pathological consequences including amyloidogenic APP processing."},"narrative":{"teleology":[{"year":2003,"claim":"Establishing that loss-of-function mutations throughout NEU1 cause sialidosis by disrupting enzymatic activity, stability, and lysosomal targeting answered the long-standing question of genotype-phenotype relationships in this lysosomal storage disease.","evidence":"Expression of patient-derived NEU1 mutants with activity assays, immunolocalization, and structural modeling","pmids":["14517945"],"confidence":"High","gaps":["Structural model was homology-based; no experimentally determined NEU1 structure at this time","Mechanism by which individual mutations disrupt folding versus catalysis not fully delineated"]},{"year":2009,"claim":"Demonstrating that PPCA chaperones NEU1 as a heterodimer and prevents its self-oligomerization resolved how NEU1 is maintained in an active, monomeric state in lysosomes.","evidence":"Hydrodynamic analysis, mutagenesis of binding interfaces, and structural modeling of NEU1 oligomers versus PPCA–NEU1 heterodimer","pmids":["19666471"],"confidence":"High","gaps":["The activation mechanism of NEU1 by PPCA was not structurally resolved at atomic level","Whether PPCA chaperoning is also required for cell-surface NEU1 pools was unclear"]},{"year":2009,"claim":"Showing that NEU1 desialylates integrin β4 to attenuate FAK/ERK signaling and suppress colon cancer liver metastasis established NEU1 as a cell-surface signaling modulator with tumor-suppressive function.","evidence":"NEU1 overexpression/silencing in HT-29 cells with lectin blotting of integrin β4 immunoprecipitates, phosphorylation assays, and in vivo metastasis model","pmids":["19151752"],"confidence":"High","gaps":["Whether NEU1 desialylates other integrins with similar signaling consequences was untested","The sialic acid linkage specificity on integrin β4 was not defined"]},{"year":2010,"claim":"Discovering that NEU1 is constitutively associated with TLR2/3/4 and that ligand-induced NEU1 activation removes sialyl α-2,3-linked residues to enable TLR4 dimerization and MyD88-dependent NF-κB signaling established NEU1 as a gatekeeper of innate immune receptor activation.","evidence":"Co-immunoprecipitation of NEU1 with TLRs; live-cell sialidase assays; pharmacological inhibition with functional NF-κB readouts","pmids":["20864924"],"confidence":"Medium","gaps":["Single-lab finding at this stage; the steric hindrance model for TLR dimerization lacked direct structural support","Whether NEU1 also regulates TRIF-dependent TLR4 signaling was unknown"]},{"year":2010,"claim":"Identification of a cell-surface NEU1–MMP-9–TrkA complex in which NGF triggers Gαi-mediated MMP-9 activation and subsequent NEU1 sialidase activity, required for Trk receptor activation and neurite outgrowth, revealed a neurotrophin signaling module dependent on desialylation.","evidence":"Co-immunoprecipitation; live-cell sialidase assays with IC50 determination of Tamiflu; neurite outgrowth in TrkA-PC12 cells and primary neurons","pmids":["20347965"],"confidence":"High","gaps":["The precise sialylated residues on TrkA targeted by NEU1 were not identified","Whether this mechanism extends to TrkB/TrkC was not tested"]},{"year":2013,"claim":"Demonstrating that NEU1 deficiency causes oversialylation and lysosomal accumulation of APP, leading to amyloidogenic processing and Aβ secretion via lysosomal exocytosis, connected sialidase biology to Alzheimer's disease pathology.","evidence":"NEU1-deficient mouse model; APP sialylation and processing assays; lysosomal exocytosis measurements; rescue by cerebral NEU1 injection in AD mouse","pmids":["24225533"],"confidence":"High","gaps":["Whether NEU1 directly desialylates APP or acts on trafficking machinery was not fully resolved","Relevance to human sporadic AD beyond the mouse model was not established"]},{"year":2013,"claim":"Showing that elastin-derived peptides signal through the NEU1-containing elastin receptor complex to activate PI3Kγ in monocytes and drive atherosclerosis, with genetic ablation of cathepsin A–NEU1 in hematopoietic cells abolishing plaque progression, positioned NEU1 as a driver of vascular inflammation.","evidence":"Atherosclerosis mouse models with bone marrow transplant; PI3Kγ KO epistasis; in vitro monocyte ROS and migration assays with neuraminidase inhibition","pmids":["24357053"],"confidence":"High","gaps":["Whether NEU1 sialidase activity per se or its scaffolding role in the elastin receptor complex is the essential function was not distinguished"]},{"year":2015,"claim":"Revealing that NEU1 desialylates MUC1 ectodomain upon flagellin stimulation, increasing bacterial adhesion but also promoting MUC1-ED shedding as a decoy receptor, uncovered a dual role for NEU1 in airway innate defense.","evidence":"Co-immunoprecipitation of NEU1 with MUC1; Scatchard binding analysis; adhesion/invasion assays; bronchoalveolar lavage from patients","pmids":["25963144"],"confidence":"High","gaps":["The net in vivo benefit versus cost of NEU1-mediated MUC1 desialylation in infection outcome was not resolved","Whether NEU1 targets other mucins similarly was not tested"]},{"year":2018,"claim":"Demonstrating that NEU1 expression is induced by IL-1β/LPS and in turn promotes TNF-α and IL-1β production, forming a positive feedback loop for M1 macrophage polarization, extended NEU1's role from a signal enabler to a transcriptional amplifier of inflammation.","evidence":"CRISPR/Cas9 KO, siRNA, and lentiviral overexpression of NEU1 in primary human CD14+ monocytes and cell lines with cytokine readouts","pmids":["29371126"],"confidence":"High","gaps":["The transcription factors mediating NEU1 induction by IL-1β were not identified","Whether this feedback loop operates in vivo during chronic inflammation was not shown"]},{"year":2020,"claim":"Showing that NEU1 disrupts fibronectin–integrin α5β1 interaction and suppresses Akt signaling to inhibit bladder cancer proliferation and promote apoptosis broadened the integrin-based tumor-suppressive mechanism to additional integrin family members.","evidence":"NEU1 overexpression and siRNA knockdown; co-immunoprecipitation of fibronectin–integrin α5β1; in vivo xenograft in nude mice","pmids":["32164705"],"confidence":"High","gaps":["Whether NEU1 directly desialylates integrin α5, β1, or fibronectin was not distinguished","The specific sialylated glycan sites were not mapped"]},{"year":2021,"claim":"Identifying that NEU1 and siglec-E reciprocally regulate TLR4 sialylation — NEU1 promoting MyD88-dependent and siglec-E promoting TRIF-dependent signaling — and that NEU1 prevents Triad3A-mediated TLR4 ubiquitination and degradation, established a sialylation-based toggle between two branches of TLR4 signaling.","evidence":"NEU1 overexpression and siglec-E silencing; co-IP; phosphorylation and ubiquitination assays; cytokine and IFN-β measurements","pmids":["33763070"],"confidence":"Medium","gaps":["Single-lab study; reciprocal regulation model not validated in vivo","Direct evidence that sialylation state of TLR4 determines MyD88 vs. TRIF pathway choice at endogenous expression levels is lacking"]},{"year":2022,"claim":"Discovering that activated microglia release NEU1 via lysosomal exocytosis and that extracellular NEU1 desialylates Trem2 and MerTK to enhance phagocytosis while also sensitizing neurons to excitotoxicity revealed a paracrine mode of NEU1 action in neuroinflammation.","evidence":"NEU1 knockdown in microglia; lysosomal exocytosis inhibitors; receptor desialylation assays; phagocytosis assays; neuron co-culture experiments","pmids":["35693885"],"confidence":"High","gaps":["Whether extracellular NEU1 is enzymatically active as a monomer or requires PPCA in the extracellular space was not addressed","In vivo relevance of paracrine NEU1 in neurodegeneration remains untested"]},{"year":2023,"claim":"Solving the cryo-EM/X-ray structure of NEU1 revealed two self-association interfaces, a wide substrate-binding cavity, and an inactive catalytic loop conformation, establishing the structural basis for PPCA-induced activation.","evidence":"Crystal and cryo-EM structure determination of murine NEU1; comparison with PPCA-bound complex","pmids":["37205763"],"confidence":"High","gaps":["Structure of the PPCA–NEU1 complex at atomic resolution has not been determined","The structural basis for NEU1's substrate selectivity (e.g., α-2,3 vs. α-2,6 sialic acid linkages) is not fully explained"]},{"year":null,"claim":"Key unresolved questions include how NEU1 is trafficked to and retained at the cell surface, whether PPCA is required for cell-surface NEU1 activity, and what determines NEU1's substrate selectivity among different sialylated receptors in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of the PPCA–NEU1 complex at atomic resolution","Mechanism of NEU1 trafficking to cell surface unknown","In vivo substrate hierarchy for cell-surface NEU1 not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,1,3,4,5,8,10,12,13,14,17]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,5,10,11,14]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[0,3,12,17]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,4,5,8,10,13,14]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[13]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[5,7,10,11]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,4,5,10,11,14]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,1,14,17]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2,12]}],"complexes":["PPCA-NEU1 heterodimer","Elastin receptor complex (ERC)","NEU1-MMP-9-TrkA complex"],"partners":["CTSA","MMP9","TLR4","NTRK1","ITGB4","MUC1","TREM2","MERTK"],"other_free_text":[]},"mechanistic_narrative":"NEU1 is a sialidase that removes sialic acid residues from glycoproteins and glycolipids in lysosomes and at the cell surface, functioning as a broad regulator of glycoprotein sialylation status with consequences for receptor signaling, innate immunity, cell adhesion, and lysosomal homeostasis. In lysosomes, NEU1 requires chaperoning by protective protein/cathepsin A (PPCA), which prevents NEU1 self-oligomerization and induces a conformational change in its catalytic loop to activate the enzyme [PMID:19666471, PMID:37205763]; loss-of-function mutations in NEU1 cause sialidosis, a lysosomal storage disease [PMID:14517945, PMID:25600812]. At the cell surface, NEU1 forms constitutive or stimulus-induced complexes with TLR2/3/4, Trk receptors, integrins, and MUC1, and its desialylation of these targets controls receptor dimerization, downstream kinase cascades (NF-κB, FAK/ERK, Akt, MAPK), and inflammatory or metastatic outcomes [PMID:20864924, PMID:20347965, PMID:19151752, PMID:25963144, PMID:31649671]. NEU1 deficiency leads to oversialylation of APP and excessive lysosomal exocytosis causing extracellular Aβ release, while activated microglia secrete NEU1 to desialylate phagocytic receptors Trem2 and MerTK, enhancing phagocytosis but also sensitizing neurons to excitotoxicity [PMID:24225533, PMID:35693885]."},"prefetch_data":{"uniprot":{"accession":"Q99519","full_name":"Sialidase-1","aliases":["Acetylneuraminyl hydrolase","G9 sialidase","Lysosomal sialidase","N-acetyl-alpha-neuraminidase 1"],"length_aa":415,"mass_kda":45.5,"function":"Catalyzes the removal of sialic acid (N-acetylneuraminic acid) moieties from glycoproteins and glycolipids. To be active, it is strictly dependent on its presence in the multienzyme complex. Appears to have a preference for alpha 2-3 and alpha 2-6 sialyl linkage","subcellular_location":"Lysosome membrane; Lysosome lumen; Cell membrane; Cytoplasmic vesicle; Lysosome","url":"https://www.uniprot.org/uniprotkb/Q99519/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NEU1","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/NEU1","total_profiled":1310},"omim":[{"mim_id":"613111","title":"CATHEPSIN A; CTSA","url":"https://www.omim.org/entry/613111"},{"mim_id":"611458","title":"GALACTOSIDASE, BETA-1; GLB1","url":"https://www.omim.org/entry/611458"},{"mim_id":"608527","title":"NEURAMINIDASE 4; NEU4","url":"https://www.omim.org/entry/608527"},{"mim_id":"608272","title":"NEURAMINIDASE 1; NEU1","url":"https://www.omim.org/entry/608272"},{"mim_id":"606107","title":"SOLUTE CARRIER FAMILY 44, MEMBER 4; SLC44A4","url":"https://www.omim.org/entry/606107"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Vesicles","reliability":"Supported"},{"location":"Cell Junctions","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/NEU1"},"hgnc":{"alias_symbol":[],"prev_symbol":["NEU"]},"alphafold":{"accession":"Q99519","domains":[{"cath_id":"2.120.10.10","chopping":"56-409","consensus_level":"medium","plddt":95.6316,"start":56,"end":409}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99519","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q99519-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q99519-F1-predicted_aligned_error_v6.png","plddt_mean":89.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NEU1","jax_strain_url":"https://www.jax.org/strain/search?query=NEU1"},"sequence":{"accession":"Q99519","fasta_url":"https://rest.uniprot.org/uniprotkb/Q99519.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q99519/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99519"}},"corpus_meta":[{"pmid":"3945311","id":"PMC_3945311","title":"The neu oncogene encodes an epidermal 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medicine","url":"https://pubmed.ncbi.nlm.nih.gov/32793668","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47211,"output_tokens":4687,"usd":0.105969},"stage2":{"model":"claude-opus-4-6","input_tokens":8222,"output_tokens":3832,"usd":0.205365},"total_usd":0.311334,"stage1_batch_id":"msgbatch_012ZiiCUBfB62NCTQd4Bk1Jo","stage2_batch_id":"msgbatch_01Xuqq9aUHoQouS7UVGZNStZ","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"NEU1 mutations cause sialidosis by disrupting lysosomal sialidase activity, intracellular localization, stability, and supramolecular organization. A structural model of the sialidase enabled localization of mutations and prediction of their impact on tertiary structure and biochemical properties.\",\n      \"method\": \"Expression of mutant NEU1 proteins with activity assays, localization studies, and structural modeling\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (activity assays, localization, structural modeling) across many patient mutations; replicated across labs\",\n      \"pmids\": [\"14517945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"NEU1 suppresses colon cancer metastasis by desialylating integrin β4, which decreases integrin phosphorylation and attenuates downstream FAK and ERK1/2 signaling, and also downregulates MMP-7.\",\n      \"method\": \"NEU1 overexpression/silencing in HT-29 cells; lectin blotting of integrin β4 immunoprecipitates; phosphorylation assays; in vivo liver metastasis model; cell surface biotinylation and immunofluorescence\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (lectin blotting, phosphorylation assays, in vivo model, immunofluorescence) in a single study with rigorous controls\",\n      \"pmids\": [\"19151752\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"NEU1 forms a heterodimeric complex with its chaperone protective protein/cathepsin A (PPCA), which prevents NEU1 from self-associating into chain-like oligomers. PPCA binds NEU1 at overlapping sites used for NEU1-NEU1 self-association, and can disassemble pre-formed NEU1 oligomers.\",\n      \"method\": \"Hydrodynamic analysis, identification of binding sites by mutagenesis, structural modeling of NEU1 oligomer and PPCA-NEU1 heterodimer\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution and structural modeling with multiple orthogonal biophysical methods\",\n      \"pmids\": [\"19666471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"NEU1 deficiency in lysosomes leads to accumulation and amyloidogenic processing of oversialylated amyloid precursor protein (APP) in lysosomes, and extracellular release of Aβ peptides via excessive lysosomal exocytosis, establishing NEU1 as a regulator of APP processing and Aβ secretion.\",\n      \"method\": \"NEU1-deficient mouse model; biochemical analysis of APP sialylation and processing; lysosomal exocytosis assays; cerebral injection of NEU1 in AD mouse model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in vivo and in vitro, including rescue experiment with NEU1 injection in AD model\",\n      \"pmids\": [\"24225533\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"NEU1 forms a complex with MMP-9 on the cell surface of neurons and TrkA-expressing cells. Nerve growth factor (NGF) binding to TrkA activates GPCR Gαi signaling and MMP-9, which in turn activates NEU1 sialidase; this NEU1 activity is required for Trk receptor activation and downstream neurotrophin signaling (neurite outgrowth).\",\n      \"method\": \"Co-immunoprecipitation of NEU1/MMP-9/TrkA complex; sialidase activity assays in live cells; Tamiflu inhibition of sialidase with IC50 determination; neurite outgrowth assays in TrkA-PC12 cells and primary neurons\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP, in-cell sialidase assay, pharmacological inhibition with functional readout, multiple cell types\",\n      \"pmids\": [\"20347965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"NEU1 sialidase is constitutively associated in a complex with TLR-2, TLR-3, and TLR-4 receptors on the cell surface, and is activated upon ligand binding. Activated NEU1 hydrolyzes sialyl α-2,3-linked β-galactosyl residues to remove steric hindrance to TLR-4 dimerization, enabling MyD88/TLR4 complex recruitment, NFκB activation, and pro-inflammatory responses.\",\n      \"method\": \"Sialidase activity assays in live cells; co-immunoprecipitation; pharmacological inhibition studies\",\n      \"journal\": \"Journal of visualized experiments\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, mechanistic model supported by sialidase assay and co-IP but limited orthogonal validation in this paper alone\",\n      \"pmids\": [\"20864924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Elastin-derived peptides signal through the elastin receptor complex (containing NEU1/neuraminidase 1) to activate PI3Kγ in monocytes, promoting ROS production, migration, and atherosclerosis development in vivo; absence of the cathepsin A–NEU1 complex in hematopoietic cells abolishes plaque progression.\",\n      \"method\": \"Mouse models of atherosclerosis with EP injection; PI3Kγ knockout bone marrow transplant; in vitro monocyte migration and ROS assays with neuraminidase inhibition; genetic ablation of cathepsin A–NEU1 complex in hematopoietic lineage\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic epistasis combined with in vitro mechanistic dissection and multiple orthogonal approaches\",\n      \"pmids\": [\"24357053\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NEU1 expression in monocytes/macrophages is induced by IL-1β and LPS, and NEU1 in turn enhances TNF-α and IL-1β expression, forming a positive feedback loop that promotes M1 macrophage polarization. CRISPR/Cas9 knockout of NEU1 attenuates LPS- and IL-1β-induced cytokine expression.\",\n      \"method\": \"Lentiviral NEU1 overexpression; CRISPR/Cas9 knockout; siRNA knockdown in primary human CD14+ monocytes; cytokine expression assays\",\n      \"journal\": \"Vascular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic perturbation methods (overexpression, CRISPR KO, siRNA) in both cell lines and primary human cells with consistent functional readouts\",\n      \"pmids\": [\"29371126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"NEU1 desialylates MUC1 ectodomain (ED) in airway epithelia in response to P. aeruginosa flagellin binding, increasing bacterial adhesion; NEU1 also promotes shedding of the desialylated MUC1-ED, which acts as a competitive decoy receptor blocking bacterial adhesion. NEU1 association with MUC1 increases upon flagellin stimulation.\",\n      \"method\": \"Scatchard binding analysis; co-immunoprecipitation of NEU1 with MUC1; NEU1-driven desialylation assays; adhesion/invasion assays; bronchoalveolar lavage analysis from patients\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (binding assay, co-IP, functional invasion assay, ex vivo patient samples) in single study\",\n      \"pmids\": [\"25963144\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"NEU1 overexpression in pulmonary fibroblasts increases collagen types I and III levels and accelerates degradation of MMP-14. NEU1 overexpression in small airway epithelial cells impairs migration, while in endothelial cells it disrupts capillary-like tube formation and increases T cell adhesion. Intratracheal delivery of NEU1 in vivo induces lymphocyte accumulation and collagen deposition consistent with lung fibrosis.\",\n      \"method\": \"Adenovirus-mediated NEU1 overexpression in primary human cells; in vivo intratracheal instillation; wounding assays; collagen assays; gene expression analysis\",\n      \"journal\": \"American journal of physiology. Lung cellular and molecular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple cell types, both in vitro and in vivo, with functional phenotypic readouts\",\n      \"pmids\": [\"26993524\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In Leishmania donovani-infected macrophages, NEU1 associates with TLR4 and its desialylation of TLR4 promotes TLR4-MyD88 association, MAP kinase signaling, NFκB nuclear translocation, and Th1 cytokine production. Impaired translocation of NEU1 from cytosol to membrane in infected cells is attributed to reduced NEU1 tyrosine-phosphorylation and diminished association with cathepsin A.\",\n      \"method\": \"Co-immunoprecipitation of NEU1 with TLR4 and cathepsin A; NEU1 overexpression/silencing; sialic acid modification analysis; downstream signaling assays; parasite burden assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (co-IP, overexpression, silencing, signaling assays) with functional outcome (parasite burden)\",\n      \"pmids\": [\"31649671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"NEU1 and siglec-E act in a reciprocal interplay using sialic acids on TLR4 as a common substrate: NEU1 desialylation of TLR4 promotes MyD88-dependent signaling, while siglec-E engagement of hypersialylated TLR4 activates TRIF-dependent signaling via SHP1/SHP2. NEU1 overexpression also prevents TLR4 ubiquitination and degradation by Triad3A.\",\n      \"method\": \"NEU1 overexpression and siglec-E silencing; co-immunoprecipitation; phosphorylation and ubiquitination assays; cytokine and IFN-β measurements\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic and biochemical methods, but single lab\",\n      \"pmids\": [\"33763070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM/X-ray structure of murine NEU1 reveals that the enzyme oligomerizes through two self-association interfaces, displays a wide substrate-binding cavity, and has a catalytic loop in an inactive conformation. A mechanism is proposed whereby binding of protective protein/cathepsin A (PPCA) induces a conformational change in the catalytic loop to activate NEU1.\",\n      \"method\": \"Crystal/cryo-EM structure determination of murine NEU1; structural analysis of oligomerization interfaces and substrate-binding cavity; comparison with PPCA-bound complex\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — three-dimensional structure with mechanistic interpretation and structural validation\",\n      \"pmids\": [\"37205763\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Activated microglia release NEU1 (along with cathepsin A) into culture medium via lysosomal exocytosis. Extracellular NEU1 desialylates microglial phagocytic receptors Trem2 and MerTK, increasing galectin-3 binding to Trem2 and enhancing phagocytosis. NEU1 released from activated microglia also desialylates neurons and sensitizes them to glutamate-induced death.\",\n      \"method\": \"Neu1 knockdown in microglia; lysosomal exocytosis inhibitors; NEU1 overexpression; neuraminidase activity assays; phagocytosis assays; receptor desialylation assays; neuron co-culture experiments\",\n      \"journal\": \"Frontiers in cellular neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches (KD, OE, pharmacological inhibition, functional phagocytosis and neuronal death assays) in single study\",\n      \"pmids\": [\"35693885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NEU1 suppresses bladder cancer cell proliferation and promotes apoptosis by disrupting fibronectin–integrin α5β1 interaction (assessed by immunoprecipitation and ELISA) and deactivating the Akt signaling pathway.\",\n      \"method\": \"NEU1 overexpression and siRNA knockdown; immunoprecipitation; ELISA; flow cytometry; western blot; in vivo xenograft in BALB/c-nu mice\",\n      \"journal\": \"Cell communication and signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (co-IP, ELISA, in vivo xenograft) with consistent functional phenotypes\",\n      \"pmids\": [\"32164705\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"miR-125b directly targets NEU1 (validated by luciferase assay), and this targeting is involved in miR-125b-mediated promotion of gastric cancer cell migration and invasion.\",\n      \"method\": \"Luciferase reporter assay for miR-125b targeting of NEU1 3'UTR; cell migration and invasion assays with NEU1 manipulation\",\n      \"journal\": \"Tumour biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, luciferase assay with functional follow-up but limited mechanistic depth for NEU1 specifically\",\n      \"pmids\": [\"27220320\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Dipsacoside B binds NEU1 protein (molecular docking, binding energy −7.86 kcal/mol) and inhibits NEU1 activity, reducing NEU1-mediated suppression of lysosomal exocytosis, thereby promoting lysosomal autophagy and protecting against acetaminophen-induced liver injury.\",\n      \"method\": \"NEU1 activity assay using fluorogenic substrate; western blot for NEU1 and autophagy markers; molecular docking; in vivo mouse model; in vitro HepG2 cells\",\n      \"journal\": \"Annals of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — enzymatic activity assay and in vivo model, but molecular docking is computational and mechanistic link for NEU1 in lysosomal exocytosis inferred rather than directly demonstrated\",\n      \"pmids\": [\"32793668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Novel missense mutation p.R347Q in NEU1 causes sialidosis with loss of lysosomal sialidase enzymatic activity, confirmed in patient-derived cells.\",\n      \"method\": \"Sequencing of NEU1 coding region in patient cells; enzymatic activity assays in patient-derived cells; structural conservation analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct enzymatic activity measurement in patient cells with novel mutation, but single study\",\n      \"pmids\": [\"25600812\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NEU1 is a lysosomal and cell-surface sialidase that removes sialic acids from glycoproteins and glycolipids; it is chaperoned to lysosomes by protective protein/cathepsin A (PPCA), which prevents premature NEU1 oligomerization and activates NEU1 via a conformational change in its catalytic loop (crystal structure established). On the cell surface, NEU1 forms complexes with multiple receptors (TLRs, Trk receptors, integrins, MUC1) and desialylates them to regulate downstream signaling: NEU1-driven desialylation of TLR4 enables TLR dimerization and MyD88-dependent NFκB activation; desialylation of Trk receptors (via MMP-9 cross-talk) is required for neurotrophin signaling; desialylation of integrin β4 attenuates FAK/ERK signaling to suppress metastasis; and desialylation of MUC1 modulates bacterial adhesion and decoy receptor shedding. NEU1 deficiency causes lysosomal storage disease (sialidosis) and promotes excessive lysosomal exocytosis with pathological consequences including amyloidogenic APP processing.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"NEU1 is a sialidase that removes sialic acid residues from glycoproteins and glycolipids in lysosomes and at the cell surface, functioning as a broad regulator of glycoprotein sialylation status with consequences for receptor signaling, innate immunity, cell adhesion, and lysosomal homeostasis. In lysosomes, NEU1 requires chaperoning by protective protein/cathepsin A (PPCA), which prevents NEU1 self-oligomerization and induces a conformational change in its catalytic loop to activate the enzyme [PMID:19666471, PMID:37205763]; loss-of-function mutations in NEU1 cause sialidosis, a lysosomal storage disease [PMID:14517945, PMID:25600812]. At the cell surface, NEU1 forms constitutive or stimulus-induced complexes with TLR2/3/4, Trk receptors, integrins, and MUC1, and its desialylation of these targets controls receptor dimerization, downstream kinase cascades (NF-κB, FAK/ERK, Akt, MAPK), and inflammatory or metastatic outcomes [PMID:20864924, PMID:20347965, PMID:19151752, PMID:25963144, PMID:31649671]. NEU1 deficiency leads to oversialylation of APP and excessive lysosomal exocytosis causing extracellular Aβ release, while activated microglia secrete NEU1 to desialylate phagocytic receptors Trem2 and MerTK, enhancing phagocytosis but also sensitizing neurons to excitotoxicity [PMID:24225533, PMID:35693885].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing that loss-of-function mutations throughout NEU1 cause sialidosis by disrupting enzymatic activity, stability, and lysosomal targeting answered the long-standing question of genotype-phenotype relationships in this lysosomal storage disease.\",\n      \"evidence\": \"Expression of patient-derived NEU1 mutants with activity assays, immunolocalization, and structural modeling\",\n      \"pmids\": [\"14517945\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model was homology-based; no experimentally determined NEU1 structure at this time\", \"Mechanism by which individual mutations disrupt folding versus catalysis not fully delineated\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrating that PPCA chaperones NEU1 as a heterodimer and prevents its self-oligomerization resolved how NEU1 is maintained in an active, monomeric state in lysosomes.\",\n      \"evidence\": \"Hydrodynamic analysis, mutagenesis of binding interfaces, and structural modeling of NEU1 oligomers versus PPCA–NEU1 heterodimer\",\n      \"pmids\": [\"19666471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The activation mechanism of NEU1 by PPCA was not structurally resolved at atomic level\", \"Whether PPCA chaperoning is also required for cell-surface NEU1 pools was unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Showing that NEU1 desialylates integrin β4 to attenuate FAK/ERK signaling and suppress colon cancer liver metastasis established NEU1 as a cell-surface signaling modulator with tumor-suppressive function.\",\n      \"evidence\": \"NEU1 overexpression/silencing in HT-29 cells with lectin blotting of integrin β4 immunoprecipitates, phosphorylation assays, and in vivo metastasis model\",\n      \"pmids\": [\"19151752\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether NEU1 desialylates other integrins with similar signaling consequences was untested\", \"The sialic acid linkage specificity on integrin β4 was not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Discovering that NEU1 is constitutively associated with TLR2/3/4 and that ligand-induced NEU1 activation removes sialyl α-2,3-linked residues to enable TLR4 dimerization and MyD88-dependent NF-κB signaling established NEU1 as a gatekeeper of innate immune receptor activation.\",\n      \"evidence\": \"Co-immunoprecipitation of NEU1 with TLRs; live-cell sialidase assays; pharmacological inhibition with functional NF-κB readouts\",\n      \"pmids\": [\"20864924\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab finding at this stage; the steric hindrance model for TLR dimerization lacked direct structural support\", \"Whether NEU1 also regulates TRIF-dependent TLR4 signaling was unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identification of a cell-surface NEU1–MMP-9–TrkA complex in which NGF triggers Gαi-mediated MMP-9 activation and subsequent NEU1 sialidase activity, required for Trk receptor activation and neurite outgrowth, revealed a neurotrophin signaling module dependent on desialylation.\",\n      \"evidence\": \"Co-immunoprecipitation; live-cell sialidase assays with IC50 determination of Tamiflu; neurite outgrowth in TrkA-PC12 cells and primary neurons\",\n      \"pmids\": [\"20347965\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The precise sialylated residues on TrkA targeted by NEU1 were not identified\", \"Whether this mechanism extends to TrkB/TrkC was not tested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Demonstrating that NEU1 deficiency causes oversialylation and lysosomal accumulation of APP, leading to amyloidogenic processing and Aβ secretion via lysosomal exocytosis, connected sialidase biology to Alzheimer's disease pathology.\",\n      \"evidence\": \"NEU1-deficient mouse model; APP sialylation and processing assays; lysosomal exocytosis measurements; rescue by cerebral NEU1 injection in AD mouse\",\n      \"pmids\": [\"24225533\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether NEU1 directly desialylates APP or acts on trafficking machinery was not fully resolved\", \"Relevance to human sporadic AD beyond the mouse model was not established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showing that elastin-derived peptides signal through the NEU1-containing elastin receptor complex to activate PI3Kγ in monocytes and drive atherosclerosis, with genetic ablation of cathepsin A–NEU1 in hematopoietic cells abolishing plaque progression, positioned NEU1 as a driver of vascular inflammation.\",\n      \"evidence\": \"Atherosclerosis mouse models with bone marrow transplant; PI3Kγ KO epistasis; in vitro monocyte ROS and migration assays with neuraminidase inhibition\",\n      \"pmids\": [\"24357053\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether NEU1 sialidase activity per se or its scaffolding role in the elastin receptor complex is the essential function was not distinguished\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Revealing that NEU1 desialylates MUC1 ectodomain upon flagellin stimulation, increasing bacterial adhesion but also promoting MUC1-ED shedding as a decoy receptor, uncovered a dual role for NEU1 in airway innate defense.\",\n      \"evidence\": \"Co-immunoprecipitation of NEU1 with MUC1; Scatchard binding analysis; adhesion/invasion assays; bronchoalveolar lavage from patients\",\n      \"pmids\": [\"25963144\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The net in vivo benefit versus cost of NEU1-mediated MUC1 desialylation in infection outcome was not resolved\", \"Whether NEU1 targets other mucins similarly was not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating that NEU1 expression is induced by IL-1β/LPS and in turn promotes TNF-α and IL-1β production, forming a positive feedback loop for M1 macrophage polarization, extended NEU1's role from a signal enabler to a transcriptional amplifier of inflammation.\",\n      \"evidence\": \"CRISPR/Cas9 KO, siRNA, and lentiviral overexpression of NEU1 in primary human CD14+ monocytes and cell lines with cytokine readouts\",\n      \"pmids\": [\"29371126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The transcription factors mediating NEU1 induction by IL-1β were not identified\", \"Whether this feedback loop operates in vivo during chronic inflammation was not shown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showing that NEU1 disrupts fibronectin–integrin α5β1 interaction and suppresses Akt signaling to inhibit bladder cancer proliferation and promote apoptosis broadened the integrin-based tumor-suppressive mechanism to additional integrin family members.\",\n      \"evidence\": \"NEU1 overexpression and siRNA knockdown; co-immunoprecipitation of fibronectin–integrin α5β1; in vivo xenograft in nude mice\",\n      \"pmids\": [\"32164705\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether NEU1 directly desialylates integrin α5, β1, or fibronectin was not distinguished\", \"The specific sialylated glycan sites were not mapped\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identifying that NEU1 and siglec-E reciprocally regulate TLR4 sialylation — NEU1 promoting MyD88-dependent and siglec-E promoting TRIF-dependent signaling — and that NEU1 prevents Triad3A-mediated TLR4 ubiquitination and degradation, established a sialylation-based toggle between two branches of TLR4 signaling.\",\n      \"evidence\": \"NEU1 overexpression and siglec-E silencing; co-IP; phosphorylation and ubiquitination assays; cytokine and IFN-β measurements\",\n      \"pmids\": [\"33763070\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study; reciprocal regulation model not validated in vivo\", \"Direct evidence that sialylation state of TLR4 determines MyD88 vs. TRIF pathway choice at endogenous expression levels is lacking\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovering that activated microglia release NEU1 via lysosomal exocytosis and that extracellular NEU1 desialylates Trem2 and MerTK to enhance phagocytosis while also sensitizing neurons to excitotoxicity revealed a paracrine mode of NEU1 action in neuroinflammation.\",\n      \"evidence\": \"NEU1 knockdown in microglia; lysosomal exocytosis inhibitors; receptor desialylation assays; phagocytosis assays; neuron co-culture experiments\",\n      \"pmids\": [\"35693885\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether extracellular NEU1 is enzymatically active as a monomer or requires PPCA in the extracellular space was not addressed\", \"In vivo relevance of paracrine NEU1 in neurodegeneration remains untested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Solving the cryo-EM/X-ray structure of NEU1 revealed two self-association interfaces, a wide substrate-binding cavity, and an inactive catalytic loop conformation, establishing the structural basis for PPCA-induced activation.\",\n      \"evidence\": \"Crystal and cryo-EM structure determination of murine NEU1; comparison with PPCA-bound complex\",\n      \"pmids\": [\"37205763\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the PPCA–NEU1 complex at atomic resolution has not been determined\", \"The structural basis for NEU1's substrate selectivity (e.g., α-2,3 vs. α-2,6 sialic acid linkages) is not fully explained\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include how NEU1 is trafficked to and retained at the cell surface, whether PPCA is required for cell-surface NEU1 activity, and what determines NEU1's substrate selectivity among different sialylated receptors in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of the PPCA–NEU1 complex at atomic resolution\", \"Mechanism of NEU1 trafficking to cell surface unknown\", \"In vivo substrate hierarchy for cell-surface NEU1 not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 1, 3, 4, 5, 8, 10, 12, 13, 14, 17]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 5, 10, 11, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [0, 3, 12, 17]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 4, 5, 8, 10, 13, 14]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [5, 7, 10, 11]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 4, 5, 10, 11, 14]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 1, 14, 17]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2, 12]}\n    ],\n    \"complexes\": [\n      \"PPCA-NEU1 heterodimer\",\n      \"Elastin receptor complex (ERC)\",\n      \"NEU1-MMP-9-TrkA complex\"\n    ],\n    \"partners\": [\n      \"CTSA\",\n      \"MMP9\",\n      \"TLR4\",\n      \"NTRK1\",\n      \"ITGB4\",\n      \"MUC1\",\n      \"TREM2\",\n      \"MERTK\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}