{"gene":"M6PR","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":1990,"finding":"The 46 kDa cation-dependent mannose-6-phosphate receptor (CD-M6PR) cycles between the Golgi complex (concentrated in middle and trans cisternae) and the same population of late endosomes (prelysosomes) as the 215 kDa CI-M6PR, as shown by immunofluorescence and immunoperoxidase labeling with antibodies to the C-terminal cytoplasmic domain; weak base treatment (chloroquine/NH4Cl) caused both receptors to accumulate in swollen multivesicular endosomes.","method":"Immunofluorescence and immunoperoxidase labeling with synthetic peptide antibodies; chloroquine/NH4Cl treatment; double-labeling of both receptors","journal":"European journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization with functional context, single lab","pmids":["1964415"],"is_preprint":false},{"year":1987,"finding":"The human IGF-II receptor is structurally identical to the cation-independent mannose-6-phosphate receptor (CI-M6PR/IGF2R), as determined from cDNA sequence, revealing a transmembrane receptor with a large extracellular domain of fifteen repeat sequences and a small fibronectin collagen-binding homology domain.","method":"cDNA cloning and sequence analysis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — cDNA sequencing establishing identity of two receptors, widely replicated foundational finding","pmids":["2957598"],"is_preprint":false},{"year":1995,"finding":"The M6P/IGF2R gene functions as a tumour suppressor in human hepatocellular carcinogenesis; 70% of hepatocellular tumours show loss of heterozygosity at the M6P/IGF2R locus, and 25% of LOH tumours carry point mutations in the remaining allele producing truncated receptor protein or significant amino acid substitutions.","method":"Loss of heterozygosity analysis; mutation screening with sequencing of tumour DNA","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — LOH plus sequencing of point mutations, replicated across multiple tumour samples","pmids":["7493029"],"is_preprint":false},{"year":1998,"finding":"TIP47, a 47 kDa cytosolic protein, binds selectively to the cytoplasmic domains of both CI-M6PR and CD-M6PR and is required for MPR transport from endosomes to the trans-Golgi network; TIP47 recognizes a phenylalanine/tryptophan signal in the cytoplasmic tail of CD-M6PR essential for proper endosomal sorting.","method":"Binding assays to cytoplasmic domain peptides; in vitro and in vivo transport assays; identification of critical sorting signal by mutagenesis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstitution of transport plus in vivo validation and signal identification, highly cited","pmids":["9590177"],"is_preprint":false},{"year":1998,"finding":"PACS-1, a cytosolic sorting protein, is required for TGN localization of the mannose-6-phosphate receptor; PACS-1 connects MPR to clathrin-sorting machinery and mediates retrieval to the TGN via binding to phosphorylated cytosolic domains.","method":"Antisense knockdown; cell-free TGN localization assays; in vitro binding assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — cell-free assays plus in vivo antisense knockdown, highly cited","pmids":["9695949"],"is_preprint":false},{"year":2001,"finding":"The GGA proteins (Golgi-localized, gamma-ear-containing, ARF-binding proteins) bind via their VHS domain to acidic-cluster-dileucine signals in the cytosolic tails of both CI-M6PR and CD-M6PR at the TGN, and mediate sorting of the receptors from the TGN onto tubulo-vesicular carriers; a dominant-negative GGA mutant blocked exit of MPRs from the TGN.","method":"VHS domain binding assays; co-localization by immunofluorescence; dominant-negative GGA expression; subcellular fractionation","journal":"Science","confidence":"High","confidence_rationale":"Tier 1–2 — direct binding assays, dominant-negative experiments, and co-fractionation; highly cited","pmids":["11387475"],"is_preprint":false},{"year":2004,"finding":"The mammalian retromer complex (containing VPS26) is required for efficient endosome-to-Golgi retrieval of CI-M6PR; loss of mVPS26 causes CI-M6PR to be either rapidly degraded or mislocalized to the plasma membrane, and mVPS26 localizes to multivesicular body endosomes by electron microscopy.","method":"VPS26 knockout/depletion; CD8 reporter chimera trafficking assays; immunoelectron microscopy; immunofluorescence","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1–2 — knockout with defined trafficking phenotype plus EM localization, highly cited","pmids":["15078902"],"is_preprint":false},{"year":2004,"finding":"The acidic cluster (Glu58, Glu59) of the CK2 site in CD-M6PR cytoplasmic tail is essential for high-affinity GGA1 binding in vitro, while phosphorylation of Ser57 by CK2 is dispensable; AP-1 binding requires a broader set of glutamates (Glu55, Glu56, Glu58, Glu59) but is also independent of Ser57 phosphorylation. GGA1 binds CD-M6PR with 2.4-fold higher affinity than AP-1, suggesting competitive regulation.","method":"In vitro binding assays with mutant CD-M6PR cytoplasmic tail peptides; site-directed mutagenesis; in vivo co-immunoprecipitation with GGA1","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro binding assays with systematic mutagenesis and quantitative affinity measurements","pmids":["15044437"],"is_preprint":false},{"year":2012,"finding":"Retromer is required for retrograde exit of CI-M6PR (chimeric CD8-CI-M6PR) from early endosomes; both CI-M6PR and Shiga toxin B pass through recycling endosomes en route to the TGN, and ablation of the recycling endosome diverts both cargos to an aberrant compartment. EHD1 is required for STxB but not CI-M6PR transport from recycling endosomes to the TGN.","method":"Retromer component knockdown; recycling endosome ablation; CD8-M6PR chimera trafficking assay; immunofluorescence co-localization","journal":"Traffic","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA knockdown with defined trafficking phenotype using chimeric reporter, single lab","pmids":["22540229"],"is_preprint":false},{"year":2018,"finding":"The luminal/extracellular domain of CI-M6PR influences its retrograde endosome-to-TGN trafficking; partial deletion or replacement of the luminal domain mistargeted the receptor to non-TGN compartments, while a short HA-hCI-M6PR-tail construct (transmembrane domain + C-terminus only) preferentially targeted to the TGN. The retromer complex, through interaction with SNX5, regulates trafficking of a luminal-truncated CI-M6PR chimera.","method":"Deletion/chimeric mutant expression; immunofluorescence localization; co-immunoprecipitation with SNX5","journal":"Journal of biomedical research","confidence":"Medium","confidence_rationale":"Tier 2–3 — multiple chimeric constructs tested in cells with localization readout, single lab","pmids":["29988026"],"is_preprint":false},{"year":2019,"finding":"GCC88, a trans-Golgi golgin tethering factor, is required for endosome-to-TGN retrograde transport of CI-M6PR; GCC88 knockout perturbs CI-M6PR retrieval, decreases its steady-state cellular level, causes improper processing of newly synthesized cathepsin-D (a CI-M6PR-dependent lysosomal hydrolase), and reduces lysosomal proteolytic capacity without impairing autophagy.","method":"GCC88 knockout (CRISPR); CI-M6PR localization by immunofluorescence; cathepsin-D processing assay; lysosomal proteolysis assay","journal":"Cell biology international","confidence":"Medium","confidence_rationale":"Tier 2 — knockout with multiple functional readouts, single lab","pmids":["30791178"],"is_preprint":false},{"year":2019,"finding":"CI-M6PR mediates ligand internalization and trafficking to endolysosomal compartments; its cellular uptake involves simultaneous binding of two receptor units forming dimers, and topological arrangement of mannose-6-phosphate glycoclusters (valency and spatial organization) determines efficiency of CI-M6PR-mediated cell uptake.","method":"Synthesis of glycoclusters with defined valency; cell uptake assays in CI-M6PR-positive cells","journal":"Bioconjugate chemistry","confidence":"Low","confidence_rationale":"Tier 3 — functional cell uptake assay with pharmacological probes, no direct receptor binding assay","pmids":["31538768"],"is_preprint":false},{"year":2020,"finding":"M6PR (CD-M6PR) facilitates release of phosphorothioate antisense oligonucleotides (PS-ASOs) from late endosomes; GCC2 recruits M6PR to late endosomes upon PS-ASO treatment, M6PR co-localizes with PS-ASOs on late endosomal membranes, and M6PR reduction impairs PS-ASO endosomal escape and activity both in human cells and in mouse liver in vivo.","method":"siRNA knockdown of M6PR and GCC2; immunofluorescence co-localization; PS-ASO activity assays; in vivo mouse subcutaneous PS-ASO treatment","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA knockdown in cells and in vivo with functional readout, multiple orthogonal assays in single lab","pmids":["31840180"],"is_preprint":false},{"year":2022,"finding":"Arl8b GTPase binds RUFY1 and controls RUFY1 endosomal localization via Rab14 interaction; RUFY1 depletion delays CI-M6PR retrieval from endosomes to the TGN, impairing delivery of newly synthesized hydrolases to lysosomes. RUFY1 interacts with the dynein-dynactin complex via its coiled-coil region and mediates dynein-dependent organelle clustering.","method":"Co-immunoprecipitation; RUFY1 siRNA depletion; CI-M6PR retrograde trafficking assay; lysosomal hydrolase delivery assay; dynein interaction pulldown","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IPs, depletion with defined trafficking phenotype, multiple orthogonal methods","pmids":["36282215"],"is_preprint":false},{"year":2022,"finding":"CD-M6PR is present in mature late endosomes containing hSCARB2, RAB9, BMP, and LAMP2, and CD-M6PR knockdown impairs EV71 (Enterovirus 71) uncoating; CI-M6PR interacts with hSCARB2 through M6P-binding sites, and CD-M6PR likely plays a role in EV71 uncoating in late endosomes.","method":"siRNA knockdown of CD-M6PR; immunofluorescence localization; viral growth/uncoating assay; EV71 infection time-course","journal":"Biology open","confidence":"Medium","confidence_rationale":"Tier 2–3 — siRNA knockdown with viral uncoating phenotype, mechanistic detail inferred rather than directly demonstrated","pmids":["35929543"],"is_preprint":false},{"year":2022,"finding":"RUNX1 transcriptionally regulates RAB31 expression by binding its promoter; RAB31 downregulation (via RUNX1 haplodeficiency or direct siRNA/CRISPR knockdown) causes striking enlargement of early endosomes and impairs early endosomal trafficking of M6PR (mannose-6-phosphate receptor), along with VWF and EGFR, in megakaryocytes.","method":"Promoter-reporter assays; siRNA/CRISPR knockdown; immunofluorescence with EEA1 and CD63 markers; iPS-derived megakaryocytes from patient","journal":"Blood advances","confidence":"Medium","confidence_rationale":"Tier 2 — multiple knockdown approaches with defined endosomal phenotype, patient-derived cells","pmids":["35839075"],"is_preprint":false},{"year":2023,"finding":"CLN3 (Batten disease protein) physically interacts with CI-M6PR and functions as a vesicular trafficking hub connecting the Golgi and lysosome; CLN3 depletion causes mis-trafficking of CI-M6PR, mis-sorting of lysosomal enzymes, and defective autophagic lysosomal reformation. CLN3 overexpression promotes formation of lysosomal tubules in a CI-M6PR- and autophagy-dependent manner generating new proto-lysosomes.","method":"Proteomic analysis (co-immunoprecipitation + mass spectrometry); CLN3 depletion and overexpression; immunofluorescence; lysosomal enzyme sorting assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — proteomics-confirmed interaction, multiple orthogonal functional assays, gain- and loss-of-function","pmids":["37400440"],"is_preprint":false},{"year":2023,"finding":"M6PR (CD-M6PR) is a critical host factor for influenza A virus (IAV) replication; the lumenal domain of M6PR interacts directly with the ectodomain of HA2 subunit of IAV hemagglutinin, and this interaction promotes fusion of the viral envelope with late endosomal membranes. M6PR knockdown inhibits nuclear accumulation of viral NP at early timepoints without affecting attachment, internalization, early endosome trafficking, or late endosome acidification.","method":"siRNA knockdown; exogenous M6PR complementation; nuclear NP accumulation assay; co-immunoprecipitation of M6PR with HA; domain mapping with lumenal-domain and HA2-ectodomain constructs; membrane fusion assay","journal":"Science China. Life sciences","confidence":"High","confidence_rationale":"Tier 1–2 — domain-mapped co-IP, complementation rescue, stepwise entry assays distinguishing fusion from other steps","pmids":["38038885"],"is_preprint":false},{"year":2025,"finding":"M6PR binds STING and sorts it into endosomes for degradation, thereby suppressing STING signaling and cellular senescence; berberine upregulates M6PR specifically in senescent cells, and M6PR knockdown prevents berberine-mediated STING degradation even when STING expression is reversed, demonstrating M6PR-dependent endosomal retention of STING.","method":"Immunoprecipitation; immunofluorescence; Western blotting; cell thermal shift assay; M6PR knockdown; STING dimerization assay; doxorubicin-induced senescence model","journal":"Phytomedicine","confidence":"Medium","confidence_rationale":"Tier 2–3 — co-IP identifying M6PR-STING interaction with functional endosomal sorting readout; single lab","pmids":["40714423"],"is_preprint":false}],"current_model":"M6PR (cation-dependent mannose-6-phosphate receptor, CD-M6PR) is a transmembrane receptor that cycles between the trans-Golgi network (TGN) and late endosomes/multivesicular bodies to deliver newly synthesized lysosomal hydrolases tagged with mannose-6-phosphate; its cytoplasmic tail contains acidic-cluster-dileucine motifs recognized sequentially by GGA proteins (for TGN exit) and TIP47/PACS-1/retromer machinery (for endosome-to-TGN retrieval), while in late endosomes its lumenal domain additionally participates in viral membrane fusion (IAV HA2 interaction), STING degradation, and PS-ASO endosomal escape, and the structurally identical CI-M6PR/IGF2R also acts as a tumour suppressor through IGF-II degradation and TGF-β activation."},"narrative":{"teleology":[{"year":1987,"claim":"Establishing that the cation-independent M6PR is identical to the IGF-II receptor resolved a long-standing question about how IGF-II clearance and lysosomal enzyme sorting are linked through one bifunctional receptor.","evidence":"cDNA cloning and complete sequence analysis of the human IGF-II receptor","pmids":["2957598"],"confidence":"High","gaps":["Functional relationship between IGF-II binding and M6P-dependent trafficking not delineated","No equivalent identity question resolved for the smaller CD-M6PR"]},{"year":1990,"claim":"Demonstrating that CD-M6PR concentrates in Golgi cisternae and cycles to the same late endosome/prelysosome population as CI-M6PR established the shared itinerary of both receptors and linked receptor trafficking to lysosomal biogenesis.","evidence":"Immunofluorescence and immunoperoxidase double-labeling; chloroquine/NH4Cl perturbation causing receptor accumulation in swollen multivesicular endosomes","pmids":["1964415"],"confidence":"Medium","gaps":["Molecular signals governing CD-M6PR cycling were unknown","Functional redundancy or division of labor between CD-M6PR and CI-M6PR not resolved"]},{"year":1995,"claim":"Identifying frequent LOH and inactivating mutations at the M6P/IGF2R locus in hepatocellular carcinoma established CI-M6PR as a tumour suppressor, linking lysosomal receptor biology to cancer genetics.","evidence":"LOH analysis and sequencing of tumour DNA from hepatocellular carcinoma samples","pmids":["7493029"],"confidence":"High","gaps":["Whether tumour suppression operates through IGF-II clearance, TGF-β activation, or both was not resolved","No equivalent tumour suppressor role demonstrated for CD-M6PR"]},{"year":1998,"claim":"Identification of TIP47 and PACS-1 as cytosolic adaptors that bind MPR cytoplasmic tails revealed the molecular machinery for endosome-to-TGN retrieval, answering how receptors are recycled after cargo delivery.","evidence":"In vitro transport reconstitution, binding assays with cytoplasmic-domain peptides, antisense knockdown, and mutagenesis identifying the Phe/Trp sorting signal","pmids":["9590177","9695949"],"confidence":"High","gaps":["Relationship between TIP47 and retromer-mediated retrieval unclear","In vivo redundancy among retrieval adaptors not tested"]},{"year":2001,"claim":"Demonstrating that GGA proteins bind acidic-cluster-dileucine motifs in MPR tails and mediate TGN exit resolved the anterograde sorting step, completing a minimal framework for the forward and return legs of MPR trafficking.","evidence":"VHS domain binding assays, dominant-negative GGA expression blocking TGN exit, co-localization and subcellular fractionation","pmids":["11387475"],"confidence":"High","gaps":["How GGA and AP-1 adaptors cooperate or compete at the TGN was not fully resolved","Cargo selectivity differences between GGA family members not determined"]},{"year":2004,"claim":"Parallel studies established that the retromer complex (via VPS26) is essential for CI-M6PR endosome-to-TGN retrieval, and that CK2-site acidic residues in the CD-M6PR tail determine differential GGA1 versus AP-1 binding affinity, refining the adaptor hierarchy.","evidence":"VPS26 depletion with CD8-M6PR chimera trafficking and immunoEM localization; in vitro binding with systematic cytoplasmic-tail mutants and quantitative affinity measurements","pmids":["15078902","15044437"],"confidence":"High","gaps":["Whether retromer directly contacts CD-M6PR or acts primarily on CI-M6PR was ambiguous","Structural basis of GGA–tail interaction not resolved at atomic level in this period"]},{"year":2012,"claim":"Tracking retromer-dependent CI-M6PR retrieval through recycling endosomes distinguished an intermediate compartment in the retrograde pathway and separated CI-M6PR retrieval from EHD1-dependent Shiga toxin transport.","evidence":"siRNA knockdown of retromer components and recycling-endosome ablation with CD8-CI-M6PR chimera trafficking","pmids":["22540229"],"confidence":"Medium","gaps":["Whether CD-M6PR uses the same recycling-endosome intermediate is untested","Single-lab study; independent replication of route segregation awaited"]},{"year":2019,"claim":"Identification of GCC88 as a TGN tethering factor required for CI-M6PR retrieval, and characterization of luminal-domain contributions to CI-M6PR trafficking, expanded the machinery beyond cytoplasmic-tail adaptors to include golgin-mediated vesicle capture and lumenal determinants.","evidence":"GCC88 CRISPR knockout with CI-M6PR localization and cathepsin-D processing assays; chimeric luminal-domain deletion constructs with immunofluorescence","pmids":["30791178","29988026"],"confidence":"Medium","gaps":["Direct physical interaction between GCC88 and M6PR-containing vesicles not shown","Relative contributions of luminal versus cytoplasmic sorting signals not quantified"]},{"year":2020,"claim":"Discovering that CD-M6PR facilitates phosphorothioate antisense oligonucleotide endosomal escape via GCC2-mediated recruitment to late endosomes revealed an unexpected non-canonical function for CD-M6PR in nucleic acid delivery.","evidence":"siRNA knockdown of M6PR and GCC2 in human cells and in vivo mouse liver; PS-ASO activity assays and immunofluorescence co-localization","pmids":["31840180"],"confidence":"Medium","gaps":["Mechanism by which M6PR promotes membrane destabilization for ASO escape is unknown","Whether this function involves M6P-ligand binding or a distinct interaction surface is unresolved"]},{"year":2022,"claim":"Multiple studies expanded the CD-M6PR and CI-M6PR interactome and trafficking regulators: RUFY1-dynein was shown to drive endosome-to-TGN retrieval, RAB31 (regulated by RUNX1) was required for early endosomal M6PR trafficking in megakaryocytes, and CD-M6PR was found to participate in EV71 uncoating in late endosomes.","evidence":"Co-IP and RUFY1 siRNA with CI-M6PR retrograde assay (JCB); CRISPR/siRNA knockdown of RAB31 in iPSC-megakaryocytes (Blood Adv); CD-M6PR siRNA with EV71 uncoating assay (Biol Open)","pmids":["36282215","35839075","35929543"],"confidence":"Medium","gaps":["RUFY1's direct versus indirect interaction with M6PR not resolved","RAB31 effect on M6PR may be indirect via general endosomal morphology","EV71 uncoating mechanism and whether CD-M6PR contacts viral capsid are unclear"]},{"year":2023,"claim":"CLN3 (Batten disease protein) was identified as a physical interactor of CI-M6PR that coordinates Golgi-lysosome trafficking and autophagic lysosomal reformation, and CD-M6PR was shown to directly bind influenza A HA2 to promote viral membrane fusion, revealing disease-relevant receptor functions beyond hydrolase sorting.","evidence":"Co-IP/mass spectrometry with CLN3 gain- and loss-of-function plus lysosomal enzyme assays (Nat Commun); domain-mapped co-IP of M6PR lumenal domain with HA2 ectodomain, siRNA knockdown with rescue and membrane fusion assay (Sci China Life Sci)","pmids":["37400440","38038885"],"confidence":"High","gaps":["Structural basis of M6PR lumenal domain–HA2 interaction unknown","Whether CLN3 interaction is M6P-dependent or mediated by a distinct surface not determined"]},{"year":2025,"claim":"M6PR was found to bind STING and sort it into endosomes for degradation, directly linking M6PR to innate immune regulation and cellular senescence control.","evidence":"Co-immunoprecipitation, M6PR knockdown preventing berberine-mediated STING degradation, cell thermal shift assay, doxorubicin-induced senescence model","pmids":["40714423"],"confidence":"Medium","gaps":["Whether M6PR-STING interaction is direct or adaptor-mediated needs reconstitution","Physiological relevance outside pharmacological (berberine) context not established","Single-lab finding awaiting independent confirmation"]},{"year":null,"claim":"Key open questions include the structural basis of CD-M6PR lumenal domain interactions with non-canonical cargoes (HA2, STING, PS-ASOs), the degree of functional redundancy between CD-M6PR and CI-M6PR in vivo, and the mechanism by which M6PR promotes endosomal membrane destabilization for cargo escape.","evidence":"","pmids":[],"confidence":"Low","gaps":["No atomic-resolution structure of full-length CD-M6PR in complex with any cargo","In vivo genetic models (CD-M6PR knockout) not extensively characterized for non-canonical functions","Crosstalk between M6PR-mediated STING degradation and lysosomal hydrolase delivery pathways untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[0,3,5,7,11,17]}],"localization":[{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[0,5,7,10]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[0,6,8,12,14]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[14,16]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[8,12]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,3,4,5,6,8,10,13]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[16]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5,7,16]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[6,10,16]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[17,18]}],"complexes":[],"partners":["TIP47","PACS1","GGA1","VPS26","RUFY1","CLN3","STING","GCC88"],"other_free_text":[]},"mechanistic_narrative":"M6PR (cation-dependent mannose-6-phosphate receptor, CD-M6PR) is a transmembrane cargo receptor that cycles between the trans-Golgi network (TGN) and late endosomes/multivesicular bodies to deliver mannose-6-phosphate-tagged lysosomal hydrolases to the endolysosomal compartment [PMID:1964415, PMID:15078902]. Anterograde exit from the TGN is driven by GGA proteins that recognize acidic-cluster-dileucine motifs in the CD-M6PR cytoplasmic tail, while retrograde retrieval from endosomes depends on TIP47, PACS-1, the retromer complex, and tethering factors such as GCC88 and RUFY1-dynein [PMID:11387475, PMID:9590177, PMID:9695949, PMID:15078902, PMID:30791178, PMID:36282215]. Beyond lysosomal biogenesis, the lumenal domain of CD-M6PR directly binds the HA2 ectodomain of influenza A hemagglutinin to promote viral membrane fusion in late endosomes, facilitates endosomal escape of phosphorothioate antisense oligonucleotides, and sorts STING into endosomes for degradation to suppress innate immune signaling and senescence [PMID:38038885, PMID:31840180, PMID:40714423]. The structurally distinct cation-independent M6PR (CI-M6PR/IGF2R) shares overlapping trafficking itineraries and acts as a tumour suppressor lost in hepatocellular carcinoma, and interacts with CLN3 (Batten disease protein) to coordinate Golgi-lysosome vesicular trafficking and autophagic lysosomal reformation [PMID:7493029, PMID:37400440]."},"prefetch_data":{"uniprot":{"accession":"P20645","full_name":"Cation-dependent mannose-6-phosphate receptor","aliases":["46 kDa mannose 6-phosphate receptor","MPR 46"],"length_aa":277,"mass_kda":31.0,"function":"Transport of phosphorylated lysosomal enzymes from the Golgi complex and the cell surface to lysosomes. Lysosomal enzymes bearing phosphomannosyl residues bind specifically to mannose-6-phosphate receptors in the Golgi apparatus and the resulting receptor-ligand complex is transported to an acidic prelyosomal compartment where the low pH mediates the dissociation of the complex","subcellular_location":"Lysosome membrane","url":"https://www.uniprot.org/uniprotkb/P20645/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/M6PR","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"ARL3","stoichiometry":0.2},{"gene":"CANX","stoichiometry":0.2},{"gene":"STX7","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/M6PR","total_profiled":1310},"omim":[{"mim_id":"616945","title":"CLAVESIN 2; CLVS2","url":"https://www.omim.org/entry/616945"},{"mim_id":"613401","title":"VPS33B-INTERACTING PROTEIN, APICAL-BASOLATERAL POLARITY REGULATOR, SPE39 HOMOLOG; VIPAS39","url":"https://www.omim.org/entry/613401"},{"mim_id":"611292","title":"CLAVESIN 1; CLVS1","url":"https://www.omim.org/entry/611292"},{"mim_id":"610893","title":"CHARGED MULTIVESICULAR BODY PROTEIN 2A; CHMP2A","url":"https://www.omim.org/entry/610893"},{"mim_id":"610223","title":"RAS AND RAB INTERACTOR 3; RIN3","url":"https://www.omim.org/entry/610223"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/M6PR"},"hgnc":{"alias_symbol":["CD-MPR","CD-M6PR"],"prev_symbol":[]},"alphafold":{"accession":"P20645","domains":[{"cath_id":"2.70.130.10","chopping":"42-177","consensus_level":"high","plddt":93.8458,"start":42,"end":177},{"cath_id":"1.10.287","chopping":"187-242","consensus_level":"high","plddt":92.0177,"start":187,"end":242}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P20645","model_url":"https://alphafold.ebi.ac.uk/files/AF-P20645-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P20645-F1-predicted_aligned_error_v6.png","plddt_mean":84.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=M6PR","jax_strain_url":"https://www.jax.org/strain/search?query=M6PR"},"sequence":{"accession":"P20645","fasta_url":"https://rest.uniprot.org/uniprotkb/P20645.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P20645/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P20645"}},"corpus_meta":[{"pmid":"22540229","id":"PMC_22540229","title":"Retromer guides STxB and CD8-M6PR from early to recycling endosomes, EHD1 guides STxB from recycling endosome to Golgi.","date":"2012","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/22540229","citation_count":53,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"31840180","id":"PMC_31840180","title":"Golgi-endosome transport mediated by M6PR facilitates release of antisense oligonucleotides from endosomes.","date":"2020","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/31840180","citation_count":43,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"37400440","id":"PMC_37400440","title":"Loss of the batten disease protein CLN3 leads to mis-trafficking of M6PR and defective autophagic-lysosomal reformation.","date":"2023","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/37400440","citation_count":39,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"1964415","id":"PMC_1964415","title":"The 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phosphorylation of Ser57 has no influence on GGA1 binding. AP-1 binding additionally requires Glu55 and Glu56. GGA1 binds CD-MPR with ~2.4-fold higher affinity than AP-1, which may regulate sequential binding of these adaptors to the partly overlapping sorting signals in the TGN.\",\n      \"method\": \"In vitro binding assays with site-directed mutagenesis of CD-MPR cytoplasmic tail; co-immunoprecipitation for in vivo interaction validation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro assay with systematic mutagenesis plus in vivo validation, multiple orthogonal methods\",\n      \"pmids\": [\"15044437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Retromer is required for retrograde exit of CI-M6PR from early endosomes, and the recycling endosome is a required intermediate compartment in the retrograde trafficking route of CI-M6PR to the TGN. EHD1 is not significantly required for CI-M6PR transit from recycling endosomes to the TGN (unlike Shiga toxin B).\",\n      \"method\": \"Knockdown of retromer components and ablation of recycling endosomes; fluorescence microscopy and trafficking assays with chimeric CD8-CI-M6PR cargo\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KD with specific trafficking phenotype readout, two orthogonal cargos tested, recycling endosome ablation as additional perturbation\",\n      \"pmids\": [\"22540229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"The 46 kDa (cation-dependent) M6PR cycles between the Golgi complex and the same population of late endosomes (prelysosomes) as the 215 kDa (cation-independent) M6PR, establishing a shared recycling itinerary for both receptors.\",\n      \"method\": \"Immunofluorescence and immunoperoxidase double-labeling with antibodies to both M6PR forms; weak-base (chloroquine/NH4Cl) treatment to trap endosomal compartments\",\n      \"journal\": \"European journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization by fractionation/immunolabeling with functional compartment perturbation, single lab\",\n      \"pmids\": [\"1964415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GCC88, a trans-Golgi golgin tethering factor, is required for endosome-to-TGN retrograde transport of CI-M6PR. GCC88 knockout perturbs CI-M6PR retrieval, reduces its steady-state cellular levels, causes improper processing of the CI-M6PR-dependent lysosomal hydrolase cathepsin-D, and reduces lysosomal proteolytic capacity.\",\n      \"method\": \"CRISPR knockout of GCC88; immunofluorescence and western blotting to assess CI-M6PR localization and cathepsin-D processing; lysosomal activity assay\",\n      \"journal\": \"Cell biology international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple downstream readouts (receptor localization, cargo processing, lysosomal function), single lab\",\n      \"pmids\": [\"30791178\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"M6PR physically co-localizes with PS-ASOs in late endosomes, binds PS-ASOs, and facilitates their vesicular escape from late endosomes. GCC2 acts upstream of M6PR in the same Golgi-to-late endosome transport pathway, and GCC2 relocalizes to late endosomes upon PS-ASO treatment. M6PR knockdown impairs PS-ASO release and activity in human and mouse cells in vivo.\",\n      \"method\": \"siRNA knockdown of GCC2 and M6PR; co-localization by confocal microscopy; in vivo mouse subcutaneous ASO treatment; epistasis established by showing GCC2 and M6PR act in the same pathway\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with specific functional readout, co-localization, in vivo validation, single lab\",\n      \"pmids\": [\"31840180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RUFY1 interacts with Arl8b and Rab14 to localize to recycling endosomes, where it promotes dynein-dynactin-dependent retrograde retrieval of CI-M6PR from endosomes to the TGN; RUFY1 depletion delays CI-M6PR retrieval and impairs delivery of newly synthesized hydrolases to lysosomes.\",\n      \"method\": \"Co-immunoprecipitation; siRNA knockdown with CI-M6PR trafficking assay (CD8α-CI-M6PR chimera); lysosomal hydrolase delivery assay; dynein-dynactin interaction by pulldown\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, clean KD with specific trafficking phenotype, multiple orthogonal methods, single lab with rigorous controls\",\n      \"pmids\": [\"36282215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CLN3 interacts with CI-M6PR (identified by proteomic analysis) and functions as a vesicular trafficking hub connecting Golgi and lysosomes. CLN3 depletion causes mis-trafficking of CI-M6PR, mis-sorting of lysosomal enzymes, and defective autophagic lysosomal reformation. CLN3 overexpression promotes formation of CI-M6PR-dependent lysosomal tubules generating proto-lysosomes.\",\n      \"method\": \"Proteomic co-immunoprecipitation; siRNA knockdown and overexpression with immunofluorescence, lysosomal enzyme sorting assays, and live imaging of lysosomal tubules\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — MS-based interactome plus KD and OE with multiple orthogonal functional readouts, single study with rigorous controls\",\n      \"pmids\": [\"37400440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CD-M6PR is present in mature late endosomes (RAB9-, LAMP2-, BMP-positive) along with hSCARB2, and CD-M6PR knockdown impairs EV71 uncoating in those compartments. The lumenal mannose-6-phosphate-binding site of CD-M6PR is implicated, as hSCARB2 interacts with CI-M6PR through M6P-binding sites.\",\n      \"method\": \"siRNA knockdown of CD-M6PR; immunofluorescence co-localization; infection assay measuring EV71 uncoating efficiency\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with specific uncoating phenotype and localization evidence, mechanistic inference partially indirect\",\n      \"pmids\": [\"35929543\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"M6PR (CD-M6PR) interacts with the HA2 ectodomain of influenza A virus hemagglutinin via its lumenal domain in late endosomes, directly promoting fusion of the viral envelope with the late endosomal membrane to facilitate IAV replication; M6PR knockdown blocks nuclear accumulation of viral NP without affecting attachment, internalization, early endosome trafficking, or late endosome acidification.\",\n      \"method\": \"siRNA knockdown; co-immunoprecipitation between M6PR lumenal domain and HA2 ectodomain; viral NP nuclear accumulation assay; membrane fusion assay\",\n      \"journal\": \"Science China. Life sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with specific step-resolved phenotype plus direct protein interaction by Co-IP and domain mapping, single lab\",\n      \"pmids\": [\"38038885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The luminal domain of CI-M6PR influences its retrograde endosome-to-TGN sorting: partial deletion or replacement of the luminal domain misdirects the receptor, while a short HA-tagged construct bearing only the transmembrane domain and C-terminal cytoplasmic tail retains proper TGN targeting. The retromer complex regulates trafficking of luminal-truncated CI-M6PR via interaction with SNX5.\",\n      \"method\": \"Deletion and chimeric mutant expression; immunofluorescence localization; co-immunoprecipitation with SNX5\",\n      \"journal\": \"Journal of biomedical research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — domain-mapping mutagenesis with localization readout plus single Co-IP, single lab\",\n      \"pmids\": [\"29988026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RUNX1 transcriptionally regulates RAB31, and RAB31 controls early endosomal trafficking of M6PR (mannose-6-phosphate receptor) in megakaryocytes; knockdown of RUNX1 or RAB31 causes striking enlargement of early endosomes and impairs M6PR trafficking at the early endosome level.\",\n      \"method\": \"siRNA and CRISPR/Cas9 knockdown; immunofluorescence with endosomal markers; patient-derived iPSC-megakaryocyte model; promoter-reporter assay\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO/KD with specific endosomal phenotype and multiple patient-derived validation models\",\n      \"pmids\": [\"35839075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"M6PR binds to STING and sorts it into endosomes for degradation, thereby suppressing STING signaling and senescence-associated secretory phenotypes. Berberine upregulates M6PR selectively in senescent cells, and M6PR knockdown abolishes the anti-senescence effect of berberine even when STING expression is reversed.\",\n      \"method\": \"Immunoprecipitation; immunofluorescence co-localization; cell thermal shift assay; siRNA knockdown with STING signaling and senescence readouts; Western blotting\",\n      \"journal\": \"Phytomedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct protein interaction by Co-IP and CETSA, KD epistasis establishing pathway position, single lab\",\n      \"pmids\": [\"40714423\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"M6PR (both cation-dependent CD-M6PR and cation-independent CI-M6PR) cycles between the trans-Golgi network and late endosomes/lysosomes, where it delivers mannose-6-phosphate-tagged lysosomal hydrolases; retrograde retrieval from early/recycling endosomes to the TGN depends on retromer (via SNX5), RUFY1–Arl8b–dynein machinery, and golgin tethers GCC88 and GCC2, while TGN export relies on GGA1 and AP-1 binding to acidic-cluster/DXXLL motifs in the cytoplasmic tail; additionally, M6PR participates in endosomal escape of antisense oligonucleotides, viral membrane fusion (IAV HA2, EV71), STING degradation, and CLN3-dependent lysosomal reformation.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"The 46 kDa cation-dependent mannose-6-phosphate receptor (CD-M6PR) cycles between the Golgi complex (concentrated in middle and trans cisternae) and the same population of late endosomes (prelysosomes) as the 215 kDa CI-M6PR, as shown by immunofluorescence and immunoperoxidase labeling with antibodies to the C-terminal cytoplasmic domain; weak base treatment (chloroquine/NH4Cl) caused both receptors to accumulate in swollen multivesicular endosomes.\",\n      \"method\": \"Immunofluorescence and immunoperoxidase labeling with synthetic peptide antibodies; chloroquine/NH4Cl treatment; double-labeling of both receptors\",\n      \"journal\": \"European journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with functional context, single lab\",\n      \"pmids\": [\"1964415\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"The human IGF-II receptor is structurally identical to the cation-independent mannose-6-phosphate receptor (CI-M6PR/IGF2R), as determined from cDNA sequence, revealing a transmembrane receptor with a large extracellular domain of fifteen repeat sequences and a small fibronectin collagen-binding homology domain.\",\n      \"method\": \"cDNA cloning and sequence analysis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cDNA sequencing establishing identity of two receptors, widely replicated foundational finding\",\n      \"pmids\": [\"2957598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The M6P/IGF2R gene functions as a tumour suppressor in human hepatocellular carcinogenesis; 70% of hepatocellular tumours show loss of heterozygosity at the M6P/IGF2R locus, and 25% of LOH tumours carry point mutations in the remaining allele producing truncated receptor protein or significant amino acid substitutions.\",\n      \"method\": \"Loss of heterozygosity analysis; mutation screening with sequencing of tumour DNA\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — LOH plus sequencing of point mutations, replicated across multiple tumour samples\",\n      \"pmids\": [\"7493029\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"TIP47, a 47 kDa cytosolic protein, binds selectively to the cytoplasmic domains of both CI-M6PR and CD-M6PR and is required for MPR transport from endosomes to the trans-Golgi network; TIP47 recognizes a phenylalanine/tryptophan signal in the cytoplasmic tail of CD-M6PR essential for proper endosomal sorting.\",\n      \"method\": \"Binding assays to cytoplasmic domain peptides; in vitro and in vivo transport assays; identification of critical sorting signal by mutagenesis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstitution of transport plus in vivo validation and signal identification, highly cited\",\n      \"pmids\": [\"9590177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"PACS-1, a cytosolic sorting protein, is required for TGN localization of the mannose-6-phosphate receptor; PACS-1 connects MPR to clathrin-sorting machinery and mediates retrieval to the TGN via binding to phosphorylated cytosolic domains.\",\n      \"method\": \"Antisense knockdown; cell-free TGN localization assays; in vitro binding assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — cell-free assays plus in vivo antisense knockdown, highly cited\",\n      \"pmids\": [\"9695949\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The GGA proteins (Golgi-localized, gamma-ear-containing, ARF-binding proteins) bind via their VHS domain to acidic-cluster-dileucine signals in the cytosolic tails of both CI-M6PR and CD-M6PR at the TGN, and mediate sorting of the receptors from the TGN onto tubulo-vesicular carriers; a dominant-negative GGA mutant blocked exit of MPRs from the TGN.\",\n      \"method\": \"VHS domain binding assays; co-localization by immunofluorescence; dominant-negative GGA expression; subcellular fractionation\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — direct binding assays, dominant-negative experiments, and co-fractionation; highly cited\",\n      \"pmids\": [\"11387475\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The mammalian retromer complex (containing VPS26) is required for efficient endosome-to-Golgi retrieval of CI-M6PR; loss of mVPS26 causes CI-M6PR to be either rapidly degraded or mislocalized to the plasma membrane, and mVPS26 localizes to multivesicular body endosomes by electron microscopy.\",\n      \"method\": \"VPS26 knockout/depletion; CD8 reporter chimera trafficking assays; immunoelectron microscopy; immunofluorescence\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — knockout with defined trafficking phenotype plus EM localization, highly cited\",\n      \"pmids\": [\"15078902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The acidic cluster (Glu58, Glu59) of the CK2 site in CD-M6PR cytoplasmic tail is essential for high-affinity GGA1 binding in vitro, while phosphorylation of Ser57 by CK2 is dispensable; AP-1 binding requires a broader set of glutamates (Glu55, Glu56, Glu58, Glu59) but is also independent of Ser57 phosphorylation. GGA1 binds CD-M6PR with 2.4-fold higher affinity than AP-1, suggesting competitive regulation.\",\n      \"method\": \"In vitro binding assays with mutant CD-M6PR cytoplasmic tail peptides; site-directed mutagenesis; in vivo co-immunoprecipitation with GGA1\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro binding assays with systematic mutagenesis and quantitative affinity measurements\",\n      \"pmids\": [\"15044437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Retromer is required for retrograde exit of CI-M6PR (chimeric CD8-CI-M6PR) from early endosomes; both CI-M6PR and Shiga toxin B pass through recycling endosomes en route to the TGN, and ablation of the recycling endosome diverts both cargos to an aberrant compartment. EHD1 is required for STxB but not CI-M6PR transport from recycling endosomes to the TGN.\",\n      \"method\": \"Retromer component knockdown; recycling endosome ablation; CD8-M6PR chimera trafficking assay; immunofluorescence co-localization\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA knockdown with defined trafficking phenotype using chimeric reporter, single lab\",\n      \"pmids\": [\"22540229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The luminal/extracellular domain of CI-M6PR influences its retrograde endosome-to-TGN trafficking; partial deletion or replacement of the luminal domain mistargeted the receptor to non-TGN compartments, while a short HA-hCI-M6PR-tail construct (transmembrane domain + C-terminus only) preferentially targeted to the TGN. The retromer complex, through interaction with SNX5, regulates trafficking of a luminal-truncated CI-M6PR chimera.\",\n      \"method\": \"Deletion/chimeric mutant expression; immunofluorescence localization; co-immunoprecipitation with SNX5\",\n      \"journal\": \"Journal of biomedical research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple chimeric constructs tested in cells with localization readout, single lab\",\n      \"pmids\": [\"29988026\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"GCC88, a trans-Golgi golgin tethering factor, is required for endosome-to-TGN retrograde transport of CI-M6PR; GCC88 knockout perturbs CI-M6PR retrieval, decreases its steady-state cellular level, causes improper processing of newly synthesized cathepsin-D (a CI-M6PR-dependent lysosomal hydrolase), and reduces lysosomal proteolytic capacity without impairing autophagy.\",\n      \"method\": \"GCC88 knockout (CRISPR); CI-M6PR localization by immunofluorescence; cathepsin-D processing assay; lysosomal proteolysis assay\",\n      \"journal\": \"Cell biology international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — knockout with multiple functional readouts, single lab\",\n      \"pmids\": [\"30791178\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CI-M6PR mediates ligand internalization and trafficking to endolysosomal compartments; its cellular uptake involves simultaneous binding of two receptor units forming dimers, and topological arrangement of mannose-6-phosphate glycoclusters (valency and spatial organization) determines efficiency of CI-M6PR-mediated cell uptake.\",\n      \"method\": \"Synthesis of glycoclusters with defined valency; cell uptake assays in CI-M6PR-positive cells\",\n      \"journal\": \"Bioconjugate chemistry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — functional cell uptake assay with pharmacological probes, no direct receptor binding assay\",\n      \"pmids\": [\"31538768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"M6PR (CD-M6PR) facilitates release of phosphorothioate antisense oligonucleotides (PS-ASOs) from late endosomes; GCC2 recruits M6PR to late endosomes upon PS-ASO treatment, M6PR co-localizes with PS-ASOs on late endosomal membranes, and M6PR reduction impairs PS-ASO endosomal escape and activity both in human cells and in mouse liver in vivo.\",\n      \"method\": \"siRNA knockdown of M6PR and GCC2; immunofluorescence co-localization; PS-ASO activity assays; in vivo mouse subcutaneous PS-ASO treatment\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA knockdown in cells and in vivo with functional readout, multiple orthogonal assays in single lab\",\n      \"pmids\": [\"31840180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Arl8b GTPase binds RUFY1 and controls RUFY1 endosomal localization via Rab14 interaction; RUFY1 depletion delays CI-M6PR retrieval from endosomes to the TGN, impairing delivery of newly synthesized hydrolases to lysosomes. RUFY1 interacts with the dynein-dynactin complex via its coiled-coil region and mediates dynein-dependent organelle clustering.\",\n      \"method\": \"Co-immunoprecipitation; RUFY1 siRNA depletion; CI-M6PR retrograde trafficking assay; lysosomal hydrolase delivery assay; dynein interaction pulldown\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IPs, depletion with defined trafficking phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"36282215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CD-M6PR is present in mature late endosomes containing hSCARB2, RAB9, BMP, and LAMP2, and CD-M6PR knockdown impairs EV71 (Enterovirus 71) uncoating; CI-M6PR interacts with hSCARB2 through M6P-binding sites, and CD-M6PR likely plays a role in EV71 uncoating in late endosomes.\",\n      \"method\": \"siRNA knockdown of CD-M6PR; immunofluorescence localization; viral growth/uncoating assay; EV71 infection time-course\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — siRNA knockdown with viral uncoating phenotype, mechanistic detail inferred rather than directly demonstrated\",\n      \"pmids\": [\"35929543\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RUNX1 transcriptionally regulates RAB31 expression by binding its promoter; RAB31 downregulation (via RUNX1 haplodeficiency or direct siRNA/CRISPR knockdown) causes striking enlargement of early endosomes and impairs early endosomal trafficking of M6PR (mannose-6-phosphate receptor), along with VWF and EGFR, in megakaryocytes.\",\n      \"method\": \"Promoter-reporter assays; siRNA/CRISPR knockdown; immunofluorescence with EEA1 and CD63 markers; iPS-derived megakaryocytes from patient\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple knockdown approaches with defined endosomal phenotype, patient-derived cells\",\n      \"pmids\": [\"35839075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CLN3 (Batten disease protein) physically interacts with CI-M6PR and functions as a vesicular trafficking hub connecting the Golgi and lysosome; CLN3 depletion causes mis-trafficking of CI-M6PR, mis-sorting of lysosomal enzymes, and defective autophagic lysosomal reformation. CLN3 overexpression promotes formation of lysosomal tubules in a CI-M6PR- and autophagy-dependent manner generating new proto-lysosomes.\",\n      \"method\": \"Proteomic analysis (co-immunoprecipitation + mass spectrometry); CLN3 depletion and overexpression; immunofluorescence; lysosomal enzyme sorting assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — proteomics-confirmed interaction, multiple orthogonal functional assays, gain- and loss-of-function\",\n      \"pmids\": [\"37400440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"M6PR (CD-M6PR) is a critical host factor for influenza A virus (IAV) replication; the lumenal domain of M6PR interacts directly with the ectodomain of HA2 subunit of IAV hemagglutinin, and this interaction promotes fusion of the viral envelope with late endosomal membranes. M6PR knockdown inhibits nuclear accumulation of viral NP at early timepoints without affecting attachment, internalization, early endosome trafficking, or late endosome acidification.\",\n      \"method\": \"siRNA knockdown; exogenous M6PR complementation; nuclear NP accumulation assay; co-immunoprecipitation of M6PR with HA; domain mapping with lumenal-domain and HA2-ectodomain constructs; membrane fusion assay\",\n      \"journal\": \"Science China. Life sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — domain-mapped co-IP, complementation rescue, stepwise entry assays distinguishing fusion from other steps\",\n      \"pmids\": [\"38038885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"M6PR binds STING and sorts it into endosomes for degradation, thereby suppressing STING signaling and cellular senescence; berberine upregulates M6PR specifically in senescent cells, and M6PR knockdown prevents berberine-mediated STING degradation even when STING expression is reversed, demonstrating M6PR-dependent endosomal retention of STING.\",\n      \"method\": \"Immunoprecipitation; immunofluorescence; Western blotting; cell thermal shift assay; M6PR knockdown; STING dimerization assay; doxorubicin-induced senescence model\",\n      \"journal\": \"Phytomedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — co-IP identifying M6PR-STING interaction with functional endosomal sorting readout; single lab\",\n      \"pmids\": [\"40714423\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"M6PR (cation-dependent mannose-6-phosphate receptor, CD-M6PR) is a transmembrane receptor that cycles between the trans-Golgi network (TGN) and late endosomes/multivesicular bodies to deliver newly synthesized lysosomal hydrolases tagged with mannose-6-phosphate; its cytoplasmic tail contains acidic-cluster-dileucine motifs recognized sequentially by GGA proteins (for TGN exit) and TIP47/PACS-1/retromer machinery (for endosome-to-TGN retrieval), while in late endosomes its lumenal domain additionally participates in viral membrane fusion (IAV HA2 interaction), STING degradation, and PS-ASO endosomal escape, and the structurally identical CI-M6PR/IGF2R also acts as a tumour suppressor through IGF-II degradation and TGF-β activation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"M6PR encodes the cation-dependent mannose-6-phosphate receptor (CD-M6PR), a type I transmembrane protein that cycles between the trans-Golgi network (TGN) and late endosomes/lysosomes to deliver mannose-6-phosphate-tagged lysosomal hydrolases, sharing this recycling itinerary with the cation-independent CI-M6PR [PMID:1964415]. Anterograde TGN export is governed by overlapping cytoplasmic-tail sorting signals that bind GGA1 (via an acidic-cluster motif) and AP-1, with GGA1 exhibiting higher affinity, while retrograde endosome-to-TGN retrieval requires retromer–SNX5, transit through recycling endosomes, the RUFY1–Arl8b–dynein machinery, and golgin tethers GCC88 and GCC2 [PMID:15044437, PMID:22540229, PMID:36282215, PMID:30791178, PMID:29988026]. Beyond hydrolase sorting, M6PR facilitates endosomal escape of phosphorothioate antisense oligonucleotides, promotes influenza A virus HA2-dependent membrane fusion and enterovirus 71 uncoating in late endosomes via its lumenal mannose-6-phosphate-binding site, cooperates with CLN3 in autophagic lysosomal reformation, and directs STING to endosomes for degradation to suppress senescence-associated signaling [PMID:31840180, PMID:38038885, PMID:35929543, PMID:37400440, PMID:40714423].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Establishing that CD-M6PR and CI-M6PR share the same late-endosomal recycling compartment answered the fundamental question of whether both receptors use a common itinerary, unifying their biology.\",\n      \"evidence\": \"Immunofluorescence and immunoperoxidase double-labeling with weak-base perturbation in cultured cells\",\n      \"pmids\": [\"1964415\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab observation without independent replication at that time\",\n        \"Molecular determinants directing both receptors to the same compartment were unknown\",\n        \"Recycling dynamics and kinetics were not resolved\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Systematic mutagenesis of the CD-M6PR cytoplasmic tail resolved how GGA1 and AP-1 bind overlapping but distinct acidic-cluster motifs, with GGA1 showing higher affinity, establishing a sequential adaptor-binding model for TGN sorting.\",\n      \"evidence\": \"In vitro binding assays with site-directed CD-M6PR tail mutants; co-immunoprecipitation for in vivo validation\",\n      \"pmids\": [\"15044437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether the sequential GGA1→AP-1 binding model operates in vivo during vesicle budding was not tested\",\n        \"Structural basis of the differential affinity was not determined\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrating that retromer is required for CI-M6PR exit from early endosomes and that the recycling endosome is a required intermediate station defined the compartmental itinerary of retrograde retrieval.\",\n      \"evidence\": \"Retromer component knockdown and recycling endosome ablation with chimeric CD8-CI-M6PR trafficking assays\",\n      \"pmids\": [\"22540229\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Identity of the cargo receptor for CI-M6PR at the recycling endosome-to-TGN step was unclear\",\n        \"Motor machinery driving retrograde transport was not identified\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing that the CI-M6PR luminal domain influences retrograde sorting and that retromer acts via SNX5 interaction revealed that both luminal and cytoplasmic determinants coordinate endosome-to-TGN retrieval.\",\n      \"evidence\": \"Domain deletion/chimera expression with immunofluorescence localization; co-immunoprecipitation with SNX5\",\n      \"pmids\": [\"29988026\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single Co-IP for SNX5 without reciprocal validation\",\n        \"Structural basis for luminal domain contribution to sorting was not resolved\",\n        \"Whether luminal ligand occupancy modulates sorting was untested\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identifying golgin GCC88 as required for CI-M6PR endosome-to-TGN tethering linked receptor retrieval to a specific TGN-resident tethering factor and showed functional consequences for lysosomal hydrolase maturation.\",\n      \"evidence\": \"CRISPR knockout of GCC88 with CI-M6PR localization, cathepsin-D processing, and lysosomal activity assays\",\n      \"pmids\": [\"30791178\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct physical interaction between GCC88 and CI-M6PR-containing vesicles was not demonstrated\",\n        \"Relationship between GCC88 and GCC2 in the same retrieval pathway was undefined\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showing that M6PR binds phosphorothioate antisense oligonucleotides in late endosomes and facilitates their vesicular escape, with GCC2 acting upstream, expanded M6PR function beyond hydrolase sorting to nucleic acid delivery.\",\n      \"evidence\": \"siRNA knockdown of GCC2 and M6PR; confocal co-localization; in vivo mouse ASO treatment with functional readout\",\n      \"pmids\": [\"31840180\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which M6PR promotes membrane destabilization for ASO escape is unknown\",\n        \"Whether ASO binding occurs at the M6P-binding site or a distinct interface was not defined\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Multiple 2022 studies collectively defined how CI-M6PR retrograde transport depends on RUFY1–Arl8b–dynein at recycling endosomes, how RAB31 (regulated by RUNX1) controls M6PR early endosomal trafficking in megakaryocytes, and how CLN3 interacts with CI-M6PR to coordinate lysosomal enzyme delivery and autophagic lysosomal reformation.\",\n      \"evidence\": \"Co-IP, siRNA/CRISPR knockdowns, chimeric cargo trafficking assays, proteomic interactome, patient-derived iPSC-megakaryocytes, live imaging of lysosomal tubules\",\n      \"pmids\": [\"36282215\", \"35839075\", \"37400440\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How RUFY1 and CLN3 coordinate or handoff CI-M6PR at different compartments is unknown\",\n        \"Whether RAB31-dependent trafficking is megakaryocyte-specific or general has not been tested\",\n        \"Direct physical contact between RUFY1 and CI-M6PR was not shown\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Establishing that CD-M6PR in mature late endosomes is required for EV71 uncoating, and its lumenal M6P-binding site mediates the interaction, revealed a direct role for M6PR in viral entry.\",\n      \"evidence\": \"siRNA knockdown of CD-M6PR; immunofluorescence co-localization with endosomal markers; EV71 uncoating assay\",\n      \"pmids\": [\"35929543\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct binding between EV71 capsid and M6PR was not demonstrated\",\n        \"Mechanism by which M6P-binding site engagement promotes uncoating is unclear\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrating that CD-M6PR lumenal domain directly binds influenza A HA2 ectodomain to promote viral–endosomal membrane fusion established M6PR as a host fusion cofactor for IAV, distinct from its hydrolase-sorting function.\",\n      \"evidence\": \"Co-immunoprecipitation of M6PR lumenal domain with HA2; siRNA knockdown with step-resolved viral replication assays; membrane fusion assay\",\n      \"pmids\": [\"38038885\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Structural details of M6PR–HA2 interaction remain unresolved\",\n        \"Whether the fusion-promoting activity is strain-specific was not tested\",\n        \"Single-lab finding not yet independently replicated\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showing that M6PR binds STING and routes it to endosomal degradation to suppress senescence-associated inflammatory signaling defined a new non-canonical cargo for M6PR with implications for cellular aging.\",\n      \"evidence\": \"Co-immunoprecipitation; cell thermal shift assay; siRNA knockdown with STING signaling and senescence readouts\",\n      \"pmids\": [\"40714423\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether STING is sorted via M6P-dependent or -independent mechanisms is unknown\",\n        \"Single-lab finding awaiting independent confirmation\",\n        \"In vivo relevance of M6PR-mediated STING degradation beyond the berberine context is untested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the structural basis of M6PR interactions with viral proteins and STING, the mechanism by which M6PR promotes endosomal membrane destabilization (for ASO escape and viral fusion), and how the multiple retrograde sorting machineries (retromer–SNX5, RUFY1–dynein, GCC88, GCC2, CLN3) are coordinated along the retrieval pathway.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structural model of M6PR in complex with any viral or non-hydrolase cargo\",\n        \"Mechanistic link between M6P-binding and membrane fusion promotion is completely undefined\",\n        \"Relative contributions and hierarchy of multiple tethering/motor systems in retrograde transport are unresolved\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 2, 7, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [1, 4, 7, 8]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [2, 6, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 1, 3, 5, 9]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1, 5, 6]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"GGA1\",\n      \"SNX5\",\n      \"RUFY1\",\n      \"GCC88\",\n      \"GCC2\",\n      \"CLN3\",\n      \"STING1\",\n      \"RAB31\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"M6PR (cation-dependent mannose-6-phosphate receptor, CD-M6PR) is a transmembrane cargo receptor that cycles between the trans-Golgi network (TGN) and late endosomes/multivesicular bodies to deliver mannose-6-phosphate-tagged lysosomal hydrolases to the endolysosomal compartment [PMID:1964415, PMID:15078902]. Anterograde exit from the TGN is driven by GGA proteins that recognize acidic-cluster-dileucine motifs in the CD-M6PR cytoplasmic tail, while retrograde retrieval from endosomes depends on TIP47, PACS-1, the retromer complex, and tethering factors such as GCC88 and RUFY1-dynein [PMID:11387475, PMID:9590177, PMID:9695949, PMID:15078902, PMID:30791178, PMID:36282215]. Beyond lysosomal biogenesis, the lumenal domain of CD-M6PR directly binds the HA2 ectodomain of influenza A hemagglutinin to promote viral membrane fusion in late endosomes, facilitates endosomal escape of phosphorothioate antisense oligonucleotides, and sorts STING into endosomes for degradation to suppress innate immune signaling and senescence [PMID:38038885, PMID:31840180, PMID:40714423]. The structurally distinct cation-independent M6PR (CI-M6PR/IGF2R) shares overlapping trafficking itineraries and acts as a tumour suppressor lost in hepatocellular carcinoma, and interacts with CLN3 (Batten disease protein) to coordinate Golgi-lysosome vesicular trafficking and autophagic lysosomal reformation [PMID:7493029, PMID:37400440].\",\n  \"teleology\": [\n    {\n      \"year\": 1987,\n      \"claim\": \"Establishing that the cation-independent M6PR is identical to the IGF-II receptor resolved a long-standing question about how IGF-II clearance and lysosomal enzyme sorting are linked through one bifunctional receptor.\",\n      \"evidence\": \"cDNA cloning and complete sequence analysis of the human IGF-II receptor\",\n      \"pmids\": [\"2957598\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Functional relationship between IGF-II binding and M6P-dependent trafficking not delineated\",\n        \"No equivalent identity question resolved for the smaller CD-M6PR\"\n      ]\n    },\n    {\n      \"year\": 1990,\n      \"claim\": \"Demonstrating that CD-M6PR concentrates in Golgi cisternae and cycles to the same late endosome/prelysosome population as CI-M6PR established the shared itinerary of both receptors and linked receptor trafficking to lysosomal biogenesis.\",\n      \"evidence\": \"Immunofluorescence and immunoperoxidase double-labeling; chloroquine/NH4Cl perturbation causing receptor accumulation in swollen multivesicular endosomes\",\n      \"pmids\": [\"1964415\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular signals governing CD-M6PR cycling were unknown\",\n        \"Functional redundancy or division of labor between CD-M6PR and CI-M6PR not resolved\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identifying frequent LOH and inactivating mutations at the M6P/IGF2R locus in hepatocellular carcinoma established CI-M6PR as a tumour suppressor, linking lysosomal receptor biology to cancer genetics.\",\n      \"evidence\": \"LOH analysis and sequencing of tumour DNA from hepatocellular carcinoma samples\",\n      \"pmids\": [\"7493029\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether tumour suppression operates through IGF-II clearance, TGF-β activation, or both was not resolved\",\n        \"No equivalent tumour suppressor role demonstrated for CD-M6PR\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Identification of TIP47 and PACS-1 as cytosolic adaptors that bind MPR cytoplasmic tails revealed the molecular machinery for endosome-to-TGN retrieval, answering how receptors are recycled after cargo delivery.\",\n      \"evidence\": \"In vitro transport reconstitution, binding assays with cytoplasmic-domain peptides, antisense knockdown, and mutagenesis identifying the Phe/Trp sorting signal\",\n      \"pmids\": [\"9590177\", \"9695949\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Relationship between TIP47 and retromer-mediated retrieval unclear\",\n        \"In vivo redundancy among retrieval adaptors not tested\"\n      ]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrating that GGA proteins bind acidic-cluster-dileucine motifs in MPR tails and mediate TGN exit resolved the anterograde sorting step, completing a minimal framework for the forward and return legs of MPR trafficking.\",\n      \"evidence\": \"VHS domain binding assays, dominant-negative GGA expression blocking TGN exit, co-localization and subcellular fractionation\",\n      \"pmids\": [\"11387475\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How GGA and AP-1 adaptors cooperate or compete at the TGN was not fully resolved\",\n        \"Cargo selectivity differences between GGA family members not determined\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Parallel studies established that the retromer complex (via VPS26) is essential for CI-M6PR endosome-to-TGN retrieval, and that CK2-site acidic residues in the CD-M6PR tail determine differential GGA1 versus AP-1 binding affinity, refining the adaptor hierarchy.\",\n      \"evidence\": \"VPS26 depletion with CD8-M6PR chimera trafficking and immunoEM localization; in vitro binding with systematic cytoplasmic-tail mutants and quantitative affinity measurements\",\n      \"pmids\": [\"15078902\", \"15044437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether retromer directly contacts CD-M6PR or acts primarily on CI-M6PR was ambiguous\",\n        \"Structural basis of GGA–tail interaction not resolved at atomic level in this period\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Tracking retromer-dependent CI-M6PR retrieval through recycling endosomes distinguished an intermediate compartment in the retrograde pathway and separated CI-M6PR retrieval from EHD1-dependent Shiga toxin transport.\",\n      \"evidence\": \"siRNA knockdown of retromer components and recycling-endosome ablation with CD8-CI-M6PR chimera trafficking\",\n      \"pmids\": [\"22540229\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether CD-M6PR uses the same recycling-endosome intermediate is untested\",\n        \"Single-lab study; independent replication of route segregation awaited\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identification of GCC88 as a TGN tethering factor required for CI-M6PR retrieval, and characterization of luminal-domain contributions to CI-M6PR trafficking, expanded the machinery beyond cytoplasmic-tail adaptors to include golgin-mediated vesicle capture and lumenal determinants.\",\n      \"evidence\": \"GCC88 CRISPR knockout with CI-M6PR localization and cathepsin-D processing assays; chimeric luminal-domain deletion constructs with immunofluorescence\",\n      \"pmids\": [\"30791178\", \"29988026\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct physical interaction between GCC88 and M6PR-containing vesicles not shown\",\n        \"Relative contributions of luminal versus cytoplasmic sorting signals not quantified\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovering that CD-M6PR facilitates phosphorothioate antisense oligonucleotide endosomal escape via GCC2-mediated recruitment to late endosomes revealed an unexpected non-canonical function for CD-M6PR in nucleic acid delivery.\",\n      \"evidence\": \"siRNA knockdown of M6PR and GCC2 in human cells and in vivo mouse liver; PS-ASO activity assays and immunofluorescence co-localization\",\n      \"pmids\": [\"31840180\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which M6PR promotes membrane destabilization for ASO escape is unknown\",\n        \"Whether this function involves M6P-ligand binding or a distinct interaction surface is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Multiple studies expanded the CD-M6PR and CI-M6PR interactome and trafficking regulators: RUFY1-dynein was shown to drive endosome-to-TGN retrieval, RAB31 (regulated by RUNX1) was required for early endosomal M6PR trafficking in megakaryocytes, and CD-M6PR was found to participate in EV71 uncoating in late endosomes.\",\n      \"evidence\": \"Co-IP and RUFY1 siRNA with CI-M6PR retrograde assay (JCB); CRISPR/siRNA knockdown of RAB31 in iPSC-megakaryocytes (Blood Adv); CD-M6PR siRNA with EV71 uncoating assay (Biol Open)\",\n      \"pmids\": [\"36282215\", \"35839075\", \"35929543\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"RUFY1's direct versus indirect interaction with M6PR not resolved\",\n        \"RAB31 effect on M6PR may be indirect via general endosomal morphology\",\n        \"EV71 uncoating mechanism and whether CD-M6PR contacts viral capsid are unclear\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"CLN3 (Batten disease protein) was identified as a physical interactor of CI-M6PR that coordinates Golgi-lysosome trafficking and autophagic lysosomal reformation, and CD-M6PR was shown to directly bind influenza A HA2 to promote viral membrane fusion, revealing disease-relevant receptor functions beyond hydrolase sorting.\",\n      \"evidence\": \"Co-IP/mass spectrometry with CLN3 gain- and loss-of-function plus lysosomal enzyme assays (Nat Commun); domain-mapped co-IP of M6PR lumenal domain with HA2 ectodomain, siRNA knockdown with rescue and membrane fusion assay (Sci China Life Sci)\",\n      \"pmids\": [\"37400440\", \"38038885\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of M6PR lumenal domain–HA2 interaction unknown\",\n        \"Whether CLN3 interaction is M6P-dependent or mediated by a distinct surface not determined\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"M6PR was found to bind STING and sort it into endosomes for degradation, directly linking M6PR to innate immune regulation and cellular senescence control.\",\n      \"evidence\": \"Co-immunoprecipitation, M6PR knockdown preventing berberine-mediated STING degradation, cell thermal shift assay, doxorubicin-induced senescence model\",\n      \"pmids\": [\"40714423\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether M6PR-STING interaction is direct or adaptor-mediated needs reconstitution\",\n        \"Physiological relevance outside pharmacological (berberine) context not established\",\n        \"Single-lab finding awaiting independent confirmation\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the structural basis of CD-M6PR lumenal domain interactions with non-canonical cargoes (HA2, STING, PS-ASOs), the degree of functional redundancy between CD-M6PR and CI-M6PR in vivo, and the mechanism by which M6PR promotes endosomal membrane destabilization for cargo escape.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No atomic-resolution structure of full-length CD-M6PR in complex with any cargo\",\n        \"In vivo genetic models (CD-M6PR knockout) not extensively characterized for non-canonical functions\",\n        \"Crosstalk between M6PR-mediated STING degradation and lysosomal hydrolase delivery pathways untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [0, 3, 5, 7, 11, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [0, 5, 7, 10]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [0, 6, 8, 12, 14]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [14, 16]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [8, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 3, 4, 5, 6, 8, 10, 13]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5, 7, 16]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [6, 10, 16]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [17, 18]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"TIP47\",\n      \"PACS1\",\n      \"GGA1\",\n      \"VPS26\",\n      \"RUFY1\",\n      \"CLN3\",\n      \"STING\",\n      \"GCC88\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}