{"gene":"C8G","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":1987,"finding":"C8G (gamma subunit) is encoded by a separate gene distinct from C8A and C8B, producing a distinct ~1.0 kb mRNA in liver, and the three subunits undergo both covalent (disulfide) and noncovalent association to form the mature C8 oligomer.","method":"cDNA cloning, Northern blot analysis of poly(A) RNA from baboon and rat liver using subunit-specific probes","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — direct molecular characterization with multiple orthogonal methods, foundational study","pmids":["3676249"],"is_preprint":false},{"year":1989,"finding":"C8G maps to chromosome 9q (distinct from C8A and C8B which are physically linked on chromosome 1p), and C8A and C8B are oriented 5' alpha–beta 3' within 2.5 kb of each other; genetic linkage of alpha-gamma to beta is determined solely by the alpha subunit.","method":"Somatic cell hybrid panel analysis with subunit-specific cDNA probes; genomic DNA restriction digest probing","journal":"Genomics","confidence":"High","confidence_rationale":"Tier 1 — direct chromosomal assignment with multiple probes and hybrid panel","pmids":["2613233"],"is_preprint":false},{"year":1991,"finding":"C8G is a lipocalin family member; Cys40 of C8G forms the disulfide bond to Cys164 of C8A; C8G binds retinol and retinoic acid (in the presence of 2 M NaCl), and molecular modeling predicts a hydrophobic binding pocket formed by eight antiparallel beta-strands conserved with beta-lactoglobulin and retinol-binding protein.","method":"Disulfide-bond mapping by peptide sequencing; retinol/retinoic acid binding assay; molecular modeling based on beta-lactoglobulin crystal structure","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2/3 — direct binding assay and disulfide mapping, but ligand binding required non-physiological salt and no crystal structure","pmids":["1707134"],"is_preprint":false},{"year":1994,"finding":"The human C8G gene spans ~1.8 kb and contains seven exons; exon boundaries correspond closely to those of other lipocalin genes, confirming ancestral relationship to the lipocalin family. Putative regulatory elements include SP1 binding sites, glucocorticoid response elements, and SV40 enhancer core sequences upstream of the transcription initiation site.","method":"Genomic cloning, S1 nuclease mapping, anchored PCR for transcription start site; comparison of exon structure to lipocalin genes","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — direct genomic structural characterization with multiple methods","pmids":["8172891"],"is_preprint":false},{"year":1995,"finding":"Expression of C8A, C8B, and C8G in the human hepatoma cell line HepG2 is upregulated by IL-6, characterizing C8 as a positive acute-phase protein in vitro; additionally, post-transcriptional regulation of C8B (but not C8A or C8G) was detected by comparing relative mRNA and protein levels.","method":"Immunoprecipitation and SDS-PAGE of biosynthetically labeled subunits from IL-6-treated HepG2 cells; transcript level comparison","journal":"Experimental and clinical immunogenetics","confidence":"Medium","confidence_rationale":"Tier 2 — direct protein biosynthesis assay with defined cytokine treatment, single lab","pmids":["7710765"],"is_preprint":false},{"year":2002,"finding":"C8G is not required for complement-mediated lysis of erythrocytes or killing of Gram-negative bacteria; C8A and C8B are the essential subunits for MAC bactericidal activity, while C8G enhances but is not essential for bactericidal function.","method":"Reconstitution of MAC activity using purified individual subunits (C8 alpha-gamma, C8 alpha, C8 beta, C8 gamma) in erythrocyte lysis and bacterial killing assays","journal":"Molecular immunology","confidence":"High","confidence_rationale":"Tier 1 — direct reconstitution with purified components and defined functional readouts","pmids":["12413696"],"is_preprint":false},{"year":2002,"finding":"The crystal structure of recombinant C8G at 1.2 Å resolution reveals a typical lipocalin fold (calyx) with a distinct, deep hydrophobic binding pocket larger than that of NGAL, with Cys40 located in a partially disordered loop (loop 1, residues 38–52) near the calyx opening; this architecture is consistent with ligand-binding function, and the loop position suggests access to the calyx may be regulated by conformational changes in C8A.","method":"X-ray crystallography of recombinant C8G at 1.2 Å resolution","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure with structural comparison to other lipocalins","pmids":["12033936"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of the C8 alpha MACPF domain disulfide-linked to C8G (alphaMACPF-gamma) at 2.15 Å reveals two regions of interaction: (1) a 19-residue indel in C8A fills the entrance to the C8G ligand-binding calyx, blocking ligand access; (2) a hydrophobic pocket in C8G makes contact with the side of the C8G beta-barrel, inducing conformational changes in alphaMACPF likely important for C8 function.","method":"X-ray crystallography of alphaMACPF-gamma co-crystal at 2.15 Å resolution; structural comparison","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — high-resolution co-crystal structure defining intermolecular contacts between C8A and C8G","pmids":["18440555"],"is_preprint":false},{"year":2021,"finding":"C8G acts as a neuroinflammation inhibitor: C8G is expressed in brain astrocytes, and recombinant C8G protein inhibits glial hyperactivation and neuroinflammation, while shRNA-mediated knockdown exacerbates neuroinflammation and cognitive decline in mouse models. C8G interacts with sphingosine-1-phosphate receptor 2 (S1PR2) and antagonizes the pro-inflammatory action of S1P in microglia.","method":"Recombinant protein administration; shRNA knockdown; co-immunoprecipitation identifying S1PR2 as C8G binding partner; in vivo acute and chronic Alzheimer's disease mouse models with behavioral readouts","journal":"Brain","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (recombinant protein gain-of-function, shRNA loss-of-function, Co-IP, in vivo models)","pmids":["33382892"],"is_preprint":false},{"year":2021,"finding":"C8G is localized to perivascular astrocytes and protects blood-brain barrier (BBB) integrity: intracerebroventricular recombinant C8G preserved BBB integrity in LPS-induced neuroinflammation, while C8G knockdown enhanced BBB permeability and neutrophil infiltration. C8G antagonizes S1PR2 (expressed on endothelial cells) to inhibit inflammatory activation of endothelial cells, maintaining endothelial integrity in an in vitro BBB model.","method":"Immunofluorescence localization; intracerebroventricular administration of recombinant C8G; shRNA knockdown; pharmacological S1PR2 agonist/antagonist; in vitro BBB permeability assay","journal":"Frontiers in physiology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods with defined functional readouts, builds on prior S1PR2 interaction finding","pmids":["34149451"],"is_preprint":false},{"year":2024,"finding":"Hepatic expression of C8G is positively regulated by the nuclear receptor HNF4α: C8G mRNA and protein are markedly decreased in liver-specific Hnf4a-null mice; forced expression of HNF4α in immortalized human hepatocytes induces C8G expression; and transactivation assays show C8G induction is dependent on HNF4α binding sites, identifying C8G as a direct transcriptional target of HNF4α.","method":"Liver-specific Hnf4a-knockout mouse models; hepatoma cell line HNF4α knockdown; forced HNF4α expression in hepatocytes; transactivation assay with HNF4α binding site mutations","journal":"In vitro cellular & developmental biology. Animal","confidence":"High","confidence_rationale":"Tier 1/2 — loss-of-function in vivo and in vitro with transactivation assay and binding site validation","pmids":["39285151"],"is_preprint":false}],"current_model":"C8G is a lipocalin-family subunit of complement C8 encoded on chromosome 9q34.3, disulfide-bonded via Cys40 to Cys164 of C8A to form the alpha-gamma dimer; its 1.2 Å crystal structure reveals a deep hydrophobic calyx whose entrance is occluded by a C8A indel in the co-complex structure, suggesting ligand access is regulated by C8A conformational changes during MAC assembly; while C8G is dispensable for MAC-mediated lysis and bacterial killing (with C8A and C8B being the essential subunits), it enhances bactericidal activity and, independently of the MAC, functions as a neuroinflammation inhibitor expressed in perivascular astrocytes that antagonizes S1PR2 to suppress S1P-driven microglial inflammation and protect blood-brain barrier integrity; hepatic C8G expression is directly transcriptionally regulated by HNF4α."},"narrative":{"teleology":[{"year":1987,"claim":"Establishing that C8G is the product of a distinct gene resolved how the three C8 subunits are independently synthesized and then assembled via covalent and noncovalent interactions in the liver.","evidence":"cDNA cloning and Northern blot of baboon and rat liver poly(A) RNA with subunit-specific probes","pmids":["3676249"],"confidence":"High","gaps":["Mechanism of intracellular assembly of C8 trimer not defined","Signals directing C8G secretion not characterized"]},{"year":1989,"claim":"Mapping C8G to chromosome 9q, separate from the linked C8A/C8B locus on 1p, demonstrated that alpha-gamma disulfide linkage occurs post-translationally from independently regulated loci.","evidence":"Somatic cell hybrid panel with subunit-specific cDNA probes and genomic restriction analysis","pmids":["2613233"],"confidence":"High","gaps":["Regulatory consequences of separate chromosomal locations not explored"]},{"year":1991,"claim":"Identifying C8G as a lipocalin family member and mapping the Cys40 disulfide to C8A Cys164 defined the structural basis for alpha-gamma dimer formation and raised the possibility of small hydrophobic ligand binding.","evidence":"Disulfide-bond mapping by peptide sequencing; retinol/retinoic acid binding assay; molecular modeling","pmids":["1707134"],"confidence":"Medium","gaps":["Ligand binding required non-physiological 2 M NaCl, casting doubt on in vivo relevance","No crystal structure available at the time","Physiological ligand identity unresolved"]},{"year":1994,"claim":"Characterization of the C8G gene structure (7 exons, ~1.8 kb) and identification of upstream SP1, glucocorticoid response elements, and SV40 enhancer core motifs provided a framework for understanding transcriptional regulation.","evidence":"Genomic cloning, S1 nuclease mapping, anchored PCR for transcription start site","pmids":["8172891"],"confidence":"High","gaps":["Functional validation of putative regulatory elements not performed at this stage"]},{"year":1995,"claim":"Demonstrating IL-6-mediated upregulation of C8G in HepG2 cells established C8 as a positive acute-phase reactant, linking complement MAC assembly to inflammatory signaling.","evidence":"Immunoprecipitation and SDS-PAGE of biosynthetically labeled subunits from IL-6-treated HepG2 cells","pmids":["7710765"],"confidence":"Medium","gaps":["In vivo acute-phase regulation not confirmed","Mechanism of IL-6 transcriptional activation of C8G not dissected"]},{"year":2002,"claim":"Reconstitution experiments showed that C8G is dispensable for MAC hemolytic and bactericidal activity but enhances bacterial killing, redefining C8G's role from essential structural subunit to modulatory enhancer of MAC function.","evidence":"Purified subunit reconstitution in erythrocyte lysis and bacterial killing assays","pmids":["12413696"],"confidence":"High","gaps":["Molecular mechanism by which C8G enhances bactericidal activity not determined","Whether C8G modulates MAC pore stoichiometry or kinetics unknown"]},{"year":2002,"claim":"The 1.2 Å crystal structure of free C8G revealed a deep hydrophobic calyx larger than NGAL's and a partially disordered loop 1 near the calyx entrance, providing the first atomic-resolution view of a potential ligand-binding site.","evidence":"X-ray crystallography of recombinant C8G at 1.2 Å resolution","pmids":["12033936"],"confidence":"High","gaps":["No ligand co-crystal obtained","Physiological ligand identity still unknown"]},{"year":2008,"claim":"The 2.15 Å co-crystal structure of C8A-MACPF with C8G showed that a 19-residue C8A indel occludes the C8G calyx entrance, explaining why ligand binding may be regulated by conformational changes during MAC assembly.","evidence":"X-ray crystallography of alphaMACPF-gamma complex at 2.15 Å resolution","pmids":["18440555"],"confidence":"High","gaps":["Whether calyx opening occurs during MAC insertion into target membranes is untested","Identity of any physiological calyx ligand remains unknown"]},{"year":2021,"claim":"Discovery that astrocyte-expressed C8G antagonizes S1PR2 to suppress microglial neuroinflammation and protect blood–brain barrier integrity established a complement-independent neuroimmune function for C8G.","evidence":"Recombinant C8G gain-of-function and shRNA loss-of-function in mouse neuroinflammation models; Co-IP identifying S1PR2 interaction; in vitro BBB permeability assays","pmids":["33382892","34149451"],"confidence":"High","gaps":["Binding interface between C8G and S1PR2 not structurally resolved","Source of brain C8G (local astrocyte production vs. plasma-derived) not fully distinguished","Relevance to human neurological disease not demonstrated"]},{"year":2024,"claim":"Identification of HNF4α as a direct transcriptional activator of hepatic C8G expression connected C8G regulation to a master hepatocyte transcription factor, explaining liver-predominant expression.","evidence":"Liver-specific Hnf4a-knockout mice; forced HNF4α expression in hepatocytes; transactivation assay with binding site mutations","pmids":["39285151"],"confidence":"High","gaps":["Whether HNF4α also controls C8G expression in astrocytes unknown","Interplay with IL-6-mediated acute-phase regulation not integrated"]},{"year":null,"claim":"The physiological ligand of the C8G lipocalin calyx remains unidentified, and whether calyx ligand binding occurs in vivo—particularly during MAC pore assembly when the C8A indel might disengage—is the central unresolved mechanistic question.","evidence":"","pmids":[],"confidence":"High","gaps":["No endogenous ligand identified for the C8G calyx","Structural dynamics of calyx opening during MAC assembly untested","Contribution of C8G to MAC pore architecture at cryo-EM resolution not resolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,8,9]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[2,6]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,5]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[8,9]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,5,7]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[8,9]}],"complexes":["Complement C8 (C8A-C8G-C8B)","Membrane attack complex (MAC)"],"partners":["C8A","C8B","S1PR2"],"other_free_text":[]},"mechanistic_narrative":"C8G encodes the gamma chain of complement component C8, a lipocalin-family protein that forms a disulfide-linked heterodimer with C8A via Cys40–Cys164 and, together with C8B, assembles into the C8 heterotrimer required for membrane attack complex (MAC) formation [PMID:3676249, PMID:1707134]. High-resolution crystal structures reveal a deep hydrophobic calyx characteristic of lipocalins, whose entrance is physically occluded by a C8A indel in the assembled alpha-gamma complex, indicating that ligand access is conformationally regulated during MAC assembly [PMID:12033936, PMID:18440555]. Although C8G is dispensable for MAC-mediated erythrocyte lysis and bacterial killing, it enhances bactericidal efficiency [PMID:12413696]. Independent of the complement system, C8G is expressed in perivascular astrocytes where it antagonizes sphingosine-1-phosphate receptor 2 (S1PR2), suppressing S1P-driven microglial neuroinflammation and preserving blood–brain barrier integrity [PMID:33382892, PMID:34149451]."},"prefetch_data":{"uniprot":{"accession":"P07360","full_name":"Complement component C8 gamma chain","aliases":[],"length_aa":202,"mass_kda":22.3,"function":"Component of the membrane attack complex (MAC), a multiprotein complex activated by the complement cascade, which inserts into a target cell membrane and forms a pore, leading to target cell membrane rupture and cell lysis (PubMed:26841837, PubMed:27052168, PubMed:30552328). The MAC is initiated by proteolytic cleavage of C5 into complement C5b in response to the classical, alternative, lectin and GZMK complement pathways (PubMed:30552328, PubMed:39914456, PubMed:39814882). The complement pathways consist in a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:30552328). C8G, together with C8A and C8B, inserts into the target membrane, but does not form pores by itself (PubMed:30552328). During MAC assembly, associates with C5b, C6 and C7 to form the C5b8 intermediate complex that inserts into the target membrane and traverses the bilayer increasing membrane rigidity (PubMed:30552328, PubMed:6833260)","subcellular_location":"Secreted; Target cell membrane","url":"https://www.uniprot.org/uniprotkb/P07360/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C8G","classification":"Not Classified","n_dependent_lines":14,"n_total_lines":1208,"dependency_fraction":0.011589403973509934},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C8G","total_profiled":1310},"omim":[{"mim_id":"613790","title":"COMPLEMENT COMPONENT 8 DEFICIENCY, TYPE I; C8D1","url":"https://www.omim.org/entry/613790"},{"mim_id":"613789","title":"COMPLEMENT COMPONENT 8 DEFICIENCY, TYPE II; C8D2","url":"https://www.omim.org/entry/613789"},{"mim_id":"120960","title":"COMPLEMENT COMPONENT 8, BETA SUBUNIT; C8B","url":"https://www.omim.org/entry/120960"},{"mim_id":"120950","title":"COMPLEMENT COMPONENT 8, ALPHA SUBUNIT; C8A","url":"https://www.omim.org/entry/120950"},{"mim_id":"120930","title":"COMPLEMENT COMPONENT 8, GAMMA SUBUNIT; C8G","url":"https://www.omim.org/entry/120930"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"liver","ntpm":522.4}],"url":"https://www.proteinatlas.org/search/C8G"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P07360","domains":[{"cath_id":"2.40.128.20","chopping":"36-199","consensus_level":"high","plddt":95.5774,"start":36,"end":199}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P07360","model_url":"https://alphafold.ebi.ac.uk/files/AF-P07360-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P07360-F1-predicted_aligned_error_v6.png","plddt_mean":89.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C8G","jax_strain_url":"https://www.jax.org/strain/search?query=C8G"},"sequence":{"accession":"P07360","fasta_url":"https://rest.uniprot.org/uniprotkb/P07360.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P07360/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P07360"}},"corpus_meta":[{"pmid":"7829063","id":"PMC_7829063","title":"Comparative mapping of lipocalin genes in human and mouse: the four genes for complement C8 gamma chain, prostaglandin-D-synthase, oncogene-24p3, and progestagen-associated endometrial protein map to HSA9 and MMU2.","date":"1994","source":"Genomics","url":"https://pubmed.ncbi.nlm.nih.gov/7829063","citation_count":54,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"20167877","id":"PMC_20167877","title":"Serum proteomic analysis of diet-induced steatohepatitis and metabolic syndrome in the Ossabaw miniature swine.","date":"2010","source":"American journal of physiology. 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and C8B) are physically linked on chromosome 1p. The alpha-gamma subunit exists as a disulfide-linked dimer, and the beta chain is non-covalently associated with it.\",\n      \"method\": \"Somatic cell hybrid analysis with cDNA probes specific to each subunit; Southern blot analysis of genomic DNA digests\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct chromosomal assignment with multiple cDNA probes and somatic cell hybrids, establishing distinct gene loci and subunit arrangement\",\n      \"pmids\": [\"2613233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"C8G is a member of the lipocalin superfamily, a family of carrier proteins for small hydrophobic ligands. The C8G gene maps to HSA9q34 in humans and to proximal MMU2 in mouse, clustering with other lipocalin genes.\",\n      \"method\": \"Linkage analysis in interspecific backcross progeny; in situ hybridization\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct chromosomal mapping by linkage and in situ hybridization, supported by sequence-based lipocalin family classification\",\n      \"pmids\": [\"7829063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IL-6 induces expression of C8 alpha-gamma and beta subunits in HepG2 hepatoma cells, characterizing C8 (including the gamma chain encoded by C8G) as a positive acute-phase protein. Evidence for post-transcriptional regulation of the C8 beta subunit was also obtained.\",\n      \"method\": \"Immunoprecipitation and SDS-PAGE of biosynthetically labeled C8 subunits from HepG2 cells treated with IL-6\",\n      \"journal\": \"Experimental and clinical immunogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct in vitro protein labeling and immunoprecipitation in a cell model, but single lab study\",\n      \"pmids\": [\"7710765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The human C8G gene is located at chromosome 9q34.3, in a chromosomal region containing at least four other lipocalin genes. The gamma chain's functional role as a lipocalin (binding small hydrophobic ligands) is noted, though its specific ligand remained undefined.\",\n      \"method\": \"PCR-based genotyping with CEPH reference families; multipoint linkage analysis placing C8G distal to D9S207\",\n      \"journal\": \"Annals of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct genetic mapping confirming chromosomal location, single study\",\n      \"pmids\": [\"8865989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"C8G inhibits neuroinflammation by interacting with sphingosine-1-phosphate receptor 2 (S1PR2), antagonizing the pro-inflammatory action of S1P in microglia. C8G is mainly localized to brain astrocytes. Recombinant C8G protein inhibited glial hyperactivation and cognitive decline in mouse models of Alzheimer's disease, while shRNA-mediated knockdown exacerbated neuroinflammation.\",\n      \"method\": \"Co-immunoprecipitation to identify S1PR2 as C8G binding partner; recombinant protein administration and shRNA knockdown in acute and chronic mouse models; behavioral testing; immunofluorescence localization\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding partner identification combined with loss-of-function (shRNA) and gain-of-function (recombinant protein) experiments with defined phenotypic readouts in multiple models\",\n      \"pmids\": [\"33382892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"C8G is localized to perivascular astrocytes in the brain, while its receptor S1PR2 is expressed on endothelial cells. Astrocytic C8G protects blood-brain barrier integrity by antagonizing S1PR2 on endothelial cells. In an LPS-induced neuroinflammation model, intracerebroventricular recombinant C8G preserved BBB integrity, while shRNA-mediated knockdown enhanced BBB permeability and neutrophil infiltration.\",\n      \"method\": \"Immunofluorescence localization; intracerebroventricular injection of recombinant protein; shRNA knockdown; pharmacological agonists/antagonists of S1PR2; in vitro BBB model with transendothelial electrical resistance measurements\",\n      \"journal\": \"Frontiers in physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with functional consequence, multiple orthogonal methods including KD, recombinant protein, and pharmacological tools confirming S1PR2-mediated mechanism\",\n      \"pmids\": [\"34149451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Hepatic expression of C8G is positively regulated by the nuclear receptor HNF4α. C8G expression was markedly decreased in liver-specific Hnf4a-null mice and in human hepatoma cell lines with suppressed HNF4α, and was induced by forced HNF4α expression in immortalized hepatocytes. HNF4α binding sites in the C8g promoter are required for its transactivation, identifying C8G as a direct HNF4α target gene.\",\n      \"method\": \"Liver-specific Hnf4a knockout mouse models; siRNA-mediated HNF4α knockdown in human hepatoma cells; forced HNF4α expression in immortalized hepatocytes; promoter transactivation assays with HNF4α binding site mutations; RT-PCR and Western blot\",\n      \"journal\": \"In vitro cellular & developmental biology. Animal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal approaches (KO mouse, KD and overexpression in cell lines, promoter mutagenesis) establishing HNF4α as direct transcriptional regulator of C8G\",\n      \"pmids\": [\"39285151\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C8G encodes the gamma chain of complement component C8, a lipocalin-family protein that is part of the disulfide-linked C8 alpha-gamma dimer (non-covalently associated with the C8 beta chain); it is produced in the liver under transcriptional control of HNF4α, functions as an acute-phase protein induced by IL-6, and in the brain is expressed by astrocytes where it inhibits neuroinflammation and protects blood-brain barrier integrity by binding to and antagonizing sphingosine-1-phosphate receptor 2 (S1PR2) on microglia and endothelial cells.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1987,\n      \"finding\": \"C8G (gamma subunit) is encoded by a separate gene distinct from C8A and C8B, producing a distinct ~1.0 kb mRNA in liver, and the three subunits undergo both covalent (disulfide) and noncovalent association to form the mature C8 oligomer.\",\n      \"method\": \"cDNA cloning, Northern blot analysis of poly(A) RNA from baboon and rat liver using subunit-specific probes\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct molecular characterization with multiple orthogonal methods, foundational study\",\n      \"pmids\": [\"3676249\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1989,\n      \"finding\": \"C8G maps to chromosome 9q (distinct from C8A and C8B which are physically linked on chromosome 1p), and C8A and C8B are oriented 5' alpha–beta 3' within 2.5 kb of each other; genetic linkage of alpha-gamma to beta is determined solely by the alpha subunit.\",\n      \"method\": \"Somatic cell hybrid panel analysis with subunit-specific cDNA probes; genomic DNA restriction digest probing\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct chromosomal assignment with multiple probes and hybrid panel\",\n      \"pmids\": [\"2613233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"C8G is a lipocalin family member; Cys40 of C8G forms the disulfide bond to Cys164 of C8A; C8G binds retinol and retinoic acid (in the presence of 2 M NaCl), and molecular modeling predicts a hydrophobic binding pocket formed by eight antiparallel beta-strands conserved with beta-lactoglobulin and retinol-binding protein.\",\n      \"method\": \"Disulfide-bond mapping by peptide sequencing; retinol/retinoic acid binding assay; molecular modeling based on beta-lactoglobulin crystal structure\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — direct binding assay and disulfide mapping, but ligand binding required non-physiological salt and no crystal structure\",\n      \"pmids\": [\"1707134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The human C8G gene spans ~1.8 kb and contains seven exons; exon boundaries correspond closely to those of other lipocalin genes, confirming ancestral relationship to the lipocalin family. Putative regulatory elements include SP1 binding sites, glucocorticoid response elements, and SV40 enhancer core sequences upstream of the transcription initiation site.\",\n      \"method\": \"Genomic cloning, S1 nuclease mapping, anchored PCR for transcription start site; comparison of exon structure to lipocalin genes\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct genomic structural characterization with multiple methods\",\n      \"pmids\": [\"8172891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Expression of C8A, C8B, and C8G in the human hepatoma cell line HepG2 is upregulated by IL-6, characterizing C8 as a positive acute-phase protein in vitro; additionally, post-transcriptional regulation of C8B (but not C8A or C8G) was detected by comparing relative mRNA and protein levels.\",\n      \"method\": \"Immunoprecipitation and SDS-PAGE of biosynthetically labeled subunits from IL-6-treated HepG2 cells; transcript level comparison\",\n      \"journal\": \"Experimental and clinical immunogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct protein biosynthesis assay with defined cytokine treatment, single lab\",\n      \"pmids\": [\"7710765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"C8G is not required for complement-mediated lysis of erythrocytes or killing of Gram-negative bacteria; C8A and C8B are the essential subunits for MAC bactericidal activity, while C8G enhances but is not essential for bactericidal function.\",\n      \"method\": \"Reconstitution of MAC activity using purified individual subunits (C8 alpha-gamma, C8 alpha, C8 beta, C8 gamma) in erythrocyte lysis and bacterial killing assays\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct reconstitution with purified components and defined functional readouts\",\n      \"pmids\": [\"12413696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The crystal structure of recombinant C8G at 1.2 Å resolution reveals a typical lipocalin fold (calyx) with a distinct, deep hydrophobic binding pocket larger than that of NGAL, with Cys40 located in a partially disordered loop (loop 1, residues 38–52) near the calyx opening; this architecture is consistent with ligand-binding function, and the loop position suggests access to the calyx may be regulated by conformational changes in C8A.\",\n      \"method\": \"X-ray crystallography of recombinant C8G at 1.2 Å resolution\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure with structural comparison to other lipocalins\",\n      \"pmids\": [\"12033936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of the C8 alpha MACPF domain disulfide-linked to C8G (alphaMACPF-gamma) at 2.15 Å reveals two regions of interaction: (1) a 19-residue indel in C8A fills the entrance to the C8G ligand-binding calyx, blocking ligand access; (2) a hydrophobic pocket in C8G makes contact with the side of the C8G beta-barrel, inducing conformational changes in alphaMACPF likely important for C8 function.\",\n      \"method\": \"X-ray crystallography of alphaMACPF-gamma co-crystal at 2.15 Å resolution; structural comparison\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution co-crystal structure defining intermolecular contacts between C8A and C8G\",\n      \"pmids\": [\"18440555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"C8G acts as a neuroinflammation inhibitor: C8G is expressed in brain astrocytes, and recombinant C8G protein inhibits glial hyperactivation and neuroinflammation, while shRNA-mediated knockdown exacerbates neuroinflammation and cognitive decline in mouse models. C8G interacts with sphingosine-1-phosphate receptor 2 (S1PR2) and antagonizes the pro-inflammatory action of S1P in microglia.\",\n      \"method\": \"Recombinant protein administration; shRNA knockdown; co-immunoprecipitation identifying S1PR2 as C8G binding partner; in vivo acute and chronic Alzheimer's disease mouse models with behavioral readouts\",\n      \"journal\": \"Brain\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (recombinant protein gain-of-function, shRNA loss-of-function, Co-IP, in vivo models)\",\n      \"pmids\": [\"33382892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"C8G is localized to perivascular astrocytes and protects blood-brain barrier (BBB) integrity: intracerebroventricular recombinant C8G preserved BBB integrity in LPS-induced neuroinflammation, while C8G knockdown enhanced BBB permeability and neutrophil infiltration. C8G antagonizes S1PR2 (expressed on endothelial cells) to inhibit inflammatory activation of endothelial cells, maintaining endothelial integrity in an in vitro BBB model.\",\n      \"method\": \"Immunofluorescence localization; intracerebroventricular administration of recombinant C8G; shRNA knockdown; pharmacological S1PR2 agonist/antagonist; in vitro BBB permeability assay\",\n      \"journal\": \"Frontiers in physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods with defined functional readouts, builds on prior S1PR2 interaction finding\",\n      \"pmids\": [\"34149451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Hepatic expression of C8G is positively regulated by the nuclear receptor HNF4α: C8G mRNA and protein are markedly decreased in liver-specific Hnf4a-null mice; forced expression of HNF4α in immortalized human hepatocytes induces C8G expression; and transactivation assays show C8G induction is dependent on HNF4α binding sites, identifying C8G as a direct transcriptional target of HNF4α.\",\n      \"method\": \"Liver-specific Hnf4a-knockout mouse models; hepatoma cell line HNF4α knockdown; forced HNF4α expression in hepatocytes; transactivation assay with HNF4α binding site mutations\",\n      \"journal\": \"In vitro cellular & developmental biology. Animal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — loss-of-function in vivo and in vitro with transactivation assay and binding site validation\",\n      \"pmids\": [\"39285151\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"C8G is a lipocalin-family subunit of complement C8 encoded on chromosome 9q34.3, disulfide-bonded via Cys40 to Cys164 of C8A to form the alpha-gamma dimer; its 1.2 Å crystal structure reveals a deep hydrophobic calyx whose entrance is occluded by a C8A indel in the co-complex structure, suggesting ligand access is regulated by C8A conformational changes during MAC assembly; while C8G is dispensable for MAC-mediated lysis and bacterial killing (with C8A and C8B being the essential subunits), it enhances bactericidal activity and, independently of the MAC, functions as a neuroinflammation inhibitor expressed in perivascular astrocytes that antagonizes S1PR2 to suppress S1P-driven microglial inflammation and protect blood-brain barrier integrity; hepatic C8G expression is directly transcriptionally regulated by HNF4α.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C8G encodes the gamma chain of complement component C8, a lipocalin-family protein that forms a disulfide-linked dimer with the C8 alpha chain and associates non-covalently with the C8 beta chain to constitute the functional C8 complex [PMID:2613233, PMID:7829063]. In the liver, C8G is a direct transcriptional target of HNF4α and functions as a positive acute-phase protein induced by IL-6 [PMID:39285151, PMID:7710765]. In the brain, C8G is expressed by perivascular astrocytes and inhibits neuroinflammation and protects blood–brain barrier integrity by binding and antagonizing sphingosine-1-phosphate receptor 2 (S1PR2) on microglia and endothelial cells; recombinant C8G suppresses glial hyperactivation and cognitive decline in Alzheimer's disease mouse models, while its knockdown exacerbates inflammation and barrier permeability [PMID:33382892, PMID:34149451].\",\n  \"teleology\": [\n    {\n      \"year\": 1989,\n      \"claim\": \"Establishing that C8G is encoded by a separate gene on chromosome 9q—distinct from the chromosome 1–linked C8A/C8B locus—resolved how the three-subunit C8 complex is assembled from independently encoded polypeptides.\",\n      \"evidence\": \"Somatic cell hybrid analysis with subunit-specific cDNA probes and Southern blot of genomic DNA\",\n      \"pmids\": [\"2613233\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No functional role for the gamma chain beyond structural incorporation into C8 was defined\",\n        \"Mechanism of disulfide-linked alpha-gamma dimerization not characterized\"\n      ]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Classification of C8G as a lipocalin-superfamily member, mapping to a lipocalin gene cluster at 9q34, raised the hypothesis that the gamma chain functions as a carrier for small hydrophobic ligands rather than serving a purely structural role in MAC formation.\",\n      \"evidence\": \"Linkage analysis in interspecific backcross progeny and fluorescence in situ hybridization\",\n      \"pmids\": [\"7829063\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The specific hydrophobic ligand of C8G remained unidentified\",\n        \"Whether lipocalin function is relevant inside or outside the complement pathway was unknown\"\n      ]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Demonstration that IL-6 induces C8 alpha-gamma expression in HepG2 cells established C8G as a positive acute-phase reactant, linking it to innate immune signaling beyond the complement cascade.\",\n      \"evidence\": \"Immunoprecipitation and SDS-PAGE of biosynthetically labeled C8 subunits from IL-6-treated HepG2 hepatoma cells\",\n      \"pmids\": [\"7710765\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab HepG2 study; not confirmed in primary hepatocytes or in vivo\",\n        \"Signaling pathway downstream of IL-6 responsible for C8G induction was not dissected\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of S1PR2 as a direct binding partner of C8G on microglia and endothelial cells, and demonstration that astrocyte-derived C8G antagonizes S1PR2-mediated neuroinflammation and blood–brain barrier disruption, revealed a complement-independent neuroprotective function for C8G.\",\n      \"evidence\": \"Co-immunoprecipitation for binding-partner identification; recombinant C8G administration and shRNA knockdown in LPS and Alzheimer's disease mouse models; in vitro BBB transendothelial electrical resistance assays; immunofluorescence localization to perivascular astrocytes\",\n      \"pmids\": [\"33382892\", \"34149451\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether C8G functions as an orthosteric or allosteric S1PR2 antagonist is undefined\",\n        \"Structural basis of C8G–S1PR2 interaction not resolved\",\n        \"Relative contribution of free C8G versus C8 complex-bound gamma chain in the brain not determined\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identification of HNF4α as a direct transcriptional activator of C8G—via promoter binding sites required for transactivation—defined the upstream regulatory axis controlling hepatic C8G expression.\",\n      \"evidence\": \"Liver-specific Hnf4a knockout mice; siRNA knockdown and forced expression of HNF4α in hepatoma and immortalized hepatocyte lines; promoter mutagenesis and transactivation assays\",\n      \"pmids\": [\"39285151\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether HNF4α cooperates with IL-6-responsive transcription factors (e.g., STAT3) at the C8G promoter is untested\",\n        \"Transcriptional regulation of C8G in astrocytes has not been characterized\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The endogenous lipocalin ligand of C8G, whether it contributes to MAC pore formation beyond a structural role, and the transcriptional control of C8G in astrocytes remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No endogenous small hydrophobic ligand has been identified for the lipocalin pocket of C8G\",\n        \"Functional contribution of C8G to MAC assembly versus non-complement roles is unpartitioned\",\n        \"Astrocyte-specific transcriptional regulation of C8G is unknown\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 5]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 2, 4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 2, 4, 5]}\n    ],\n    \"complexes\": [\n      \"complement C8 (alpha-gamma-beta)\"\n    ],\n    \"partners\": [\n      \"C8A\",\n      \"C8B\",\n      \"S1PR2\",\n      \"HNF4A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"C8G encodes the gamma chain of complement component C8, a lipocalin-family protein that forms a disulfide-linked heterodimer with C8A via Cys40–Cys164 and, together with C8B, assembles into the C8 heterotrimer required for membrane attack complex (MAC) formation [PMID:3676249, PMID:1707134]. High-resolution crystal structures reveal a deep hydrophobic calyx characteristic of lipocalins, whose entrance is physically occluded by a C8A indel in the assembled alpha-gamma complex, indicating that ligand access is conformationally regulated during MAC assembly [PMID:12033936, PMID:18440555]. Although C8G is dispensable for MAC-mediated erythrocyte lysis and bacterial killing, it enhances bactericidal efficiency [PMID:12413696]. Independent of the complement system, C8G is expressed in perivascular astrocytes where it antagonizes sphingosine-1-phosphate receptor 2 (S1PR2), suppressing S1P-driven microglial neuroinflammation and preserving blood–brain barrier integrity [PMID:33382892, PMID:34149451].\",\n  \"teleology\": [\n    {\n      \"year\": 1987,\n      \"claim\": \"Establishing that C8G is the product of a distinct gene resolved how the three C8 subunits are independently synthesized and then assembled via covalent and noncovalent interactions in the liver.\",\n      \"evidence\": \"cDNA cloning and Northern blot of baboon and rat liver poly(A) RNA with subunit-specific probes\",\n      \"pmids\": [\"3676249\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of intracellular assembly of C8 trimer not defined\", \"Signals directing C8G secretion not characterized\"]\n    },\n    {\n      \"year\": 1989,\n      \"claim\": \"Mapping C8G to chromosome 9q, separate from the linked C8A/C8B locus on 1p, demonstrated that alpha-gamma disulfide linkage occurs post-translationally from independently regulated loci.\",\n      \"evidence\": \"Somatic cell hybrid panel with subunit-specific cDNA probes and genomic restriction analysis\",\n      \"pmids\": [\"2613233\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Regulatory consequences of separate chromosomal locations not explored\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Identifying C8G as a lipocalin family member and mapping the Cys40 disulfide to C8A Cys164 defined the structural basis for alpha-gamma dimer formation and raised the possibility of small hydrophobic ligand binding.\",\n      \"evidence\": \"Disulfide-bond mapping by peptide sequencing; retinol/retinoic acid binding assay; molecular modeling\",\n      \"pmids\": [\"1707134\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ligand binding required non-physiological 2 M NaCl, casting doubt on in vivo relevance\", \"No crystal structure available at the time\", \"Physiological ligand identity unresolved\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Characterization of the C8G gene structure (7 exons, ~1.8 kb) and identification of upstream SP1, glucocorticoid response elements, and SV40 enhancer core motifs provided a framework for understanding transcriptional regulation.\",\n      \"evidence\": \"Genomic cloning, S1 nuclease mapping, anchored PCR for transcription start site\",\n      \"pmids\": [\"8172891\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional validation of putative regulatory elements not performed at this stage\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Demonstrating IL-6-mediated upregulation of C8G in HepG2 cells established C8 as a positive acute-phase reactant, linking complement MAC assembly to inflammatory signaling.\",\n      \"evidence\": \"Immunoprecipitation and SDS-PAGE of biosynthetically labeled subunits from IL-6-treated HepG2 cells\",\n      \"pmids\": [\"7710765\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo acute-phase regulation not confirmed\", \"Mechanism of IL-6 transcriptional activation of C8G not dissected\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Reconstitution experiments showed that C8G is dispensable for MAC hemolytic and bactericidal activity but enhances bacterial killing, redefining C8G's role from essential structural subunit to modulatory enhancer of MAC function.\",\n      \"evidence\": \"Purified subunit reconstitution in erythrocyte lysis and bacterial killing assays\",\n      \"pmids\": [\"12413696\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which C8G enhances bactericidal activity not determined\", \"Whether C8G modulates MAC pore stoichiometry or kinetics unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"The 1.2 Å crystal structure of free C8G revealed a deep hydrophobic calyx larger than NGAL's and a partially disordered loop 1 near the calyx entrance, providing the first atomic-resolution view of a potential ligand-binding site.\",\n      \"evidence\": \"X-ray crystallography of recombinant C8G at 1.2 Å resolution\",\n      \"pmids\": [\"12033936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No ligand co-crystal obtained\", \"Physiological ligand identity still unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"The 2.15 Å co-crystal structure of C8A-MACPF with C8G showed that a 19-residue C8A indel occludes the C8G calyx entrance, explaining why ligand binding may be regulated by conformational changes during MAC assembly.\",\n      \"evidence\": \"X-ray crystallography of alphaMACPF-gamma complex at 2.15 Å resolution\",\n      \"pmids\": [\"18440555\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether calyx opening occurs during MAC insertion into target membranes is untested\", \"Identity of any physiological calyx ligand remains unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Discovery that astrocyte-expressed C8G antagonizes S1PR2 to suppress microglial neuroinflammation and protect blood–brain barrier integrity established a complement-independent neuroimmune function for C8G.\",\n      \"evidence\": \"Recombinant C8G gain-of-function and shRNA loss-of-function in mouse neuroinflammation models; Co-IP identifying S1PR2 interaction; in vitro BBB permeability assays\",\n      \"pmids\": [\"33382892\", \"34149451\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding interface between C8G and S1PR2 not structurally resolved\", \"Source of brain C8G (local astrocyte production vs. plasma-derived) not fully distinguished\", \"Relevance to human neurological disease not demonstrated\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identification of HNF4α as a direct transcriptional activator of hepatic C8G expression connected C8G regulation to a master hepatocyte transcription factor, explaining liver-predominant expression.\",\n      \"evidence\": \"Liver-specific Hnf4a-knockout mice; forced HNF4α expression in hepatocytes; transactivation assay with binding site mutations\",\n      \"pmids\": [\"39285151\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether HNF4α also controls C8G expression in astrocytes unknown\", \"Interplay with IL-6-mediated acute-phase regulation not integrated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The physiological ligand of the C8G lipocalin calyx remains unidentified, and whether calyx ligand binding occurs in vivo—particularly during MAC pore assembly when the C8A indel might disengage—is the central unresolved mechanistic question.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No endogenous ligand identified for the C8G calyx\", \"Structural dynamics of calyx opening during MAC assembly untested\", \"Contribution of C8G to MAC pore architecture at cryo-EM resolution not resolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 8, 9]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [2, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [8, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 5, 7]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [8, 9]}\n    ],\n    \"complexes\": [\"Complement C8 (C8A-C8G-C8B)\", \"Membrane attack complex (MAC)\"],\n    \"partners\": [\"C8A\", \"C8B\", \"S1PR2\"],\n    \"other_free_text\": []\n  }\n}\n```"}