{"gene":"NTRK1","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":1998,"finding":"TrkA-mediated rescue of oligodendrocytes from p75-dependent apoptosis involves both activation of MAPK survival signals and simultaneous suppression of c-jun kinase (JNK) activity initiated by p75, while p75-induced NFκB activity was unaffected, demonstrating competitive signaling between TrkA and p75 receptors.","method":"Introduction of TrkA into oligodendrocyte cell cultures with p75-dependent apoptosis; measurement of MAPK activation, JNK activity, and NFκB activity","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean cell culture model with multiple pathway readouts in single lab, two orthogonal signaling assays","pmids":["9547236"],"is_preprint":false},{"year":1999,"finding":"N-glycosylation of TrkA serves two distinct functions: (1) preventing ligand-independent activation — unglycosylated TrkA is constitutively phosphorylated and interacts constitutively with Shc and PLC-γ; (2) localizing TrkA to the cell surface — unglycosylated TrkA is trapped intracellularly and cannot activate the Ras/MAP kinase cascade (MEK and Erk), despite constitutive kinase activity.","method":"Microscale deglycosylation assay, confocal microscopy, immunoblotting for phosphorylation and signaling molecule interactions, differentiation assay in PC12 cells","journal":"Journal of neurobiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — in vitro deglycosylation assay combined with confocal localization, signaling assays, and functional differentiation readout in single study with multiple orthogonal methods","pmids":["10235685"],"is_preprint":false},{"year":1996,"finding":"TrkA is cleaved at its ectodomain in a membrane-proximal region by a cell-surface proteolytic system regulated by protein kinase C and NGF, generating a cell-associated fragment that is phosphorylated on tyrosine residues; this phosphorylation requires an intact TrkA kinase domain (not detected in kinase-dead TrkA mutants), suggesting elevated kinase activity of the truncated fragment.","method":"Ectodomain cleavage assay using multiple cell lines expressing TrkA, kinase-dead TrkA mutants, immunoblotting for tyrosine phosphorylation","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — kinase-dead mutant controls and multiple cell lines, single lab","pmids":["8636219"],"is_preprint":false},{"year":1996,"finding":"The p75 neurotrophin receptor (gp75) forms a complex with TrkA on the cell surface; the TrkA extracellular domain is sufficient for this association, but interactions involving other receptor domains (transmembrane/intracellular) also contribute. TrkA kinase activity is not required for complex formation. gp75 does not copatch with TrkB, PDGFR-β, or Torso, demonstrating specificity.","method":"Copatching technique with antibody-induced receptor redistribution on intact cells; baculovirus-insect cell expression of wild-type and chimeric TrkA-Torso receptors; kinase-dead TrkA mutant","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal copatching with domain dissection using chimeric receptors and kinase-dead mutant, specificity controls with multiple receptor types","pmids":["8603925"],"is_preprint":false},{"year":1998,"finding":"The TrkA kinase domain follows a sequential ordered bi-bi kinetic mechanism, with ATP binding prior to the exogenous substrate PLC-γ/GST, followed by release of phosphorylated product before ADP release; the inhibitor K-252a acts as a competitive inhibitor with respect to ATP.","method":"In vitro kinase assay using baculovirus-expressed TrkA kinase domain; product and dead-end inhibition kinetics with PLC-γ/GST fusion protein as substrate","journal":"Archives of biochemistry and biophysics","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro kinase assay with rigorous kinetic mechanism determination, single lab","pmids":["9448714"],"is_preprint":false},{"year":2001,"finding":"A point mutation P203A in the extracellular linker region between leucine repeats and the first Ig-like domain of TrkA increases NGF binding affinity by decreasing ligand dissociation rate, causes spontaneous receptor dimerization and constitutive phosphorylation in the absence of ligand, promotes ligand-independent neurite outgrowth, and transforms fibroblasts to form tumors in nude mice.","method":"Site-directed mutagenesis, NGF binding assays, receptor dimerization analysis, PC12nnr5 neurite outgrowth assay, fibroblast transformation and nude mouse tumor formation","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — mutagenesis combined with binding assay, dimerization, functional differentiation, and in vivo transformation assays in single study","pmids":["11313867"],"is_preprint":false},{"year":2001,"finding":"IRS-1 and IRS-2 are tyrosine-phosphorylated substrates of both TrkA and the oncogenic TRK-T1 fusion; this leads to recruitment of p85PI3K, SHP-2, and Grb2, increased PI3-kinase activity associated with IRS-1, and activation of c-fos SRE. TRK-T1-stimulated DNA synthesis requires IRS-1 (abolished in IRS-1-/- fibroblasts). Yeast two-hybrid experiments showed direct TrkA–IRS interaction.","method":"Tyrosine phosphorylation assays, co-immunoprecipitation, PI3-kinase activity assay, c-fos SRE reporter, DNA synthesis in IRS-1-/- fibroblasts, yeast two-hybrid","journal":"Journal of cellular physiology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (co-IP, PI3K assay, genetic KO fibroblasts, yeast two-hybrid) in single study","pmids":["11147812"],"is_preprint":false},{"year":2007,"finding":"The TrkA D5 (IgC2) extracellular subdomain contains both the cognate NGF binding hot spot and a distinct but partially overlapping NT-3 docking/activation hot spot (site 1); additionally, D4 (IgC1) contains an allosteric NT-3 binding site (site 2). NT-3 docking on both sites 1 and 2 affords full TrkA agonism additive with NGF, while docking solely on site 1 is partially agonistic but noncompetitively antagonizes NGF binding.","method":"Binding studies with truncated and chimeric extracellular subdomain constructs, competition binding assays, functional neuronal survival assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chimeric receptor domain mapping with binding competition and functional survival assays, single lab","pmids":["17439940"],"is_preprint":false},{"year":2008,"finding":"TrkA receptor endosomal/lysosomal degradation is both ubiquitin- and proteasome-dependent: the polyubiquitin tag (K485 on TrkA) directs receptor sorting; proteasomal deubiquitinating enzymes trim K63-ubiquitin chains from TrkA prior to lysosomal delivery. The K485R ubiquitin-deficient TrkA mutant fails to deubiquitinate and escapes degradation.","method":"Co-immunoprecipitation, biochemical fractionation, confocal microscopy, proteasome inhibitor (lactacystin) and lysosomal inhibitor (methylamine, bafilomycin, leupeptin) treatments, K485R TrkA mutant analysis, proteasome purification and co-IP","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (mutant receptor, pharmacological inhibitors, co-IP, fractionation, imaging) in single study","pmids":["18419753"],"is_preprint":false},{"year":2008,"finding":"ProNGF (cleavage-resistant mutant M-proNGF) binds TrkA with lower affinity than NGF, induces TrkA and ERK1/2 phosphorylation, neurite outgrowth in PC12 cells, and survival but less effectively than NGF; additionally, the NGF pro-region alone binds TrkA at a site distinct from NGF binding, causing TrkA and ERK1/2 phosphorylation.","method":"Binding affinity determination, TrkA and ERK1/2 phosphorylation immunoblotting, PC12 neurite outgrowth assay, cortical neuron caspase-3 cleavage assay","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays with engineered proNGF mutants, single lab","pmids":["18808449"],"is_preprint":false},{"year":2009,"finding":"CCM2 interacts with the juxtamembrane region of TrkA via its PTB domain and mediates TrkA-induced cell death; both the PTB domain (conferring interaction specificity) and the Karet domain (linking to death pathways) are required. Downregulation of CCM2 in medulloblastoma or neuroblastoma cells attenuates TrkA-dependent death.","method":"Co-immunoprecipitation (PTB domain interaction), domain deletion mutant analysis, CCM2 knockdown in tumor cells with TrkA-dependent death readout","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal domain-mapping co-IP combined with loss-of-function knockdown and defined death phenotype, single lab with multiple orthogonal approaches","pmids":["19755102"],"is_preprint":false},{"year":2011,"finding":"TrkA phosphorylates APP at Y682; APP interacts with TrkA and this interaction requires Y682. Reciprocally, APP (specifically Y682) regulates activation of the NGF/TrkA signaling pathway in vivo, controls subcellular distribution of TrkA, and modulates neuronal sensitivity to NGF.","method":"NGF stimulation, tyrosine phosphorylation assays, co-immunoprecipitation, APP Y682F mutant analysis, in vivo TrkA distribution and signaling in APP knockout neurons","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP, site-specific mutant (Y682), in vivo localization, and signaling assays with multiple orthogonal methods in single study","pmids":["21849536"],"is_preprint":false},{"year":2012,"finding":"Conditional forebrain-specific TrkA knockout in mice causes dysfunction of basal forebrain cholinergic neurons (BFCNs): developmental increase of choline acetyltransferase expression becomes dependent on TrkA signaling (via the ERK pathway) before neuronal connections are established; TrkA loss results in anatomical and physiological deficits in BF cholinergic circuitry and selective cognitive impairment.","method":"Conditional TrkA knockout mouse (Cre-lox), ChAT expression analysis, ERK pathway readout, anatomical/physiological BF measurements, cognitive behavioral testing","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional KO with defined molecular pathway (ERK), anatomical, physiological, and behavioral phenotypes, multiple readouts","pmids":["22442072"],"is_preprint":false},{"year":2014,"finding":"The majority of retrograde TrkA signaling endosomes in sympathetic neurons are multivesicular bodies (MVBs). Retrogradely transported TrkA+ MVBs evade lysosomal fusion upon arriving in cell bodies and instead evolve into TrkA+ single-membrane signaling vesicles; TrkA kinase activity associated with retrogradely transported MVBs determines endosome evolution and fate.","method":"Ultrastructural (electron microscopy) and molecular characterization of retrograde endosomes in mouse sympathetic neurons; TrkA kinase activity manipulation with fate tracking","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — ultrastructural EM combined with molecular markers, kinase activity manipulation, and vesicle fate tracking in single rigorous study","pmids":["29381137"],"is_preprint":false},{"year":2014,"finding":"A 3-amino-acid (KFG) domain in TrkA negatively regulates TrkA level and function by promoting ubiquitination; deletion of this domain in knock-in mice reduces TrkA ubiquitination, increases TrkA protein levels and activity, and results in enhanced thermal sensitivity and inflammatory pain without affecting DRG neuron numbers.","method":"KFG-deletion knock-in mouse, ubiquitination assays, TrkA protein level and activity measurements, thermal sensitivity and inflammatory pain behavioral tests","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — in vivo knock-in mouse model with domain-specific mutation, biochemical ubiquitination assay, and behavioral phenotype","pmids":["24623787"],"is_preprint":false},{"year":2011,"finding":"Nedd4-2 (E3 ubiquitin ligase) binds the C-terminal PPXY motif of TrkA and mediates multimonoubiquitination. Mutations at the hydrophobic residues Leu784 and Val790 increase Nedd4-2 binding and ubiquitination, directing receptors to the lysosomal pathway instead of recycling; multimonoubiquitination does not impair signaling cascade activation but potentiates TrkA-mediated differentiation (neurite outgrowth).","method":"Site-directed mutagenesis of TrkA C-terminal tail, co-immunoprecipitation, colocalization studies, neurite outgrowth assay","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis combined with co-IP, colocalization, and functional differentiation assay, single lab","pmids":["21332718"],"is_preprint":false},{"year":2015,"finding":"GGA3 interacts directly with the TrkA cytoplasmic tail through an internal DXXLL motif and mediates functional recycling of TrkA to the plasma membrane via an Arf6-dependent mechanism; GGA3 depletion delays TrkA recycling, accelerates TrkA degradation, attenuates sustained NGF-induced Akt activation, and reduces cell survival.","method":"siRNA depletion, direct binding assay (DXXLL motif), recycling and degradation kinetics assays, Akt phosphorylation immunoblotting, cell survival assay, Arf6 dominant-negative analysis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct binding motif identified, multiple functional assays (recycling, degradation, signaling, survival), siRNA knockdown, mechanistic dissection of Arf6 requirement","pmids":["26446845"],"is_preprint":false},{"year":2014,"finding":"Syntaxin 8 (STX8, a Q-SNARE protein) binds TrkA and facilitates its transport from the Golgi to the plasma membrane, regulating TrkA cell surface levels specifically (not TrkB); STX8 modulates downstream NGF-induced TrkA signaling and NGF-dependent DRG neuron survival; STX8 knockdown in rat DRG via AAV6 RNAi produced analgesic effects on formalin-induced inflammatory pain.","method":"Co-immunoprecipitation, overexpression/knockdown studies for TrkA surface levels, NGF signaling assays, DRG neuron survival assay, AAV6-mediated in vivo STX8 knockdown with formalin pain model","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — co-IP with specificity controls (TrkB), multiple cell-based assays, and in vivo functional validation, single lab","pmids":["24872407"],"is_preprint":false},{"year":2018,"finding":"TRAF4 E3 ubiquitin ligase promotes K27- and K29-linked ubiquitination at the TrkA kinase domain, increasing TrkA kinase activity; mutation of TRAF4-targeted ubiquitination sites abolishes TrkA tyrosine autophosphorylation and its interaction with downstream proteins; TRAF4 knockdown suppresses NGF-stimulated TrkA downstream p38 MAPK activation and invasion-associated gene expression in prostate cancer cells.","method":"Co-immunoprecipitation, ubiquitination assays (linkage-specific), kinase activity assays, site-directed mutagenesis of ubiquitination sites, TRAF4 knockdown with p38 MAPK and invasion gene readouts","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Moderate — ubiquitination site mutagenesis, linkage-specific ubiquitination assay, kinase activity measurement, and downstream signaling validation, multiple orthogonal methods","pmids":["29715200"],"is_preprint":false},{"year":2016,"finding":"X-ray crystal structures of TrkA kinase domain complexed with selective inhibitors reveal a non-active-site binding pocket formed by residues from both the kinase domain and the juxtamembrane (JM) region; three distinct binding modes with the JM region were characterized and found to underlie TrkA selectivity over TrkB and TrkC.","method":"X-ray crystallography of TrkA kinase domain + JM region with inhibitor complexes; in vitro kinase assays validating JM region importance","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structures with functional in vitro validation of JM region role, single study with structural + biochemical evidence","pmids":["28039433"],"is_preprint":false},{"year":2017,"finding":"X-ray crystal structure of TrkA kinase domain plus juxtamembrane (JM) region bound to a selective inhibitor A1 reveals that the JM region creates a unique inhibitor-binding pocket conferring potency and selectivity over TrkB and TrkC; in vitro assays validated the importance of the JM region for inhibitor potency.","method":"X-ray crystallography, in vitro kinase inhibition assays with JM-region mutants/constructs","journal":"Bioorganic & medicinal chemistry letters","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure combined with in vitro functional assay validating JM region importance, single study","pmids":["28159414"],"is_preprint":false},{"year":2012,"finding":"TrkA has two established intracellular docking sites (Y490 and Y785) directly involved in signal propagation; phosphoproteomic dissection using Y490F and Y785F TrkA mutants identified a clear subset of downstream phosphorylation events not dependent on either docking site.","method":"Phosphoproteomics (mass spectrometry) of TrkA signaling using Y490F and Y785F receptor mutants","journal":"Advances in biological regulation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — phosphoproteomic approach with receptor mutants, single lab","pmids":["23266087"],"is_preprint":false},{"year":2004,"finding":"p75NTR enhances TrkA signaling by specifically augmenting phosphorylation of the 46- and 52-kDa isoforms of Shc during NGF-induced TrkA activation; p75NTR physically co-immunoprecipitates with Shc; Akt serine phosphorylation downstream of Shc is also p75NTR-dependent; p75NTR does not enhance tyrosine phosphorylation of other TrkA substrates.","method":"Antisense knockdown of p75NTR and TrkA, phosphorylation immunoblotting of Shc isoforms and Akt, co-immunoprecipitation of p75NTR and Shc","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — antisense knockdown with substrate-specific phosphorylation analysis and co-IP, single lab","pmids":["15056278"],"is_preprint":false},{"year":2005,"finding":"TrkA induces apoptosis of neuroblastoma cells via a p53-dependent mechanism: TrkA increases p53 target protein expression; kinase-inactive TrkA or p53 inactivation (dominant inhibitory p53, E1B55K, or p53 mutation) prevents TrkA-induced apoptosis; caspase inhibitor or Bcl-XL overexpression also prevents TrkA apoptosis. Conversely, TrkA overexpression in non-transformed sympathetic neurons suppresses p53 and enhances survival.","method":"TrkA expression in neuroblastoma cell lines, p53 dominant-negative and E1B55K expression, caspase inhibitor treatment, Bcl-XL overexpression, p53 target protein immunoblotting, apoptosis assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic and pharmacological interventions with defined molecular pathway, single lab","pmids":["15961390"],"is_preprint":false},{"year":2019,"finding":"The ganglioside GM1 oligosaccharide directly contacts TrkA at the cell surface to promote neuroblastoma differentiation: photoactivatable cross-linking with GM1 derivatives bearing the photoactivable group on the oligosaccharide (but not the ceramide) generated cross-linked TrkA-GM1 complexes. GM1 resides in detergent-resistant raft fractions while TrkA is in the soluble fraction, suggesting TrkA interacts with GM1 by extending its extracellular domain toward the membrane.","method":"Photoactivatable cross-linking with three radiolabeled GM1 derivatives, PAGE separation, radioimaging and immunoblotting, plasma membrane lipid raft isolation","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chemical cross-linking with domain-specific probes combined with fractionation, single lab","pmids":["30776097"],"is_preprint":false},{"year":2015,"finding":"NGF stimulation induces CD44 binding to TrkA at the plasma membrane, activating the p115RhoGEF/RhoA/ROCK1 pathway to stimulate breast cancer cell invasion independently of TrkA kinase activity; this TrkA kinase-independent CD44 signaling contributes to resistance to the TrkA kinase inhibitor lestaurtinib.","method":"Mass spectrometry proteomics, co-immunoprecipitation, proximity ligation assays, siRNA knockdown of CD44, in vitro invasion assays, mouse xenograft tumor model","journal":"Oncotarget","confidence":"High","confidence_rationale":"Tier 2 / Moderate — proteomics, co-IP, PLA, genetic knockdown, and in vivo xenograft validation, multiple orthogonal methods in single study","pmids":["25840418"],"is_preprint":false},{"year":2019,"finding":"ProNGF binding to sortilin induces sequential formation of a sortilin/TrkA/EphA2 complex at the plasma membrane, leading to TrkA phosphorylation-dependent Akt activation and EphA2-dependent Src activation; EphA2 inhibition abolishes proNGF-stimulated clonogenic growth of breast cancer cells.","method":"Proteomic analysis, proximity ligation assays, co-immunoprecipitation, siRNA knockdown of EphA2, clonogenic growth assays, in vivo primary tumor and metastasis models","journal":"Cancer letters","confidence":"High","confidence_rationale":"Tier 2 / Moderate — proteomics, PLA, co-IP, genetic knockdown with functional readouts, in vivo validation, single lab","pmids":["30771434"],"is_preprint":false},{"year":2006,"finding":"Brn3a and Klf7 transcription factors cooperate to control TrkA expression in sensory neurons: in vitro, they synergistically activate the TrkA enhancer; in vivo, TrkA expression is severely reduced in Brn3a-/-;Klf7-/- double-mutant trigeminal ganglia compared to single mutants, and all Trk+ neurons are lost by birth in double mutants.","method":"In vitro TrkA enhancer reporter assays, genetic epistasis with Brn3a and Klf7 single and double knockout mice, in vivo TrkA expression analysis","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with double KO mouse, enhancer assay, and in vivo expression phenotype, multiple orthogonal methods","pmids":["17011544"],"is_preprint":false},{"year":2018,"finding":"EZH2 represses NTRK1 (TrkA) transcription via H3K27me3 histone modifications at the NTRK1 P1 promoter region; EZH2 knockdown or inhibition de-represses NTRK1 expression and induces neuroblastoma cell differentiation (neurite extension); depletion of NTRK1 cancels EZH2 knockdown-induced differentiation, establishing NTRK1 as a downstream effector.","method":"EZH2 knockdown by lentivirus, EZH2 inhibitor treatment, transcriptome analysis, chromatin immunoprecipitation (ChIP) for H3K27me3 at NTRK1 promoters, methylome analysis, NTRK1 siRNA epistasis","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 / Moderate — ChIP, methylome, transcriptome, epistasis knockdown all in single study with multiple orthogonal methods","pmids":["29507419"],"is_preprint":false},{"year":2018,"finding":"Light-inducible activation of TrkA intracellular domain homo-interaction (using cryptochrome 2 optogenetics) in the absence of NGF activates PI3K/AKT and Raf/ERK signaling pathways, promotes neurite growth in PC12 cells, and supports survival of dorsal root ganglion neurons, demonstrating that kinase domain dimerization/interaction is sufficient for TrkA downstream signaling.","method":"Optogenetic TrkA activation using CRY2-based homo-interaction; PI3K/AKT and Raf/ERK phosphorylation assays; neurite outgrowth in PC12 cells; DRG neuron survival assay","journal":"ACS synthetic biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — synthetic optogenetic approach with multiple signaling and functional readouts, single lab","pmids":["29975841"],"is_preprint":false},{"year":2020,"finding":"Site-dependent phosphorylation of individual intracellular tyrosines in TrkA controls MAPK/ERK signaling: using light-sensitive tyrosine analogues (p-azido-L-phenylalanine and caged-tyrosine via amber codon suppression), specific TrkA tyrosine mutants were identified that can activate the ERK pathway in the absence of NGF upon light illumination, revealing which phosphorylation sites drive defined downstream signaling.","method":"Genetic code expansion with amber codon suppression, light-sensitive unnatural amino acid incorporation at specific TrkA tyrosines, ERK activation assays with light-controlled phosphorylation","journal":"Communications biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel site-specific phosphorylation control method with ERK signaling readout, single lab","pmids":["33239753"],"is_preprint":false},{"year":2014,"finding":"TrkA promotes MDM2-mediated ubiquitination and degradation of AGPS (alkylglyceronephosphate synthase): TrkA phosphorylates AGPS at Y451, promoting AGPS-MDM2 interaction and proteasomal degradation of AGPS, thereby suppressing ferroptosis in prostate cancer cells; TrkA inhibitor larotrectinib increases susceptibility of prostate cancer cells to ferroptosis.","method":"Label-free mass spectrometry, co-immunoprecipitation, GST pull-down (in vivo and in vitro), ubiquitination assays, site-specific mutagenesis (Y451), xenograft model","journal":"Journal of experimental & clinical cancer research","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — GST pull-down in vitro reconstitution, co-IP, mutagenesis at phosphorylation site, and in vivo xenograft validation with multiple orthogonal methods","pmids":["38200609"],"is_preprint":false},{"year":2006,"finding":"P2Y2 GPCR and TrkA receptor tyrosine kinase interact via Src family kinases (SFK): SFK inhibitors block P2Y2-mediated enhancement of TrkA signaling and neuronal differentiation in PC12 cells and DRG neurons, and abrogate co-immunoprecipitation of TrkA, P2Y2, and SFK, identifying SFK as a convergence point for GPCR-RTK crosstalk.","method":"SFK inhibitor treatment, co-immunoprecipitation of TrkA/P2Y2/SFK complex, neurite outgrowth in PC12 cells and primary DRG neurons","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — co-IP and pharmacological inhibition with functional readout, single lab","pmids":["16842754"],"is_preprint":false},{"year":2018,"finding":"NTRK1 inhibition induces phosphorylation of LATS1 and controls YAP subcellular localization, suppressing YAP-driven transcription, cancer cell proliferation and migration; NTRK1 regulates YAP oncogenic activity in vivo in mouse xenograft models, establishing crosstalk between the NGF-NTRK1 and Hippo pathways.","method":"Targeted kinase inhibitor screen, LATS1 phosphorylation assay, YAP localization analysis, cell proliferation/migration assays, mouse xenograft model","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — kinase screen followed by LATS1 phosphorylation, YAP localization, and in vivo xenograft validation, single lab","pmids":["30542115"],"is_preprint":false}],"current_model":"NTRK1/TrkA is an NGF-responsive receptor tyrosine kinase that, upon ligand binding and dimerization, undergoes autophosphorylation at key intracellular tyrosines (notably Y490 and Y785) following an ordered sequential kinetic mechanism (ATP before substrate), recruiting adaptors Shc, IRS-1/2, and PLC-γ to activate PI3K/Akt and Ras/MAPK/ERK survival and differentiation pathways; receptor activity, surface localization, and fate are regulated by N-glycosylation (required for surface targeting and prevention of constitutive activation), ubiquitination via TRAF4 (K27/K29-linked, activating) and Nedd4-2/KFG-domain-directed (degradative) mechanisms, ectodomain shedding, and post-endocytic sorting through MVB-mediated retrograde signaling endosomes, GGA3-mediated recycling, and STX8-dependent Golgi-to-plasma membrane trafficking; death signaling is mediated through the CCM2 juxtamembrane interactor and p53, while survival/death outcomes are modulated by competitive cross-talk with p75NTR (which enhances Shc phosphorylation) and additional partners including APP (mutual regulation), CD44 (kinase-independent invasion signaling), and EphA2; non-catalytic regulation includes GM1 oligosaccharide interaction at the cell surface, EZH2-mediated H3K27me3 epigenetic repression of the NTRK1 P1 promoter, and transcriptional co-activation by Brn3a and Klf7."},"narrative":{"mechanistic_narrative":"NTRK1/TrkA is a nerve growth factor (NGF)-responsive receptor tyrosine kinase that transduces neurotrophin signals into survival, differentiation, and death decisions in neurons and tumor cells [PMID:11313867, PMID:22442072]. Ligand engagement is governed by its extracellular Ig-like subdomains, where the D5 (IgC2) region contains the NGF-binding hot spot and an overlapping NT-3 docking site, while D4 (IgC1) provides an allosteric NT-3 site that tunes agonism [PMID:17439940]; proNGF and the NGF pro-region also bind and activate TrkA, the latter at a distinct site [PMID:18808449]. Receptor dimerization of the intracellular kinase domain is itself sufficient to drive downstream signaling, as shown by optogenetic homo-interaction that activates PI3K/AKT and Raf/ERK in the absence of NGF [PMID:29975841], and a constitutively dimerizing extracellular linker mutation (P203A) produces ligand-independent phosphorylation and cellular transformation [PMID:11313867]. The kinase operates by an ordered sequential bi-bi mechanism with ATP binding before substrate [PMID:9448714], and autophosphorylation at the Y490 and Y785 docking sites couples the receptor to adaptors and effectors including IRS-1/IRS-2, which recruit p85-PI3K, SHP-2, and Grb2 to activate PI3K and SRE-driven transcription [PMID:11147812, PMID:23266087]. TrkA surface presentation and signaling competence are tightly controlled by N-glycosylation, which both prevents constitutive activation and is required for cell-surface targeting [PMID:10235685], and by trafficking machinery: STX8 directs Golgi-to-plasma-membrane transport [PMID:24872407], GGA3 mediates Arf6-dependent recycling that sustains NGF-induced Akt and survival [PMID:26446845], and retrograde signaling endosomes evolve from multivesicular bodies in a kinase-activity-dependent manner [PMID:29381137]. Receptor abundance and activity are further set by opposing ubiquitination events — TRAF4-mediated K27/K29-linked ubiquitination of the kinase domain enhances activity [PMID:29715200], whereas a KFG juxtamembrane element and Nedd4-2 binding to the C-terminal PPXY motif drive degradative sorting [PMID:24623787, PMID:21332718]. TrkA delivers divergent outputs depending on context: it promotes p53-dependent apoptosis of neuroblastoma cells and engages the CCM2 juxtamembrane adaptor to mediate cell death [PMID:19755102, PMID:15961390], while supporting neuronal survival and basal forebrain cholinergic development through ERK signaling [PMID:22442072]. Its activity is modulated by co-receptor and cross-talk partners including p75NTR, which selectively augments Shc phosphorylation [PMID:9547236, PMID:15056278], APP (reciprocal regulation and Y682 phosphorylation) [PMID:21849536], and oncogenic interactions with CD44 and EphA2 [PMID:25840418, PMID:30771434]. NTRK1 transcription is positively controlled by Brn3a and Klf7 in sensory neurons [PMID:17011544] and epigenetically repressed by EZH2-deposited H3K27me3 at the P1 promoter [PMID:29507419].","teleology":[{"year":1996,"claim":"Established that TrkA partners physically with the p75 neurotrophin receptor and undergoes regulated ectodomain shedding, defining co-receptor and proteolytic layers of control before intracellular signaling was dissected.","evidence":"Copatching with chimeric TrkA-Torso receptors and kinase-dead mutants; ectodomain cleavage assays across cell lines","pmids":["8603925","8636219"],"confidence":"High","gaps":["Functional consequence of the shed cell-associated fragment in vivo not resolved","Identity of the ectodomain protease not defined"]},{"year":1998,"claim":"Defined the enzymatic logic of the TrkA kinase and showed it counteracts p75 death signaling, framing TrkA as a survival kinase that opposes JNK.","evidence":"In vitro kinase kinetics with PLC-γ/GST substrate; introduction of TrkA into p75-dependent apoptotic oligodendrocytes with MAPK/JNK/NFκB readouts","pmids":["9448714","9547236"],"confidence":"High","gaps":["Kinetic mechanism determined for isolated kinase domain, not full-length receptor","Mechanism by which TrkA suppresses JNK not molecularly defined"]},{"year":1999,"claim":"Showed that N-glycosylation gates receptor activation and surface delivery, explaining how an active kinase is prevented from firing prematurely.","evidence":"Deglycosylation assay, confocal localization, and signaling/differentiation readouts in PC12 cells","pmids":["10235685"],"confidence":"High","gaps":["Specific glycan sites responsible not mapped","Link between glycosylation state and physiological receptor regulation unclear"]},{"year":2001,"claim":"Identified the IRS-1/IRS-2 adaptor arm as essential for TrkA mitogenic signaling and a constitutively activating extracellular mutation, connecting receptor structure to PI3K activation and transformation.","evidence":"Yeast two-hybrid, co-IP, PI3K assays, IRS-1-/- fibroblasts; P203A mutagenesis with dimerization, neurite, and nude-mouse tumor assays","pmids":["11147812","11313867"],"confidence":"High","gaps":["Relative contribution of IRS versus Shc arms in neurons not resolved","Structural basis of P203A-induced dimerization not determined"]},{"year":2004,"claim":"Refined the TrkA–p75 relationship by showing p75NTR selectively boosts Shc-isoform phosphorylation and downstream Akt, distinguishing substrate-specific cross-talk from general signal amplification.","evidence":"Antisense knockdown with isoform-resolved phosphorylation immunoblotting and p75NTR–Shc co-IP","pmids":["15056278"],"confidence":"Medium","gaps":["Single lab; mechanism of selective Shc enhancement unknown","Physiological context where this dominates not established"]},{"year":2005,"claim":"Revealed TrkA's death function through a p53-dependent apoptotic program in neuroblastoma, contrasting with its survival role in normal neurons.","evidence":"TrkA expression in neuroblastoma lines with dominant-negative p53, E1B55K, caspase inhibitors, and Bcl-XL","pmids":["15961390"],"confidence":"Medium","gaps":["How an active kinase engages p53 not molecularly defined","Determinants switching survival versus death output unresolved"]},{"year":2006,"claim":"Established transcriptional and cross-talk inputs: Brn3a/Klf7 drive NTRK1 expression in sensory neurons, and P2Y2 GPCR signaling potentiates TrkA via Src family kinases.","evidence":"Enhancer reporters and Brn3a/Klf7 double-knockout epistasis; SFK inhibition and TrkA/P2Y2/SFK co-IP with neurite assays","pmids":["17011544","16842754"],"confidence":"Medium","gaps":["Direct versus indirect binding of Brn3a/Klf7 to the enhancer not fully resolved","P2Y2-TrkA SFK study is single lab with pharmacological inhibition"]},{"year":2009,"claim":"Identified CCM2 as the juxtamembrane adaptor that couples TrkA to cell death pathways, providing a molecular handle on the death-versus-survival switch.","evidence":"PTB-domain co-IP, domain-deletion mutants, and CCM2 knockdown in medulloblastoma/neuroblastoma death assays","pmids":["19755102"],"confidence":"High","gaps":["Downstream effectors of the CCM2 Karet domain not defined","Relationship to the p53 death program not integrated"]},{"year":2011,"claim":"Defined the ubiquitin-degradative arm via Nedd4-2 and the reciprocal APP interaction, linking receptor turnover and a cross-regulatory partner to signal output and localization.","evidence":"PPXY-motif mutagenesis with Nedd4-2 co-IP and neurite assays; reciprocal TrkA–APP co-IP, Y682 mutants, and APP-knockout neuron signaling","pmids":["21332718","21849536"],"confidence":"High","gaps":["How multimonoubiquitination potentiates differentiation mechanistically unclear (Nedd4-2 study Medium confidence)","Physiological significance of APP–TrkA cross-regulation beyond model systems unresolved"]},{"year":2012,"claim":"Established the in vivo requirement of TrkA-ERK signaling for basal forebrain cholinergic development and cognition, and mapped the two intracellular docking tyrosines plus docking-independent phospho-events.","evidence":"Conditional forebrain TrkA knockout with ChAT/ERK, anatomical, and behavioral phenotyping; phosphoproteomics with Y490F/Y785F mutants","pmids":["22442072","23266087"],"confidence":"High","gaps":["Identity and function of docking-independent phosphorylation events not pursued (phosphoproteomics study Medium confidence)","How ERK couples to ChAT regulation not detailed"]},{"year":2014,"claim":"Resolved post-endocytic fate and degradative control, showing MVB-derived retrograde signaling endosomes, STX8-dependent surface delivery, and KFG-domain-driven ubiquitination set receptor levels and pain sensitivity.","evidence":"EM and kinase-manipulation endosome fate tracking in sympathetic neurons; STX8 co-IP with DRG survival and AAV6 in vivo pain model; KFG-deletion knock-in mouse with ubiquitination and pain phenotypes; AGPS phosphorylation/degradation study","pmids":["29381137","24872407","24623787","38200609"],"confidence":"High","gaps":["Molecular trigger that diverts MVBs from lysosomal fusion not defined","The E3 ligase acting through the KFG domain not identified in this work"]},{"year":2015,"claim":"Demonstrated recycling and kinase-independent oncogenic signaling, showing GGA3-Arf6 recycling sustains survival and CD44 mediates invasion bypassing kinase inhibition.","evidence":"DXXLL-motif binding, recycling/degradation kinetics, Akt and survival readouts with Arf6 dominant-negative; mass spec, PLA, CD44 knockdown, and xenograft invasion assays","pmids":["26446845","25840418"],"confidence":"High","gaps":["How GGA3 sorting decisions are regulated by signaling not resolved","Generality of CD44 kinase-independent route beyond breast cancer unclear"]},{"year":2016,"claim":"Provided structural basis for TrkA-selective inhibition, showing the juxtamembrane region forms a unique non-active-site pocket conferring selectivity over TrkB/TrkC.","evidence":"X-ray crystallography of kinase domain plus JM region with inhibitor complexes and in vitro kinase validation","pmids":["28039433","28159414"],"confidence":"High","gaps":["Conformational role of the JM region in normal activation not addressed","Structures are of isolated kinase/JM, not the full receptor"]},{"year":2018,"claim":"Established opposing ubiquitin and epigenetic controls plus Hippo-pathway crosstalk: TRAF4 K27/K29 ubiquitination activates the kinase, EZH2 represses NTRK1 transcription, and NTRK1 modulates LATS1/YAP.","evidence":"Linkage-specific ubiquitination and site mutagenesis with kinase/invasion readouts; ChIP/methylome/transcriptome with NTRK1 epistasis; kinase inhibitor screen with LATS1/YAP assays and xenografts","pmids":["29715200","29507419","30542115"],"confidence":"High","gaps":["How activating versus degradative ubiquitin codes are coordinated unresolved","Mechanism linking NTRK1 activity to LATS1 phosphorylation not defined (YAP study Medium confidence)"]},{"year":2019,"claim":"Defined surface ganglioside and proNGF/sortilin/EphA2 complexes as non-canonical activation inputs, broadening the TrkA interactome at the plasma membrane.","evidence":"Photoactivatable GM1 cross-linking with raft fractionation; proteomics, PLA, co-IP, EphA2 knockdown, and in vivo tumor/metastasis models for the sortilin/TrkA/EphA2 axis","pmids":["30776097","30771434"],"confidence":"High","gaps":["Functional consequence of GM1–TrkA contact for downstream signaling not fully mapped (GM1 study Medium confidence)","Stoichiometry and assembly order of the sortilin/TrkA/EphA2 complex in vivo unclear"]},{"year":2020,"claim":"Used engineered light-controlled phosphorylation to causally link individual intracellular tyrosines to defined ERK output, dissecting site-specific signaling beyond the canonical docking sites.","evidence":"Genetic code expansion with light-sensitive tyrosine analogues and amber suppression, with ERK activation readouts; optogenetic CRY2 homo-interaction (2018)","pmids":["33239753","29975841"],"confidence":"Medium","gaps":["Synthetic activation may not recapitulate native ligand-driven kinetics","Comprehensive map of each tyrosine to specific effector branches incomplete"]},{"year":null,"claim":"How the competing activating (TRAF4) and degradative (Nedd4-2/KFG) ubiquitin signals, trafficking decisions, and partner context are integrated to switch TrkA between survival and death outputs remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking ubiquitin code, endosomal fate, and apoptotic versus survival signaling","Determinants selecting CCM2/p53 death versus PI3K/ERK survival programs not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[4,6,11,18,31]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[4,11,31]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[4]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[5,7,9,29]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[7,9]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,3,17,24,25,26]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[8,13,16]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[17]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[8,13]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[4,6,21,29,30]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[13,16,17]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[8,14,15,18]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[10,23]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[12,27]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[28]}],"complexes":[],"partners":["NGFR","SHC1","IRS1","IRS2","CCM2","APP","CD44","NEDD4L"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P04629","full_name":"High affinity nerve growth factor receptor","aliases":["Neurotrophic tyrosine kinase receptor type 1","TRK1-transforming tyrosine kinase protein","Tropomyosin-related kinase A","Tyrosine kinase receptor","Tyrosine kinase receptor A","Trk-A","gp140trk","p140-TrkA"],"length_aa":796,"mass_kda":87.5,"function":"Receptor tyrosine kinase involved in the development and the maturation of the central and peripheral nervous systems through regulation of proliferation, differentiation and survival of sympathetic and nervous neurons. High affinity receptor for NGF which is its primary ligand (PubMed:1281417, PubMed:15488758, PubMed:17196528, PubMed:1849459, PubMed:1850821, PubMed:22649032, PubMed:27445338, PubMed:8325889). Can also bind and be activated by NTF3/neurotrophin-3. However, NTF3 only supports axonal extension through NTRK1 but has no effect on neuron survival (By similarity). Upon dimeric NGF ligand-binding, undergoes homodimerization, autophosphorylation and activation (PubMed:1281417). Recruits, phosphorylates and/or activates several downstream effectors including SHC1, FRS2, SH2B1, SH2B2 and PLCG1 that regulate distinct overlapping signaling cascades driving cell survival and differentiation. Through SHC1 and FRS2 activates a GRB2-Ras-MAPK cascade that regulates cell differentiation and survival. Through PLCG1 controls NF-Kappa-B activation and the transcription of genes involved in cell survival. Through SHC1 and SH2B1 controls a Ras-PI3 kinase-AKT1 signaling cascade that is also regulating survival. In absence of ligand and activation, may promote cell death, making the survival of neurons dependent on trophic factors Resistant to NGF, it constitutively activates AKT1 and NF-kappa-B and is unable to activate the Ras-MAPK signaling cascade. Antagonizes the anti-proliferative NGF-NTRK1 signaling that promotes neuronal precursors differentiation. Isoform TrkA-III promotes angiogenesis and has oncogenic activity when overexpressed","subcellular_location":"Cell membrane; Early endosome membrane; Late endosome membrane; Recycling endosome membrane","url":"https://www.uniprot.org/uniprotkb/P04629/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NTRK1","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/NTRK1","total_profiled":1310},"omim":[{"mim_id":"619706","title":"LEUCINE-RICH REPEAT NEURONAL PROTEIN 4; LRRN4","url":"https://www.omim.org/entry/619706"},{"mim_id":"619501","title":"VENTRICULOMEGALY AND ARTHROGRYPOSIS; VENARG","url":"https://www.omim.org/entry/619501"},{"mim_id":"618368","title":"DUAL-SPECIFICITY PHOSPHATASE 26; DUSP26","url":"https://www.omim.org/entry/618368"},{"mim_id":"616720","title":"MYASTHENIC SYNDROME, CONGENITAL, 19; CMS19","url":"https://www.omim.org/entry/616720"},{"mim_id":"615759","title":"KINASE D-INTERACTING SUBSTRATE, 220-KD; KIDINS220","url":"https://www.omim.org/entry/615759"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"adrenal gland","ntpm":6.6}],"url":"https://www.proteinatlas.org/search/NTRK1"},"hgnc":{"alias_symbol":["TRK","TRKA","MTC"],"prev_symbol":[]},"alphafold":{"accession":"P04629","domains":[{"cath_id":"3.80.10.10","chopping":"40-192","consensus_level":"medium","plddt":86.4974,"start":40,"end":192},{"cath_id":"2.60.40.10","chopping":"196-281","consensus_level":"medium","plddt":87.3088,"start":196,"end":281},{"cath_id":"2.60.40.10","chopping":"284-377","consensus_level":"medium","plddt":87.7676,"start":284,"end":377},{"cath_id":"3.30.200.20","chopping":"501-591","consensus_level":"high","plddt":84.223,"start":501,"end":591},{"cath_id":"1.10.510.10","chopping":"597-787","consensus_level":"high","plddt":88.4993,"start":597,"end":787}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P04629","model_url":"https://alphafold.ebi.ac.uk/files/AF-P04629-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P04629-F1-predicted_aligned_error_v6.png","plddt_mean":78.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NTRK1","jax_strain_url":"https://www.jax.org/strain/search?query=NTRK1"},"sequence":{"accession":"P04629","fasta_url":"https://rest.uniprot.org/uniprotkb/P04629.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P04629/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P04629"}},"corpus_meta":[{"pmid":"9547236","id":"PMC_9547236","title":"Competitive 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measurement of MAPK activation, JNK activity, and NFκB activity\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean cell culture model with multiple pathway readouts in single lab, two orthogonal signaling assays\",\n      \"pmids\": [\"9547236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"N-glycosylation of TrkA serves two distinct functions: (1) preventing ligand-independent activation — unglycosylated TrkA is constitutively phosphorylated and interacts constitutively with Shc and PLC-γ; (2) localizing TrkA to the cell surface — unglycosylated TrkA is trapped intracellularly and cannot activate the Ras/MAP kinase cascade (MEK and Erk), despite constitutive kinase activity.\",\n      \"method\": \"Microscale deglycosylation assay, confocal microscopy, immunoblotting for phosphorylation and signaling molecule interactions, differentiation assay in PC12 cells\",\n      \"journal\": \"Journal of neurobiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — in vitro deglycosylation assay combined with confocal localization, signaling assays, and functional differentiation readout in single study with multiple orthogonal methods\",\n      \"pmids\": [\"10235685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"TrkA is cleaved at its ectodomain in a membrane-proximal region by a cell-surface proteolytic system regulated by protein kinase C and NGF, generating a cell-associated fragment that is phosphorylated on tyrosine residues; this phosphorylation requires an intact TrkA kinase domain (not detected in kinase-dead TrkA mutants), suggesting elevated kinase activity of the truncated fragment.\",\n      \"method\": \"Ectodomain cleavage assay using multiple cell lines expressing TrkA, kinase-dead TrkA mutants, immunoblotting for tyrosine phosphorylation\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — kinase-dead mutant controls and multiple cell lines, single lab\",\n      \"pmids\": [\"8636219\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The p75 neurotrophin receptor (gp75) forms a complex with TrkA on the cell surface; the TrkA extracellular domain is sufficient for this association, but interactions involving other receptor domains (transmembrane/intracellular) also contribute. TrkA kinase activity is not required for complex formation. gp75 does not copatch with TrkB, PDGFR-β, or Torso, demonstrating specificity.\",\n      \"method\": \"Copatching technique with antibody-induced receptor redistribution on intact cells; baculovirus-insect cell expression of wild-type and chimeric TrkA-Torso receptors; kinase-dead TrkA mutant\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal copatching with domain dissection using chimeric receptors and kinase-dead mutant, specificity controls with multiple receptor types\",\n      \"pmids\": [\"8603925\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The TrkA kinase domain follows a sequential ordered bi-bi kinetic mechanism, with ATP binding prior to the exogenous substrate PLC-γ/GST, followed by release of phosphorylated product before ADP release; the inhibitor K-252a acts as a competitive inhibitor with respect to ATP.\",\n      \"method\": \"In vitro kinase assay using baculovirus-expressed TrkA kinase domain; product and dead-end inhibition kinetics with PLC-γ/GST fusion protein as substrate\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro kinase assay with rigorous kinetic mechanism determination, single lab\",\n      \"pmids\": [\"9448714\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A point mutation P203A in the extracellular linker region between leucine repeats and the first Ig-like domain of TrkA increases NGF binding affinity by decreasing ligand dissociation rate, causes spontaneous receptor dimerization and constitutive phosphorylation in the absence of ligand, promotes ligand-independent neurite outgrowth, and transforms fibroblasts to form tumors in nude mice.\",\n      \"method\": \"Site-directed mutagenesis, NGF binding assays, receptor dimerization analysis, PC12nnr5 neurite outgrowth assay, fibroblast transformation and nude mouse tumor formation\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — mutagenesis combined with binding assay, dimerization, functional differentiation, and in vivo transformation assays in single study\",\n      \"pmids\": [\"11313867\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"IRS-1 and IRS-2 are tyrosine-phosphorylated substrates of both TrkA and the oncogenic TRK-T1 fusion; this leads to recruitment of p85PI3K, SHP-2, and Grb2, increased PI3-kinase activity associated with IRS-1, and activation of c-fos SRE. TRK-T1-stimulated DNA synthesis requires IRS-1 (abolished in IRS-1-/- fibroblasts). Yeast two-hybrid experiments showed direct TrkA–IRS interaction.\",\n      \"method\": \"Tyrosine phosphorylation assays, co-immunoprecipitation, PI3-kinase activity assay, c-fos SRE reporter, DNA synthesis in IRS-1-/- fibroblasts, yeast two-hybrid\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (co-IP, PI3K assay, genetic KO fibroblasts, yeast two-hybrid) in single study\",\n      \"pmids\": [\"11147812\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The TrkA D5 (IgC2) extracellular subdomain contains both the cognate NGF binding hot spot and a distinct but partially overlapping NT-3 docking/activation hot spot (site 1); additionally, D4 (IgC1) contains an allosteric NT-3 binding site (site 2). NT-3 docking on both sites 1 and 2 affords full TrkA agonism additive with NGF, while docking solely on site 1 is partially agonistic but noncompetitively antagonizes NGF binding.\",\n      \"method\": \"Binding studies with truncated and chimeric extracellular subdomain constructs, competition binding assays, functional neuronal survival assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chimeric receptor domain mapping with binding competition and functional survival assays, single lab\",\n      \"pmids\": [\"17439940\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"TrkA receptor endosomal/lysosomal degradation is both ubiquitin- and proteasome-dependent: the polyubiquitin tag (K485 on TrkA) directs receptor sorting; proteasomal deubiquitinating enzymes trim K63-ubiquitin chains from TrkA prior to lysosomal delivery. The K485R ubiquitin-deficient TrkA mutant fails to deubiquitinate and escapes degradation.\",\n      \"method\": \"Co-immunoprecipitation, biochemical fractionation, confocal microscopy, proteasome inhibitor (lactacystin) and lysosomal inhibitor (methylamine, bafilomycin, leupeptin) treatments, K485R TrkA mutant analysis, proteasome purification and co-IP\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (mutant receptor, pharmacological inhibitors, co-IP, fractionation, imaging) in single study\",\n      \"pmids\": [\"18419753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"ProNGF (cleavage-resistant mutant M-proNGF) binds TrkA with lower affinity than NGF, induces TrkA and ERK1/2 phosphorylation, neurite outgrowth in PC12 cells, and survival but less effectively than NGF; additionally, the NGF pro-region alone binds TrkA at a site distinct from NGF binding, causing TrkA and ERK1/2 phosphorylation.\",\n      \"method\": \"Binding affinity determination, TrkA and ERK1/2 phosphorylation immunoblotting, PC12 neurite outgrowth assay, cortical neuron caspase-3 cleavage assay\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays with engineered proNGF mutants, single lab\",\n      \"pmids\": [\"18808449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CCM2 interacts with the juxtamembrane region of TrkA via its PTB domain and mediates TrkA-induced cell death; both the PTB domain (conferring interaction specificity) and the Karet domain (linking to death pathways) are required. Downregulation of CCM2 in medulloblastoma or neuroblastoma cells attenuates TrkA-dependent death.\",\n      \"method\": \"Co-immunoprecipitation (PTB domain interaction), domain deletion mutant analysis, CCM2 knockdown in tumor cells with TrkA-dependent death readout\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal domain-mapping co-IP combined with loss-of-function knockdown and defined death phenotype, single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"19755102\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"TrkA phosphorylates APP at Y682; APP interacts with TrkA and this interaction requires Y682. Reciprocally, APP (specifically Y682) regulates activation of the NGF/TrkA signaling pathway in vivo, controls subcellular distribution of TrkA, and modulates neuronal sensitivity to NGF.\",\n      \"method\": \"NGF stimulation, tyrosine phosphorylation assays, co-immunoprecipitation, APP Y682F mutant analysis, in vivo TrkA distribution and signaling in APP knockout neurons\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP, site-specific mutant (Y682), in vivo localization, and signaling assays with multiple orthogonal methods in single study\",\n      \"pmids\": [\"21849536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Conditional forebrain-specific TrkA knockout in mice causes dysfunction of basal forebrain cholinergic neurons (BFCNs): developmental increase of choline acetyltransferase expression becomes dependent on TrkA signaling (via the ERK pathway) before neuronal connections are established; TrkA loss results in anatomical and physiological deficits in BF cholinergic circuitry and selective cognitive impairment.\",\n      \"method\": \"Conditional TrkA knockout mouse (Cre-lox), ChAT expression analysis, ERK pathway readout, anatomical/physiological BF measurements, cognitive behavioral testing\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional KO with defined molecular pathway (ERK), anatomical, physiological, and behavioral phenotypes, multiple readouts\",\n      \"pmids\": [\"22442072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The majority of retrograde TrkA signaling endosomes in sympathetic neurons are multivesicular bodies (MVBs). Retrogradely transported TrkA+ MVBs evade lysosomal fusion upon arriving in cell bodies and instead evolve into TrkA+ single-membrane signaling vesicles; TrkA kinase activity associated with retrogradely transported MVBs determines endosome evolution and fate.\",\n      \"method\": \"Ultrastructural (electron microscopy) and molecular characterization of retrograde endosomes in mouse sympathetic neurons; TrkA kinase activity manipulation with fate tracking\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — ultrastructural EM combined with molecular markers, kinase activity manipulation, and vesicle fate tracking in single rigorous study\",\n      \"pmids\": [\"29381137\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A 3-amino-acid (KFG) domain in TrkA negatively regulates TrkA level and function by promoting ubiquitination; deletion of this domain in knock-in mice reduces TrkA ubiquitination, increases TrkA protein levels and activity, and results in enhanced thermal sensitivity and inflammatory pain without affecting DRG neuron numbers.\",\n      \"method\": \"KFG-deletion knock-in mouse, ubiquitination assays, TrkA protein level and activity measurements, thermal sensitivity and inflammatory pain behavioral tests\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo knock-in mouse model with domain-specific mutation, biochemical ubiquitination assay, and behavioral phenotype\",\n      \"pmids\": [\"24623787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Nedd4-2 (E3 ubiquitin ligase) binds the C-terminal PPXY motif of TrkA and mediates multimonoubiquitination. Mutations at the hydrophobic residues Leu784 and Val790 increase Nedd4-2 binding and ubiquitination, directing receptors to the lysosomal pathway instead of recycling; multimonoubiquitination does not impair signaling cascade activation but potentiates TrkA-mediated differentiation (neurite outgrowth).\",\n      \"method\": \"Site-directed mutagenesis of TrkA C-terminal tail, co-immunoprecipitation, colocalization studies, neurite outgrowth assay\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis combined with co-IP, colocalization, and functional differentiation assay, single lab\",\n      \"pmids\": [\"21332718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GGA3 interacts directly with the TrkA cytoplasmic tail through an internal DXXLL motif and mediates functional recycling of TrkA to the plasma membrane via an Arf6-dependent mechanism; GGA3 depletion delays TrkA recycling, accelerates TrkA degradation, attenuates sustained NGF-induced Akt activation, and reduces cell survival.\",\n      \"method\": \"siRNA depletion, direct binding assay (DXXLL motif), recycling and degradation kinetics assays, Akt phosphorylation immunoblotting, cell survival assay, Arf6 dominant-negative analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding motif identified, multiple functional assays (recycling, degradation, signaling, survival), siRNA knockdown, mechanistic dissection of Arf6 requirement\",\n      \"pmids\": [\"26446845\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Syntaxin 8 (STX8, a Q-SNARE protein) binds TrkA and facilitates its transport from the Golgi to the plasma membrane, regulating TrkA cell surface levels specifically (not TrkB); STX8 modulates downstream NGF-induced TrkA signaling and NGF-dependent DRG neuron survival; STX8 knockdown in rat DRG via AAV6 RNAi produced analgesic effects on formalin-induced inflammatory pain.\",\n      \"method\": \"Co-immunoprecipitation, overexpression/knockdown studies for TrkA surface levels, NGF signaling assays, DRG neuron survival assay, AAV6-mediated in vivo STX8 knockdown with formalin pain model\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP with specificity controls (TrkB), multiple cell-based assays, and in vivo functional validation, single lab\",\n      \"pmids\": [\"24872407\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TRAF4 E3 ubiquitin ligase promotes K27- and K29-linked ubiquitination at the TrkA kinase domain, increasing TrkA kinase activity; mutation of TRAF4-targeted ubiquitination sites abolishes TrkA tyrosine autophosphorylation and its interaction with downstream proteins; TRAF4 knockdown suppresses NGF-stimulated TrkA downstream p38 MAPK activation and invasion-associated gene expression in prostate cancer cells.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays (linkage-specific), kinase activity assays, site-directed mutagenesis of ubiquitination sites, TRAF4 knockdown with p38 MAPK and invasion gene readouts\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ubiquitination site mutagenesis, linkage-specific ubiquitination assay, kinase activity measurement, and downstream signaling validation, multiple orthogonal methods\",\n      \"pmids\": [\"29715200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"X-ray crystal structures of TrkA kinase domain complexed with selective inhibitors reveal a non-active-site binding pocket formed by residues from both the kinase domain and the juxtamembrane (JM) region; three distinct binding modes with the JM region were characterized and found to underlie TrkA selectivity over TrkB and TrkC.\",\n      \"method\": \"X-ray crystallography of TrkA kinase domain + JM region with inhibitor complexes; in vitro kinase assays validating JM region importance\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structures with functional in vitro validation of JM region role, single study with structural + biochemical evidence\",\n      \"pmids\": [\"28039433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"X-ray crystal structure of TrkA kinase domain plus juxtamembrane (JM) region bound to a selective inhibitor A1 reveals that the JM region creates a unique inhibitor-binding pocket conferring potency and selectivity over TrkB and TrkC; in vitro assays validated the importance of the JM region for inhibitor potency.\",\n      \"method\": \"X-ray crystallography, in vitro kinase inhibition assays with JM-region mutants/constructs\",\n      \"journal\": \"Bioorganic & medicinal chemistry letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure combined with in vitro functional assay validating JM region importance, single study\",\n      \"pmids\": [\"28159414\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"TrkA has two established intracellular docking sites (Y490 and Y785) directly involved in signal propagation; phosphoproteomic dissection using Y490F and Y785F TrkA mutants identified a clear subset of downstream phosphorylation events not dependent on either docking site.\",\n      \"method\": \"Phosphoproteomics (mass spectrometry) of TrkA signaling using Y490F and Y785F receptor mutants\",\n      \"journal\": \"Advances in biological regulation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — phosphoproteomic approach with receptor mutants, single lab\",\n      \"pmids\": [\"23266087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"p75NTR enhances TrkA signaling by specifically augmenting phosphorylation of the 46- and 52-kDa isoforms of Shc during NGF-induced TrkA activation; p75NTR physically co-immunoprecipitates with Shc; Akt serine phosphorylation downstream of Shc is also p75NTR-dependent; p75NTR does not enhance tyrosine phosphorylation of other TrkA substrates.\",\n      \"method\": \"Antisense knockdown of p75NTR and TrkA, phosphorylation immunoblotting of Shc isoforms and Akt, co-immunoprecipitation of p75NTR and Shc\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — antisense knockdown with substrate-specific phosphorylation analysis and co-IP, single lab\",\n      \"pmids\": [\"15056278\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"TrkA induces apoptosis of neuroblastoma cells via a p53-dependent mechanism: TrkA increases p53 target protein expression; kinase-inactive TrkA or p53 inactivation (dominant inhibitory p53, E1B55K, or p53 mutation) prevents TrkA-induced apoptosis; caspase inhibitor or Bcl-XL overexpression also prevents TrkA apoptosis. Conversely, TrkA overexpression in non-transformed sympathetic neurons suppresses p53 and enhances survival.\",\n      \"method\": \"TrkA expression in neuroblastoma cell lines, p53 dominant-negative and E1B55K expression, caspase inhibitor treatment, Bcl-XL overexpression, p53 target protein immunoblotting, apoptosis assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic and pharmacological interventions with defined molecular pathway, single lab\",\n      \"pmids\": [\"15961390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The ganglioside GM1 oligosaccharide directly contacts TrkA at the cell surface to promote neuroblastoma differentiation: photoactivatable cross-linking with GM1 derivatives bearing the photoactivable group on the oligosaccharide (but not the ceramide) generated cross-linked TrkA-GM1 complexes. GM1 resides in detergent-resistant raft fractions while TrkA is in the soluble fraction, suggesting TrkA interacts with GM1 by extending its extracellular domain toward the membrane.\",\n      \"method\": \"Photoactivatable cross-linking with three radiolabeled GM1 derivatives, PAGE separation, radioimaging and immunoblotting, plasma membrane lipid raft isolation\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chemical cross-linking with domain-specific probes combined with fractionation, single lab\",\n      \"pmids\": [\"30776097\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"NGF stimulation induces CD44 binding to TrkA at the plasma membrane, activating the p115RhoGEF/RhoA/ROCK1 pathway to stimulate breast cancer cell invasion independently of TrkA kinase activity; this TrkA kinase-independent CD44 signaling contributes to resistance to the TrkA kinase inhibitor lestaurtinib.\",\n      \"method\": \"Mass spectrometry proteomics, co-immunoprecipitation, proximity ligation assays, siRNA knockdown of CD44, in vitro invasion assays, mouse xenograft tumor model\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomics, co-IP, PLA, genetic knockdown, and in vivo xenograft validation, multiple orthogonal methods in single study\",\n      \"pmids\": [\"25840418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"ProNGF binding to sortilin induces sequential formation of a sortilin/TrkA/EphA2 complex at the plasma membrane, leading to TrkA phosphorylation-dependent Akt activation and EphA2-dependent Src activation; EphA2 inhibition abolishes proNGF-stimulated clonogenic growth of breast cancer cells.\",\n      \"method\": \"Proteomic analysis, proximity ligation assays, co-immunoprecipitation, siRNA knockdown of EphA2, clonogenic growth assays, in vivo primary tumor and metastasis models\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomics, PLA, co-IP, genetic knockdown with functional readouts, in vivo validation, single lab\",\n      \"pmids\": [\"30771434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Brn3a and Klf7 transcription factors cooperate to control TrkA expression in sensory neurons: in vitro, they synergistically activate the TrkA enhancer; in vivo, TrkA expression is severely reduced in Brn3a-/-;Klf7-/- double-mutant trigeminal ganglia compared to single mutants, and all Trk+ neurons are lost by birth in double mutants.\",\n      \"method\": \"In vitro TrkA enhancer reporter assays, genetic epistasis with Brn3a and Klf7 single and double knockout mice, in vivo TrkA expression analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with double KO mouse, enhancer assay, and in vivo expression phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"17011544\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"EZH2 represses NTRK1 (TrkA) transcription via H3K27me3 histone modifications at the NTRK1 P1 promoter region; EZH2 knockdown or inhibition de-represses NTRK1 expression and induces neuroblastoma cell differentiation (neurite extension); depletion of NTRK1 cancels EZH2 knockdown-induced differentiation, establishing NTRK1 as a downstream effector.\",\n      \"method\": \"EZH2 knockdown by lentivirus, EZH2 inhibitor treatment, transcriptome analysis, chromatin immunoprecipitation (ChIP) for H3K27me3 at NTRK1 promoters, methylome analysis, NTRK1 siRNA epistasis\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP, methylome, transcriptome, epistasis knockdown all in single study with multiple orthogonal methods\",\n      \"pmids\": [\"29507419\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Light-inducible activation of TrkA intracellular domain homo-interaction (using cryptochrome 2 optogenetics) in the absence of NGF activates PI3K/AKT and Raf/ERK signaling pathways, promotes neurite growth in PC12 cells, and supports survival of dorsal root ganglion neurons, demonstrating that kinase domain dimerization/interaction is sufficient for TrkA downstream signaling.\",\n      \"method\": \"Optogenetic TrkA activation using CRY2-based homo-interaction; PI3K/AKT and Raf/ERK phosphorylation assays; neurite outgrowth in PC12 cells; DRG neuron survival assay\",\n      \"journal\": \"ACS synthetic biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — synthetic optogenetic approach with multiple signaling and functional readouts, single lab\",\n      \"pmids\": [\"29975841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Site-dependent phosphorylation of individual intracellular tyrosines in TrkA controls MAPK/ERK signaling: using light-sensitive tyrosine analogues (p-azido-L-phenylalanine and caged-tyrosine via amber codon suppression), specific TrkA tyrosine mutants were identified that can activate the ERK pathway in the absence of NGF upon light illumination, revealing which phosphorylation sites drive defined downstream signaling.\",\n      \"method\": \"Genetic code expansion with amber codon suppression, light-sensitive unnatural amino acid incorporation at specific TrkA tyrosines, ERK activation assays with light-controlled phosphorylation\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel site-specific phosphorylation control method with ERK signaling readout, single lab\",\n      \"pmids\": [\"33239753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"TrkA promotes MDM2-mediated ubiquitination and degradation of AGPS (alkylglyceronephosphate synthase): TrkA phosphorylates AGPS at Y451, promoting AGPS-MDM2 interaction and proteasomal degradation of AGPS, thereby suppressing ferroptosis in prostate cancer cells; TrkA inhibitor larotrectinib increases susceptibility of prostate cancer cells to ferroptosis.\",\n      \"method\": \"Label-free mass spectrometry, co-immunoprecipitation, GST pull-down (in vivo and in vitro), ubiquitination assays, site-specific mutagenesis (Y451), xenograft model\",\n      \"journal\": \"Journal of experimental & clinical cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — GST pull-down in vitro reconstitution, co-IP, mutagenesis at phosphorylation site, and in vivo xenograft validation with multiple orthogonal methods\",\n      \"pmids\": [\"38200609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"P2Y2 GPCR and TrkA receptor tyrosine kinase interact via Src family kinases (SFK): SFK inhibitors block P2Y2-mediated enhancement of TrkA signaling and neuronal differentiation in PC12 cells and DRG neurons, and abrogate co-immunoprecipitation of TrkA, P2Y2, and SFK, identifying SFK as a convergence point for GPCR-RTK crosstalk.\",\n      \"method\": \"SFK inhibitor treatment, co-immunoprecipitation of TrkA/P2Y2/SFK complex, neurite outgrowth in PC12 cells and primary DRG neurons\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — co-IP and pharmacological inhibition with functional readout, single lab\",\n      \"pmids\": [\"16842754\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NTRK1 inhibition induces phosphorylation of LATS1 and controls YAP subcellular localization, suppressing YAP-driven transcription, cancer cell proliferation and migration; NTRK1 regulates YAP oncogenic activity in vivo in mouse xenograft models, establishing crosstalk between the NGF-NTRK1 and Hippo pathways.\",\n      \"method\": \"Targeted kinase inhibitor screen, LATS1 phosphorylation assay, YAP localization analysis, cell proliferation/migration assays, mouse xenograft model\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — kinase screen followed by LATS1 phosphorylation, YAP localization, and in vivo xenograft validation, single lab\",\n      \"pmids\": [\"30542115\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NTRK1/TrkA is an NGF-responsive receptor tyrosine kinase that, upon ligand binding and dimerization, undergoes autophosphorylation at key intracellular tyrosines (notably Y490 and Y785) following an ordered sequential kinetic mechanism (ATP before substrate), recruiting adaptors Shc, IRS-1/2, and PLC-γ to activate PI3K/Akt and Ras/MAPK/ERK survival and differentiation pathways; receptor activity, surface localization, and fate are regulated by N-glycosylation (required for surface targeting and prevention of constitutive activation), ubiquitination via TRAF4 (K27/K29-linked, activating) and Nedd4-2/KFG-domain-directed (degradative) mechanisms, ectodomain shedding, and post-endocytic sorting through MVB-mediated retrograde signaling endosomes, GGA3-mediated recycling, and STX8-dependent Golgi-to-plasma membrane trafficking; death signaling is mediated through the CCM2 juxtamembrane interactor and p53, while survival/death outcomes are modulated by competitive cross-talk with p75NTR (which enhances Shc phosphorylation) and additional partners including APP (mutual regulation), CD44 (kinase-independent invasion signaling), and EphA2; non-catalytic regulation includes GM1 oligosaccharide interaction at the cell surface, EZH2-mediated H3K27me3 epigenetic repression of the NTRK1 P1 promoter, and transcriptional co-activation by Brn3a and Klf7.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"NTRK1/TrkA is a nerve growth factor (NGF)-responsive receptor tyrosine kinase that transduces neurotrophin signals into survival, differentiation, and death decisions in neurons and tumor cells [#5, #12]. Ligand engagement is governed by its extracellular Ig-like subdomains, where the D5 (IgC2) region contains the NGF-binding hot spot and an overlapping NT-3 docking site, while D4 (IgC1) provides an allosteric NT-3 site that tunes agonism [#7]; proNGF and the NGF pro-region also bind and activate TrkA, the latter at a distinct site [#9]. Receptor dimerization of the intracellular kinase domain is itself sufficient to drive downstream signaling, as shown by optogenetic homo-interaction that activates PI3K/AKT and Raf/ERK in the absence of NGF [#29], and a constitutively dimerizing extracellular linker mutation (P203A) produces ligand-independent phosphorylation and cellular transformation [#5]. The kinase operates by an ordered sequential bi-bi mechanism with ATP binding before substrate [#4], and autophosphorylation at the Y490 and Y785 docking sites couples the receptor to adaptors and effectors including IRS-1/IRS-2, which recruit p85-PI3K, SHP-2, and Grb2 to activate PI3K and SRE-driven transcription [#6, #21]. TrkA surface presentation and signaling competence are tightly controlled by N-glycosylation, which both prevents constitutive activation and is required for cell-surface targeting [#1], and by trafficking machinery: STX8 directs Golgi-to-plasma-membrane transport [#17], GGA3 mediates Arf6-dependent recycling that sustains NGF-induced Akt and survival [#16], and retrograde signaling endosomes evolve from multivesicular bodies in a kinase-activity-dependent manner [#13]. Receptor abundance and activity are further set by opposing ubiquitination events — TRAF4-mediated K27/K29-linked ubiquitination of the kinase domain enhances activity [#18], whereas a KFG juxtamembrane element and Nedd4-2 binding to the C-terminal PPXY motif drive degradative sorting [#14, #15]. TrkA delivers divergent outputs depending on context: it promotes p53-dependent apoptosis of neuroblastoma cells and engages the CCM2 juxtamembrane adaptor to mediate cell death [#10, #23], while supporting neuronal survival and basal forebrain cholinergic development through ERK signaling [#12]. Its activity is modulated by co-receptor and cross-talk partners including p75NTR, which selectively augments Shc phosphorylation [#0, #22], APP (reciprocal regulation and Y682 phosphorylation) [#11], and oncogenic interactions with CD44 and EphA2 [#25, #26]. NTRK1 transcription is positively controlled by Brn3a and Klf7 in sensory neurons [#27] and epigenetically repressed by EZH2-deposited H3K27me3 at the P1 promoter [#28].\"\n  ,\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that TrkA partners physically with the p75 neurotrophin receptor and undergoes regulated ectodomain shedding, defining co-receptor and proteolytic layers of control before intracellular signaling was dissected.\",\n      \"evidence\": \"Copatching with chimeric TrkA-Torso receptors and kinase-dead mutants; ectodomain cleavage assays across cell lines\",\n      \"pmids\": [\"8603925\", \"8636219\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the shed cell-associated fragment in vivo not resolved\", \"Identity of the ectodomain protease not defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Defined the enzymatic logic of the TrkA kinase and showed it counteracts p75 death signaling, framing TrkA as a survival kinase that opposes JNK.\",\n      \"evidence\": \"In vitro kinase kinetics with PLC-γ/GST substrate; introduction of TrkA into p75-dependent apoptotic oligodendrocytes with MAPK/JNK/NFκB readouts\",\n      \"pmids\": [\"9448714\", \"9547236\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetic mechanism determined for isolated kinase domain, not full-length receptor\", \"Mechanism by which TrkA suppresses JNK not molecularly defined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Showed that N-glycosylation gates receptor activation and surface delivery, explaining how an active kinase is prevented from firing prematurely.\",\n      \"evidence\": \"Deglycosylation assay, confocal localization, and signaling/differentiation readouts in PC12 cells\",\n      \"pmids\": [\"10235685\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific glycan sites responsible not mapped\", \"Link between glycosylation state and physiological receptor regulation unclear\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified the IRS-1/IRS-2 adaptor arm as essential for TrkA mitogenic signaling and a constitutively activating extracellular mutation, connecting receptor structure to PI3K activation and transformation.\",\n      \"evidence\": \"Yeast two-hybrid, co-IP, PI3K assays, IRS-1-/- fibroblasts; P203A mutagenesis with dimerization, neurite, and nude-mouse tumor assays\",\n      \"pmids\": [\"11147812\", \"11313867\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of IRS versus Shc arms in neurons not resolved\", \"Structural basis of P203A-induced dimerization not determined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Refined the TrkA–p75 relationship by showing p75NTR selectively boosts Shc-isoform phosphorylation and downstream Akt, distinguishing substrate-specific cross-talk from general signal amplification.\",\n      \"evidence\": \"Antisense knockdown with isoform-resolved phosphorylation immunoblotting and p75NTR–Shc co-IP\",\n      \"pmids\": [\"15056278\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; mechanism of selective Shc enhancement unknown\", \"Physiological context where this dominates not established\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Revealed TrkA's death function through a p53-dependent apoptotic program in neuroblastoma, contrasting with its survival role in normal neurons.\",\n      \"evidence\": \"TrkA expression in neuroblastoma lines with dominant-negative p53, E1B55K, caspase inhibitors, and Bcl-XL\",\n      \"pmids\": [\"15961390\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How an active kinase engages p53 not molecularly defined\", \"Determinants switching survival versus death output unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Established transcriptional and cross-talk inputs: Brn3a/Klf7 drive NTRK1 expression in sensory neurons, and P2Y2 GPCR signaling potentiates TrkA via Src family kinases.\",\n      \"evidence\": \"Enhancer reporters and Brn3a/Klf7 double-knockout epistasis; SFK inhibition and TrkA/P2Y2/SFK co-IP with neurite assays\",\n      \"pmids\": [\"17011544\", \"16842754\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct versus indirect binding of Brn3a/Klf7 to the enhancer not fully resolved\", \"P2Y2-TrkA SFK study is single lab with pharmacological inhibition\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identified CCM2 as the juxtamembrane adaptor that couples TrkA to cell death pathways, providing a molecular handle on the death-versus-survival switch.\",\n      \"evidence\": \"PTB-domain co-IP, domain-deletion mutants, and CCM2 knockdown in medulloblastoma/neuroblastoma death assays\",\n      \"pmids\": [\"19755102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors of the CCM2 Karet domain not defined\", \"Relationship to the p53 death program not integrated\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined the ubiquitin-degradative arm via Nedd4-2 and the reciprocal APP interaction, linking receptor turnover and a cross-regulatory partner to signal output and localization.\",\n      \"evidence\": \"PPXY-motif mutagenesis with Nedd4-2 co-IP and neurite assays; reciprocal TrkA–APP co-IP, Y682 mutants, and APP-knockout neuron signaling\",\n      \"pmids\": [\"21332718\", \"21849536\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How multimonoubiquitination potentiates differentiation mechanistically unclear (Nedd4-2 study Medium confidence)\", \"Physiological significance of APP–TrkA cross-regulation beyond model systems unresolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Established the in vivo requirement of TrkA-ERK signaling for basal forebrain cholinergic development and cognition, and mapped the two intracellular docking tyrosines plus docking-independent phospho-events.\",\n      \"evidence\": \"Conditional forebrain TrkA knockout with ChAT/ERK, anatomical, and behavioral phenotyping; phosphoproteomics with Y490F/Y785F mutants\",\n      \"pmids\": [\"22442072\", \"23266087\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity and function of docking-independent phosphorylation events not pursued (phosphoproteomics study Medium confidence)\", \"How ERK couples to ChAT regulation not detailed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Resolved post-endocytic fate and degradative control, showing MVB-derived retrograde signaling endosomes, STX8-dependent surface delivery, and KFG-domain-driven ubiquitination set receptor levels and pain sensitivity.\",\n      \"evidence\": \"EM and kinase-manipulation endosome fate tracking in sympathetic neurons; STX8 co-IP with DRG survival and AAV6 in vivo pain model; KFG-deletion knock-in mouse with ubiquitination and pain phenotypes; AGPS phosphorylation/degradation study\",\n      \"pmids\": [\"29381137\", \"24872407\", \"24623787\", \"38200609\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular trigger that diverts MVBs from lysosomal fusion not defined\", \"The E3 ligase acting through the KFG domain not identified in this work\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated recycling and kinase-independent oncogenic signaling, showing GGA3-Arf6 recycling sustains survival and CD44 mediates invasion bypassing kinase inhibition.\",\n      \"evidence\": \"DXXLL-motif binding, recycling/degradation kinetics, Akt and survival readouts with Arf6 dominant-negative; mass spec, PLA, CD44 knockdown, and xenograft invasion assays\",\n      \"pmids\": [\"26446845\", \"25840418\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How GGA3 sorting decisions are regulated by signaling not resolved\", \"Generality of CD44 kinase-independent route beyond breast cancer unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Provided structural basis for TrkA-selective inhibition, showing the juxtamembrane region forms a unique non-active-site pocket conferring selectivity over TrkB/TrkC.\",\n      \"evidence\": \"X-ray crystallography of kinase domain plus JM region with inhibitor complexes and in vitro kinase validation\",\n      \"pmids\": [\"28039433\", \"28159414\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational role of the JM region in normal activation not addressed\", \"Structures are of isolated kinase/JM, not the full receptor\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Established opposing ubiquitin and epigenetic controls plus Hippo-pathway crosstalk: TRAF4 K27/K29 ubiquitination activates the kinase, EZH2 represses NTRK1 transcription, and NTRK1 modulates LATS1/YAP.\",\n      \"evidence\": \"Linkage-specific ubiquitination and site mutagenesis with kinase/invasion readouts; ChIP/methylome/transcriptome with NTRK1 epistasis; kinase inhibitor screen with LATS1/YAP assays and xenografts\",\n      \"pmids\": [\"29715200\", \"29507419\", \"30542115\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How activating versus degradative ubiquitin codes are coordinated unresolved\", \"Mechanism linking NTRK1 activity to LATS1 phosphorylation not defined (YAP study Medium confidence)\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Defined surface ganglioside and proNGF/sortilin/EphA2 complexes as non-canonical activation inputs, broadening the TrkA interactome at the plasma membrane.\",\n      \"evidence\": \"Photoactivatable GM1 cross-linking with raft fractionation; proteomics, PLA, co-IP, EphA2 knockdown, and in vivo tumor/metastasis models for the sortilin/TrkA/EphA2 axis\",\n      \"pmids\": [\"30776097\", \"30771434\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of GM1–TrkA contact for downstream signaling not fully mapped (GM1 study Medium confidence)\", \"Stoichiometry and assembly order of the sortilin/TrkA/EphA2 complex in vivo unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Used engineered light-controlled phosphorylation to causally link individual intracellular tyrosines to defined ERK output, dissecting site-specific signaling beyond the canonical docking sites.\",\n      \"evidence\": \"Genetic code expansion with light-sensitive tyrosine analogues and amber suppression, with ERK activation readouts; optogenetic CRY2 homo-interaction (2018)\",\n      \"pmids\": [\"33239753\", \"29975841\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Synthetic activation may not recapitulate native ligand-driven kinetics\", \"Comprehensive map of each tyrosine to specific effector branches incomplete\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the competing activating (TRAF4) and degradative (Nedd4-2/KFG) ubiquitin signals, trafficking decisions, and partner context are integrated to switch TrkA between survival and death outputs remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking ubiquitin code, endosomal fate, and apoptotic versus survival signaling\", \"Determinants selecting CCM2/p53 death versus PI3K/ERK survival programs not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [4, 6, 11, 18, 31]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [4, 11, 31]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [5, 7, 9, 29]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [7, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 3, 17, 24, 25, 26]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [8, 13, 16]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [17]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [8, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 6, 21, 29, 30]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [13, 16, 17]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [8, 14, 15, 18]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [10, 23]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [12, 27]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [28]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"NGFR\", \"SHC1\", \"IRS1\", \"IRS2\", \"CCM2\", \"APP\", \"CD44\", \"NEDD4L\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}