{"gene":"CPN2","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":2004,"finding":"Carboxypeptidase N (CPN) is a heterotetrameric plasma zinc metalloprotease consisting of two enzymatically active small subunits (CPN1) and two large subunits (CPN2); the CPN2 large subunits protect CPN1 from degradation in the bloodstream. CPN enzymatically cleaves C-terminal arginines and lysines from complement anaphylatoxins (C3a, C5a), kinins, and other peptides, thereby modulating their biological activity and receptor binding.","method":"Biochemical characterization and literature synthesis of CPN subunit composition and substrate specificity","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical characterization of complex composition and enzymatic activity, established across multiple studies, but this paper is a review synthesizing prior work rather than presenting new primary experiments","pmids":["14687935"],"is_preprint":false},{"year":2006,"finding":"Crystal structure of the human CPN1 catalytic domain (2.1 Å) revealed that in the native CPN tetramer, each CPN1 subunit interacts with the leucine-rich repeat (LRR) tandem domain of its cognate CPN2 subunit via a unique hydrophobic surface patch at the catalytic domain–transthyretin (TT) domain interface, exposing the two active centers. This structural model explains how CPN2 stabilizes CPN1 in circulation.","method":"X-ray crystallography of recombinant truncated CPN1 at 2.1 Å; structural modeling of CPN1–CPN2 interactions within the tetramer","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure at 2.1 Å with modeling of subunit interactions; single rigorous paper with direct structural evidence for CPN1–CPN2 interface","pmids":["17157876"],"is_preprint":false},{"year":2009,"finding":"Targeted disruption of CPN1 (the catalytic small subunit) in mice produced complete CPN deficiency, demonstrating that CPN2 alone is insufficient for enzymatic activity and that intact CPN (requiring both CPN1 and CPN2) is required in vivo to inactivate C5a anaphylatoxin and protect against lethal anaphylactic shock. CPN1−/− mice showed 100% mortality upon i.v. C5a challenge, rescued by antihistamine pre-treatment, establishing that CPN's role in C5a inactivation is non-redundant.","method":"Gene knockout (targeted disruption of CPN1 in mice); in vivo cobra venom factor and C5a/C3a challenge; genetic epistasis with C5aR−/− and C3aR−/− double knockouts","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with defined in vivo phenotype, multiple orthogonal genetic epistasis experiments (double KO mice), replicated with pharmacologic rescue","pmids":["19414808"],"is_preprint":false},{"year":2025,"finding":"CPN2 (carboxypeptidase N subunit 2) suppresses NF-κB pathway activation in testicular cells; CPN2 overexpression reduced inflammatory cytokine production, preserved spermatogonial stem cell proliferation, and reduced apoptosis in a di-n-butyl phthalate-induced cryptorchidism rat model, while CRISPR/Cas9-mediated CPN2 knockout reversed these effects. NF-κB activation abolished the protective effects of CPN2, confirming a CPN2/NF-κB regulatory axis.","method":"CRISPR/Cas9 knockout rats; DBP-induced cryptorchidism rat model; Western blotting; histological analysis; co-culture assays; NF-κB pathway activation/inhibition","journal":"Autoimmunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR knockout and overexpression with defined cellular phenotypes and pathway placement, single lab, multiple orthogonal methods","pmids":["40720213"],"is_preprint":false}],"current_model":"CPN2 (carboxypeptidase N subunit 2) is the large regulatory subunit of the heterotetrameric plasma zinc metalloprotease CPN; it stabilizes the catalytic CPN1 subunits from proteolytic degradation in blood via hydrophobic interactions at the CPN1 catalytic domain–transthyretin interface (established by crystal structure), enabling CPN to cleave C-terminal Arg/Lys residues from complement anaphylatoxins (C5a, C3a) and kinins, with CPN's C5a-inactivating function shown to be essential in vivo for protection against anaphylactic shock; additionally, CPN2 has been shown to suppress NF-κB-mediated inflammation in testicular tissue via a CPN2/NF-κB regulatory axis."},"narrative":{"mechanistic_narrative":"CPN2 is the large regulatory subunit of carboxypeptidase N (CPN), a heterotetrameric plasma zinc metalloprotease composed of two catalytic CPN1 subunits and two CPN2 subunits, in which CPN2 protects CPN1 from proteolytic degradation in the bloodstream [PMID:14687935]. Crystallographic analysis of the CPN1 catalytic domain shows that each CPN1 subunit engages the leucine-rich repeat tandem domain of its cognate CPN2 subunit through a hydrophobic surface patch at the catalytic domain–transthyretin interface, exposing the active centers and explaining how CPN2 stabilizes CPN1 within the tetramer [PMID:17157876]. The assembled enzyme cleaves C-terminal arginines and lysines from complement anaphylatoxins (C3a, C5a) and kinins to modulate their activity; genetic disruption of the catalytic subunit demonstrates that intact CPN is non-redundantly required in vivo to inactivate C5a and protect against lethal anaphylactic shock, and that CPN2 alone cannot confer enzymatic activity [PMID:19414808]. Independently, CPN2 suppresses NF-κB pathway activation in testicular cells, where it limits inflammatory cytokine production, preserves spermatogonial stem cell proliferation, and reduces apoptosis through a CPN2/NF-κB regulatory axis [PMID:40720213].","teleology":[{"year":2004,"claim":"Established the subunit architecture and enzymatic role of CPN, defining CPN2 as the large subunit that stabilizes the catalytic CPN1 subunit and placing the complex in anaphylatoxin and kinin regulation.","evidence":"Biochemical characterization and synthesis of CPN subunit composition and substrate specificity","pmids":["14687935"],"confidence":"Medium","gaps":["Review synthesis rather than new primary data","Molecular basis of CPN2-mediated CPN1 stabilization not resolved at this stage"]},{"year":2006,"claim":"Resolved the structural basis for how CPN2 stabilizes CPN1, showing the LRR tandem domain of CPN2 binds a hydrophobic patch at the CPN1 catalytic domain–transthyretin interface while leaving the active centers exposed.","evidence":"X-ray crystallography of recombinant truncated CPN1 at 2.1 Å with modeling of subunit interactions","pmids":["17157876"],"confidence":"High","gaps":["Structure is of CPN1; CPN2 interaction is modeled rather than directly crystallized","Full tetramer assembly not experimentally solved"]},{"year":2009,"claim":"Demonstrated that intact CPN is physiologically essential and non-redundant for C5a inactivation in vivo, and that CPN2 alone is insufficient for enzymatic function.","evidence":"Targeted CPN1 knockout mice with in vivo C5a/C3a challenge and genetic epistasis using C5aR/C3aR double knockouts plus pharmacologic rescue","pmids":["19414808"],"confidence":"High","gaps":["Phenotype derives from CPN1 disruption, not direct CPN2 knockout","Does not isolate CPN2-specific functional contributions"]},{"year":2025,"claim":"Identified a function for CPN2 beyond the CPN protease complex, placing it as a suppressor of NF-κB-mediated inflammation that protects testicular tissue and spermatogonial stem cells.","evidence":"CRISPR/Cas9 knockout rats and overexpression in a DBP-induced cryptorchidism model with Western blotting, histology, co-culture, and NF-κB activation/inhibition","pmids":["40720213"],"confidence":"Medium","gaps":["Single lab and single disease model","Molecular mechanism linking CPN2 to NF-κB suppression unresolved","Relationship between this role and the CPN protease function unknown"]},{"year":null,"claim":"How CPN2 mechanistically suppresses NF-κB signaling, and whether this activity is connected to its role as the CPN protease regulatory subunit, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No direct molecular partner linking CPN2 to NF-κB identified","No structural or biochemical model for CPN2 outside the CPN tetramer"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[2]}],"complexes":["Carboxypeptidase N (CPN)"],"partners":["CPN1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P22792","full_name":"Carboxypeptidase N subunit 2","aliases":["Carboxypeptidase N 83 kDa chain","Carboxypeptidase N large subunit","Carboxypeptidase N polypeptide 2","Carboxypeptidase N regulatory subunit"],"length_aa":545,"mass_kda":60.6,"function":"The 83 kDa subunit binds and stabilizes the catalytic subunit at 37 degrees Celsius and keeps it in circulation. Under some circumstances it may be an allosteric modifier of the catalytic subunit","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P22792/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CPN2","classification":"Not Classified","n_dependent_lines":13,"n_total_lines":1208,"dependency_fraction":0.01076158940397351},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CPN2","total_profiled":1310},"omim":[{"mim_id":"604206","title":"COPINE II; CPNE2","url":"https://www.omim.org/entry/604206"},{"mim_id":"603104","title":"CARBOXYPEPTIDASE N, POLYPEPTIDE 2, 83-KD; CPN2","url":"https://www.omim.org/entry/603104"},{"mim_id":"603101","title":"CARBOXYPEPTIDASE B2, PLASMA; CPB2","url":"https://www.omim.org/entry/603101"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"liver","ntpm":204.5}],"url":"https://www.proteinatlas.org/search/CPN2"},"hgnc":{"alias_symbol":[],"prev_symbol":["ACBP"]},"alphafold":{"accession":"P22792","domains":[{"cath_id":"3.80.10.10","chopping":"278-448","consensus_level":"medium","plddt":92.2759,"start":278,"end":448},{"cath_id":"-","chopping":"480-506_521-540","consensus_level":"medium","plddt":70.946,"start":480,"end":540}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P22792","model_url":"https://alphafold.ebi.ac.uk/files/AF-P22792-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P22792-F1-predicted_aligned_error_v6.png","plddt_mean":85.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CPN2","jax_strain_url":"https://www.jax.org/strain/search?query=CPN2"},"sequence":{"accession":"P22792","fasta_url":"https://rest.uniprot.org/uniprotkb/P22792.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P22792/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P22792"}},"corpus_meta":[{"pmid":"7665619","id":"PMC_7665619","title":"Mapping of functional domains in eukaryotic protein synthesis initiation factor 4G (eIF4G) with picornaviral proteases. Implications for cap-dependent and cap-independent translational initiation.","date":"1995","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/7665619","citation_count":500,"is_preprint":false},{"pmid":"9430674","id":"PMC_9430674","title":"The copines, a novel class of C2 domain-containing, calcium-dependent, phospholipid-binding proteins conserved from Paramecium to humans.","date":"1998","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9430674","citation_count":194,"is_preprint":false},{"pmid":"14687935","id":"PMC_14687935","title":"Carboxypeptidase N: a pleiotropic regulator of inflammation.","date":"2004","source":"Molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/14687935","citation_count":131,"is_preprint":false},{"pmid":"19383910","id":"PMC_19383910","title":"Caspase-8 association with the focal adhesion complex promotes tumor cell migration and metastasis.","date":"2009","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/19383910","citation_count":120,"is_preprint":false},{"pmid":"25685243","id":"PMC_25685243","title":"Identification of novel long non-coding RNAs in clear cell renal cell carcinoma.","date":"2015","source":"Clinical epigenetics","url":"https://pubmed.ncbi.nlm.nih.gov/25685243","citation_count":72,"is_preprint":false},{"pmid":"15601944","id":"PMC_15601944","title":"Different sensory systems share projection neurons but elicit distinct motor patterns.","date":"2004","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/15601944","citation_count":71,"is_preprint":false},{"pmid":"7823079","id":"PMC_7823079","title":"Recruitment of a projection neuron determines gastric mill motor pattern selection in the stomatogastric nervous system of the crab, Cancer borealis.","date":"1994","source":"Journal of neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/7823079","citation_count":69,"is_preprint":false},{"pmid":"31411917","id":"PMC_31411917","title":"Inhibition of the ubiquitous calpains protects complex I activity and enables improved mitophagy in the heart following ischemia-reperfusion.","date":"2019","source":"American journal of physiology. Cell physiology","url":"https://pubmed.ncbi.nlm.nih.gov/31411917","citation_count":58,"is_preprint":false},{"pmid":"26501113","id":"PMC_26501113","title":"Host Protein Biomarkers Identify Active Tuberculosis in HIV Uninfected and Co-infected Individuals.","date":"2015","source":"EBioMedicine","url":"https://pubmed.ncbi.nlm.nih.gov/26501113","citation_count":46,"is_preprint":false},{"pmid":"19414808","id":"PMC_19414808","title":"Targeted disruption of the gene encoding the murine small subunit of carboxypeptidase N (CPN1) causes susceptibility to C5a anaphylatoxin-mediated shock.","date":"2009","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/19414808","citation_count":38,"is_preprint":false},{"pmid":"18160638","id":"PMC_18160638","title":"Mechanosensory gating of proprioceptor input to modulatory projection neurons.","date":"2007","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/18160638","citation_count":32,"is_preprint":false},{"pmid":"17157876","id":"PMC_17157876","title":"Crystal structure of the human carboxypeptidase N (kininase I) catalytic domain.","date":"2006","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/17157876","citation_count":32,"is_preprint":false},{"pmid":"19741101","id":"PMC_19741101","title":"Differential activation of projection neurons by two sensory pathways contributes to motor pattern selection.","date":"2009","source":"Journal of neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/19741101","citation_count":28,"is_preprint":false},{"pmid":"29068691","id":"PMC_29068691","title":"Myocardial Injury Is Distinguished from Stable Angina by a Set of Candidate Plasma Biomarkers Identified Using iTRAQ/MRM-Based Approach.","date":"2017","source":"Journal of proteome research","url":"https://pubmed.ncbi.nlm.nih.gov/29068691","citation_count":20,"is_preprint":false},{"pmid":"34997008","id":"PMC_34997008","title":"Calpain-mediated protein targets in cardiac mitochondria following ischemia-reperfusion.","date":"2022","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/34997008","citation_count":17,"is_preprint":false},{"pmid":"3023044","id":"PMC_3023044","title":"Serum angiotensin-I-converting enzyme and carboxypeptidase N in Crohn's disease and ulcerative colitis.","date":"1986","source":"Enzyme","url":"https://pubmed.ncbi.nlm.nih.gov/3023044","citation_count":17,"is_preprint":false},{"pmid":"24667622","id":"PMC_24667622","title":"Identification of potential biomarkers by serum proteomics analysis in rats with sepsis.","date":"2014","source":"Shock (Augusta, Ga.)","url":"https://pubmed.ncbi.nlm.nih.gov/24667622","citation_count":15,"is_preprint":false},{"pmid":"28469000","id":"PMC_28469000","title":"Circuit feedback increases activity level of a circuit input through interactions with intrinsic properties.","date":"2017","source":"Journal of neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/28469000","citation_count":13,"is_preprint":false},{"pmid":"36294892","id":"PMC_36294892","title":"Discovering Breast Cancer Biomarkers Candidates through mRNA Expression Analysis Based on The Cancer Genome Atlas Database.","date":"2022","source":"Journal of personalized medicine","url":"https://pubmed.ncbi.nlm.nih.gov/36294892","citation_count":13,"is_preprint":false},{"pmid":"34975707","id":"PMC_34975707","title":"Proteomics-Based Approach to Identify Novel Blood Biomarker Candidates for Differentiating Intracerebral Hemorrhage From Ischemic Stroke-A Pilot Study.","date":"2021","source":"Frontiers in neurology","url":"https://pubmed.ncbi.nlm.nih.gov/34975707","citation_count":12,"is_preprint":false},{"pmid":"38521921","id":"PMC_38521921","title":"Integrative multi-omics analysis identifies genetically supported druggable targets and immune cell specificity for myasthenia gravis.","date":"2024","source":"Journal of translational medicine","url":"https://pubmed.ncbi.nlm.nih.gov/38521921","citation_count":9,"is_preprint":false},{"pmid":"39207047","id":"PMC_39207047","title":"The trajectory of vesicular proteomic signatures from HBV-HCC by chitosan-magnetic bead-based separation and DIA-proteomic analysis.","date":"2024","source":"Journal of extracellular vesicles","url":"https://pubmed.ncbi.nlm.nih.gov/39207047","citation_count":9,"is_preprint":false},{"pmid":"35117778","id":"PMC_35117778","title":"Comparison of whole exome sequencing in circulating tumor cells of primitive and metastatic nasopharyngeal carcinoma.","date":"2020","source":"Translational cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/35117778","citation_count":7,"is_preprint":false},{"pmid":"35352885","id":"PMC_35352885","title":"Genetic Variants Associated with Adverse Events after Angiotensin-Converting Enzyme Inhibitor Use: Replication after GWAS-Based Discovery.","date":"2022","source":"Yonsei medical journal","url":"https://pubmed.ncbi.nlm.nih.gov/35352885","citation_count":7,"is_preprint":false},{"pmid":"23575750","id":"PMC_23575750","title":"Replication of Caucasian loci associated with bone mineral density in Koreans.","date":"2013","source":"Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA","url":"https://pubmed.ncbi.nlm.nih.gov/23575750","citation_count":6,"is_preprint":false},{"pmid":"25419188","id":"PMC_25419188","title":"A modeling exploration of how synaptic feedback to descending projection neurons shapes the activity of an oscillatory network.","date":"2014","source":"SIAM journal on applied dynamical systems","url":"https://pubmed.ncbi.nlm.nih.gov/25419188","citation_count":4,"is_preprint":false},{"pmid":"21699972","id":"PMC_21699972","title":"Autolytic Mycobacterium leprae Hsp65 fragments may act as biological markers for autoimmune diseases.","date":"2011","source":"Microbial pathogenesis","url":"https://pubmed.ncbi.nlm.nih.gov/21699972","citation_count":4,"is_preprint":false},{"pmid":"36115450","id":"PMC_36115450","title":"UPLC-HDMSE to discover serum biomarkers in adults with type 1 diabetes.","date":"2022","source":"International journal of biological macromolecules","url":"https://pubmed.ncbi.nlm.nih.gov/36115450","citation_count":2,"is_preprint":false},{"pmid":"39265992","id":"PMC_39265992","title":"Proteome and Metabolome Profiling of Anticoagulant Disorders Induced by Familial Protein S Deficiency.","date":"2024","source":"Journal of proteome research","url":"https://pubmed.ncbi.nlm.nih.gov/39265992","citation_count":1,"is_preprint":false},{"pmid":"40659816","id":"PMC_40659816","title":"Properdin, transcortin and HGFAC are novel plasma biomarkers in canine chronic inflammatory enteropathies from active disease to remission.","date":"2025","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/40659816","citation_count":1,"is_preprint":false},{"pmid":"40720213","id":"PMC_40720213","title":"CPN2 alleviates cryptorchidism by inhibiting the NF-κB pathway and regulating immune responses.","date":"2025","source":"Autoimmunity","url":"https://pubmed.ncbi.nlm.nih.gov/40720213","citation_count":0,"is_preprint":false},{"pmid":"41959118","id":"PMC_41959118","title":"Functional Analysis of Late-Onset Alzheimer's Disease Risk Genes in Caenorhabditis elegans Identifies Regulators of Neuronal Aging.","date":"2026","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/41959118","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":18903,"output_tokens":1701,"usd":0.041112,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8097,"output_tokens":1886,"usd":0.043818,"stage2_stop_reason":"end_turn"},"total_usd":0.08493,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"Looking at the corpus, I need to identify which papers are about the canonical human/mammalian CPN2 gene (Carboxypeptidase N subunit 2, the large regulatory subunit of carboxypeptidase N). I need to distinguish this from:\n- CPN2 as a neuronal commissural projection neuron in crabs (papers 6, 7, 11, 13, 18, 26) → NOT the gene\n- cpn2 as Mycobacterium leprae chaperonin 2 (paper 27) → NOT the gene\n- cpn-2 as a C. elegans homolog of CNN2 (paper 32) → symbol collision\n- lnc-CPN2-1 (paper 5) → non-coding RNA, not the protein\n- eIF4G cpN2 fragment (paper 1) → different gene entirely\n\nPapers about the canonical CPN2 protein (carboxypeptidase N subunit 2): 3, 10, 12, 16, 31, and papers where CPN2 appears as a detected protein (biomarker papers 9, 14, 17, 20, 22, 24, 25, 28, 29, 30) — but biomarker/expression papers are excluded.\n\nPapers with actual mechanistic findings about CPN2 (the regulatory large subunit of carboxypeptidase N): 3, 10, 12, 31.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2004,\n      \"finding\": \"Carboxypeptidase N (CPN) is a heterotetrameric plasma zinc metalloprotease consisting of two enzymatically active small subunits (CPN1) and two large subunits (CPN2); the CPN2 large subunits protect CPN1 from degradation in the bloodstream. CPN enzymatically cleaves C-terminal arginines and lysines from complement anaphylatoxins (C3a, C5a), kinins, and other peptides, thereby modulating their biological activity and receptor binding.\",\n      \"method\": \"Biochemical characterization and literature synthesis of CPN subunit composition and substrate specificity\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical characterization of complex composition and enzymatic activity, established across multiple studies, but this paper is a review synthesizing prior work rather than presenting new primary experiments\",\n      \"pmids\": [\"14687935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Crystal structure of the human CPN1 catalytic domain (2.1 Å) revealed that in the native CPN tetramer, each CPN1 subunit interacts with the leucine-rich repeat (LRR) tandem domain of its cognate CPN2 subunit via a unique hydrophobic surface patch at the catalytic domain–transthyretin (TT) domain interface, exposing the two active centers. This structural model explains how CPN2 stabilizes CPN1 in circulation.\",\n      \"method\": \"X-ray crystallography of recombinant truncated CPN1 at 2.1 Å; structural modeling of CPN1–CPN2 interactions within the tetramer\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure at 2.1 Å with modeling of subunit interactions; single rigorous paper with direct structural evidence for CPN1–CPN2 interface\",\n      \"pmids\": [\"17157876\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Targeted disruption of CPN1 (the catalytic small subunit) in mice produced complete CPN deficiency, demonstrating that CPN2 alone is insufficient for enzymatic activity and that intact CPN (requiring both CPN1 and CPN2) is required in vivo to inactivate C5a anaphylatoxin and protect against lethal anaphylactic shock. CPN1−/− mice showed 100% mortality upon i.v. C5a challenge, rescued by antihistamine pre-treatment, establishing that CPN's role in C5a inactivation is non-redundant.\",\n      \"method\": \"Gene knockout (targeted disruption of CPN1 in mice); in vivo cobra venom factor and C5a/C3a challenge; genetic epistasis with C5aR−/− and C3aR−/− double knockouts\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with defined in vivo phenotype, multiple orthogonal genetic epistasis experiments (double KO mice), replicated with pharmacologic rescue\",\n      \"pmids\": [\"19414808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CPN2 (carboxypeptidase N subunit 2) suppresses NF-κB pathway activation in testicular cells; CPN2 overexpression reduced inflammatory cytokine production, preserved spermatogonial stem cell proliferation, and reduced apoptosis in a di-n-butyl phthalate-induced cryptorchidism rat model, while CRISPR/Cas9-mediated CPN2 knockout reversed these effects. NF-κB activation abolished the protective effects of CPN2, confirming a CPN2/NF-κB regulatory axis.\",\n      \"method\": \"CRISPR/Cas9 knockout rats; DBP-induced cryptorchidism rat model; Western blotting; histological analysis; co-culture assays; NF-κB pathway activation/inhibition\",\n      \"journal\": \"Autoimmunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR knockout and overexpression with defined cellular phenotypes and pathway placement, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"40720213\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CPN2 (carboxypeptidase N subunit 2) is the large regulatory subunit of the heterotetrameric plasma zinc metalloprotease CPN; it stabilizes the catalytic CPN1 subunits from proteolytic degradation in blood via hydrophobic interactions at the CPN1 catalytic domain–transthyretin interface (established by crystal structure), enabling CPN to cleave C-terminal Arg/Lys residues from complement anaphylatoxins (C5a, C3a) and kinins, with CPN's C5a-inactivating function shown to be essential in vivo for protection against anaphylactic shock; additionally, CPN2 has been shown to suppress NF-κB-mediated inflammation in testicular tissue via a CPN2/NF-κB regulatory axis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CPN2 is the large regulatory subunit of carboxypeptidase N (CPN), a heterotetrameric plasma zinc metalloprotease composed of two catalytic CPN1 subunits and two CPN2 subunits, in which CPN2 protects CPN1 from proteolytic degradation in the bloodstream [#0]. Crystallographic analysis of the CPN1 catalytic domain shows that each CPN1 subunit engages the leucine-rich repeat tandem domain of its cognate CPN2 subunit through a hydrophobic surface patch at the catalytic domain–transthyretin interface, exposing the active centers and explaining how CPN2 stabilizes CPN1 within the tetramer [#1]. The assembled enzyme cleaves C-terminal arginines and lysines from complement anaphylatoxins (C3a, C5a) and kinins to modulate their activity; genetic disruption of the catalytic subunit demonstrates that intact CPN is non-redundantly required in vivo to inactivate C5a and protect against lethal anaphylactic shock, and that CPN2 alone cannot confer enzymatic activity [#2]. Independently, CPN2 suppresses NF-κB pathway activation in testicular cells, where it limits inflammatory cytokine production, preserves spermatogonial stem cell proliferation, and reduces apoptosis through a CPN2/NF-κB regulatory axis [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Established the subunit architecture and enzymatic role of CPN, defining CPN2 as the large subunit that stabilizes the catalytic CPN1 subunit and placing the complex in anaphylatoxin and kinin regulation.\",\n      \"evidence\": \"Biochemical characterization and synthesis of CPN subunit composition and substrate specificity\",\n      \"pmids\": [\"14687935\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Review synthesis rather than new primary data\",\n        \"Molecular basis of CPN2-mediated CPN1 stabilization not resolved at this stage\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Resolved the structural basis for how CPN2 stabilizes CPN1, showing the LRR tandem domain of CPN2 binds a hydrophobic patch at the CPN1 catalytic domain–transthyretin interface while leaving the active centers exposed.\",\n      \"evidence\": \"X-ray crystallography of recombinant truncated CPN1 at 2.1 Å with modeling of subunit interactions\",\n      \"pmids\": [\"17157876\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structure is of CPN1; CPN2 interaction is modeled rather than directly crystallized\",\n        \"Full tetramer assembly not experimentally solved\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrated that intact CPN is physiologically essential and non-redundant for C5a inactivation in vivo, and that CPN2 alone is insufficient for enzymatic function.\",\n      \"evidence\": \"Targeted CPN1 knockout mice with in vivo C5a/C3a challenge and genetic epistasis using C5aR/C3aR double knockouts plus pharmacologic rescue\",\n      \"pmids\": [\"19414808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Phenotype derives from CPN1 disruption, not direct CPN2 knockout\",\n        \"Does not isolate CPN2-specific functional contributions\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified a function for CPN2 beyond the CPN protease complex, placing it as a suppressor of NF-κB-mediated inflammation that protects testicular tissue and spermatogonial stem cells.\",\n      \"evidence\": \"CRISPR/Cas9 knockout rats and overexpression in a DBP-induced cryptorchidism model with Western blotting, histology, co-culture, and NF-κB activation/inhibition\",\n      \"pmids\": [\"40720213\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single lab and single disease model\",\n        \"Molecular mechanism linking CPN2 to NF-κB suppression unresolved\",\n        \"Relationship between this role and the CPN protease function unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CPN2 mechanistically suppresses NF-κB signaling, and whether this activity is connected to its role as the CPN protease regulatory subunit, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No direct molecular partner linking CPN2 to NF-κB identified\",\n        \"No structural or biochemical model for CPN2 outside the CPN tetramer\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"complexes\": [\"Carboxypeptidase N (CPN)\"],\n    \"partners\": [\"CPN1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}