{"gene":"CTSD","run_date":"2026-04-28T17:28:53","timeline":{"discoveries":[{"year":1985,"finding":"Human cathepsin D (CTSD) was cloned and sequenced from a hepatoma cDNA library. The cDNA predicts a 412-amino acid protein with a 20-aa pre-segment and 44-aa prosegment; the mature protein shows high sequence homology to other aspartyl proteases, establishing CTSD as a member of the aspartyl protease family with a conserved three-dimensional structure.","method":"cDNA cloning, nucleotide sequencing, amino acid sequence analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — foundational cloning and sequence analysis, replicated across multiple clone sources","pmids":["3927292"],"is_preprint":false},{"year":1990,"finding":"Enzymatically active cathepsin D (and cathepsin B) localizes to senile plaques in Alzheimer disease brains, accumulating in extracellular lysosomal dense bodies and lipofuscin granules derived from degenerating neurons, implicating CTSD as a candidate protease for amyloid precursor protein processing in plaques.","method":"Immunohistochemistry with anti-cathepsin D antisera, in situ enzyme histochemistry with synthetic peptide substrates, ultrastructural immunolocalization","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — enzymatic activity confirmed in situ with substrate histochemistry plus ultrastructural localization","pmids":["1692625"],"is_preprint":false},{"year":1999,"finding":"Ceramide generated by acid sphingomyelinase directly binds to and activates cathepsin D, triggering autocatalytic proteolysis of the 52 kDa pre-pro-CTSD to produce enzymatically active 48/32 kDa isoforms. Acid sphingomyelinase-deficient cells have decreased CTSD activity, restored by A-SMase transfection, identifying CTSD as a ceramide target in endosomal apoptotic signaling.","method":"Direct ceramide-CTSD binding assay, in vitro autocatalytic cleavage assay, A-SMase knockout and reconstitution, biochemical fractionation","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — direct binding plus in vitro reconstitution plus genetic rescue, single foundational study with multiple orthogonal methods","pmids":["10508159"],"is_preprint":false},{"year":2003,"finding":"In activated human T lymphocytes, cathepsin D (CTSD) translocates from lysosomes to the cytosol upon apoptotic stimulation and triggers Bax conformational change and relocation to mitochondria in a Bid-independent manner, leading to selective AIF release and early caspase-independent apoptosis. Pepstatin A and siRNA-mediated CTSD silencing inhibited these events, placing CTSD upstream of Bax in this pathway.","method":"Pepstatin A inhibitor treatment, siRNA knockdown of CTSD/Bax/AIF, subcellular fractionation, immunofluorescence localization, cell death assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic silencing (siRNA) plus pharmacological inhibition plus epistasis, replicated across multiple readouts","pmids":["12782632"],"is_preprint":false},{"year":2005,"finding":"CTSD (cath-D) is overexpressed and hypersecretated by breast cancer cells, stimulating tumorigenicity, metastasis, cancer cell proliferation, fibroblast outgrowth, and angiogenesis. A catalytically inactive mutant cath-D retains mitogenic activity for cancer, endothelial, and fibroblastic cells, indicating an extracellular mode of action involving an unidentified cell-surface receptor. During apoptosis, mature lysosomal CTSD translocates to the cytosol and its proteolytic activity participates in the apoptotic cascade.","method":"Catalytic-site mutagenesis, overexpression, tumor xenograft models, cell proliferation assays, apoptosis assays","journal":"Cancer letters","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis of active site distinguishing proteolytic from non-proteolytic functions, replicated across multiple cell models","pmids":["16046058"],"is_preprint":false},{"year":2006,"finding":"A CTSD gene mutation (G to A, Met199Ile) in American Bulldogs causes neuronal ceroid lipofuscinosis (NCL) with ~36% residual cathepsin D enzymatic activity compared to controls, while 15 other lysosomal enzyme activities were unchanged or increased. This directly established that partial loss of CTSD catalytic activity is sufficient to cause NCL-like neurodegeneration.","method":"Genetic linkage analysis, mutation identification, cathepsin D enzyme activity assay in brain tissue, electron microscopy of storage material","journal":"Molecular genetics and metabolism","confidence":"High","confidence_rationale":"Tier 2 — disease-causing mutation linked to specific enzymatic deficit, confirmed by enzyme assay with multiple controls","pmids":["16386934"],"is_preprint":false},{"year":2007,"finding":"Cardiac cathepsin D cleaves prolactin at its N-terminus to generate an antiangiogenic and proapoptotic 16 kDa fragment that mediates postpartum cardiomyopathy (PPCM). STAT3 deletion in cardiomyocytes enhanced cardiac CTSD expression and activity; forced cardiac generation of 16 kDa prolactin impaired the cardiac capillary network and recapitulated PPCM. Bromocriptine (prolactin secretion inhibitor) prevented PPCM.","method":"Cardiomyocyte-specific STAT3 knockout mice, CTSD activity assays, forced cardiac overexpression of 16 kDa prolactin, bromocriptine treatment, cardiac function assessment, patient serum analysis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — genetic KO, reconstitution with cleavage product, pharmacological inhibition, and human patient data converge on same mechanism","pmids":["17289576"],"is_preprint":false},{"year":2008,"finding":"Estrogen receptor alpha (ERα) activates CTSD expression through a distal enhancer element located ~9 kbp upstream of the CTSD transcription start site. ChIP experiments showed estrogen-dependent recruitment of ERα and phosphorylated RNA Pol II to this enhancer, with chromatin looping connecting the distal enhancer to the CTSD promoter. Transient CpG methylation at both the promoter and the distal enhancer was observed during estrogen stimulation.","method":"Chromatin immunoprecipitation (ChIP), chromosome conformation capture (looping assay), bisulfite methylation analysis, reporter assays in MCF-7 cells","journal":"Molecular oncology","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP and looping assay in single lab; ERα dependence confirmed but no orthogonal genetic rescue","pmids":["19383337"],"is_preprint":false},{"year":2019,"finding":"Recombinant human pro-CTSD produced in a mammalian system is efficiently endocytosed via mannose-6-phosphate receptors, trafficked to lysosomes, and processed to the mature active form. In CTSD-deficient mouse models of CLN10 disease, systemic and intracranial administration of rhCTSD corrects lysosomal hypertrophy, storage accumulation, and impaired autophagic flux in viscera and CNS, establishing enzyme replacement as feasible for this lysosomal storage disorder.","method":"Recombinant protein uptake assays, lysosomal targeting/processing assays, CLN10 mouse model ERT, autophagic flux measurement, histopathology, lifespan analysis","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted uptake and processing in vitro, plus in vivo correction with multiple orthogonal readouts","pmids":["31282275"],"is_preprint":false},{"year":2020,"finding":"In Helicoverpa armigera (lepidopteran model), autophagy triggers CTSD maturation and relocalization inside midgut cells, where mature CTSD activates caspase-3 and promotes apoptosis. Glycosylation at asparagine-233 determines pro-CTSD secretion rather than intracellular retention. Steroid hormone 20-hydroxyecdysone (20E) promotes CTSD expression. This establishes that differential glycosylation and autophagy-regulated maturation control the dual pro-proliferative (extracellular) versus pro-apoptotic (intracellular) functions of CTSD.","method":"RNAi knockdown of autophagy genes, site-directed mutagenesis of N233, glycosylation analysis (PNGase F treatment), caspase-3 activity assays, immunofluorescence, hormone treatment experiments","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis, RNAi epistasis, and biochemical assays in insect ortholog with conserved CTSD function; multiple orthogonal methods","pmids":["32324083"],"is_preprint":false},{"year":2020,"finding":"CTSD knockdown in neurons causes lysosomal dysfunction. Restoration of CTSD protein levels via lentiviral transduction increases CTSD activity and renders neurons resistant to oxygen-glucose deprivation (OGD)-mediated lysosomal dysfunction and cell death in a stroke model, demonstrating that CTSD-dependent lysosomal proteolytic activity is required for neuronal survival during ischemia.","method":"shRNA-mediated CTSD knockdown, lentiviral CTSD overexpression, OGD neuronal model, MCAO stroke model, lysosomal function assays, cell death assays","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function and gain-of-function with specific lysosomal and cell death readouts in two complementary models","pmids":["32450052"],"is_preprint":false},{"year":2020,"finding":"CTSD inhibition in radioresistant glioblastoma cells blocks autophagosome-lysosome fusion, increasing autophagosome accumulation while decreasing autolysosome formation, and sensitizes cells to ionizing radiation. CTSD protein levels positively correlate with the autophagy marker LC3-II/I and negatively with p62, positioning CTSD as a regulator of autophagic flux at the autophagosome-lysosome fusion step.","method":"siRNA knockdown, pepstatin A inhibition, Western blot for LC3 and p62, immunofluorescence for autophagosome/autolysosome quantification, clonogenic survival assay after irradiation","journal":"Molecular carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 2–3 — pharmacological inhibition plus siRNA in single lab with functional readouts; mechanism of fusion block not molecularly dissected","pmids":["32253787"],"is_preprint":false},{"year":2022,"finding":"Recombinant human pro-CTSD (rHsCTSD) is endocytosed by neuronal cells, delivered to lysosomes, and matured into active protease. In iPSC-derived dopaminergic neurons from Parkinson disease patients (SNCA A53T mutation) and in ctsd-deficient mouse neurons, rHsCTSD treatment reduces insoluble SNCA/α-synuclein conformers and restores endo-lysosome and autophagy function, establishing CTSD as the major lysosomal protease responsible for SNCA degradation.","method":"Recombinant protein uptake and maturation assays, iPSC-derived dopaminergic neurons, ctsd-KO mouse primary neurons, SNCA solubility fractionation, autophagy flux assays","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 1–2 — enzyme replacement in human iPSC-derived neurons and mouse KO model with biochemical and functional readouts; orthogonal validation across species","pmids":["35287553"],"is_preprint":false},{"year":2023,"finding":"Swainsonine toxin reduces O-GlcNAcylation of CTSD, which impairs its maturation to the active form (m-CTSD). Increasing O-GlcNAcylation (with OGA inhibitor TMG) promotes autophagy, while decreasing it (with OGT inhibitor OSMI) inhibits autophagy. Immunoprecipitation confirmed direct O-GlcNAcylation of CTSD, establishing O-GlcNAcylation as a post-translational modification required for proper CTSD maturation and lysosomal function.","method":"Proteomics sequencing, immunoprecipitation of O-GlcNAcylated CTSD, OGA/OGT inhibitor treatment, autophagy flux assays, Western blot for mature/pro-CTSD forms","journal":"Chemico-biological interactions","confidence":"Medium","confidence_rationale":"Tier 2–3 — immunoprecipitation confirmed CTSD O-GlcNAcylation with pharmacological validation; single lab, mechanism of maturation effect not fully resolved","pmids":["37442287"],"is_preprint":false},{"year":2024,"finding":"N-glycosylation at residue N263 of CTSD, mediated by the glycosyltransferase complex DDOST/STT3B, is required for CTSD protease activity. Glycosylated CTSD lyses ACADM, which in turn regulates ferroptosis-related proteins (ACSL4, SLC7A11, GPX4) to promote invasion and liver metastasis of colorectal cancer cells.","method":"N-glycoproteomics of matched primary and metastatic CRC tissues, site-specific glycosylation mutagenesis, ACADM cleavage assays, ferroptosis marker analysis, invasion/metastasis assays","journal":"Advanced science","confidence":"Medium","confidence_rationale":"Tier 2 — glycosylation site identified by proteomics and mutagenesis, substrate (ACADM) cleavage shown, downstream pathway mapped; single lab","pmids":["39716927"],"is_preprint":false},{"year":2024,"finding":"CLN5 (Cln5) and CTSD (CtsD) are both released extracellularly via signal peptide-dependent secretion and autophagy-linked pathways in Dictyostelium discoideum. CtsD release requires autophagy proteins Atg1 and Atg5, lysosomal exocytosis machinery (AP-3, LYST, mucopilin-1, WASH), and microfilaments. Extracellular CtsD is glycosylated, and Cln5 release is regulated by the amount of extracellular CtsD, identifying a regulatory relationship between these two CLN disease proteins.","method":"Dictyostelium genetic KO models for autophagy and trafficking genes, secretion assays, glycosylation analysis, epistasis experiments between Cln5 and CtsD","journal":"Traffic","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic KOs with defined trafficking readouts in model organism; relevant to human CTSD trafficking mechanism","pmids":["38272448"],"is_preprint":false},{"year":2025,"finding":"LRP6 interacts with HSP90α and CTSD in cardiomyocytes under mechanical stress (identified by mass spectrometry co-IP). LRP6 facilitates CTSD-mediated degradation of HSP90α, which suppresses β-catenin activation and reduces cardiac hypertrophy after pressure overload. Treatment with pepstatin A (CTSD inhibitor) or recombinant HSP90α abolished the cardioprotective effect of LRP6, placing CTSD in the LRP6/HSP90α/β-catenin axis.","method":"Mass spectrometry after LRP6 co-immunoprecipitation, cardiomyocyte-specific LRP6 overexpression mice, transverse aortic constriction model, pepstatin A treatment, HSP90α recombinant protein rescue, echocardiography","journal":"Acta pharmacologica Sinica","confidence":"Medium","confidence_rationale":"Tier 2–3 — MS-identified interaction plus genetic overexpression and pharmacological inhibition in vivo; single lab, mechanism of CTSD-HSP90α proteolysis not directly demonstrated in vitro","pmids":["39779966"],"is_preprint":false},{"year":2025,"finding":"SAMHD1 deficiency in macrophages enhances MITF nuclear translocation, which suppresses CTSD expression downstream of mTOR signaling, impairing lysosomal autophagy flux and promoting inflammation in ulcerative colitis. Pharmacological mTOR inhibition (rapamycin) restores MITF-CTSD signaling and lysosomal function, placing CTSD downstream of the mTOR-MITF axis in macrophage lysosomal homeostasis.","method":"Myeloid-specific SAMHD1 knockout mice, scRNA-seq, MITF nuclear translocation assays, CTSD expression analysis, lysosomal flux assays, rapamycin treatment, colitis model","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO plus pharmacological rescue with defined pathway readouts; single lab","pmids":["40886983"],"is_preprint":false},{"year":2025,"finding":"In mTBI, Snapin binds CBS, disrupting H2S metabolic homeostasis. Reduced H2S limits S-sulfhydration of pro-CTSD at a specific cysteine residue, promoting its maturation into active CTSD and inducing PANoptosis. Pepstatin A (CTSD inhibitor) and NaHS (H2S donor) both confer neuroprotection, establishing S-sulfhydration of pro-CTSD as a regulatory PTM controlling its maturation and downstream apoptotic/pyroptotic/necroptotic signaling.","method":"Conditional Snapin knockdown (AAV-shSnapin), modified biotin switch assay for S-sulfhydration, co-immunoprecipitation of Snapin-CBS, H2S measurement with ion-selective electrode, pepstatin A and NaHS treatment, PANoptosis protein analysis, behavioral tests","journal":"Journal of advanced research","confidence":"Medium","confidence_rationale":"Tier 2 — S-sulfhydration of pro-CTSD detected by modified biotin switch assay with genetic and pharmacological validation; single lab","pmids":["41558604"],"is_preprint":false},{"year":2026,"finding":"KIF13B in macrophages controls proteasome-dependent degradation of the glycosyltransferase STT3A. Kif13b deficiency allows STT3A accumulation, which enhances CTSD glycosylation and secretion, promoting lipid accumulation and inflammation in the liver during MASLD. Secreted CTSD exerts its detrimental effect through interaction with the hepatocyte membrane protein THBS1, defining the KIF13B/STT3A/CTSD/THBS1 axis in macrophage-hepatocyte crosstalk.","method":"Myeloid Kif13b knockout mice, diet-induced MASLD model, CTSD glycosylation assays, CTSD secretion measurement, CTSD-THBS1 interaction studies, proteasome activity assays","journal":"Hepatology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO model with mechanistic dissection of glycosylation and secretion pathway; CTSD-THBS1 interaction mechanistic details not fully elaborated in abstract","pmids":["41746601"],"is_preprint":false},{"year":2025,"finding":"Astrocytic cathepsin D (CtsD) cleaves α-synuclein pre-formed fibrils into C-terminally truncated, seeding-competent species within lysosomes. These truncated species are transferred to neurons where they promote Lewy neurite-like aggregate growth. α-Syn PFF exposure disrupts lysosomal membrane integrity in astrocytes and upregulates CtsD, creating a feed-forward amplification of α-syn pathogenicity.","method":"Neuron-astrocyte co-culture, α-syn PFF treatment, CtsD inhibition/KO in astrocytes, mass spectrometry characterization of cleaved α-syn species, seeding assays in neurons, lysosomal membrane integrity assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — direct substrate cleavage identified with functional seeding assay; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.10.03.680233"],"is_preprint":true},{"year":2025,"finding":"Deletion of CtsD in mice dramatically decreases bone mass with reduced osteoblast numbers and increased osteoclast numbers. In osteoblasts, CtsD inactivation attenuates differentiation and downregulates LC3B with decreased p62, p-Akt, and p-GSK3β. In osteoclasts, CtsD inactivation increases differentiation with decreased LC3B but elevated p62, demonstrating that CtsD-mediated autophagy plays opposing roles in osteoblasts versus osteoclasts to regulate bone homeostasis.","method":"CtsD conditional knockout mice, microCT bone analysis, histomorphometry, siRNA knockdown in MC3T3E1 and RAW264.7 cells, LC3B/p62/Akt/GSK3β Western blot, osteoblast and osteoclast differentiation assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO plus siRNA with multiple cellular and molecular readouts; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.04.09.645406"],"is_preprint":true}],"current_model":"CTSD encodes a lysosomal aspartic protease that, after mannose-6-phosphate-dependent lysosomal targeting and autophagy-regulated maturation (controlled by glycosylation at specific asparagines and S-sulfhydration of the pro-form), degrades diverse substrates including α-synuclein, prolactin (generating an antiangiogenic 16 kDa fragment that causes postpartum cardiomyopathy), HSP90α (suppressing β-catenin/cardiac hypertrophy), and ACADM (regulating ferroptosis); upon lysosomal membrane permeabilization, cytosolic CTSD acts upstream of Bax to trigger mitochondrial AIF release and caspase-independent apoptosis, and is directly activated by ceramide via acid sphingomyelinase, while loss-of-function mutations cause the lysosomal storage disorder CLN10 (neuronal ceroid lipofuscinosis)."},"narrative":{"teleology":[{"year":1985,"claim":"Cloning and sequencing of human CTSD established it as an aspartyl protease with a pre-pro-mature domain architecture conserved with other members of the pepsin family, providing the molecular identity needed for all subsequent mechanistic work.","evidence":"cDNA cloning from hepatoma library with full-length sequencing and homology analysis","pmids":["3927292"],"confidence":"High","gaps":["No enzymatic characterization of recombinant protein in this study","Cellular substrates unknown","Post-translational processing intermediates not resolved"]},{"year":1990,"claim":"Detection of enzymatically active CTSD in extracellular senile plaques of Alzheimer disease brains provided the first evidence that CTSD functions outside the lysosomal lumen in a disease context, raising the question of how it reaches extracellular compartments.","evidence":"Immunohistochemistry and in situ enzyme histochemistry with synthetic peptide substrates on AD brain tissue","pmids":["1692625"],"confidence":"High","gaps":["No direct demonstration that CTSD cleaves amyloid precursor protein or Aβ","Mechanism of CTSD externalization not determined","Causal role versus bystander in plaque formation unresolved"]},{"year":1999,"claim":"The discovery that ceramide generated by acid sphingomyelinase directly binds and activates pro-CTSD autocatalytic processing revealed a lipid-based activation mechanism linking sphingolipid signaling to lysosomal protease activity in apoptotic pathways.","evidence":"Direct ceramide-CTSD binding assay, in vitro autocatalytic cleavage reconstitution, A-SMase knockout and rescue","pmids":["10508159"],"confidence":"High","gaps":["Binding site on CTSD not mapped","In vivo significance of ceramide-CTSD axis in non-immune cells not tested","Structural basis for ceramide-induced autoactivation unknown"]},{"year":2003,"claim":"Demonstration that cytosolic CTSD (released from permeabilized lysosomes) activates Bax in a Bid-independent manner to trigger mitochondrial AIF release established CTSD as a proximal executor of caspase-independent apoptosis, resolving how lysosomal proteases intersect mitochondrial death pathways.","evidence":"siRNA knockdown and pepstatin A inhibition in human T lymphocytes with subcellular fractionation and epistasis analysis","pmids":["12782632"],"confidence":"High","gaps":["Direct CTSD-Bax interaction not biochemically demonstrated","Whether CTSD cleaves Bax or acts via an intermediate is unresolved","Generalizability beyond T lymphocytes not shown in this study"]},{"year":2005,"claim":"Showing that a catalytically inactive CTSD mutant retains mitogenic activity in breast cancer cells revealed a protease-independent extracellular function, separating CTSD's intracellular pro-apoptotic role from a receptor-mediated proliferative role in the tumor microenvironment.","evidence":"Active-site mutagenesis, tumor xenografts, and proliferation assays across cancer, endothelial, and fibroblast cell types","pmids":["16046058"],"confidence":"High","gaps":["The putative cell-surface receptor for secreted CTSD remains unidentified","Structural determinants of protease-independent signaling not mapped","In vivo relevance of proteolytic versus non-proteolytic functions not separated"]},{"year":2006,"claim":"Identification of a CTSD missense mutation (Met199Ile) causing neuronal ceroid lipofuscinosis (CLN10) with ~36% residual enzyme activity directly linked partial CTSD loss-of-function to lysosomal storage neurodegeneration, defining CTSD as the CLN10 disease gene.","evidence":"Genetic linkage, mutation sequencing, and cathepsin D enzyme activity assays in American Bulldog brain tissue","pmids":["16386934"],"confidence":"High","gaps":["Human CLN10-causing mutations not characterized in this study","Threshold of CTSD activity required for neuronal survival not defined","Specific substrates whose failure to degrade causes NCL pathology not identified"]},{"year":2007,"claim":"The discovery that cardiac CTSD cleaves prolactin to generate a 16 kDa antiangiogenic fragment that drives postpartum cardiomyopathy identified a specific pathophysiological substrate and established CTSD as a key mediator of PPCM downstream of STAT3 loss.","evidence":"Cardiomyocyte-specific STAT3 KO mice, forced 16 kDa prolactin overexpression, bromocriptine rescue, and human patient serum analysis","pmids":["17289576"],"confidence":"High","gaps":["Precise cleavage site on prolactin not mapped","Whether other cardiac proteases contribute to 16 kDa prolactin generation not excluded","Long-term efficacy of bromocriptine in human PPCM not established here"]},{"year":2008,"claim":"Mapping the ERα-dependent distal enhancer ~9 kb upstream of CTSD with chromatin looping to the promoter explained how estrogen drives CTSD overexpression in breast cancer, connecting transcriptional regulation to the known CTSD overexpression phenotype.","evidence":"ChIP, chromosome conformation capture, and bisulfite methylation analysis in MCF-7 cells","pmids":["19383337"],"confidence":"Medium","gaps":["No genetic deletion of the enhancer to confirm necessity","Contribution of this enhancer relative to other regulatory elements not quantified","Looping mechanism not confirmed in non-breast cancer cell types"]},{"year":2019,"claim":"Successful enzyme replacement therapy with recombinant pro-CTSD in CLN10 mouse models—showing M6PR-dependent uptake, lysosomal maturation, and correction of storage pathology and autophagic flux—provided proof-of-concept that exogenous CTSD can functionally replace the endogenous enzyme in vivo.","evidence":"Recombinant pro-CTSD uptake/processing assays, systemic and intracranial ERT in CTSD-deficient mice with histopathology and lifespan analysis","pmids":["31282275"],"confidence":"High","gaps":["Blood-brain barrier penetrance of systemic ERT limited","Long-term dosing and immunogenicity not fully assessed","Whether ERT fully rescues neurological phenotype unclear"]},{"year":2020,"claim":"Convergent studies established that CTSD is essential for autophagic flux and neuronal survival: CTSD knockdown causes lysosomal dysfunction and sensitivity to ischemic injury, while CTSD inhibition blocks autophagosome-lysosome fusion in glioblastoma cells, positioning CTSD as rate-limiting for late-stage autophagy.","evidence":"shRNA knockdown and lentiviral rescue in neurons with OGD/MCAO models; siRNA and pepstatin A in glioblastoma with LC3/p62 quantification","pmids":["32450052","32253787"],"confidence":"High","gaps":["Molecular mechanism by which CTSD promotes autophagosome-lysosome fusion not identified","Whether CTSD acts on fusion machinery directly or via substrate clearance is unknown","Cell-type specificity of CTSD dependence in autophagy not systematically tested"]},{"year":2020,"claim":"Glycosylation at N233 in insect CTSD was shown to determine whether pro-CTSD is secreted (glycosylated) or retained intracellularly for autophagy-dependent maturation and caspase-3 activation, revealing a glycosylation-dependent sorting switch that governs the dual extracellular/intracellular functions of CTSD.","evidence":"Site-directed mutagenesis of N233, autophagy gene RNAi epistasis, and PNGase F treatment in Helicoverpa armigera midgut cells","pmids":["32324083"],"confidence":"High","gaps":["Conservation of N233 glycosylation sorting switch in mammalian CTSD not demonstrated","Whether this mechanism operates in human cancer secretion of CTSD untested","Receptor for extracellular insect CTSD not identified"]},{"year":2022,"claim":"Recombinant CTSD treatment of iPSC-derived dopaminergic neurons from Parkinson disease patients (SNCA A53T) and CTSD-deficient mouse neurons reduced insoluble α-synuclein and restored endo-lysosomal function, establishing CTSD as the major lysosomal protease responsible for α-synuclein degradation.","evidence":"rHsCTSD uptake and maturation assays in iPSC-derived neurons and ctsd-KO mouse neurons with SNCA solubility fractionation and autophagy flux measurement","pmids":["35287553"],"confidence":"High","gaps":["Cleavage sites on α-synuclein not mapped","Whether CTSD can clear established Lewy body-like inclusions in vivo unknown","Contribution of other lysosomal proteases (e.g., cathepsin B/L) to α-synuclein clearance not excluded"]},{"year":2023,"claim":"Identification of O-GlcNAcylation as a post-translational modification required for CTSD maturation added a new regulatory layer, showing that perturbation of O-GlcNAc cycling (as by swainsonine) impairs conversion of pro-CTSD to mature active enzyme and consequently disrupts autophagy.","evidence":"Immunoprecipitation of O-GlcNAcylated CTSD, OGA/OGT inhibitor treatment, autophagy flux assays","pmids":["37442287"],"confidence":"Medium","gaps":["Specific O-GlcNAcylation sites on CTSD not mapped","Whether O-GlcNAcylation affects CTSD folding, trafficking, or catalytic competence is unresolved","Single lab finding; independent confirmation needed"]},{"year":2024,"claim":"N-glycosylation at N263 by the DDOST/STT3B complex was shown to be required for CTSD protease activity, and glycosylated CTSD cleaves ACADM to modulate ferroptosis-related proteins (ACSL4, SLC7A11, GPX4), linking CTSD to ferroptosis regulation and colorectal cancer liver metastasis.","evidence":"N-glycoproteomics of matched CRC tissues, site-directed mutagenesis of N263, ACADM cleavage assays, ferroptosis marker analysis","pmids":["39716927"],"confidence":"Medium","gaps":["Direct in vitro cleavage of ACADM by purified CTSD not demonstrated","How ACADM degradation mechanistically alters GPX4/SLC7A11 levels unclear","Whether N263 glycosylation is rate-limiting in non-cancer contexts unknown"]},{"year":2024,"claim":"Studies in Dictyostelium revealed that CTSD extracellular release depends on autophagy machinery (Atg1/Atg5), lysosomal exocytosis components (AP-3, LYST, mucolipin-1, WASH), and microfilaments, and that CLN5 protein secretion is regulated by extracellular CTSD levels, defining a trafficking pathway and functional link between two NCL disease proteins.","evidence":"Genetic KO of autophagy and trafficking genes in Dictyostelium, secretion assays, glycosylation analysis, Cln5-CtsD epistasis","pmids":["38272448"],"confidence":"Medium","gaps":["Conservation of this secretory pathway in mammalian cells not confirmed","Molecular basis of Cln5-CtsD regulatory interaction not defined","Whether WASH complex role is direct or indirect unclear"]},{"year":2025,"claim":"Multiple studies expanded CTSD's mechanistic landscape: LRP6 facilitates CTSD-mediated HSP90α degradation to suppress β-catenin-driven cardiac hypertrophy; S-sulfhydration of pro-CTSD inhibits its maturation and downstream PANoptosis in traumatic brain injury; CTSD is positioned downstream of mTOR-MITF signaling for macrophage lysosomal homeostasis; and secreted glycosylated CTSD (regulated by KIF13B/STT3A) drives hepatocyte lipid accumulation via THBS1 interaction in MASLD.","evidence":"LRP6 co-IP/MS and cardiomyocyte-specific overexpression with TAC model; modified biotin switch for S-sulfhydration with AAV-shSnapin; myeloid SAMHD1 KO with rapamycin rescue; myeloid Kif13b KO with MASLD model","pmids":["39779966","41558604","40886983","41746601"],"confidence":"Medium","gaps":["Direct in vitro cleavage of HSP90α by CTSD not shown","Specific cysteine residue(s) S-sulfhydrated on pro-CTSD not identified by mutagenesis","CTSD-THBS1 interaction interface not characterized","Each finding from a single laboratory"]},{"year":null,"claim":"Key unresolved questions include: the identity of the cell-surface receptor mediating protease-independent CTSD mitogenic signaling in cancer; the structural basis for ceramide-induced CTSD autoactivation; the precise mechanism by which cytosolic CTSD activates Bax; and whether CTSD-generated truncated α-synuclein species drive Parkinson disease pathology in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["Putative CTSD cell-surface receptor unidentified after 20 years","No crystal structure of ceramide-bound CTSD","In vivo validation of astrocyte-to-neuron α-synuclein seeding via CTSD cleavage pending"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2,3,4,6,12,14,16]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,2,5,6,12,14]}],"localization":[{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[2,3,8,9,10,11,12,13]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,4]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[1,4,15,19]}],"pathway":[{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[8,9,10,11,12,13,17]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[2,3,4,9,18]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,8,13,14]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[5,6,8,12]}],"complexes":[],"partners":["ASAH1","BAX","SNCA","LRP6","HSP90AA1","ACADM","THBS1","CLN5"],"other_free_text":[]},"mechanistic_narrative":"Cathepsin D (CTSD) is a lysosomal aspartic endoprotease that serves as a principal effector of lysosomal proteolysis, with critical roles in autophagic flux, regulated cell death, and substrate-specific cleavage events across multiple tissues. After mannose-6-phosphate receptor-mediated lysosomal targeting and maturation—controlled by N-glycosylation (notably at N263, via DDOST/STT3B), O-GlcNAcylation, and S-sulfhydration of the pro-form—mature CTSD degrades substrates including α-synuclein, prolactin (generating an antiangiogenic 16 kDa fragment that causes postpartum cardiomyopathy), HSP90α (suppressing β-catenin-driven cardiac hypertrophy), and ACADM (modulating ferroptosis) [PMID:3927292, PMID:35287553, PMID:17289576, PMID:39716927, PMID:39779966]. Upon lysosomal membrane permeabilization, cytosolic CTSD—activated by ceramide via acid sphingomyelinase—triggers Bax-dependent, caspase-independent apoptosis through mitochondrial AIF release [PMID:10508159, PMID:12782632]. Loss-of-function mutations in CTSD cause the lysosomal storage disorder CLN10 (neuronal ceroid lipofuscinosis), and enzyme replacement with recombinant pro-CTSD corrects lysosomal storage and autophagic dysfunction in CLN10 models [PMID:16386934, PMID:31282275]."},"prefetch_data":{"uniprot":{"accession":"P07339","full_name":"Cathepsin D","aliases":[],"length_aa":412,"mass_kda":44.6,"function":"Acid protease active in intracellular protein breakdown. Plays a role in APP processing following cleavage and activation by ADAM30 which leads to APP degradation (PubMed:27333034). 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dual roles\",\n      \"pmids\": [\"32324083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Recombinant human pro-CTSD is taken up by cells, correctly targeted to lysosomes via mannose-6-phosphate receptor, and processed to the active mature protease; restoration of CTSD activity corrects lysosomal hypertrophy, storage accumulation, and impaired autophagic flux in CTSD-deficient models.\",\n      \"method\": \"Enzyme replacement therapy in vitro and in vivo, lysosomal fractionation, autophagic flux assays, immunofluorescence, Western blot in murine CLN10 model\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal in vitro and in vivo methods with rigorous controls demonstrating trafficking, maturation, and functional rescue\",\n      \"pmids\": [\"31282275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CTSD is the major lysosomal protease responsible for SNCA/α-synuclein degradation; recombinant human pro-CTSD is endocytosed by neuronal cells, trafficked to lysosomes, matured to enzymatically active form, and reduces insoluble SNCA conformers in PD patient iPSC-derived neurons and CTSD-deficient mouse brains.\",\n      \"method\": \"Enzyme replacement in iPSC-derived dopaminergic neurons (PD patients, A53T mutation), Western blot for insoluble/soluble SNCA fractions, immunofluorescence, structured illumination microscopy, in vivo dosing in ctsd-knockout mice\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods across human iPSC and mouse models, replicated across systems\",\n      \"pmids\": [\"35287553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CTSD is required for lysosomal proteolytic function in neurons; shRNA-mediated knockdown of CTSD in neurons causes lysosomal dysfunction, and restoration of CTSD via lentiviral transduction rescues lysosomal function and neuronal survival following oxygen-glucose deprivation.\",\n      \"method\": \"shRNA knockdown, lentiviral overexpression, lysosomal activity assays, cell viability assays in mouse cortical neurons and MCAO stroke model\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function and gain-of-function with defined lysosomal and cell death phenotypes, in vitro and in vivo\",\n      \"pmids\": [\"32450052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CTSD regulates radiosensitivity in glioblastoma by controlling autophagosome-lysosome fusion; inhibition of CTSD (by siRNA or pepstatin A) increases autophagosome formation but decreases autolysosome formation, attenuating autophagic flux and sensitizing cells to ionizing radiation.\",\n      \"method\": \"siRNA knockdown, pepstatin A inhibition, Western blot, immunofluorescence for LC3 and autophagy markers in human glioma cells\",\n      \"journal\": \"Molecular carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined autophagy phenotype, single lab with two orthogonal methods\",\n      \"pmids\": [\"32253787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Estrogen receptor alpha (ERα) activates CTSD transcription through long-distance looping between a distal ERE located 9 kb upstream of the CTSD transcription start site and the proximal promoter; ERα and phosphorylated RNA Pol II are recruited to this distal enhancer, and CpG methylation at the enhancer is transiently regulated during estrogen stimulation.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), chromosome conformation capture (3C/looping assay), methylation analysis in MCF-7 cells\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and looping assay provide direct mechanistic evidence; single lab, moderate evidence\",\n      \"pmids\": [\"19383337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"N-glycosylation of CTSD at residue N263, mediated by the glycosyltransferase complex DDOST/STT3B, regulates CTSD protease activity; glycosylated CTSD lyses ACADM, which in turn regulates ferroptosis-related proteins (ACSL4, SLC7A11, GPX4) to control invasion and metastasis of colorectal cancer cells.\",\n      \"method\": \"N-glycoproteomic profiling, site-directed mutagenesis of N263, co-immunoprecipitation, siRNA knockdown of DDOST/STT3B, invasion/metastasis assays in colorectal cancer cells\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — site-specific glycosylation mapped and functionally validated with downstream substrate identified; single lab\",\n      \"pmids\": [\"39716927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Swainsonine reduces O-GlcNAcylation of CTSD, impairing its maturation to the active form and causing lysosomal dysfunction; inhibition of OGA (promoting O-GlcNAcylation) promotes autophagy while inhibition of OGT (reducing O-GlcNAcylation) inhibits autophagy, demonstrating that CTSD O-GlcNAcylation is required for its maturation and lysosomal function.\",\n      \"method\": \"Immunoprecipitation, OGA/OGT pharmacological inhibition, proteomics, Western blot for mature vs. pro-CTSD in cell culture\",\n      \"journal\": \"Chemico-biological interactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — IP and pharmacological modulation with functional readout; single lab, moderate evidence\",\n      \"pmids\": [\"37442287\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LRP6 interacts with HSP90α and CTSD in cardiomyocytes under mechanical stress; LRP6 facilitates CTSD-mediated degradation of HSP90α, which suppresses β-catenin activation and reduces cardiac hypertrophy; inhibition of CTSD with pepstatin A abolishes the cardioprotective effect of LRP6 overexpression.\",\n      \"method\": \"Mass spectrometry (co-IP), cardiomyocyte-specific LRP6 overexpression mouse model (TAC), pepstatin A inhibition, recombinant HSP90α rescue experiment, echocardiography\",\n      \"journal\": \"Acta pharmacologica Sinica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — MS-identified interaction validated in vivo with loss-of-function and rescue; single lab\",\n      \"pmids\": [\"39779966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SAMHD1 deficiency impairs macrophage autophagy-lysosomal flux by enhancing MITF nuclear translocation, which suppresses CTSD expression (a downstream MITF target); pharmacological inhibition of PI3K/AKT/mTOR pathway restores MITF-CTSD signaling and lysosomal function.\",\n      \"method\": \"myeloid-specific SAMHD1-knockout mice, single-cell RNA-seq, rapamycin treatment, Western blot, lysosomal function assays\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO mouse model with pathway rescue; single lab\",\n      \"pmids\": [\"40886983\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CLN5 (Cln5) and CTSD (CtsD) homologs in Dictyostelium discoideum are secreted via signal peptides and released through pathways linked to autophagy; Cln5 release requires autophagy proteins Atg1, Atg5, Atg9 and autophagosomal-lysosomal fusion; CtsD release requires Atg1 and Atg5. CtsD is glycosylated extracellularly and its extracellular levels regulate Cln5 release. Lysosomal exocytosis also facilitates CtsD release, mediated by AP-3, LYST, mucolipin-1, and WASH.\",\n      \"method\": \"Genetic knockout of autophagy proteins, secretion assays, glycosylation analysis, lysosomal exocytosis assays in D. discoideum\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic knockouts with defined trafficking phenotypes in D. discoideum model; relevant to human CLN10 disease mechanism\",\n      \"pmids\": [\"38272448\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Snapin binds CBS (cystathionine β-synthase) after TBI, disrupting H2S metabolic homeostasis and reducing endogenous H2S levels; reduced H2S limits S-sulfhydration of pro-CTSD, promoting its maturation into active CTSD and inducing neuronal PANoptosis; pepstatin A (CTSD inhibitor) or NaHS (H2S donor) treatment reduces neuronal death.\",\n      \"method\": \"AAV-shRNA knockdown of Snapin, co-immunoprecipitation, modified biotin switch assay for S-sulfhydration, sulfide ion-selective electrode, controlled cortical impact TBI model in mice\",\n      \"journal\": \"Journal of advanced research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical S-sulfhydration assay, co-IP, and in vivo KD with functional rescue; single lab, novel finding\",\n      \"pmids\": [\"41558604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"KIF13B deficiency impairs proteasome-dependent degradation of the glycosyltransferase STT3A in macrophages, thereby enhancing CTSD glycosylation and secretion; secreted CTSD interacts with hepatocyte membrane protein THBS1 to promote lipid accumulation and inflammatory responses in the liver during MASLD.\",\n      \"method\": \"Myeloid-specific Kif13b-knockout mice, co-immunoprecipitation, mass spectrometry, glycosylation analysis, proteasome inhibition experiments, diet-induced MASLD model\",\n      \"journal\": \"Hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO mouse model with defined molecular mechanism and co-IP validated protein interactions; single lab\",\n      \"pmids\": [\"41746601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A missense mutation M199I in CTSD reduces cathepsin D enzymatic activity to 36% of control levels in the brain, causing neuronal ceroid lipofuscinosis (NCL/CLN10) in American Bulldogs, establishing that partial loss of CTSD protease activity is sufficient to cause lysosomal storage disease.\",\n      \"method\": \"Enzymatic activity assays for cathepsin D and 15 other lysosomal enzymes, genotyping, pedigree analysis, electron microscopy in affected dog brains\",\n      \"journal\": \"Molecular genetics and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct enzymatic activity measurement with mutagenesis context, replicated across tissues\",\n      \"pmids\": [\"16386934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Dexamethasone increases H3K9 acetylation and miR-1912-3p expression via glucocorticoid receptor activation in fetal chondrocytes; miR-1912-3p decreases CTSD expression, which inhibits autophagy flux; overexpression of CTSD rescues autophagic flux inhibited by dexamethasone, placing CTSD downstream of GR/miR-1912-3p in autophagy regulation.\",\n      \"method\": \"siRNA, overexpression constructs, miRNA mimic/inhibitor, ChIP for H3K9ac, autophagic flux assays in fetal chondrocytes; in vivo prenatal dexamethasone rat model\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis and rescue experiments with defined pathway placement; single lab\",\n      \"pmids\": [\"37249374\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Vasoinhibin (a prolactin fragment) is generated by CTSD cleavage of prolactin in the retina of neonatal mice; in CTSD-null mice, PRL cleavage to vasoinhibin was reduced but not abolished (renin also contributes), as demonstrated by pepstatin A (CTSD inhibitor) and CTSD-null retinal extracts.\",\n      \"method\": \"CTSD-null mouse retinal extracts, pepstatin A inhibition, selective renin inhibitor VTP-27999, recombinant renin cleavage assay, Western blot for 14 kDa vasoinhibin\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CTSD-null model and inhibitor-based dissection of protease contribution; replicated with recombinant enzyme; preprint not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Astrocytic CTSD (CtsD) cleaves internalized α-synuclein pre-formed fibrils into C-terminally truncated, seeding-competent species in lysosomes; these truncated species are transferred to neurons and promote Lewy neurite-like aggregate growth; α-syn PFF exposure disrupts lysosomal membrane integrity in astrocytes, upregulating CtsD expression in a feed-forward mechanism.\",\n      \"method\": \"Neuron-astrocyte co-culture system, pharmacological CtsD inhibition, immunofluorescence for lysosomal markers and α-syn species, Western blot for truncated α-syn, seed amplification assay\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct mechanistic dissection with CtsD inhibition and co-culture transfer assay; novel finding, preprint only\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CTSD deletion in mice dramatically decreases bone mass due to decreased osteoblast number and mineral apposition rate and increased osteoclast number; siRNA-mediated CTSD inactivation in osteoblastic cells (MC3T3E1) attenuates osteoblastic differentiation and downregulates LC3B and p-Akt/p-GSK3β; CTSD inactivation in osteoclast precursors (RAW264.7) increases osteoclast differentiation with decreased LC3B but upregulated p62, demonstrating distinct autophagy pathway roles in each cell type.\",\n      \"method\": \"Conditional CTSD-knockout mice, microCT, histomorphometry, siRNA knockdown in MC3T3E1 and RAW264.7 cells, Western blot for autophagy markers\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO and siRNA with defined cell-type-specific phenotypes; preprint, single lab\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"N-glycosylation of CTSD at residue N258 (CTSD-N258A mutant study) regulates its lysosomal localization, maturation, and ability to permeabilize the lysosomal membrane; CTSD-N258A mutation inhibits BMSC apoptosis and promotes lysosomal localization of CTSD, reducing cytoplasmic CTSD and apoptosis-related proteins (BID, caspase-3, caspase-1).\",\n      \"method\": \"CTSD N258A site-directed mutagenesis, flow cytometry for apoptosis, AO staining for lysosomal membrane permeability, confocal colocalization, Western blot in BMSC cell model\",\n      \"journal\": \"PLoS ONE\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — site-directed mutagenesis of specific glycosylation site with functional apoptosis and lysosomal readouts; single lab\",\n      \"pmids\": [\"41931502\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CTSD (cathepsin D) is a lysosomal aspartic protease that is synthesized as a glycosylated pro-enzyme, trafficked to lysosomes via mannose-6-phosphate receptors, and processed to its active mature form; autophagy promotes its maturation and intracellular retention, while N-glycosylation (including at N233/N258/N263) regulates its secretion, lysosomal localization, and enzymatic activity; mature CTSD degrades substrates including α-synuclein, HSP90α, ACADM, and prolactin, and its activity is essential for lysosomal homeostasis, autophagic flux, and cell fate decisions (apoptosis vs. proliferation), with loss-of-function causing lysosomal storage disease (CLN10/neuronal ceroid lipofuscinosis) and its transcription being regulated by estrogen receptor α via long-range enhancer looping.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1985,\n      \"finding\": \"Human cathepsin D (CTSD) was cloned and sequenced from a hepatoma cDNA library. The cDNA predicts a 412-amino acid protein with a 20-aa pre-segment and 44-aa prosegment; the mature protein shows high sequence homology to other aspartyl proteases, establishing CTSD as a member of the aspartyl protease family with a conserved three-dimensional structure.\",\n      \"method\": \"cDNA cloning, nucleotide sequencing, amino acid sequence analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — foundational cloning and sequence analysis, replicated across multiple clone sources\",\n      \"pmids\": [\"3927292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"Enzymatically active cathepsin D (and cathepsin B) localizes to senile plaques in Alzheimer disease brains, accumulating in extracellular lysosomal dense bodies and lipofuscin granules derived from degenerating neurons, implicating CTSD as a candidate protease for amyloid precursor protein processing in plaques.\",\n      \"method\": \"Immunohistochemistry with anti-cathepsin D antisera, in situ enzyme histochemistry with synthetic peptide substrates, ultrastructural immunolocalization\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — enzymatic activity confirmed in situ with substrate histochemistry plus ultrastructural localization\",\n      \"pmids\": [\"1692625\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Ceramide generated by acid sphingomyelinase directly binds to and activates cathepsin D, triggering autocatalytic proteolysis of the 52 kDa pre-pro-CTSD to produce enzymatically active 48/32 kDa isoforms. Acid sphingomyelinase-deficient cells have decreased CTSD activity, restored by A-SMase transfection, identifying CTSD as a ceramide target in endosomal apoptotic signaling.\",\n      \"method\": \"Direct ceramide-CTSD binding assay, in vitro autocatalytic cleavage assay, A-SMase knockout and reconstitution, biochemical fractionation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding plus in vitro reconstitution plus genetic rescue, single foundational study with multiple orthogonal methods\",\n      \"pmids\": [\"10508159\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In activated human T lymphocytes, cathepsin D (CTSD) translocates from lysosomes to the cytosol upon apoptotic stimulation and triggers Bax conformational change and relocation to mitochondria in a Bid-independent manner, leading to selective AIF release and early caspase-independent apoptosis. Pepstatin A and siRNA-mediated CTSD silencing inhibited these events, placing CTSD upstream of Bax in this pathway.\",\n      \"method\": \"Pepstatin A inhibitor treatment, siRNA knockdown of CTSD/Bax/AIF, subcellular fractionation, immunofluorescence localization, cell death assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic silencing (siRNA) plus pharmacological inhibition plus epistasis, replicated across multiple readouts\",\n      \"pmids\": [\"12782632\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CTSD (cath-D) is overexpressed and hypersecretated by breast cancer cells, stimulating tumorigenicity, metastasis, cancer cell proliferation, fibroblast outgrowth, and angiogenesis. A catalytically inactive mutant cath-D retains mitogenic activity for cancer, endothelial, and fibroblastic cells, indicating an extracellular mode of action involving an unidentified cell-surface receptor. During apoptosis, mature lysosomal CTSD translocates to the cytosol and its proteolytic activity participates in the apoptotic cascade.\",\n      \"method\": \"Catalytic-site mutagenesis, overexpression, tumor xenograft models, cell proliferation assays, apoptosis assays\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis of active site distinguishing proteolytic from non-proteolytic functions, replicated across multiple cell models\",\n      \"pmids\": [\"16046058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A CTSD gene mutation (G to A, Met199Ile) in American Bulldogs causes neuronal ceroid lipofuscinosis (NCL) with ~36% residual cathepsin D enzymatic activity compared to controls, while 15 other lysosomal enzyme activities were unchanged or increased. This directly established that partial loss of CTSD catalytic activity is sufficient to cause NCL-like neurodegeneration.\",\n      \"method\": \"Genetic linkage analysis, mutation identification, cathepsin D enzyme activity assay in brain tissue, electron microscopy of storage material\",\n      \"journal\": \"Molecular genetics and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — disease-causing mutation linked to specific enzymatic deficit, confirmed by enzyme assay with multiple controls\",\n      \"pmids\": [\"16386934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Cardiac cathepsin D cleaves prolactin at its N-terminus to generate an antiangiogenic and proapoptotic 16 kDa fragment that mediates postpartum cardiomyopathy (PPCM). STAT3 deletion in cardiomyocytes enhanced cardiac CTSD expression and activity; forced cardiac generation of 16 kDa prolactin impaired the cardiac capillary network and recapitulated PPCM. Bromocriptine (prolactin secretion inhibitor) prevented PPCM.\",\n      \"method\": \"Cardiomyocyte-specific STAT3 knockout mice, CTSD activity assays, forced cardiac overexpression of 16 kDa prolactin, bromocriptine treatment, cardiac function assessment, patient serum analysis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic KO, reconstitution with cleavage product, pharmacological inhibition, and human patient data converge on same mechanism\",\n      \"pmids\": [\"17289576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Estrogen receptor alpha (ERα) activates CTSD expression through a distal enhancer element located ~9 kbp upstream of the CTSD transcription start site. ChIP experiments showed estrogen-dependent recruitment of ERα and phosphorylated RNA Pol II to this enhancer, with chromatin looping connecting the distal enhancer to the CTSD promoter. Transient CpG methylation at both the promoter and the distal enhancer was observed during estrogen stimulation.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), chromosome conformation capture (looping assay), bisulfite methylation analysis, reporter assays in MCF-7 cells\",\n      \"journal\": \"Molecular oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and looping assay in single lab; ERα dependence confirmed but no orthogonal genetic rescue\",\n      \"pmids\": [\"19383337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Recombinant human pro-CTSD produced in a mammalian system is efficiently endocytosed via mannose-6-phosphate receptors, trafficked to lysosomes, and processed to the mature active form. In CTSD-deficient mouse models of CLN10 disease, systemic and intracranial administration of rhCTSD corrects lysosomal hypertrophy, storage accumulation, and impaired autophagic flux in viscera and CNS, establishing enzyme replacement as feasible for this lysosomal storage disorder.\",\n      \"method\": \"Recombinant protein uptake assays, lysosomal targeting/processing assays, CLN10 mouse model ERT, autophagic flux measurement, histopathology, lifespan analysis\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted uptake and processing in vitro, plus in vivo correction with multiple orthogonal readouts\",\n      \"pmids\": [\"31282275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In Helicoverpa armigera (lepidopteran model), autophagy triggers CTSD maturation and relocalization inside midgut cells, where mature CTSD activates caspase-3 and promotes apoptosis. Glycosylation at asparagine-233 determines pro-CTSD secretion rather than intracellular retention. Steroid hormone 20-hydroxyecdysone (20E) promotes CTSD expression. This establishes that differential glycosylation and autophagy-regulated maturation control the dual pro-proliferative (extracellular) versus pro-apoptotic (intracellular) functions of CTSD.\",\n      \"method\": \"RNAi knockdown of autophagy genes, site-directed mutagenesis of N233, glycosylation analysis (PNGase F treatment), caspase-3 activity assays, immunofluorescence, hormone treatment experiments\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis, RNAi epistasis, and biochemical assays in insect ortholog with conserved CTSD function; multiple orthogonal methods\",\n      \"pmids\": [\"32324083\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CTSD knockdown in neurons causes lysosomal dysfunction. Restoration of CTSD protein levels via lentiviral transduction increases CTSD activity and renders neurons resistant to oxygen-glucose deprivation (OGD)-mediated lysosomal dysfunction and cell death in a stroke model, demonstrating that CTSD-dependent lysosomal proteolytic activity is required for neuronal survival during ischemia.\",\n      \"method\": \"shRNA-mediated CTSD knockdown, lentiviral CTSD overexpression, OGD neuronal model, MCAO stroke model, lysosomal function assays, cell death assays\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function and gain-of-function with specific lysosomal and cell death readouts in two complementary models\",\n      \"pmids\": [\"32450052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CTSD inhibition in radioresistant glioblastoma cells blocks autophagosome-lysosome fusion, increasing autophagosome accumulation while decreasing autolysosome formation, and sensitizes cells to ionizing radiation. CTSD protein levels positively correlate with the autophagy marker LC3-II/I and negatively with p62, positioning CTSD as a regulator of autophagic flux at the autophagosome-lysosome fusion step.\",\n      \"method\": \"siRNA knockdown, pepstatin A inhibition, Western blot for LC3 and p62, immunofluorescence for autophagosome/autolysosome quantification, clonogenic survival assay after irradiation\",\n      \"journal\": \"Molecular carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — pharmacological inhibition plus siRNA in single lab with functional readouts; mechanism of fusion block not molecularly dissected\",\n      \"pmids\": [\"32253787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Recombinant human pro-CTSD (rHsCTSD) is endocytosed by neuronal cells, delivered to lysosomes, and matured into active protease. In iPSC-derived dopaminergic neurons from Parkinson disease patients (SNCA A53T mutation) and in ctsd-deficient mouse neurons, rHsCTSD treatment reduces insoluble SNCA/α-synuclein conformers and restores endo-lysosome and autophagy function, establishing CTSD as the major lysosomal protease responsible for SNCA degradation.\",\n      \"method\": \"Recombinant protein uptake and maturation assays, iPSC-derived dopaminergic neurons, ctsd-KO mouse primary neurons, SNCA solubility fractionation, autophagy flux assays\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — enzyme replacement in human iPSC-derived neurons and mouse KO model with biochemical and functional readouts; orthogonal validation across species\",\n      \"pmids\": [\"35287553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Swainsonine toxin reduces O-GlcNAcylation of CTSD, which impairs its maturation to the active form (m-CTSD). Increasing O-GlcNAcylation (with OGA inhibitor TMG) promotes autophagy, while decreasing it (with OGT inhibitor OSMI) inhibits autophagy. Immunoprecipitation confirmed direct O-GlcNAcylation of CTSD, establishing O-GlcNAcylation as a post-translational modification required for proper CTSD maturation and lysosomal function.\",\n      \"method\": \"Proteomics sequencing, immunoprecipitation of O-GlcNAcylated CTSD, OGA/OGT inhibitor treatment, autophagy flux assays, Western blot for mature/pro-CTSD forms\",\n      \"journal\": \"Chemico-biological interactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — immunoprecipitation confirmed CTSD O-GlcNAcylation with pharmacological validation; single lab, mechanism of maturation effect not fully resolved\",\n      \"pmids\": [\"37442287\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"N-glycosylation at residue N263 of CTSD, mediated by the glycosyltransferase complex DDOST/STT3B, is required for CTSD protease activity. Glycosylated CTSD lyses ACADM, which in turn regulates ferroptosis-related proteins (ACSL4, SLC7A11, GPX4) to promote invasion and liver metastasis of colorectal cancer cells.\",\n      \"method\": \"N-glycoproteomics of matched primary and metastatic CRC tissues, site-specific glycosylation mutagenesis, ACADM cleavage assays, ferroptosis marker analysis, invasion/metastasis assays\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — glycosylation site identified by proteomics and mutagenesis, substrate (ACADM) cleavage shown, downstream pathway mapped; single lab\",\n      \"pmids\": [\"39716927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CLN5 (Cln5) and CTSD (CtsD) are both released extracellularly via signal peptide-dependent secretion and autophagy-linked pathways in Dictyostelium discoideum. CtsD release requires autophagy proteins Atg1 and Atg5, lysosomal exocytosis machinery (AP-3, LYST, mucopilin-1, WASH), and microfilaments. Extracellular CtsD is glycosylated, and Cln5 release is regulated by the amount of extracellular CtsD, identifying a regulatory relationship between these two CLN disease proteins.\",\n      \"method\": \"Dictyostelium genetic KO models for autophagy and trafficking genes, secretion assays, glycosylation analysis, epistasis experiments between Cln5 and CtsD\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic KOs with defined trafficking readouts in model organism; relevant to human CTSD trafficking mechanism\",\n      \"pmids\": [\"38272448\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LRP6 interacts with HSP90α and CTSD in cardiomyocytes under mechanical stress (identified by mass spectrometry co-IP). LRP6 facilitates CTSD-mediated degradation of HSP90α, which suppresses β-catenin activation and reduces cardiac hypertrophy after pressure overload. Treatment with pepstatin A (CTSD inhibitor) or recombinant HSP90α abolished the cardioprotective effect of LRP6, placing CTSD in the LRP6/HSP90α/β-catenin axis.\",\n      \"method\": \"Mass spectrometry after LRP6 co-immunoprecipitation, cardiomyocyte-specific LRP6 overexpression mice, transverse aortic constriction model, pepstatin A treatment, HSP90α recombinant protein rescue, echocardiography\",\n      \"journal\": \"Acta pharmacologica Sinica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — MS-identified interaction plus genetic overexpression and pharmacological inhibition in vivo; single lab, mechanism of CTSD-HSP90α proteolysis not directly demonstrated in vitro\",\n      \"pmids\": [\"39779966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SAMHD1 deficiency in macrophages enhances MITF nuclear translocation, which suppresses CTSD expression downstream of mTOR signaling, impairing lysosomal autophagy flux and promoting inflammation in ulcerative colitis. Pharmacological mTOR inhibition (rapamycin) restores MITF-CTSD signaling and lysosomal function, placing CTSD downstream of the mTOR-MITF axis in macrophage lysosomal homeostasis.\",\n      \"method\": \"Myeloid-specific SAMHD1 knockout mice, scRNA-seq, MITF nuclear translocation assays, CTSD expression analysis, lysosomal flux assays, rapamycin treatment, colitis model\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus pharmacological rescue with defined pathway readouts; single lab\",\n      \"pmids\": [\"40886983\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In mTBI, Snapin binds CBS, disrupting H2S metabolic homeostasis. Reduced H2S limits S-sulfhydration of pro-CTSD at a specific cysteine residue, promoting its maturation into active CTSD and inducing PANoptosis. Pepstatin A (CTSD inhibitor) and NaHS (H2S donor) both confer neuroprotection, establishing S-sulfhydration of pro-CTSD as a regulatory PTM controlling its maturation and downstream apoptotic/pyroptotic/necroptotic signaling.\",\n      \"method\": \"Conditional Snapin knockdown (AAV-shSnapin), modified biotin switch assay for S-sulfhydration, co-immunoprecipitation of Snapin-CBS, H2S measurement with ion-selective electrode, pepstatin A and NaHS treatment, PANoptosis protein analysis, behavioral tests\",\n      \"journal\": \"Journal of advanced research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — S-sulfhydration of pro-CTSD detected by modified biotin switch assay with genetic and pharmacological validation; single lab\",\n      \"pmids\": [\"41558604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"KIF13B in macrophages controls proteasome-dependent degradation of the glycosyltransferase STT3A. Kif13b deficiency allows STT3A accumulation, which enhances CTSD glycosylation and secretion, promoting lipid accumulation and inflammation in the liver during MASLD. Secreted CTSD exerts its detrimental effect through interaction with the hepatocyte membrane protein THBS1, defining the KIF13B/STT3A/CTSD/THBS1 axis in macrophage-hepatocyte crosstalk.\",\n      \"method\": \"Myeloid Kif13b knockout mice, diet-induced MASLD model, CTSD glycosylation assays, CTSD secretion measurement, CTSD-THBS1 interaction studies, proteasome activity assays\",\n      \"journal\": \"Hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO model with mechanistic dissection of glycosylation and secretion pathway; CTSD-THBS1 interaction mechanistic details not fully elaborated in abstract\",\n      \"pmids\": [\"41746601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Astrocytic cathepsin D (CtsD) cleaves α-synuclein pre-formed fibrils into C-terminally truncated, seeding-competent species within lysosomes. These truncated species are transferred to neurons where they promote Lewy neurite-like aggregate growth. α-Syn PFF exposure disrupts lysosomal membrane integrity in astrocytes and upregulates CtsD, creating a feed-forward amplification of α-syn pathogenicity.\",\n      \"method\": \"Neuron-astrocyte co-culture, α-syn PFF treatment, CtsD inhibition/KO in astrocytes, mass spectrometry characterization of cleaved α-syn species, seeding assays in neurons, lysosomal membrane integrity assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct substrate cleavage identified with functional seeding assay; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.10.03.680233\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Deletion of CtsD in mice dramatically decreases bone mass with reduced osteoblast numbers and increased osteoclast numbers. In osteoblasts, CtsD inactivation attenuates differentiation and downregulates LC3B with decreased p62, p-Akt, and p-GSK3β. In osteoclasts, CtsD inactivation increases differentiation with decreased LC3B but elevated p62, demonstrating that CtsD-mediated autophagy plays opposing roles in osteoblasts versus osteoclasts to regulate bone homeostasis.\",\n      \"method\": \"CtsD conditional knockout mice, microCT bone analysis, histomorphometry, siRNA knockdown in MC3T3E1 and RAW264.7 cells, LC3B/p62/Akt/GSK3β Western blot, osteoblast and osteoclast differentiation assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO plus siRNA with multiple cellular and molecular readouts; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.04.09.645406\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"CTSD encodes a lysosomal aspartic protease that, after mannose-6-phosphate-dependent lysosomal targeting and autophagy-regulated maturation (controlled by glycosylation at specific asparagines and S-sulfhydration of the pro-form), degrades diverse substrates including α-synuclein, prolactin (generating an antiangiogenic 16 kDa fragment that causes postpartum cardiomyopathy), HSP90α (suppressing β-catenin/cardiac hypertrophy), and ACADM (regulating ferroptosis); upon lysosomal membrane permeabilization, cytosolic CTSD acts upstream of Bax to trigger mitochondrial AIF release and caspase-independent apoptosis, and is directly activated by ceramide via acid sphingomyelinase, while loss-of-function mutations cause the lysosomal storage disorder CLN10 (neuronal ceroid lipofuscinosis).\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CTSD (cathepsin D) is a lysosomal aspartic endoprotease that functions as a central regulator of lysosomal proteolysis, autophagic flux, and cell fate decisions. Synthesized as a glycosylated pro-enzyme, CTSD is trafficked to lysosomes via mannose-6-phosphate receptors where it is processed to an enzymatically active mature form; N-glycosylation at specific residues (N233, N258, N263) and O-GlcNAcylation regulate its maturation, lysosomal retention versus secretion, and protease activity, while autophagy promotes its intracellular maturation and activation of caspase-3-dependent apoptosis [PMID:32324083, PMID:41931502, PMID:39716927, PMID:37442287]. Within lysosomes, mature CTSD degrades key substrates including α-synuclein and HSP90α, and its activity is essential for maintaining lysosomal homeostasis, autophagic flux, and neuronal survival; loss of CTSD function causes neuronal ceroid lipofuscinosis (CLN10), a lysosomal storage disease, as demonstrated by causative mutations that reduce enzymatic activity [PMID:35287553, PMID:39779966, PMID:31282275, PMID:16386934]. CTSD transcription is regulated by estrogen receptor α through long-range enhancer looping and by the MITF transcription factor downstream of PI3K/AKT/mTOR signaling [PMID:19383337, PMID:40886983].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Establishing that partial loss of CTSD enzymatic activity is sufficient to cause lysosomal storage disease resolved the question of whether CTSD is a disease-critical lysosomal protease.\",\n      \"evidence\": \"Enzymatic activity assays and pedigree analysis of M199I missense mutation in American Bulldogs with neuronal ceroid lipofuscinosis\",\n      \"pmids\": [\"16386934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human causative mutations not yet demonstrated in this study\", \"Mechanism by which reduced activity leads to storage accumulation not defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of a distal estrogen-responsive enhancer 9 kb upstream that loops to the CTSD promoter established a specific transcriptional mechanism for ERα-dependent CTSD regulation.\",\n      \"evidence\": \"ChIP and chromosome conformation capture (3C) in MCF-7 breast cancer cells\",\n      \"pmids\": [\"19383337\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional requirement of the distal ERE not tested by deletion/mutation\", \"Whether this looping mechanism operates in non-breast cell types is unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstration that recombinant pro-CTSD is taken up, correctly targeted to lysosomes, and processed to active mature form — rescuing lysosomal hypertrophy and impaired autophagic flux in CTSD-deficient models — established CTSD as the rate-limiting protease for lysosomal homeostasis and a viable enzyme replacement therapy target.\",\n      \"evidence\": \"Enzyme replacement therapy with lysosomal fractionation and autophagic flux assays in murine CLN10 model\",\n      \"pmids\": [\"31282275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term in vivo efficacy and CNS penetration not fully characterized\", \"Identity of accumulated storage substrates not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Autophagy was shown to trigger CTSD maturation and caspase-3 activation intracellularly, while N233 glycosylation diverts pro-CTSD to secretion where it promotes proliferation — resolving the dual cell fate roles of CTSD as pro-apoptotic (intracellular) versus pro-proliferative (extracellular).\",\n      \"evidence\": \"RNAi, glycosylation inhibition, autophagy modulation, and caspase assays in Helicoverpa armigera model\",\n      \"pmids\": [\"32324083\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Extracellular pro-CTSD receptor/signaling pathway not identified\", \"Conservation of N233 glycosylation-dependent secretion switch in mammalian systems not confirmed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Loss-of-function and rescue experiments in neurons established that CTSD is required for lysosomal proteolytic function and neuronal survival under ischemic stress, and that CTSD inhibition blocks autophagosome-lysosome fusion.\",\n      \"evidence\": \"shRNA knockdown and lentiviral rescue in cortical neurons/MCAO model; siRNA and pepstatin A in glioblastoma cells with autophagy flux markers\",\n      \"pmids\": [\"32450052\", \"32253787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CTSD directly mediates autophagosome-lysosome fusion or acts indirectly through proteolysis is unclear\", \"Specific substrates whose accumulation impairs fusion not identified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of α-synuclein as a major CTSD substrate in lysosomes, with recombinant CTSD reducing insoluble α-syn conformers in PD patient neurons, established a direct link between CTSD activity and Parkinson's disease-relevant proteostasis.\",\n      \"evidence\": \"Enzyme replacement in iPSC-derived dopaminergic neurons from PD patients and CTSD-knockout mice\",\n      \"pmids\": [\"35287553\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific cleavage sites on α-synuclein not mapped\", \"Whether CTSD-generated α-syn fragments are themselves pathogenic was not addressed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"O-GlcNAcylation was shown to be required for CTSD maturation, adding a second post-translational glycan modification axis — beyond N-glycosylation — that controls CTSD processing and lysosomal function.\",\n      \"evidence\": \"OGA/OGT pharmacological inhibition with immunoprecipitation and Western blot for pro- vs. mature CTSD\",\n      \"pmids\": [\"37442287\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific O-GlcNAcylation sites on CTSD not mapped\", \"Direct versus indirect effects of OGA/OGT modulation not fully resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"N-glycosylation at N263 mediated by DDOST/STT3B was shown to regulate CTSD protease activity and its ability to cleave ACADM, linking CTSD glycosylation to ferroptosis regulation in colorectal cancer.\",\n      \"evidence\": \"N-glycoproteomics, N263 site-directed mutagenesis, co-IP, and invasion assays in colorectal cancer cells\",\n      \"pmids\": [\"39716927\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether N263 glycosylation regulates CTSD in non-cancer contexts unknown\", \"Structural basis for glycosylation-dependent activity change not determined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"CTSD was placed downstream of MITF transcriptional control and the PI3K/AKT/mTOR axis in macrophages, while separately shown to mediate LRP6-dependent HSP90α degradation in cardiomyocytes, expanding the substrate repertoire and upstream regulatory inputs of CTSD.\",\n      \"evidence\": \"SAMHD1-knockout mice with scRNA-seq and rapamycin rescue; LRP6 overexpression mouse model with co-IP and pepstatin A\",\n      \"pmids\": [\"40886983\", \"39779966\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether MITF directly binds the CTSD promoter not shown by direct ChIP\", \"Structural basis for LRP6-facilitated CTSD-HSP90α interaction unknown\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"N-glycosylation at N258 was shown to regulate CTSD lysosomal membrane permeabilization and apoptosis, while S-sulfhydration of pro-CTSD was identified as a post-translational brake on CTSD maturation after traumatic brain injury, revealing new post-translational control mechanisms.\",\n      \"evidence\": \"N258A mutagenesis with apoptosis and lysosomal permeability assays in BMSCs; modified biotin switch assay for S-sulfhydration and AAV-shRNA Snapin knockdown in TBI mouse model\",\n      \"pmids\": [\"41931502\", \"41558604\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How N258 glycosylation specifically affects lysosomal membrane interactions not structurally resolved\", \"S-sulfhydration site(s) on pro-CTSD not mapped\", \"Whether S-sulfhydration regulation occurs under physiological (non-TBI) conditions unknown\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"KIF13B deficiency was shown to enhance CTSD glycosylation via STT3A stabilization, increasing CTSD secretion; secreted CTSD interacts with THBS1 on hepatocytes to drive lipid accumulation, establishing a paracrine signaling role for extracellular CTSD.\",\n      \"evidence\": \"Myeloid-specific Kif13b-knockout mice, co-IP/mass spectrometry, proteasome inhibition, diet-induced MASLD model\",\n      \"pmids\": [\"41746601\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Binding domain on THBS1 for CTSD not mapped\", \"Whether CTSD acts as protease or ligand in THBS1 interaction not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full substrate repertoire of mature CTSD in different cell types, the structural basis by which site-specific glycosylation controls activity and sorting, whether extracellular pro-CTSD signals through a dedicated receptor, and the relative contributions of CTSD-generated α-synuclein fragments to prion-like propagation versus clearance.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of glycosylated CTSD forms available\", \"Extracellular pro-CTSD receptor unidentified\", \"Comprehensive substrate profiling across tissues not performed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [2, 6, 8, 13, 15, 16]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [1, 2, 3, 6, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [1, 2, 3, 7, 18]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 10, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [1, 3, 4, 7, 14]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [0, 3, 18]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 2, 6, 7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [2, 13]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SNCA\",\n      \"HSP90AA1\",\n      \"ACADM\",\n      \"LRP6\",\n      \"THBS1\",\n      \"STT3B\",\n      \"CLN5\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"Cathepsin D (CTSD) is a lysosomal aspartic endoprotease that serves as a principal effector of lysosomal proteolysis, with critical roles in autophagic flux, regulated cell death, and substrate-specific cleavage events across multiple tissues. After mannose-6-phosphate receptor-mediated lysosomal targeting and maturation—controlled by N-glycosylation (notably at N263, via DDOST/STT3B), O-GlcNAcylation, and S-sulfhydration of the pro-form—mature CTSD degrades substrates including α-synuclein, prolactin (generating an antiangiogenic 16 kDa fragment that causes postpartum cardiomyopathy), HSP90α (suppressing β-catenin-driven cardiac hypertrophy), and ACADM (modulating ferroptosis) [PMID:3927292, PMID:35287553, PMID:17289576, PMID:39716927, PMID:39779966]. Upon lysosomal membrane permeabilization, cytosolic CTSD—activated by ceramide via acid sphingomyelinase—triggers Bax-dependent, caspase-independent apoptosis through mitochondrial AIF release [PMID:10508159, PMID:12782632]. Loss-of-function mutations in CTSD cause the lysosomal storage disorder CLN10 (neuronal ceroid lipofuscinosis), and enzyme replacement with recombinant pro-CTSD corrects lysosomal storage and autophagic dysfunction in CLN10 models [PMID:16386934, PMID:31282275].\",\n  \"teleology\": [\n    {\n      \"year\": 1985,\n      \"claim\": \"Cloning and sequencing of human CTSD established it as an aspartyl protease with a pre-pro-mature domain architecture conserved with other members of the pepsin family, providing the molecular identity needed for all subsequent mechanistic work.\",\n      \"evidence\": \"cDNA cloning from hepatoma library with full-length sequencing and homology analysis\",\n      \"pmids\": [\"3927292\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No enzymatic characterization of recombinant protein in this study\", \"Cellular substrates unknown\", \"Post-translational processing intermediates not resolved\"]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Detection of enzymatically active CTSD in extracellular senile plaques of Alzheimer disease brains provided the first evidence that CTSD functions outside the lysosomal lumen in a disease context, raising the question of how it reaches extracellular compartments.\",\n      \"evidence\": \"Immunohistochemistry and in situ enzyme histochemistry with synthetic peptide substrates on AD brain tissue\",\n      \"pmids\": [\"1692625\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No direct demonstration that CTSD cleaves amyloid precursor protein or Aβ\", \"Mechanism of CTSD externalization not determined\", \"Causal role versus bystander in plaque formation unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"The discovery that ceramide generated by acid sphingomyelinase directly binds and activates pro-CTSD autocatalytic processing revealed a lipid-based activation mechanism linking sphingolipid signaling to lysosomal protease activity in apoptotic pathways.\",\n      \"evidence\": \"Direct ceramide-CTSD binding assay, in vitro autocatalytic cleavage reconstitution, A-SMase knockout and rescue\",\n      \"pmids\": [\"10508159\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding site on CTSD not mapped\", \"In vivo significance of ceramide-CTSD axis in non-immune cells not tested\", \"Structural basis for ceramide-induced autoactivation unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstration that cytosolic CTSD (released from permeabilized lysosomes) activates Bax in a Bid-independent manner to trigger mitochondrial AIF release established CTSD as a proximal executor of caspase-independent apoptosis, resolving how lysosomal proteases intersect mitochondrial death pathways.\",\n      \"evidence\": \"siRNA knockdown and pepstatin A inhibition in human T lymphocytes with subcellular fractionation and epistasis analysis\",\n      \"pmids\": [\"12782632\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct CTSD-Bax interaction not biochemically demonstrated\", \"Whether CTSD cleaves Bax or acts via an intermediate is unresolved\", \"Generalizability beyond T lymphocytes not shown in this study\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Showing that a catalytically inactive CTSD mutant retains mitogenic activity in breast cancer cells revealed a protease-independent extracellular function, separating CTSD's intracellular pro-apoptotic role from a receptor-mediated proliferative role in the tumor microenvironment.\",\n      \"evidence\": \"Active-site mutagenesis, tumor xenografts, and proliferation assays across cancer, endothelial, and fibroblast cell types\",\n      \"pmids\": [\"16046058\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The putative cell-surface receptor for secreted CTSD remains unidentified\", \"Structural determinants of protease-independent signaling not mapped\", \"In vivo relevance of proteolytic versus non-proteolytic functions not separated\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of a CTSD missense mutation (Met199Ile) causing neuronal ceroid lipofuscinosis (CLN10) with ~36% residual enzyme activity directly linked partial CTSD loss-of-function to lysosomal storage neurodegeneration, defining CTSD as the CLN10 disease gene.\",\n      \"evidence\": \"Genetic linkage, mutation sequencing, and cathepsin D enzyme activity assays in American Bulldog brain tissue\",\n      \"pmids\": [\"16386934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human CLN10-causing mutations not characterized in this study\", \"Threshold of CTSD activity required for neuronal survival not defined\", \"Specific substrates whose failure to degrade causes NCL pathology not identified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The discovery that cardiac CTSD cleaves prolactin to generate a 16 kDa antiangiogenic fragment that drives postpartum cardiomyopathy identified a specific pathophysiological substrate and established CTSD as a key mediator of PPCM downstream of STAT3 loss.\",\n      \"evidence\": \"Cardiomyocyte-specific STAT3 KO mice, forced 16 kDa prolactin overexpression, bromocriptine rescue, and human patient serum analysis\",\n      \"pmids\": [\"17289576\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise cleavage site on prolactin not mapped\", \"Whether other cardiac proteases contribute to 16 kDa prolactin generation not excluded\", \"Long-term efficacy of bromocriptine in human PPCM not established here\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Mapping the ERα-dependent distal enhancer ~9 kb upstream of CTSD with chromatin looping to the promoter explained how estrogen drives CTSD overexpression in breast cancer, connecting transcriptional regulation to the known CTSD overexpression phenotype.\",\n      \"evidence\": \"ChIP, chromosome conformation capture, and bisulfite methylation analysis in MCF-7 cells\",\n      \"pmids\": [\"19383337\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No genetic deletion of the enhancer to confirm necessity\", \"Contribution of this enhancer relative to other regulatory elements not quantified\", \"Looping mechanism not confirmed in non-breast cancer cell types\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Successful enzyme replacement therapy with recombinant pro-CTSD in CLN10 mouse models—showing M6PR-dependent uptake, lysosomal maturation, and correction of storage pathology and autophagic flux—provided proof-of-concept that exogenous CTSD can functionally replace the endogenous enzyme in vivo.\",\n      \"evidence\": \"Recombinant pro-CTSD uptake/processing assays, systemic and intracranial ERT in CTSD-deficient mice with histopathology and lifespan analysis\",\n      \"pmids\": [\"31282275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Blood-brain barrier penetrance of systemic ERT limited\", \"Long-term dosing and immunogenicity not fully assessed\", \"Whether ERT fully rescues neurological phenotype unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Convergent studies established that CTSD is essential for autophagic flux and neuronal survival: CTSD knockdown causes lysosomal dysfunction and sensitivity to ischemic injury, while CTSD inhibition blocks autophagosome-lysosome fusion in glioblastoma cells, positioning CTSD as rate-limiting for late-stage autophagy.\",\n      \"evidence\": \"shRNA knockdown and lentiviral rescue in neurons with OGD/MCAO models; siRNA and pepstatin A in glioblastoma with LC3/p62 quantification\",\n      \"pmids\": [\"32450052\", \"32253787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which CTSD promotes autophagosome-lysosome fusion not identified\", \"Whether CTSD acts on fusion machinery directly or via substrate clearance is unknown\", \"Cell-type specificity of CTSD dependence in autophagy not systematically tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Glycosylation at N233 in insect CTSD was shown to determine whether pro-CTSD is secreted (glycosylated) or retained intracellularly for autophagy-dependent maturation and caspase-3 activation, revealing a glycosylation-dependent sorting switch that governs the dual extracellular/intracellular functions of CTSD.\",\n      \"evidence\": \"Site-directed mutagenesis of N233, autophagy gene RNAi epistasis, and PNGase F treatment in Helicoverpa armigera midgut cells\",\n      \"pmids\": [\"32324083\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conservation of N233 glycosylation sorting switch in mammalian CTSD not demonstrated\", \"Whether this mechanism operates in human cancer secretion of CTSD untested\", \"Receptor for extracellular insect CTSD not identified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Recombinant CTSD treatment of iPSC-derived dopaminergic neurons from Parkinson disease patients (SNCA A53T) and CTSD-deficient mouse neurons reduced insoluble α-synuclein and restored endo-lysosomal function, establishing CTSD as the major lysosomal protease responsible for α-synuclein degradation.\",\n      \"evidence\": \"rHsCTSD uptake and maturation assays in iPSC-derived neurons and ctsd-KO mouse neurons with SNCA solubility fractionation and autophagy flux measurement\",\n      \"pmids\": [\"35287553\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cleavage sites on α-synuclein not mapped\", \"Whether CTSD can clear established Lewy body-like inclusions in vivo unknown\", \"Contribution of other lysosomal proteases (e.g., cathepsin B/L) to α-synuclein clearance not excluded\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of O-GlcNAcylation as a post-translational modification required for CTSD maturation added a new regulatory layer, showing that perturbation of O-GlcNAc cycling (as by swainsonine) impairs conversion of pro-CTSD to mature active enzyme and consequently disrupts autophagy.\",\n      \"evidence\": \"Immunoprecipitation of O-GlcNAcylated CTSD, OGA/OGT inhibitor treatment, autophagy flux assays\",\n      \"pmids\": [\"37442287\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific O-GlcNAcylation sites on CTSD not mapped\", \"Whether O-GlcNAcylation affects CTSD folding, trafficking, or catalytic competence is unresolved\", \"Single lab finding; independent confirmation needed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"N-glycosylation at N263 by the DDOST/STT3B complex was shown to be required for CTSD protease activity, and glycosylated CTSD cleaves ACADM to modulate ferroptosis-related proteins (ACSL4, SLC7A11, GPX4), linking CTSD to ferroptosis regulation and colorectal cancer liver metastasis.\",\n      \"evidence\": \"N-glycoproteomics of matched CRC tissues, site-directed mutagenesis of N263, ACADM cleavage assays, ferroptosis marker analysis\",\n      \"pmids\": [\"39716927\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct in vitro cleavage of ACADM by purified CTSD not demonstrated\", \"How ACADM degradation mechanistically alters GPX4/SLC7A11 levels unclear\", \"Whether N263 glycosylation is rate-limiting in non-cancer contexts unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Studies in Dictyostelium revealed that CTSD extracellular release depends on autophagy machinery (Atg1/Atg5), lysosomal exocytosis components (AP-3, LYST, mucolipin-1, WASH), and microfilaments, and that CLN5 protein secretion is regulated by extracellular CTSD levels, defining a trafficking pathway and functional link between two NCL disease proteins.\",\n      \"evidence\": \"Genetic KO of autophagy and trafficking genes in Dictyostelium, secretion assays, glycosylation analysis, Cln5-CtsD epistasis\",\n      \"pmids\": [\"38272448\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Conservation of this secretory pathway in mammalian cells not confirmed\", \"Molecular basis of Cln5-CtsD regulatory interaction not defined\", \"Whether WASH complex role is direct or indirect unclear\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Multiple studies expanded CTSD's mechanistic landscape: LRP6 facilitates CTSD-mediated HSP90α degradation to suppress β-catenin-driven cardiac hypertrophy; S-sulfhydration of pro-CTSD inhibits its maturation and downstream PANoptosis in traumatic brain injury; CTSD is positioned downstream of mTOR-MITF signaling for macrophage lysosomal homeostasis; and secreted glycosylated CTSD (regulated by KIF13B/STT3A) drives hepatocyte lipid accumulation via THBS1 interaction in MASLD.\",\n      \"evidence\": \"LRP6 co-IP/MS and cardiomyocyte-specific overexpression with TAC model; modified biotin switch for S-sulfhydration with AAV-shSnapin; myeloid SAMHD1 KO with rapamycin rescue; myeloid Kif13b KO with MASLD model\",\n      \"pmids\": [\"39779966\", \"41558604\", \"40886983\", \"41746601\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct in vitro cleavage of HSP90α by CTSD not shown\", \"Specific cysteine residue(s) S-sulfhydrated on pro-CTSD not identified by mutagenesis\", \"CTSD-THBS1 interaction interface not characterized\", \"Each finding from a single laboratory\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the identity of the cell-surface receptor mediating protease-independent CTSD mitogenic signaling in cancer; the structural basis for ceramide-induced CTSD autoactivation; the precise mechanism by which cytosolic CTSD activates Bax; and whether CTSD-generated truncated α-synuclein species drive Parkinson disease pathology in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Putative CTSD cell-surface receptor unidentified after 20 years\", \"No crystal structure of ceramide-bound CTSD\", \"In vivo validation of astrocyte-to-neuron α-synuclein seeding via CTSD cleavage pending\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2, 3, 4, 6, 12, 14, 16]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 2, 5, 6, 12, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [2, 3, 8, 9, 10, 11, 12, 13]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 4]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 4, 15, 19]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [8, 9, 10, 11, 12, 13, 17]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [2, 3, 4, 9, 18]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 8, 13, 14]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [5, 6, 8, 12]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ASAH1\",\n      \"BAX\",\n      \"SNCA\",\n      \"LRP6\",\n      \"HSP90AA1\",\n      \"ACADM\",\n      \"THBS1\",\n      \"CLN5\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}