{"gene":"KLK8","run_date":"2026-06-10T02:59:49","timeline":{"discoveries":[{"year":2018,"finding":"Crystal structures of KLK8 with a ligand-free active site and with the inhibitor leupeptin were determined, revealing subsite specificity and Ca2+ bound to the 75-loop. Substrate profiling using positional scanning with fluorogenic tetrapeptides and the PICS approach revealed prime-side specificity. Enzyme kinetics showed stimulation by Ca2+ and inhibition by Zn2+; variants D70K and H99A confirmed the antagonistic role of the two cation binding sites. Molecular docking and dynamics calculations provided insights into substrate binding and the dual regulation of activity by Ca2+ and Zn2+ through an allosteric surface loop network.","method":"X-ray crystallography, positional scanning with fluorogenic tetrapeptides, PICS proteomic substrate profiling, enzyme kinetics, site-directed mutagenesis (D70K, H99A), molecular docking and dynamics","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure determination combined with mutagenesis, in vitro enzyme kinetics, and substrate profiling in a single rigorous study","pmids":["30013126"],"is_preprint":false},{"year":2008,"finding":"KLK8 (neuropsin) is required for a form of synaptic tagging in the hippocampal Schaffer-collateral pathway. A single weak tetanus evokes neuropsin-dependent late associativity that signals through the integrin/actin and L-type voltage-dependent Ca2+ channel (LVDCC) pathway. A second, neuropsin-independent form of synaptic capture is triggered by two tetanic stimuli, but both forms converge on LVDCC.","method":"Electrophysiology (LTP recording in hippocampal slices), KLK8 knockout mice, pharmacological inhibition (LVDCC blockers, integrin pathway inhibitors)","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic knockout with defined electrophysiological phenotype and pharmacological pathway dissection, replicated in multiple stimulation paradigms within one rigorous study","pmids":["18216192"],"is_preprint":false},{"year":2011,"finding":"Neuropsin (KLK8) is involved in synaptic tagging during LTP at both basal and apical dendritic inputs in hippocampal pyramidal neurons, and is required for synaptic cross-tagging between LTP and LTD at apical dendritic inputs via integrin β1 and CaMKII signaling.","method":"Electrophysiology (LTP/LTD recording in hippocampal slices), KLK8 knockout mice, pharmacological inhibition of integrin β1 and CaMKII","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — replicates and extends earlier neuropsin-dependent synaptic tagging findings with genetic knockout and pharmacological pathway dissection","pmids":["21646406"],"is_preprint":false},{"year":2023,"finding":"KLK8 proteolytically cleaves the extracellular domain of NCAM1 in hippocampal neurons. Overexpression of KLK8 induces neuronal apoptosis; this is rescued by NCAM1 overexpression or NCAM1 mimetic peptide. KLK8 deficiency prevents CUMS-induced loss of NCAM1 in the hippocampus, placing KLK8-mediated NCAM1 cleavage upstream of neuronal apoptosis in a depression model.","method":"Adenovirus-mediated KLK8 overexpression and knockdown in primary hippocampal neurons and HT22 cells, transgenic/KO mice, cleavage assay, immunofluorescence, NCAM1 rescue experiments (overexpression and mimetic peptide)","journal":"Cell death & disease","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct cleavage of NCAM1 by KLK8 demonstrated with multiple orthogonal methods (gain-of-function, loss-of-function, rescue), in vitro and in vivo","pmids":["37076499"],"is_preprint":false},{"year":2025,"finding":"KLK8 cleaves pro-HGF to release active HGF, which activates the Met/Src/Btk/NF-κB signaling pathway in microglial cells, promoting microglial activation and neuroinflammation. KLK8 overexpression in BV2 microglial cells is sufficient to induce microglial activation. Co-IP coupled with mass spectrometry identified CD44 as a potential KLK8 interactor; KLK8 overexpression decreased CD44 levels.","method":"Adenovirus-mediated KLK8 overexpression, Co-immunoprecipitation plus mass spectrometry, pro-HGF cleavage assay, transcriptional profiling, pharmacological Met inhibition, STZ diabetic mouse model, KLK8 knockout mice","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — direct cleavage of pro-HGF demonstrated, mechanistic pathway (Met/Src/Btk/NF-κB) validated by inhibitor studies and KO mice, multiple orthogonal methods","pmids":["40521191"],"is_preprint":false},{"year":2012,"finding":"KLK8 can unmask a PAR2 receptor-activating sequence from a synthetic PAR2-derived peptide precursor and can signal via rat PAR2. However, KLK8 does not signal via human PAR2 in HEK or KNRK cells; instead it disarms human PAR1. KLK14, by contrast, signals via both rat and human PAR2.","method":"Calcium transient assays, MAPK activation assays, β-arrestin interaction assays, receptor internalization assays, cleavage of synthetic PAR-derived peptide sequences in PAR2-expressing HEK and KNRK cells","journal":"Biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple signaling readouts (calcium, MAPK, β-arrestin, internalization) and direct peptide cleavage, single lab","pmids":["22505524"],"is_preprint":false},{"year":2021,"finding":"KLK8 promotes colorectal cancer EMT, proliferation, migration and invasion through a PAR1-dependent pathway. PAR1 antagonist SCH79797 (but not PAR2 antagonist FSLLRY-NH2) attenuated these KLK8-induced effects in vitro and in vivo, placing PAR1 downstream of KLK8 in CRC progression.","method":"CCK-8 and colony formation assays, transwell migration/invasion, wound-healing assay, xenograft and metastasis models in nude mice, pharmacological PAR1/PAR2 antagonism, bioinformatics","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic manipulation (KLK8 overexpression/knockdown) plus pharmacological pathway dissection in vitro and in vivo, single lab","pmids":["34552064"],"is_preprint":false},{"year":2002,"finding":"Recombinant neuropsin (KLK8) applied extracellularly enhanced neurite projection from soma at 14 h and promoted neuronal aggregation with neurite fascicles at 48 h in cultured mouse hippocampal neurons, demonstrating a role for extracellular KLK8 in neurite outgrowth and fasciculation.","method":"Recombinant neuropsin application to mouse hippocampal neuron cultures, immunolocalization of endogenous neuropsin, morphological analysis","journal":"Neuroscience letters","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct application of recombinant protein with defined morphological readout, replicated across multiple time points, single lab","pmids":["11880192"],"is_preprint":false},{"year":2003,"finding":"KLK8 (neuropsin)-deficient mice show prolonged recovery of the epidermis after UV-B irradiation, with a thicker stratum corneum and delayed increase in involucrin immunoreactivity (a marker for cell envelope assembly), indicating that KLK8 participates in early epidermal differentiation and cell envelope assembly but not in migration or desquamation per se.","method":"KLK8 knockout mice, UV-B irradiation, histomorphology, in situ hybridization for KLK8 mRNA, involucrin immunostaining","journal":"The British journal of dermatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with defined histological phenotype and molecular marker, single lab","pmids":["14616360"],"is_preprint":false},{"year":2007,"finding":"A human-specific T→A mutation (c.71-127T>A) in an intronic region of KLK8 is necessary and sufficient to trigger inclusion of an additional exon, producing the human-specific type II splice form of neuropsin expressed only in human brain and absent from non-human primates.","method":"Comparative sequence analysis across primates, in vitro splicing assay, mutation assay","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro splicing assay combined with mutation-sufficiency test, single lab with defined molecular mechanism","pmids":["17487847"],"is_preprint":false},{"year":2018,"finding":"KLK8 (neuropsin) knockdown in mouse primary hippocampal neurons (via siRNA or intra-hippocampal antisense oligonucleotides) reduces MAP2c expression, dendrite length, branching and spine density, and inactivates PKA and downstream pCREB, leading to downregulation of memory-linked genes and impaired memory consolidation.","method":"siRNA knockdown in primary hippocampal neurons, intra-hippocampal antisense oligonucleotide administration in vivo, dendritic morphology analysis, Western blot for MAP2c/PKA/pCREB, behavioral memory tests","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function in vitro and in vivo with defined molecular and behavioral readouts, single lab","pmids":["29967374"],"is_preprint":false},{"year":2021,"finding":"Genetic knockdown of murine Klk8 in TgCRND8 Alzheimer's model mice reduces amyloid-β and tau pathology, shifts APP processing toward the non-amyloidogenic pathway, improves neurovascular unit integrity, and enhances microglial Aβ phagocytosis and neuronal Aβ resistance.","method":"Genetic cross of TgCRND8 mice with mKlk8+/- mice, amyloid/tau quantification, APP cleavage product analysis, primary glia Aβ phagocytosis assay, primary neuron Aβ resistance assay, behavioral tests","journal":"Neuropathology and applied neurobiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockdown with multiple defined molecular readouts and primary cell experiments, single lab","pmids":["33341972"],"is_preprint":false},{"year":2024,"finding":"SREBP1 directly transcriptionally regulates KLK8 expression in oral cancer cells (demonstrated by EMSA for SREBF1/KLK8 promoter activity). KLK8 in turn drives CCL22 secretion, promoting regulatory T-cell (Treg) chemotaxis. Ginkgolide B inhibits SREBP1, thereby reducing KLK8 transcription and CCL22 output and suppressing immune escape.","method":"EMSA (electrophoretic mobility shift assay), RNA sequencing, SREBP1/KLK8 genetic engineering (overexpression and knockdown), recombinant KLK8 protein treatment, Treg chemotaxis assay, MOC-2 mouse model","journal":"Phytomedicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EMSA demonstrating direct transcription factor binding to KLK8 promoter, supported by genetic engineering and in vivo model, single lab","pmids":["39616730"],"is_preprint":false},{"year":2024,"finding":"KLK8 promotes myocardial fibrosis in diabetic cardiomyopathy through PAR1 activation (not PAR2). KLK8 overexpression in cardiac fibroblasts promotes differentiation, collagen synthesis and migration; a PAR1 antagonist (not a PAR2 antagonist) blocks these effects and attenuates TGF-β1/Smad3 signaling.","method":"KLK8 overexpression via pEX-1 plasmid and siRNA knockdown in neonatal rat cardiac fibroblasts, PAR1/PAR2 antagonists, collagen synthesis and migration assays, DCM mouse model, molecular docking","journal":"Chinese medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gain-of-function and loss-of-function with receptor-specific pharmacological dissection, in vitro and in vivo, single lab","pmids":["39578836"],"is_preprint":false},{"year":2025,"finding":"Age-dependent upregulation of KLK8 in pulmonary endothelial cells promotes endothelial senescence by inactivating the fibronectin/focal adhesion kinase (FAK) pathway, increasing susceptibility of aged mice to ventilator-induced lung injury.","method":"KLK8 knockout mice, intra-pulmonary KLK8-overexpressing mice, KLK8 overexpression/knockdown in mouse lung vascular endothelial cells, transcriptome sequencing, senescence assays, FAK pathway analysis, VILI model","journal":"Aging cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic gain- and loss-of-function in vivo and in vitro with defined molecular pathway (fibronectin/FAK), single lab","pmids":["41310943"],"is_preprint":false},{"year":2025,"finding":"Endothelial KLK8 cleaves syndecan-4 (SDC4), contributing to loss of glycocalyx integrity in glomerular endothelial cells. KLK8 also promotes LIFR upregulation, and LIFR signaling mediates glomerular endothelial dysfunction and mesangial cell activation in diabetic nephropathy. Endothelial-specific Klk8 knockout mice show improved albuminuria and glomerulosclerosis in STZ diabetic models.","method":"Endothelial-specific KLK8 knockout (Klk8ΔEC) and global KO mice, STZ diabetic model, scRNA-seq, proteomics, SDC4 cleavage assay, KLK8 siRNA in cultured glomerular endothelial cells, lentivirus-Lifr shRNA in vivo, Co-IP","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct SDC4 cleavage and LIFR pathway demonstrated by multiple biochemical and genetic methods in vitro and in vivo, preprint not yet peer-reviewed","pmids":[],"is_preprint":true},{"year":2003,"finding":"Mouse mast cells express neuropsin (KLK8/Prss19) mRNA and protein, storing it in secretory granules. Expression is upregulated in mucosal mast cells during helminth infection, representing the first identification of neuropsin in an immune cell.","method":"cDNA library sequence analysis of BALB/c bone marrow-derived mast cells, kinetic expression studies, immunohistochemistry","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — localization by immunohistochemistry and mRNA analysis, single lab, no functional consequence established","pmids":["12646205"],"is_preprint":false},{"year":2022,"finding":"KLK8 overexpression in hypoxia-induced H9c2 cardiomyocytes promotes hypertrophy marker expression (ANP, BNP, MHC7), an effect blocked by the p38 MAPK inhibitor SB202190, placing KLK8 upstream of the p38 MAPK/p53 pathway in right ventricular hypertrophy.","method":"KLK8 overexpression and siRNA knockdown in H9c2 cardiomyocytes, pharmacological p38 MAPK inhibition (SB202190), Western blot for hypertrophy markers, HPH rat model","journal":"Tissue & cell","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, pharmacological pathway placement without direct biochemical substrate identification","pmids":["35994918"],"is_preprint":false},{"year":2026,"finding":"KLK8 directly interacts with IκBα and promotes its cytoplasmic degradation, leading to p65 nuclear translocation and enhanced NF-κB activity, and stimulates NLRP3 inflammasome activation (increased NLRP3, ASC, cleaved Caspase-1) in IL-1β-stimulated chondrocytes, promoting cartilage degradation and pyroptosis in osteoarthritis.","method":"KLK8 siRNA knockdown in IL-1β-stimulated chondrocytes, direct interaction assay with IκBα, Western blot for NF-κB pathway components and NLRP3 inflammasome, flow cytometry for pyroptosis, ELISA for cytokines, MLI-induced OA mouse model","journal":"Tissue & cell","confidence":"Low","confidence_rationale":"Tier 3 / Weak — direct KLK8-IκBα interaction shown but method details limited in abstract; single lab, single paper","pmids":["41793791"],"is_preprint":false},{"year":2002,"finding":"KLK8 mRNA expression is upregulated in hyperkeratotic skin conditions (psoriasis vulgaris, seborrheic keratosis, lichen planus, squamous cell carcinoma) and correlates with keratinocyte terminal differentiation induced by high calcium in cell culture, indicating a role in epidermal terminal differentiation.","method":"Northern blot, in situ hybridization of normal and pathological human skin samples, calcium-induced differentiation in keratinocyte cultures","journal":"Molecular pathology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — mRNA localization with in situ hybridization, no direct functional loss-of-function experiment","pmids":["12147714"],"is_preprint":false}],"current_model":"KLK8 (neuropsin) is a secreted trypsin-like serine protease whose crystal structure reveals Ca2+-stimulated and Zn2+-inhibited activity via an allosteric loop network; in the hippocampus it cleaves extracellular substrates (including NCAM1) to regulate synaptic tagging, LTP associativity, dendritic morphology, and PKA-CREB signaling, while in peripheral tissues it activates PAR1 (not PAR2) to promote fibrosis, cleaves pro-HGF to activate Met/Src/Btk/NF-κB neuroinflammatory signaling, cleaves SDC4 to disrupt glycocalyx integrity in renal endothelium, inactivates the fibronectin/FAK pathway to induce endothelial senescence, interacts with IκBα to sustain NF-κB activity in chondrocytes, and promotes epithelial differentiation/desquamation in skin."},"narrative":{"mechanistic_narrative":"KLK8 (neuropsin) is a secreted, Ca2+-stimulated and Zn2+-inhibited trypsin-like serine protease whose activity is governed by an allosteric surface-loop network that couples its two antagonistic cation-binding sites to substrate selectivity [PMID:30013126]. In the hippocampus, extracellular KLK8 shapes synaptic plasticity and circuit remodeling: it is required for late synaptic tagging and cross-tagging between LTP and LTD through integrin β1/CaMKII and L-type voltage-dependent Ca2+ channel signaling [PMID:18216192, PMID:21646406], supports neurite outgrowth and fasciculation [PMID:11880192], and sustains dendritic architecture, spine density, and memory consolidation via PKA–CREB signaling [PMID:29967374]. A key neural substrate is the extracellular domain of NCAM1, whose KLK8-mediated cleavage drives neuronal apoptosis and is engaged in a chronic-stress depression model [PMID:37076499]. Across peripheral tissues, KLK8 acts through proteolytic activation of receptor and matrix substrates: it cleaves pro-HGF to trigger Met/Src/Btk/NF-κB signaling and microglial neuroinflammation [PMID:40521191], and it activates protease-activated receptor PAR1 — while disarming rather than activating human PAR1/PAR2 in some contexts — to promote epithelial–mesenchymal transition in colorectal cancer and myocardial fibrosis via TGF-β1/Smad3 [PMID:22505524, PMID:34552064, PMID:39578836]. KLK8 also participates in epidermal terminal differentiation and cell-envelope assembly [PMID:14616360]. A human-specific intronic mutation generates a brain-restricted type II splice form of neuropsin found only in humans [PMID:17487847].","teleology":[{"year":2002,"claim":"Established that extracellular KLK8 protease activity has direct morphogenic effects on neurons and links to epidermal differentiation, framing it as a tissue-remodeling protease rather than a passive secretory product.","evidence":"Recombinant neuropsin application to hippocampal neuron cultures with morphological readout; Northern/in situ analysis of human skin and calcium-induced keratinocyte differentiation","pmids":["11880192","12147714"],"confidence":"Medium","gaps":["No substrate identified for the neurite-outgrowth or epidermal effects","Skin data are correlative mRNA expression without loss-of-function"]},{"year":2003,"claim":"Genetic loss-of-function established that KLK8 contributes to early epidermal differentiation and cell-envelope assembly, and revealed expression in immune (mast) cells.","evidence":"KLK8 knockout mice with UV-B injury and involucrin staining; mast cell cDNA/IHC during helminth infection","pmids":["14616360","12646205"],"confidence":"Medium","gaps":["Mast cell expression has no established functional consequence","Direct epidermal substrate not defined"]},{"year":2008,"claim":"Resolved whether KLK8 has a defined role in synaptic plasticity by showing it is specifically required for tetanus-evoked late synaptic associativity converging on integrin/actin and LVDCC pathways.","evidence":"LTP recording in hippocampal slices from KLK8 knockout mice with pharmacological pathway dissection","pmids":["18216192"],"confidence":"High","gaps":["Molecular substrate cleaved during tagging not identified","Link between protease activity and integrin/LVDCC signaling unresolved"]},{"year":2007,"claim":"Defined the molecular basis of a human-specific neuropsin isoform, showing a single intronic mutation is sufficient to create a brain-restricted type II splice form.","evidence":"Comparative primate sequencing, in vitro splicing assay, mutation-sufficiency test","pmids":["17487847"],"confidence":"Medium","gaps":["Functional consequence of the type II isoform not established","Activity difference between isoforms unknown"]},{"year":2011,"claim":"Extended the plasticity role by showing KLK8 mediates synaptic cross-tagging between LTP and LTD via integrin β1 and CaMKII.","evidence":"LTP/LTD recording in hippocampal slices from KLK8 knockout mice with integrin β1 and CaMKII inhibition","pmids":["21646406"],"confidence":"High","gaps":["Proteolytic substrate underlying cross-tagging not identified"]},{"year":2012,"claim":"Clarified KLK8 receptor specificity, showing it disarms human PAR1 and signals via rat but not human PAR2, distinguishing it from KLK14.","evidence":"Calcium, MAPK, β-arrestin and internalization assays plus synthetic PAR-peptide cleavage in PAR-expressing cells","pmids":["22505524"],"confidence":"Medium","gaps":["Species-specific receptor behavior complicates extrapolation to human tissue","Single lab without reciprocal validation"]},{"year":2018,"claim":"Provided the structural and biochemical foundation, defining KLK8 subsite specificity and the dual allosteric regulation of activity by Ca2+ (stimulatory) and Zn2+ (inhibitory).","evidence":"X-ray crystallography (ligand-free and leupeptin-bound), positional scanning and PICS substrate profiling, kinetics, D70K/H99A mutagenesis, docking/MD","pmids":["30013126"],"confidence":"High","gaps":["Physiological substrates cleaved in vivo not enumerated by structure alone","In vivo relevance of Ca2+/Zn2+ switch untested"]},{"year":2018,"claim":"Connected KLK8 to memory at the molecular level, showing it maintains dendritic morphology and PKA–CREB-dependent memory gene expression.","evidence":"siRNA and antisense knockdown in primary neurons and in vivo, dendritic/spine morphometry, Western blot for MAP2c/PKA/pCREB, behavioral tests","pmids":["29967374"],"confidence":"Medium","gaps":["Substrate linking KLK8 to PKA/CREB not defined","Direct vs indirect effect on signaling unresolved"]},{"year":2021,"claim":"Identified PAR1 as the operative receptor for KLK8 in cancer progression and demonstrated a contributory role in Alzheimer-type pathology.","evidence":"KLK8 gain/loss-of-function in colorectal cancer with PAR1/PAR2 antagonists and xenografts; genetic Klk8 reduction in TgCRND8 mice with amyloid/tau and APP-processing readouts","pmids":["34552064","33341972"],"confidence":"Medium","gaps":["Direct cleavage event upstream of PAR1 activation not shown in CRC","Mechanism linking KLK8 to APP processing not biochemically defined"]},{"year":2023,"claim":"Identified NCAM1 extracellular domain as a direct KLK8 substrate placing proteolysis upstream of neuronal apoptosis in depression.","evidence":"Adenoviral overexpression/knockdown in neurons and HT22 cells, cleavage assay, NCAM1 rescue (overexpression and mimetic peptide), CUMS model","pmids":["37076499"],"confidence":"High","gaps":["Cleavage site and fragment fate not mapped","Relationship to the synaptic-tagging substrate unclear"]},{"year":2024,"claim":"Defined KLK8 as both a transcriptional target of SREBP1 and a driver of fibrosis/immune escape, broadening it into metabolic and immunomodulatory programs.","evidence":"EMSA for SREBF1/KLK8 promoter, Treg chemotaxis and CCL22 readouts in oral cancer; KLK8/PAR1 in cardiac fibroblasts with TGF-β1/Smad3 and DCM model","pmids":["39616730","39578836"],"confidence":"Medium","gaps":["Direct substrate driving CCL22 secretion not identified","PAR1 cleavage by KLK8 in cardiac context inferred pharmacologically"]},{"year":2025,"claim":"Established KLK8 substrate cleavage events in vascular and inflammatory disease: pro-HGF activation driving Met/Src/Btk/NF-κB neuroinflammation, fibronectin/FAK inactivation in endothelial senescence, and SDC4 cleavage disrupting endothelial glycocalyx.","evidence":"Adenoviral overexpression, Co-IP/MS, pro-HGF cleavage assay and Met inhibition in microglia; KLK8 KO/overexpression in lung endothelium with FAK analysis; endothelial-specific Klk8 KO with SDC4 cleavage and LIFR pathway in diabetic nephropathy (one preprint)","pmids":["40521191","41310943"],"confidence":"Medium","gaps":["CD44 interaction identified by Co-IP/MS not functionally validated","SDC4/LIFR findings remain in preprint form"]},{"year":null,"claim":"How the structurally defined Ca2+/Zn2+ allosteric switch selects among the diverse documented substrates (NCAM1, pro-HGF, SDC4, fibronectin, PAR receptors) in different tissue microenvironments remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No unified substrate-selection model across tissues","Endogenous activation/inhibition of KLK8 in each context undefined","Direct cleavage sites for most non-NCAM1/pro-HGF substrates unmapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,3,4,5,15]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,7]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,6,13]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,3,4]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[1,2,10]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,18]}],"complexes":[],"partners":["NCAM1","HGF","SDC4","PAR1","F2R","IKBA","CD44","FN1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O60259","full_name":"Kallikrein-8","aliases":["Neuropsin","NP","Ovasin","Serine protease 19","Serine protease TADG-14","Tumor-associated differentially expressed gene 14 protein"],"length_aa":260,"mass_kda":28.0,"function":"Serine protease which is capable of degrading a number of proteins such as casein, fibrinogen, kininogen, fibronectin and collagen type IV. Also cleaves L1CAM in response to increased neural activity. Induces neurite outgrowth and fasciculation of cultured hippocampal neurons. Plays a role in the formation and maturation of orphan and small synaptic boutons in the Schaffer-collateral pathway, regulates Schaffer-collateral long-term potentiation in the hippocampus and is required for memory acquisition and synaptic plasticity. Involved in skin desquamation and keratinocyte proliferation. Plays a role in the secondary phase of pathogenesis following spinal cord injury","subcellular_location":"Secreted; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/O60259/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/KLK8","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/KLK8","total_profiled":1310},"omim":[{"mim_id":"615868","title":"SERINE PEPTIDASE INHIBITOR, KAZAL-TYPE, 6; SPINK6","url":"https://www.omim.org/entry/615868"},{"mim_id":"613511","title":"SERINE PROTEASE INHIBITOR, KAZAL-TYPE, 9; SPINK9","url":"https://www.omim.org/entry/613511"},{"mim_id":"605644","title":"KALLIKREIN-RELATED PEPTIDASE 8; KLK8","url":"https://www.omim.org/entry/605644"},{"mim_id":"604438","title":"KALLIKREIN-RELATED PEPTIDASE 7; KLK7","url":"https://www.omim.org/entry/604438"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"cervix","ntpm":41.5},{"tissue":"esophagus","ntpm":115.9},{"tissue":"skin 1","ntpm":96.5},{"tissue":"vagina","ntpm":47.8}],"url":"https://www.proteinatlas.org/search/KLK8"},"hgnc":{"alias_symbol":["HNP","TADG14","neuropsin","ovasin"],"prev_symbol":["PRSS19"]},"alphafold":{"accession":"O60259","domains":[{"cath_id":"2.40.10.10","chopping":"36-258","consensus_level":"medium","plddt":95.4562,"start":36,"end":258}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O60259","model_url":"https://alphafold.ebi.ac.uk/files/AF-O60259-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O60259-F1-predicted_aligned_error_v6.png","plddt_mean":88.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=KLK8","jax_strain_url":"https://www.jax.org/strain/search?query=KLK8"},"sequence":{"accession":"O60259","fasta_url":"https://rest.uniprot.org/uniprotkb/O60259.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O60259/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O60259"}},"corpus_meta":[{"pmid":"11495691","id":"PMC_11495691","title":"Human alpha-defensin 1 (HNP-1) inhibits adenoviral infection in vitro.","date":"2001","source":"Regulatory peptides","url":"https://pubmed.ncbi.nlm.nih.gov/11495691","citation_count":132,"is_preprint":false},{"pmid":"15656915","id":"PMC_15656915","title":"Upregulated expression of human neutrophil peptides 1, 2 and 3 (HNP 1-3) in colon cancer serum and tumours: a biomarker study.","date":"2005","source":"BMC cancer","url":"https://pubmed.ncbi.nlm.nih.gov/15656915","citation_count":127,"is_preprint":false},{"pmid":"8601627","id":"PMC_8601627","title":"Intramolecular inhibition of human defensin HNP-1 by its propiece.","date":"1996","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/8601627","citation_count":111,"is_preprint":false},{"pmid":"9063901","id":"PMC_9063901","title":"Differential scanning microcalorimetry indicates that human defensin, HNP-2, interacts specifically with biomembrane mimetic 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Substrate profiling using positional scanning with fluorogenic tetrapeptides and the PICS approach revealed prime-side specificity. Enzyme kinetics showed stimulation by Ca2+ and inhibition by Zn2+; variants D70K and H99A confirmed the antagonistic role of the two cation binding sites. Molecular docking and dynamics calculations provided insights into substrate binding and the dual regulation of activity by Ca2+ and Zn2+ through an allosteric surface loop network.\",\n      \"method\": \"X-ray crystallography, positional scanning with fluorogenic tetrapeptides, PICS proteomic substrate profiling, enzyme kinetics, site-directed mutagenesis (D70K, H99A), molecular docking and dynamics\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure determination combined with mutagenesis, in vitro enzyme kinetics, and substrate profiling in a single rigorous study\",\n      \"pmids\": [\"30013126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"KLK8 (neuropsin) is required for a form of synaptic tagging in the hippocampal Schaffer-collateral pathway. A single weak tetanus evokes neuropsin-dependent late associativity that signals through the integrin/actin and L-type voltage-dependent Ca2+ channel (LVDCC) pathway. A second, neuropsin-independent form of synaptic capture is triggered by two tetanic stimuli, but both forms converge on LVDCC.\",\n      \"method\": \"Electrophysiology (LTP recording in hippocampal slices), KLK8 knockout mice, pharmacological inhibition (LVDCC blockers, integrin pathway inhibitors)\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout with defined electrophysiological phenotype and pharmacological pathway dissection, replicated in multiple stimulation paradigms within one rigorous study\",\n      \"pmids\": [\"18216192\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Neuropsin (KLK8) is involved in synaptic tagging during LTP at both basal and apical dendritic inputs in hippocampal pyramidal neurons, and is required for synaptic cross-tagging between LTP and LTD at apical dendritic inputs via integrin β1 and CaMKII signaling.\",\n      \"method\": \"Electrophysiology (LTP/LTD recording in hippocampal slices), KLK8 knockout mice, pharmacological inhibition of integrin β1 and CaMKII\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — replicates and extends earlier neuropsin-dependent synaptic tagging findings with genetic knockout and pharmacological pathway dissection\",\n      \"pmids\": [\"21646406\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"KLK8 proteolytically cleaves the extracellular domain of NCAM1 in hippocampal neurons. Overexpression of KLK8 induces neuronal apoptosis; this is rescued by NCAM1 overexpression or NCAM1 mimetic peptide. KLK8 deficiency prevents CUMS-induced loss of NCAM1 in the hippocampus, placing KLK8-mediated NCAM1 cleavage upstream of neuronal apoptosis in a depression model.\",\n      \"method\": \"Adenovirus-mediated KLK8 overexpression and knockdown in primary hippocampal neurons and HT22 cells, transgenic/KO mice, cleavage assay, immunofluorescence, NCAM1 rescue experiments (overexpression and mimetic peptide)\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct cleavage of NCAM1 by KLK8 demonstrated with multiple orthogonal methods (gain-of-function, loss-of-function, rescue), in vitro and in vivo\",\n      \"pmids\": [\"37076499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"KLK8 cleaves pro-HGF to release active HGF, which activates the Met/Src/Btk/NF-κB signaling pathway in microglial cells, promoting microglial activation and neuroinflammation. KLK8 overexpression in BV2 microglial cells is sufficient to induce microglial activation. Co-IP coupled with mass spectrometry identified CD44 as a potential KLK8 interactor; KLK8 overexpression decreased CD44 levels.\",\n      \"method\": \"Adenovirus-mediated KLK8 overexpression, Co-immunoprecipitation plus mass spectrometry, pro-HGF cleavage assay, transcriptional profiling, pharmacological Met inhibition, STZ diabetic mouse model, KLK8 knockout mice\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — direct cleavage of pro-HGF demonstrated, mechanistic pathway (Met/Src/Btk/NF-κB) validated by inhibitor studies and KO mice, multiple orthogonal methods\",\n      \"pmids\": [\"40521191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"KLK8 can unmask a PAR2 receptor-activating sequence from a synthetic PAR2-derived peptide precursor and can signal via rat PAR2. However, KLK8 does not signal via human PAR2 in HEK or KNRK cells; instead it disarms human PAR1. KLK14, by contrast, signals via both rat and human PAR2.\",\n      \"method\": \"Calcium transient assays, MAPK activation assays, β-arrestin interaction assays, receptor internalization assays, cleavage of synthetic PAR-derived peptide sequences in PAR2-expressing HEK and KNRK cells\",\n      \"journal\": \"Biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple signaling readouts (calcium, MAPK, β-arrestin, internalization) and direct peptide cleavage, single lab\",\n      \"pmids\": [\"22505524\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"KLK8 promotes colorectal cancer EMT, proliferation, migration and invasion through a PAR1-dependent pathway. PAR1 antagonist SCH79797 (but not PAR2 antagonist FSLLRY-NH2) attenuated these KLK8-induced effects in vitro and in vivo, placing PAR1 downstream of KLK8 in CRC progression.\",\n      \"method\": \"CCK-8 and colony formation assays, transwell migration/invasion, wound-healing assay, xenograft and metastasis models in nude mice, pharmacological PAR1/PAR2 antagonism, bioinformatics\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic manipulation (KLK8 overexpression/knockdown) plus pharmacological pathway dissection in vitro and in vivo, single lab\",\n      \"pmids\": [\"34552064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Recombinant neuropsin (KLK8) applied extracellularly enhanced neurite projection from soma at 14 h and promoted neuronal aggregation with neurite fascicles at 48 h in cultured mouse hippocampal neurons, demonstrating a role for extracellular KLK8 in neurite outgrowth and fasciculation.\",\n      \"method\": \"Recombinant neuropsin application to mouse hippocampal neuron cultures, immunolocalization of endogenous neuropsin, morphological analysis\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct application of recombinant protein with defined morphological readout, replicated across multiple time points, single lab\",\n      \"pmids\": [\"11880192\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"KLK8 (neuropsin)-deficient mice show prolonged recovery of the epidermis after UV-B irradiation, with a thicker stratum corneum and delayed increase in involucrin immunoreactivity (a marker for cell envelope assembly), indicating that KLK8 participates in early epidermal differentiation and cell envelope assembly but not in migration or desquamation per se.\",\n      \"method\": \"KLK8 knockout mice, UV-B irradiation, histomorphology, in situ hybridization for KLK8 mRNA, involucrin immunostaining\",\n      \"journal\": \"The British journal of dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with defined histological phenotype and molecular marker, single lab\",\n      \"pmids\": [\"14616360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A human-specific T→A mutation (c.71-127T>A) in an intronic region of KLK8 is necessary and sufficient to trigger inclusion of an additional exon, producing the human-specific type II splice form of neuropsin expressed only in human brain and absent from non-human primates.\",\n      \"method\": \"Comparative sequence analysis across primates, in vitro splicing assay, mutation assay\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro splicing assay combined with mutation-sufficiency test, single lab with defined molecular mechanism\",\n      \"pmids\": [\"17487847\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"KLK8 (neuropsin) knockdown in mouse primary hippocampal neurons (via siRNA or intra-hippocampal antisense oligonucleotides) reduces MAP2c expression, dendrite length, branching and spine density, and inactivates PKA and downstream pCREB, leading to downregulation of memory-linked genes and impaired memory consolidation.\",\n      \"method\": \"siRNA knockdown in primary hippocampal neurons, intra-hippocampal antisense oligonucleotide administration in vivo, dendritic morphology analysis, Western blot for MAP2c/PKA/pCREB, behavioral memory tests\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function in vitro and in vivo with defined molecular and behavioral readouts, single lab\",\n      \"pmids\": [\"29967374\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Genetic knockdown of murine Klk8 in TgCRND8 Alzheimer's model mice reduces amyloid-β and tau pathology, shifts APP processing toward the non-amyloidogenic pathway, improves neurovascular unit integrity, and enhances microglial Aβ phagocytosis and neuronal Aβ resistance.\",\n      \"method\": \"Genetic cross of TgCRND8 mice with mKlk8+/- mice, amyloid/tau quantification, APP cleavage product analysis, primary glia Aβ phagocytosis assay, primary neuron Aβ resistance assay, behavioral tests\",\n      \"journal\": \"Neuropathology and applied neurobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockdown with multiple defined molecular readouts and primary cell experiments, single lab\",\n      \"pmids\": [\"33341972\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SREBP1 directly transcriptionally regulates KLK8 expression in oral cancer cells (demonstrated by EMSA for SREBF1/KLK8 promoter activity). KLK8 in turn drives CCL22 secretion, promoting regulatory T-cell (Treg) chemotaxis. Ginkgolide B inhibits SREBP1, thereby reducing KLK8 transcription and CCL22 output and suppressing immune escape.\",\n      \"method\": \"EMSA (electrophoretic mobility shift assay), RNA sequencing, SREBP1/KLK8 genetic engineering (overexpression and knockdown), recombinant KLK8 protein treatment, Treg chemotaxis assay, MOC-2 mouse model\",\n      \"journal\": \"Phytomedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EMSA demonstrating direct transcription factor binding to KLK8 promoter, supported by genetic engineering and in vivo model, single lab\",\n      \"pmids\": [\"39616730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KLK8 promotes myocardial fibrosis in diabetic cardiomyopathy through PAR1 activation (not PAR2). KLK8 overexpression in cardiac fibroblasts promotes differentiation, collagen synthesis and migration; a PAR1 antagonist (not a PAR2 antagonist) blocks these effects and attenuates TGF-β1/Smad3 signaling.\",\n      \"method\": \"KLK8 overexpression via pEX-1 plasmid and siRNA knockdown in neonatal rat cardiac fibroblasts, PAR1/PAR2 antagonists, collagen synthesis and migration assays, DCM mouse model, molecular docking\",\n      \"journal\": \"Chinese medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gain-of-function and loss-of-function with receptor-specific pharmacological dissection, in vitro and in vivo, single lab\",\n      \"pmids\": [\"39578836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Age-dependent upregulation of KLK8 in pulmonary endothelial cells promotes endothelial senescence by inactivating the fibronectin/focal adhesion kinase (FAK) pathway, increasing susceptibility of aged mice to ventilator-induced lung injury.\",\n      \"method\": \"KLK8 knockout mice, intra-pulmonary KLK8-overexpressing mice, KLK8 overexpression/knockdown in mouse lung vascular endothelial cells, transcriptome sequencing, senescence assays, FAK pathway analysis, VILI model\",\n      \"journal\": \"Aging cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic gain- and loss-of-function in vivo and in vitro with defined molecular pathway (fibronectin/FAK), single lab\",\n      \"pmids\": [\"41310943\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Endothelial KLK8 cleaves syndecan-4 (SDC4), contributing to loss of glycocalyx integrity in glomerular endothelial cells. KLK8 also promotes LIFR upregulation, and LIFR signaling mediates glomerular endothelial dysfunction and mesangial cell activation in diabetic nephropathy. Endothelial-specific Klk8 knockout mice show improved albuminuria and glomerulosclerosis in STZ diabetic models.\",\n      \"method\": \"Endothelial-specific KLK8 knockout (Klk8ΔEC) and global KO mice, STZ diabetic model, scRNA-seq, proteomics, SDC4 cleavage assay, KLK8 siRNA in cultured glomerular endothelial cells, lentivirus-Lifr shRNA in vivo, Co-IP\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct SDC4 cleavage and LIFR pathway demonstrated by multiple biochemical and genetic methods in vitro and in vivo, preprint not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mouse mast cells express neuropsin (KLK8/Prss19) mRNA and protein, storing it in secretory granules. Expression is upregulated in mucosal mast cells during helminth infection, representing the first identification of neuropsin in an immune cell.\",\n      \"method\": \"cDNA library sequence analysis of BALB/c bone marrow-derived mast cells, kinetic expression studies, immunohistochemistry\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — localization by immunohistochemistry and mRNA analysis, single lab, no functional consequence established\",\n      \"pmids\": [\"12646205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"KLK8 overexpression in hypoxia-induced H9c2 cardiomyocytes promotes hypertrophy marker expression (ANP, BNP, MHC7), an effect blocked by the p38 MAPK inhibitor SB202190, placing KLK8 upstream of the p38 MAPK/p53 pathway in right ventricular hypertrophy.\",\n      \"method\": \"KLK8 overexpression and siRNA knockdown in H9c2 cardiomyocytes, pharmacological p38 MAPK inhibition (SB202190), Western blot for hypertrophy markers, HPH rat model\",\n      \"journal\": \"Tissue & cell\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, pharmacological pathway placement without direct biochemical substrate identification\",\n      \"pmids\": [\"35994918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"KLK8 directly interacts with IκBα and promotes its cytoplasmic degradation, leading to p65 nuclear translocation and enhanced NF-κB activity, and stimulates NLRP3 inflammasome activation (increased NLRP3, ASC, cleaved Caspase-1) in IL-1β-stimulated chondrocytes, promoting cartilage degradation and pyroptosis in osteoarthritis.\",\n      \"method\": \"KLK8 siRNA knockdown in IL-1β-stimulated chondrocytes, direct interaction assay with IκBα, Western blot for NF-κB pathway components and NLRP3 inflammasome, flow cytometry for pyroptosis, ELISA for cytokines, MLI-induced OA mouse model\",\n      \"journal\": \"Tissue & cell\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — direct KLK8-IκBα interaction shown but method details limited in abstract; single lab, single paper\",\n      \"pmids\": [\"41793791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"KLK8 mRNA expression is upregulated in hyperkeratotic skin conditions (psoriasis vulgaris, seborrheic keratosis, lichen planus, squamous cell carcinoma) and correlates with keratinocyte terminal differentiation induced by high calcium in cell culture, indicating a role in epidermal terminal differentiation.\",\n      \"method\": \"Northern blot, in situ hybridization of normal and pathological human skin samples, calcium-induced differentiation in keratinocyte cultures\",\n      \"journal\": \"Molecular pathology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — mRNA localization with in situ hybridization, no direct functional loss-of-function experiment\",\n      \"pmids\": [\"12147714\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"KLK8 (neuropsin) is a secreted trypsin-like serine protease whose crystal structure reveals Ca2+-stimulated and Zn2+-inhibited activity via an allosteric loop network; in the hippocampus it cleaves extracellular substrates (including NCAM1) to regulate synaptic tagging, LTP associativity, dendritic morphology, and PKA-CREB signaling, while in peripheral tissues it activates PAR1 (not PAR2) to promote fibrosis, cleaves pro-HGF to activate Met/Src/Btk/NF-κB neuroinflammatory signaling, cleaves SDC4 to disrupt glycocalyx integrity in renal endothelium, inactivates the fibronectin/FAK pathway to induce endothelial senescence, interacts with IκBα to sustain NF-κB activity in chondrocytes, and promotes epithelial differentiation/desquamation in skin.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"KLK8 (neuropsin) is a secreted, Ca2+-stimulated and Zn2+-inhibited trypsin-like serine protease whose activity is governed by an allosteric surface-loop network that couples its two antagonistic cation-binding sites to substrate selectivity [#0]. In the hippocampus, extracellular KLK8 shapes synaptic plasticity and circuit remodeling: it is required for late synaptic tagging and cross-tagging between LTP and LTD through integrin β1/CaMKII and L-type voltage-dependent Ca2+ channel signaling [#1, #2], supports neurite outgrowth and fasciculation [#7], and sustains dendritic architecture, spine density, and memory consolidation via PKA–CREB signaling [#10]. A key neural substrate is the extracellular domain of NCAM1, whose KLK8-mediated cleavage drives neuronal apoptosis and is engaged in a chronic-stress depression model [#3]. Across peripheral tissues, KLK8 acts through proteolytic activation of receptor and matrix substrates: it cleaves pro-HGF to trigger Met/Src/Btk/NF-κB signaling and microglial neuroinflammation [#4], and it activates protease-activated receptor PAR1 — while disarming rather than activating human PAR1/PAR2 in some contexts — to promote epithelial–mesenchymal transition in colorectal cancer and myocardial fibrosis via TGF-β1/Smad3 [#5, #6, #13]. KLK8 also participates in epidermal terminal differentiation and cell-envelope assembly [#8]. A human-specific intronic mutation generates a brain-restricted type II splice form of neuropsin found only in humans [#9].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that extracellular KLK8 protease activity has direct morphogenic effects on neurons and links to epidermal differentiation, framing it as a tissue-remodeling protease rather than a passive secretory product.\",\n      \"evidence\": \"Recombinant neuropsin application to hippocampal neuron cultures with morphological readout; Northern/in situ analysis of human skin and calcium-induced keratinocyte differentiation\",\n      \"pmids\": [\"11880192\", \"12147714\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No substrate identified for the neurite-outgrowth or epidermal effects\", \"Skin data are correlative mRNA expression without loss-of-function\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Genetic loss-of-function established that KLK8 contributes to early epidermal differentiation and cell-envelope assembly, and revealed expression in immune (mast) cells.\",\n      \"evidence\": \"KLK8 knockout mice with UV-B injury and involucrin staining; mast cell cDNA/IHC during helminth infection\",\n      \"pmids\": [\"14616360\", \"12646205\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mast cell expression has no established functional consequence\", \"Direct epidermal substrate not defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Resolved whether KLK8 has a defined role in synaptic plasticity by showing it is specifically required for tetanus-evoked late synaptic associativity converging on integrin/actin and LVDCC pathways.\",\n      \"evidence\": \"LTP recording in hippocampal slices from KLK8 knockout mice with pharmacological pathway dissection\",\n      \"pmids\": [\"18216192\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular substrate cleaved during tagging not identified\", \"Link between protease activity and integrin/LVDCC signaling unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Defined the molecular basis of a human-specific neuropsin isoform, showing a single intronic mutation is sufficient to create a brain-restricted type II splice form.\",\n      \"evidence\": \"Comparative primate sequencing, in vitro splicing assay, mutation-sufficiency test\",\n      \"pmids\": [\"17487847\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the type II isoform not established\", \"Activity difference between isoforms unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Extended the plasticity role by showing KLK8 mediates synaptic cross-tagging between LTP and LTD via integrin β1 and CaMKII.\",\n      \"evidence\": \"LTP/LTD recording in hippocampal slices from KLK8 knockout mice with integrin β1 and CaMKII inhibition\",\n      \"pmids\": [\"21646406\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Proteolytic substrate underlying cross-tagging not identified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Clarified KLK8 receptor specificity, showing it disarms human PAR1 and signals via rat but not human PAR2, distinguishing it from KLK14.\",\n      \"evidence\": \"Calcium, MAPK, β-arrestin and internalization assays plus synthetic PAR-peptide cleavage in PAR-expressing cells\",\n      \"pmids\": [\"22505524\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Species-specific receptor behavior complicates extrapolation to human tissue\", \"Single lab without reciprocal validation\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Provided the structural and biochemical foundation, defining KLK8 subsite specificity and the dual allosteric regulation of activity by Ca2+ (stimulatory) and Zn2+ (inhibitory).\",\n      \"evidence\": \"X-ray crystallography (ligand-free and leupeptin-bound), positional scanning and PICS substrate profiling, kinetics, D70K/H99A mutagenesis, docking/MD\",\n      \"pmids\": [\"30013126\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological substrates cleaved in vivo not enumerated by structure alone\", \"In vivo relevance of Ca2+/Zn2+ switch untested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Connected KLK8 to memory at the molecular level, showing it maintains dendritic morphology and PKA–CREB-dependent memory gene expression.\",\n      \"evidence\": \"siRNA and antisense knockdown in primary neurons and in vivo, dendritic/spine morphometry, Western blot for MAP2c/PKA/pCREB, behavioral tests\",\n      \"pmids\": [\"29967374\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Substrate linking KLK8 to PKA/CREB not defined\", \"Direct vs indirect effect on signaling unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified PAR1 as the operative receptor for KLK8 in cancer progression and demonstrated a contributory role in Alzheimer-type pathology.\",\n      \"evidence\": \"KLK8 gain/loss-of-function in colorectal cancer with PAR1/PAR2 antagonists and xenografts; genetic Klk8 reduction in TgCRND8 mice with amyloid/tau and APP-processing readouts\",\n      \"pmids\": [\"34552064\", \"33341972\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct cleavage event upstream of PAR1 activation not shown in CRC\", \"Mechanism linking KLK8 to APP processing not biochemically defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified NCAM1 extracellular domain as a direct KLK8 substrate placing proteolysis upstream of neuronal apoptosis in depression.\",\n      \"evidence\": \"Adenoviral overexpression/knockdown in neurons and HT22 cells, cleavage assay, NCAM1 rescue (overexpression and mimetic peptide), CUMS model\",\n      \"pmids\": [\"37076499\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cleavage site and fragment fate not mapped\", \"Relationship to the synaptic-tagging substrate unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defined KLK8 as both a transcriptional target of SREBP1 and a driver of fibrosis/immune escape, broadening it into metabolic and immunomodulatory programs.\",\n      \"evidence\": \"EMSA for SREBF1/KLK8 promoter, Treg chemotaxis and CCL22 readouts in oral cancer; KLK8/PAR1 in cardiac fibroblasts with TGF-β1/Smad3 and DCM model\",\n      \"pmids\": [\"39616730\", \"39578836\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct substrate driving CCL22 secretion not identified\", \"PAR1 cleavage by KLK8 in cardiac context inferred pharmacologically\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established KLK8 substrate cleavage events in vascular and inflammatory disease: pro-HGF activation driving Met/Src/Btk/NF-κB neuroinflammation, fibronectin/FAK inactivation in endothelial senescence, and SDC4 cleavage disrupting endothelial glycocalyx.\",\n      \"evidence\": \"Adenoviral overexpression, Co-IP/MS, pro-HGF cleavage assay and Met inhibition in microglia; KLK8 KO/overexpression in lung endothelium with FAK analysis; endothelial-specific Klk8 KO with SDC4 cleavage and LIFR pathway in diabetic nephropathy (one preprint)\",\n      \"pmids\": [\"40521191\", \"41310943\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"CD44 interaction identified by Co-IP/MS not functionally validated\", \"SDC4/LIFR findings remain in preprint form\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the structurally defined Ca2+/Zn2+ allosteric switch selects among the diverse documented substrates (NCAM1, pro-HGF, SDC4, fibronectin, PAR receptors) in different tissue microenvironments remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unified substrate-selection model across tissues\", \"Endogenous activation/inhibition of KLK8 in each context undefined\", \"Direct cleavage sites for most non-NCAM1/pro-HGF substrates unmapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 3, 4, 5, 15]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0008233\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 6, 13]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 3, 4]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [1, 2, 10]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 18]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"NCAM1\", \"HGF\", \"SDC4\", \"PAR1\", \"F2R\", \"IkBa\", \"CD44\", \"FN1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}