{"gene":"SELENOP","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":1993,"finding":"Human selenoprotein P mRNA contains two conserved stem-loop structures in its 3' untranslated region (SECIS elements), one for each domain, establishing that a separate stem-loop is not required for each of the 10 selenocysteine residues encoded by UGA codons in the open reading frame.","method":"cDNA cloning and sequencing of human selenoprotein P from liver/heart libraries; comparative sequence analysis with rat cDNA; secondary structure prediction of 3' UTR","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — direct sequence characterization of cloned cDNA with structural prediction, foundational paper with >160 citations","pmids":["8421687"],"is_preprint":false},{"year":1994,"finding":"Selenoprotein P is a selenium-rich extracellular glycoprotein containing selenocysteine residues; purified rat selenoprotein P contains ~7.5 selenium atoms per molecule, and in selenium-replete rats it contains ~65% of plasma selenium with rapid turnover (plasma half-life of 3–4 h for 75Se).","method":"Protein purification from rat/human plasma; 75Se radiolabeling; SDS-PAGE; selenium content measurement; Northern blot for tissue expression and selenium-deficiency regulation","journal":"The Journal of nutrition","confidence":"High","confidence_rationale":"Tier 1 — direct biochemical characterization of purified protein; foundational paper >130 citations","pmids":["7931697"],"is_preprint":false},{"year":1994,"finding":"Selenoprotein P was purified from human plasma by immunoaffinity chromatography followed by heparin-agarose chromatography, yielding two bands (61 and 55 kDa) that are both glycoproteins; the protein accounts for approximately one-third of total plasma selenium and binds heparin.","method":"Immunoaffinity chromatography with monoclonal antibodies; heparin-agarose chromatography; SDS-PAGE; carbohydrate staining; enzymatic deglycosylation; 75Se-labeled SELENOP from HepG2 cells as tracer","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1 — direct protein purification and biochemical characterization; replicated across labs","pmids":["8142465"],"is_preprint":false},{"year":1999,"finding":"Purified human selenoprotein P functions as a phospholipid hydroperoxide glutathione peroxidase in extracellular fluids: it reduces phospholipid hydroperoxides (but not H2O2 or tert-butyl hydroperoxide) using glutathione and other thiol reductants via a tert-uni ping-pong kinetic mechanism.","method":"Conventional purification of selenoprotein P from human plasma; enzymatic assays with various hydroperoxide substrates and reducing agents; kinetic analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assay with kinetic characterization; foundational mechanistic paper >200 citations","pmids":["9915822"],"is_preprint":false},{"year":2002,"finding":"Selenoprotein P is the primary selenium supply protein in serum: immunodepletion of selenoprotein P (but not extracellular GPx) from human serum severely depleted cellular selenium and glutathione peroxidase/thioredoxin reductase activities in Jurkat T-lymphoma cells cultured in that serum; repletion with purified selenoprotein P restored activity.","method":"Immunodepletion of specific selenoproteins from human serum; cell culture with depleted/reconstituted serum; measurement of Se-dependent enzyme activities (GPx, TrxR); Se content assay","journal":"European journal of biochemistry","confidence":"High","confidence_rationale":"Tier 2 — immunodepletion/reconstitution experiment with functional readout; >125 citations","pmids":["12423375"],"is_preprint":false},{"year":2008,"finding":"Liver-derived circulating selenoprotein P is the main selenium transport form that supplies the kidney, testis, and brain; hepatocyte-specific transgenic expression of SEPP1 in Sepp-/- mice rescued neurological defects (ataxia, seizures) and male infertility and increased selenium and selenoenzyme activities in plasma, kidney, testis, and brain, demonstrating that plasma SeP from hepatocytes supports selenium-privileged tissues.","method":"Transgenic rescue experiment: human SEPP1 under hepatocyte-specific transthyretin promoter in Sepp-/- mice; selenium content measurement in tissues; selenoenzyme activity assays; neurological phenotyping; fertility testing","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 — genetic rescue with tissue-specific transgene, multiple functional readouts; >110 citations","pmids":["17961124"],"is_preprint":false},{"year":2010,"finding":"Selenoprotein P (SeP) is a liver-derived hepatokine that causes insulin resistance: hepatic SeP mRNA correlated with insulin resistance in humans; purified SeP impaired insulin signaling and glucose metabolism in hepatocytes and myocytes; genetic deletion or siRNA knockdown of SeP improved systemic insulin sensitivity and glucose tolerance in mice; the metabolic actions were mediated at least partly by inactivation of AMP-activated protein kinase (AMPK).","method":"SAGE and DNA chip analysis of human liver; administration of purified SeP to cultured cells and mice; SeP knockout mice; siRNA knockdown; insulin signaling assays; glucose tolerance tests; AMPK activity assays","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (human correlative, cell-based, KO mouse, siRNA) with defined molecular mechanism (AMPK inactivation); >460 citations","pmids":["21035759"],"is_preprint":false},{"year":2012,"finding":"Hepatocyte-produced selenoprotein P is central to whole-body selenium homeostasis: selective deletion of Sepp1 in hepatocytes lowered plasma Sepp1 to 10% of controls, increased urinary selenium excretion reducing whole-body selenium, and under selenium-deficient conditions caused liver selenium accumulation at the expense of extra-hepatic tissues. Additionally, Sepp1 synthesis is preferentially maintained over Gpx1 synthesis in hepatocytes under selenium limitation, directing selenium to peripheral tissues.","method":"Conditional hepatocyte-specific Sepp1 knockout mice (Sepp1c/c/alb-cre); plasma and tissue selenium measurement; urinary selenium excretion; selenoprotein mRNA quantification; clinical phenotyping under selenium deficiency","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with multiple biochemical and physiological readouts; >110 citations","pmids":["23038251"],"is_preprint":false},{"year":2012,"finding":"L8 myoblast cells take up Sepp1 via an apoER2-mediated endocytic mechanism requiring binding to heparan sulfate proteoglycans; the selenium-rich C-terminal domain of Sepp1 is required for uptake and utilization; Lrp1 is present on a Sepp1-binding membrane fraction but siRNA knockdown showed it is not required for 75Se uptake from Sepp1. Lysosomal acidification is needed for Sepp1 digestion and selenium release.","method":"Sepp1-affinity column pulldown from membrane fraction; mass spectrometry identification of receptors (apoER2, Lrp1); siRNA knockdown of receptors; 75Se radiolabeled Sepp1 uptake assays; protamine/chlorate inhibition; lysosome acidification blockade; Sepp1 isoform (Δ240-361) comparison","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — receptor identification by MS, siRNA validation, mechanistic dissection with multiple inhibitors and isoform variants","pmids":["22761431"],"is_preprint":false},{"year":2014,"finding":"N-terminal truncation fragments of Sepp1 (designated Sepp1UF), filtered at the glomerulus and taken up by renal proximal convoluted tubule cells via megalin-mediated endocytosis, possess peroxidase activity: they catalyze NADPH oxidation when coupled with thioredoxin reductase-1 (TrxR1) and H2O2 or tert-butylhydroperoxide as terminal electron acceptors. The single selenocysteine at position 40 (in the UYLC redox-active motif of the N-terminal thioredoxin-fold domain) is essential for this activity, as shown by Sepp1(U40S) mutagenesis.","method":"Purification of urinary Sepp1 from megalin-/- mice by monoclonal antibody; mass spectrometry identification of truncation sites; TrxR1-coupled peroxidase assay with NADPH; mutagenesis (U40S selenocysteine-to-serine); comparison of full-length vs. truncated vs. mutant Sepp1 as TrxR1 substrates","journal":"Free radical biology & medicine","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic reconstitution with active-site mutagenesis and MS characterization","pmids":["24434121"],"is_preprint":false},{"year":2016,"finding":"Selenoprotein P nomenclature was standardized: the approved HGNC symbol SELENOP replaces prior symbols (SeP, SEPP1, SELP), resolving conflicts with P-selectin (SELP) and other genes.","method":"Systematic gene nomenclature review by selenoprotein experts and HUGO Gene Nomenclature Committee","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — expert consensus nomenclature paper, directly relevant to gene identity; >220 citations","pmids":["27645994"],"is_preprint":false},{"year":2017,"finding":"SeP causes exercise resistance through its muscle receptor LRP1: SeP-deficient mice showed enhanced exercise endurance, increased ROS production, AMPK phosphorylation, and PGC-1α expression in skeletal muscle. SeP treatment suppressed H2O2-induced adaptations in cultured myotubes via LRP1, and muscle-specific LRP1 knockout blunted SeP's inhibitory effects on exercise-induced AMPK phosphorylation and Ppargc1a expression in vivo.","method":"SeP knockout mice; exercise training protocols; ROS measurement; AMPK phosphorylation assay; PGC-1α/Ppargc1a expression; NAC antioxidant treatment; LRP1-Fc blocking; muscle-specific LRP1 conditional knockout mice; cultured myotube experiments; human cohort correlation","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 1-2 — multiple genetic models (SeP KO, muscle-specific LRP1 KO), in vitro receptor blocking, and human data; >130 citations","pmids":["28263310"],"is_preprint":false},{"year":2012,"finding":"Elevated circulating SeP levels are inversely correlated with adiponectin in type 2 diabetes patients, and SeP knockout mice show increased blood adiponectin levels on both regular chow and high-fat diet, suggesting that liver-derived SeP suppresses adiponectin production.","method":"Cross-sectional clinical study with Spearman correlation and multiple regression; SeP knockout mice on different diets with adiponectin measurement","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2-3 — KO mouse functional readout combined with human correlation; single lab","pmids":["22496878"],"is_preprint":false},{"year":2020,"finding":"SeP participates in NAFLD pathogenesis through the AMPK/ACC pathway: siRNA knockdown of SEPP1 inhibited triglyceride accumulation by activating AMPK/ACC phosphorylation, while SEPP1 overexpression aggravated lipid accumulation and inhibited AMPK/ACC phosphorylation in HepG2 cells and in mouse NAFLD models.","method":"siRNA knockdown and overexpression of SEPP1 in HepG2 cells; in vivo NAFLD mouse model; triglyceride accumulation assay; Western blot for AMPK/ACC phosphorylation","journal":"Journal of cellular physiology","confidence":"Medium","confidence_rationale":"Tier 2 — gain and loss-of-function with defined signaling pathway readout; single lab","pmids":["33094480"],"is_preprint":false},{"year":2019,"finding":"Protectin DX reduces hepatic insulin resistance by suppressing SeP expression via AMPK/SIRT1 signaling: AMPK or SIRT1 siRNA abolished the suppressive effects of PDX on palmitate-induced SeP expression, and the mechanism involves FOXO1-mediated transcriptional regulation of SeP. Recombinant SeP reversed the improvement in insulin signaling conferred by PDX treatment.","method":"Human primary hepatocyte treatment with palmitate and PDX; siRNA knockdown of AMPK and SIRT1; FOXO1 ChIP qPCR; Western blot for insulin signaling; recombinant SeP reconstitution","journal":"Clinical and experimental pharmacology & physiology","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA epistasis with reconstitution experiment identifying FOXO1 as transcriptional regulator of SeP; single lab","pmids":["31246318"],"is_preprint":false},{"year":2014,"finding":"Exendin-4 (GLP-1 receptor agonist) reduces SEPP1 expression in hepatocytes via AMPK activation: AMPK activator AICAR negatively regulated SEPP1 and fetuin-A expression, and exendin-4 failed to suppress SEPP1 expression in cells transfected with AMPK siRNA, establishing AMPK as the effector linking GLP-1 signaling to SeP suppression. The mechanism involves reduction of palmitate-induced ER stress.","method":"HepG2 cell treatment with palmitate, tunicamycin, exendin-4, AICAR; AMPK siRNA knockdown; RT-PCR and Western blot for SEPP1, fetuin-A, ER stress markers","journal":"Endocrinology and metabolism (Seoul, Korea)","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA epistasis establishes AMPK as required mediator; single lab","pmids":["26194078"],"is_preprint":false},{"year":2021,"finding":"Deletion of the entire SELENOP gene in dogs causes cerebellar atrophy and ataxia (CACA) associated with severely reduced blood selenium (~30% of wild-type), demonstrating that SELENOP is essential for selenium transport into the CNS in dogs and that its loss phenocopies but exceeds the severity of Selenop-/- knockout mice.","method":"Combined linkage and homozygosity mapping; whole genome sequencing; identification of homozygous 17.3 kb deletion encompassing the entire SELENOP coding sequence; blood selenium measurement; histopathology; genotyping of >600 Belgian Shepherd dogs","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 — natural loss-of-function with complete gene deletion, confirmed in multiple affected animals, with biochemical and pathological validation","pmids":["34339417"],"is_preprint":false},{"year":2022,"finding":"SELENOP is essential for preserving parvalbumin-expressing interneuron survival under limiting selenium supply: Selenop-/- mice on the recommended dietary allowance (RDA) diet developed epileptic seizures and ataxia that were prevented by selenium supplementation or hepatocyte-specific SELENOP transgene expression; selenium supplementation restored brain glutathione peroxidase activity to control levels, and the neurological phenotype was dose- and time-dependent on selenium supply.","method":"Selenop-/- and Selenop+/- mice on varying selenium diets; video-EEG for seizure detection; behavioral phenotyping (ataxia, tremor); brain GPx activity assay; histological analysis of parvalbumin interneurons; transgenic rescue with hepatocyte-specific human SELENOP","journal":"Redox biology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with EEG-confirmed seizures, biochemical rescue, and transgenic complementation","pmids":["36182809"],"is_preprint":false},{"year":2022,"finding":"Lauric acid upregulates SELENOP expression in hepatocytes through HNF4α: luciferase promoter assays identified an HNF4α binding site in the SELENOP promoter, ChIP assay confirmed increased HNF4α binding upon lauric acid treatment, and Hnf4α siRNA knockdown abolished lauric acid-induced Selenop upregulation and the associated impairment of insulin-induced Akt phosphorylation.","method":"Luciferase reporter assay; ChIP assay; siRNA knockdown of Hnf4α and Selenop; computational transcription factor binding site analysis; Western blot for Akt phosphorylation; in vivo mouse liver validation","journal":"American journal of physiology. Endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 1-2 — promoter assay + ChIP + siRNA epistasis identifying HNF4α as transcriptional driver; replicated in vivo","pmids":["35499234"],"is_preprint":false},{"year":2023,"finding":"SELENOP modulates canonical WNT signaling in colorectal carcinogenesis through direct protein-protein interactions with the WNT co-receptors LRP5 and LRP6: SELENOP KO in tumor organoids reduced WNT target gene expression (reversible by SELENOP restoration), SELENOP increased WNT signaling activity in CRC and non-cancer cell lines, and protein-protein interaction mapping confirmed SELENOP-LRP5/6 binding as the mechanistic basis of this effect.","method":"Single-cell RNA-seq of human colon tissue; conditional intestinal Apc-deletion plus Selenop KO mouse model; tumor organoid formation assays; WNT target gene expression; SELENOP restoration experiments; protein-protein interaction mapping (SELENOP-LRP5/6); CRC cell line WNT reporter assays","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (organoids, mouse model, cell lines, PPI mapping) with mechanistic definition of LRP5/6 interaction","pmids":["37166989"],"is_preprint":false},{"year":2022,"finding":"Resveratrol is a potent suppressor of SELENOP expression in hepatocytes: high-throughput screening of 1861 FDA-approved drugs identified resveratrol, vidofludimus, and antimony potassium tartrate as the most potent suppressors (>3-fold reduction) of extracellular SELENOP concentrations in HepG2 cells, without affecting cell viability; RNA-seq revealed effects on metabolic pathways and vesicle trafficking.","method":"High-throughput ELISA-based drug screening (1861 compounds); dose-response characterization; cell viability assays; RNA-seq pathway analysis","journal":"Redox biology","confidence":"Medium","confidence_rationale":"Tier 3 — large-scale functional screen with validation, but mechanism of resveratrol action on SELENOP not fully resolved; single lab","pmids":["36586222"],"is_preprint":false}],"current_model":"SELENOP (selenoprotein P) is a liver-derived, heparin-binding extracellular glycoprotein secreted into plasma that carries up to 10 selenocysteine residues: its N-terminal thioredoxin-fold domain (with Sec40 in a UYLC motif) functions as a phospholipid hydroperoxide glutathione peroxidase and TrxR1-coupled peroxidase, while its selenium-rich C-terminal domain serves as the primary vehicle for systemic selenium transport from hepatocytes to selenium-privileged peripheral tissues (brain, testis, kidney) via receptor-mediated endocytosis through apoER2/LRP1 requiring heparan sulfate proteoglycan co-binding and lysosomal digestion; beyond selenium transport, SELENOP acts as a hepatokine that causes insulin resistance by inactivating AMPK in liver and skeletal muscle through its LRP1 muscle receptor, suppresses exercise adaptation by blocking ROS-AMPK-PGC-1α signaling via muscle LRP1, promotes lipid accumulation in NAFLD through AMPK/ACC inhibition, reduces adiponectin production, and modulates WNT/β-catenin signaling in the colon through direct protein-protein interactions with co-receptors LRP5/6; SELENOP transcription in hepatocytes is driven by HNF4α and regulated by FOXO1, AMPK/SIRT1, and dietary fatty acids."},"narrative":{"teleology":[{"year":1993,"claim":"Resolving how a single mRNA encodes 10 selenocysteine residues, cloning of human SELENOP revealed two 3′-UTR SECIS elements sufficient to direct readthrough of all in-frame UGA codons, establishing the gene's unique translational architecture.","evidence":"cDNA cloning from human liver/heart libraries with comparative sequence and secondary-structure analysis of the 3′ UTR","pmids":["8421687"],"confidence":"High","gaps":["Mechanism of ribosomal processivity through 10 consecutive UGA codons not resolved","Relative contribution of each SECIS element to decoding efficiency unknown"]},{"year":1994,"claim":"Biochemical purification established that selenoprotein P is a selenium-rich, heparin-binding extracellular glycoprotein accounting for the majority of plasma selenium, with rapid turnover consistent with a transport function.","evidence":"Purification from rat and human plasma by immunoaffinity and heparin-agarose chromatography; ⁷⁵Se radiolabeling; selenium content and half-life measurements","pmids":["7931697","8142465"],"confidence":"High","gaps":["Tissue-specific uptake mechanism unknown at this stage","Glycosylation function not determined"]},{"year":1999,"claim":"Answering whether SELENOP possesses enzymatic activity beyond selenium transport, kinetic analysis demonstrated that purified SELENOP reduces phospholipid hydroperoxides via a glutathione-dependent ping-pong mechanism, establishing it as an extracellular antioxidant enzyme.","evidence":"In vitro enzymatic assay with purified human plasma SELENOP using various hydroperoxide substrates and thiol reductants","pmids":["9915822"],"confidence":"High","gaps":["Active-site selenocysteine residue responsible for peroxidase activity not yet identified by mutagenesis","Physiological relevance of extracellular peroxidase activity in vivo undetermined"]},{"year":2002,"claim":"Immunodepletion/reconstitution experiments showed that SELENOP, not extracellular GPx, is the essential selenium supply protein in serum, as its removal abolished cellular selenoenzyme activities that were restored only by SELENOP repletion.","evidence":"Immunodepletion of SELENOP or GPx3 from human serum followed by culture of Jurkat cells and measurement of intracellular GPx and TrxR activities","pmids":["12423375"],"confidence":"High","gaps":["Receptor-mediated uptake mechanism not yet identified","Whether selenium delivery requires full-length protein or C-terminal domain alone unknown"]},{"year":2008,"claim":"Genetic rescue experiments answered whether liver is the physiological source of circulating SELENOP: hepatocyte-specific transgenic expression of SEPP1 in Sepp-null mice restored plasma selenium, rescued neurological defects, and recovered selenoenzyme activity in brain, testis, and kidney.","evidence":"Transgenic mice expressing human SEPP1 under a hepatocyte-specific promoter in Sepp⁻/⁻ background; tissue selenium and selenoenzyme activity measurements; neurological and fertility phenotyping","pmids":["17961124"],"confidence":"High","gaps":["Local (non-hepatic) SELENOP production contribution not quantified","Whether other organs can compensate under specific conditions unclear"]},{"year":2012,"claim":"Receptor identification and conditional knockout studies defined the molecular mechanism of SELENOP uptake and whole-body selenium distribution: apoER2 mediates endocytosis of SELENOP's selenium-rich C-terminal domain in muscle cells requiring heparan sulfate proteoglycans and lysosomal processing, while hepatocyte-specific Sepp1 deletion confirmed the liver as the dominant source controlling systemic selenium balance.","evidence":"Affinity pulldown with mass spectrometry receptor identification; siRNA knockdown of apoER2 and LRP1; ⁷⁵Se uptake assays with inhibitors; conditional hepatocyte-specific Sepp1 knockout mice with tissue selenium and urinary selenium measurements","pmids":["22761431","23038251"],"confidence":"High","gaps":["Brain-specific uptake receptor (apoER2 vs. megalin) hierarchy not fully resolved","Structural basis of SELENOP–apoER2 interaction unknown"]},{"year":2010,"claim":"Discovery that SELENOP acts as a hepatokine causing insulin resistance opened a new functional dimension: purified SELENOP impaired insulin signaling through AMPK inactivation, and Selenop knockout or knockdown improved glucose tolerance in mice.","evidence":"Human liver transcriptomic correlation; purified SELENOP administration to hepatocytes and myocytes; Selenop⁻/⁻ mice; siRNA knockdown; AMPK activity and insulin signaling assays","pmids":["21035759"],"confidence":"High","gaps":["Direct molecular target through which SELENOP inactivates AMPK not identified","Whether hepatokine function is selenium-dependent or protein-intrinsic not resolved"]},{"year":2014,"claim":"Active-site mutagenesis resolved the catalytic residue responsible for SELENOP peroxidase activity: Sec40 in the N-terminal UYLC motif is essential for TrxR1-coupled reduction of hydroperoxides, and truncated N-terminal fragments retain this activity.","evidence":"Purification of N-terminal fragments from megalin⁻/⁻ mouse urine; U40S mutagenesis; TrxR1-coupled NADPH oxidation assay","pmids":["24434121"],"confidence":"High","gaps":["Whether the peroxidase activity is physiologically relevant in renal tubule protection not tested in vivo"]},{"year":2017,"claim":"Identification of LRP1 as the muscle receptor mediating SELENOP's exercise-suppressive effect explained how a hepatokine impairs ROS-dependent AMPK–PGC-1α exercise adaptation: muscle-specific LRP1 knockout blunted SELENOP's inhibition of exercise-induced signaling.","evidence":"Selenop⁻/⁻ mice with exercise protocols; muscle-specific LRP1 conditional KO; LRP1-Fc blocking in myotubes; ROS, AMPK phosphorylation, and PGC-1α expression measurements","pmids":["28263310"],"confidence":"High","gaps":["Whether SELENOP-LRP1 signaling differs from selenium transport through apoER2 not mechanistically dissected","Downstream ROS target linking SELENOP to AMPK suppression not identified"]},{"year":2019,"claim":"Transcriptional regulation of SELENOP was defined: FOXO1 directly occupies the SELENOP promoter, and AMPK/SIRT1 signaling suppresses SELENOP transcription, creating a feedback loop where the hepatokine's own targets regulate its expression.","evidence":"ChIP-qPCR for FOXO1 on SELENOP promoter; AMPK and SIRT1 siRNA epistasis in palmitate-treated human primary hepatocytes; recombinant SELENOP reconstitution","pmids":["31246318"],"confidence":"Medium","gaps":["FOXO1 binding site not mapped at single-nucleotide resolution","Whether SIRT1 acts directly on FOXO1 acetylation at the SELENOP promoter not shown"]},{"year":2021,"claim":"A natural complete gene deletion of SELENOP in dogs causing cerebellar atrophy and ataxia demonstrated that SELENOP is essential for CNS selenium supply across species, extending the knockout phenotype from mice to a large-animal model.","evidence":"Linkage and homozygosity mapping; whole-genome sequencing identifying a 17.3 kb deletion; blood selenium measurement; histopathology; genotyping of >600 Belgian Shepherds","pmids":["34339417"],"confidence":"High","gaps":["Whether partial loss-of-function variants in humans cause neurological disease not yet demonstrated","Contribution of CNS-local SELENOP expression to the canine phenotype not assessed"]},{"year":2022,"claim":"Dietary selenium dose–response experiments with Selenop⁻/⁻ mice revealed that SELENOP is specifically required to preserve parvalbumin interneurons, linking selenium transport to GABAergic circuit integrity and epileptogenesis.","evidence":"Selenop⁻/⁻ mice on graded selenium diets; video-EEG seizure detection; histological quantification of parvalbumin interneurons; brain GPx activity; hepatocyte-specific SELENOP transgenic rescue","pmids":["36182809"],"confidence":"High","gaps":["Molecular mechanism of selective parvalbumin interneuron vulnerability not defined","Whether selenoprotein deficiency in these neurons is cell-autonomous or circuit-driven unknown"]},{"year":2022,"claim":"HNF4α was identified as a direct transcriptional activator of SELENOP in hepatocytes, with fatty acid (lauric acid) stimulation increasing HNF4α occupancy at the SELENOP promoter and linking dietary lipids to hepatokine production.","evidence":"Luciferase promoter assay; ChIP assay for HNF4α; Hnf4α siRNA epistasis; in vivo mouse liver validation","pmids":["35499234"],"confidence":"High","gaps":["Whether other fatty acids act through the same HNF4α mechanism not systematically tested","Interplay between HNF4α and FOXO1 on the SELENOP promoter not resolved"]},{"year":2023,"claim":"Discovery that SELENOP directly binds WNT co-receptors LRP5/LRP6 to modulate canonical WNT signaling in the colon expanded SELENOP's functional repertoire beyond selenium transport and metabolic hepatokine activity to include morphogenetic signaling in colorectal epithelium.","evidence":"Single-cell RNA-seq; conditional intestinal Apc-deletion plus Selenop KO mouse model; tumor organoid WNT target gene expression; SELENOP restoration; protein-protein interaction mapping for SELENOP–LRP5/6","pmids":["37166989"],"confidence":"High","gaps":["Whether SELENOP–LRP5/6 interaction is selenium-dependent or mediated by the protein scaffold not determined","Structural basis of SELENOP–LRP5/6 binding unknown","Effect on WNT signaling in non-intestinal tissues not explored"]},{"year":null,"claim":"Key unresolved questions include: the direct molecular mechanism by which SELENOP inactivates AMPK (whether selenium-dependent or protein-intrinsic), the structural basis of SELENOP interactions with its receptors (apoER2, LRP1, LRP5/6), whether human loss-of-function variants cause neurological disease, and how the dual antioxidant-enzyme and selenium-transport functions are coordinated in vivo.","evidence":"Open questions arising from the collective literature","pmids":[],"confidence":"High","gaps":["No crystal structure of SELENOP or SELENOP–receptor complexes available","Human genetic loss-of-function phenotype not established","Selenium-dependence of hepatokine and WNT-modulatory functions not dissected"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016209","term_label":"antioxidant activity","supporting_discovery_ids":[3,9]},{"term_id":"GO:0016491","term_label":"oxidoreductase activity","supporting_discovery_ids":[3,9]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[1,4,5,7]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[6,11]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,11,19]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[1,2,3,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[6,11,19]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[6,13]}],"complexes":[],"partners":["LRP8","LRP1","LRP5","LRP6","TXNRD1","HNF4A","FOXO1"],"other_free_text":[]},"mechanistic_narrative":"SELENOP encodes selenoprotein P, a liver-derived, heparin-binding extracellular glycoprotein that serves as the principal selenium transport protein in plasma, delivering selenium to privileged tissues—brain, testis, and kidney—via receptor-mediated endocytosis through apoER2 and LRP1, with heparan sulfate proteoglycan co-binding and lysosomal processing required for selenium release [PMID:22761431, PMID:17961124, PMID:23038251]. Its N-terminal thioredoxin-fold domain harbors a selenocysteine at position 40 (UYLC motif) that confers thioredoxin reductase 1–coupled peroxidase activity and phospholipid hydroperoxide glutathione peroxidase activity in extracellular fluids, while its selenium-rich C-terminal domain (up to 10 selenocysteine residues) constitutes the selenium cargo [PMID:9915822, PMID:24434121, PMID:8421687]. Beyond selenium homeostasis, SELENOP functions as a hepatokine that impairs insulin signaling and suppresses exercise adaptation by inactivating AMPK in liver and skeletal muscle through LRP1, promotes hepatic lipid accumulation via the AMPK/ACC axis, and modulates WNT/β-catenin signaling through direct interaction with the co-receptors LRP5/LRP6 in colonic epithelium [PMID:21035759, PMID:28263310, PMID:33094480, PMID:37166989]. Loss of SELENOP causes cerebellar atrophy, ataxia, and seizures due to failure of selenium delivery to the CNS, as demonstrated in knockout mice and a naturally occurring canine deletion [PMID:34339417, PMID:36182809]."},"prefetch_data":{"uniprot":{"accession":"P49908","full_name":"Selenoprotein P","aliases":[],"length_aa":381,"mass_kda":43.2,"function":"Might be responsible for some of the extracellular antioxidant defense properties of selenium or might be involved in the transport of selenium. 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hepatocyte-specific deletion of Sepp1 lowered plasma Sepp1 to 10% of control, increased urinary selenium excretion, and severely worsened selenium deficiency in extra-hepatic tissues, demonstrating that hepatic Sepp1 synthesis is favored over other selenoproteins under selenium-limited conditions.\",\n      \"method\": \"Conditional hepatocyte-specific knockout mice (Sepp1(c/c)/alb-cre), tissue selenium measurements, urinary selenium excretion assays, mRNA quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple orthogonal readouts (tissue selenium, urinary excretion, mRNA), replicated in selenium-deficient conditions\",\n      \"pmids\": [\"23038251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Liver-derived circulating selenoprotein P (SePP) is the main transport form of selenium supporting the kidney, testis, and brain under adequate selenium conditions; however, locally expressed SePP is required to maintain selenium content in selenium-privileged tissues such as brain and testis during dietary selenium restriction, as shown by rescue of neurological defects and male infertility in Sepp-/- mice by hepatocyte-specific transgenic human SEPP1 expression.\",\n      \"method\": \"Sepp-/- knockout mice, hepatocyte-specific transgenic rescue (transthyretin promoter-driven human SEPP1), selenium content measurements, selenoenzyme activity assays, behavioral phenotyping\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue experiment with multiple orthogonal functional readouts (fertility, neurological phenotype, tissue selenium, enzyme activity)\",\n      \"pmids\": [\"17961124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Sepp1 supplies selenium to cells via apolipoprotein E receptor-2 (apoER2)-mediated endocytosis requiring binding to heparan sulfate proteoglycans; the selenium-rich C-terminal domain (residues 240–361) is required for uptake. ApoER2 (but not LRP1) was identified as the functional receptor by siRNA knockdown. Lysosomal acidification and digestion of Sepp1 is required for selenium utilization from the protein.\",\n      \"method\": \"Rat L8 myoblast cell culture, 75Se-labeling, Sepp1 affinity column with mass spectrometry identification of receptors, siRNA knockdown of apoER2 and Lrp1, chlorate treatment, protamine addition, lysosome acidification blockade, truncated Sepp1 isoform experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (MS receptor identification, siRNA, radiolabeled selenium, isoform comparison, pharmacological inhibitors) in a single study\",\n      \"pmids\": [\"22761431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"N-terminal truncation fragments of Sepp1 (Sepp1(UF)), which contain the thioredoxin-fold domain with selenocysteine-40, function as peroxidases when coupled with thioredoxin reductase-1 (TrxR1), catalyzing NADPH oxidation with H2O2 or tert-butylhydroperoxide as terminal electron acceptors. The selenocysteine at position 40 (U40) is essential for this peroxidase activity, as the U40S mutant is not a TrxR1 substrate. Full-length Sepp1 and the C-terminal truncation Sepp1(Δ240-361) also show this activity.\",\n      \"method\": \"Purification of urinary Sepp1 fragments from megalin-/- mice, mass spectrometry, in vitro TrxR1-coupled peroxidase assay (NADPH oxidation), comparison of full-length vs. truncated vs. U40S mutant Sepp1\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic reconstitution with active-site mutagenesis (U40S) and multiple Sepp1 isoforms tested\",\n      \"pmids\": [\"24434121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Selenoprotein P (SeP) uptake in kidneys is mediated by megalin (Lrp2) receptor-dependent endocytosis, which recaptures filtered SeP from primary urine; ApoER2 mediates SeP uptake in the brain and testes. Absence of functional megalin causes significant reduction of plasma selenium due to urinary Se excretion, and ApoER2 deletion leads to lowered selenium in brain and testes, neurological dysfunction, and infertility.\",\n      \"method\": \"Review and synthesis of receptor knockout mouse studies (megalin-/- and ApoER2-/- mice), selenium measurements, phenotypic characterization\",\n      \"journal\": \"Journal of trace elements in medicine and biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review of prior KO mouse data without new primary experiments, but consistent with multiple independent KO studies\",\n      \"pmids\": [\"22683052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Deletion of the SELENOP gene in dogs causes defective selenium transport into the CNS, leading to CNS atrophy and cerebellar ataxia; blood selenium was reduced to ~30% of wildtype in homozygous mutant dogs, confirming SELENOP's essential role in selenium delivery to the nervous system.\",\n      \"method\": \"Whole-genome sequencing, linkage and homozygosity mapping, genotyping of >600 dogs, blood selenium measurement, histopathology\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — natural loss-of-function deletion with co-segregation, biochemical validation (selenium levels), and histopathological phenotype in multiple animals\",\n      \"pmids\": [\"34339417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SELENOP expression in Selenop-/- mice is essential for the survival of parvalbumin-expressing interneurons in the somatosensory cortex and hippocampus under limiting selenium supply; loss of SELENOP leads to epileptic seizures, ataxia, and tremor that can be prevented by selenium supplementation or transgenic hepatocyte-specific SELENOP expression, establishing a dose- and time-dependent requirement for SELENOP in CNS interneuron maintenance.\",\n      \"method\": \"Selenop-/- knockout mice, video-EEG, selenium supplementation experiments, transgenic hepatocyte-specific SELENOP rescue, glutathione peroxidase activity in brain, histological analysis of parvalbumin interneurons\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (EEG, enzyme activity, histology, genetic rescue, dose-response supplementation) in KO mouse model\",\n      \"pmids\": [\"36182809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Selenoprotein P (SeP) participates in NAFLD pathogenesis by inhibiting AMPK/ACC phosphorylation, promoting lipid accumulation; siRNA knockdown of SEPP1 activated AMPK/ACC signaling and inhibited triglyceride accumulation, while SEPP1 overexpression aggravated lipid accumulation and suppressed AMPK/ACC phosphorylation.\",\n      \"method\": \"siRNA knockdown and overexpression of SEPP1 in hepatocyte cell lines and in vivo NAFLD models, Western blotting for AMPK/ACC phosphorylation, triglyceride measurement\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab with gain- and loss-of-function experiments and biochemical readout, but limited mechanistic depth\",\n      \"pmids\": [\"33094480\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SELENOP interacts physically with WNT co-receptors LRP5 and LRP6 and activates canonical WNT signaling; Selenop-KO tumor organoids showed defects in organoid formation and decreased WNT target gene expression reversible by SELENOP restoration, and SELENOP-LRP5/6 protein-protein interactions were mapped and shown to contribute to WNT pathway activity in colorectal cancer and non-cancer cell lines.\",\n      \"method\": \"Human scRNA-Seq, conditional intestinal Apc-KO mouse adenoma model crossed with Selenop-KO, tumor organoid reconstitution, protein-protein interaction mapping (SELENOP-LRP5/6), WNT target gene expression, CRC cell line gain/loss-of-function\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (KO mouse model, organoid rescue, protein interaction mapping, cell line experiments) from a single rigorous study\",\n      \"pmids\": [\"37166989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Protectin DX (PDX) suppresses selenoprotein P (SeP) expression in hepatocytes via AMPK/SIRT1 activation; palmitate-induced SeP upregulation and insulin resistance were attenuated by PDX, and siRNA silencing of AMPK or SIRT1 prevented PDX's suppressive effects on SeP; recombinant SeP reversed the suppressive effects on palmitate-mediated impairment of insulin signaling, establishing SeP as a mediator of hepatic insulin resistance downstream of fatty acid signaling.\",\n      \"method\": \"Human primary hepatocytes, palmitate treatment, siRNA knockdown of AMPK and SIRT1, Western blotting, FOXO1 binding by qPCR, recombinant SeP add-back experiments\",\n      \"journal\": \"Clinical and experimental pharmacology & physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, multiple siRNA and rescue experiments, but limited structural/in vitro enzymatic validation\",\n      \"pmids\": [\"31246318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Lauric acid upregulates SELENOP expression in hepatocytes via the transcription factor HNF4α binding to the SELENOP promoter; ChIP assay confirmed increased HNF4α binding at the SELENOP promoter after lauric acid treatment; HNF4α or Selenop knockdown rescued lauric acid-induced impairment of insulin-stimulated Akt phosphorylation.\",\n      \"method\": \"Hepa1-6 hepatocytes and mouse liver, luciferase promoter assay, ChIP assay, siRNA knockdown of Hnf4α and Selenop, Akt phosphorylation Western blotting\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and promoter assay with functional siRNA rescue, single lab\",\n      \"pmids\": [\"35499234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Exendin-4 reduces SEPP1 expression in hepatocytes via AMPK activation downstream of GLP-1 receptor signaling; AMPK siRNA abolished the suppressive effect of exendin-4 on SEPP1 and fetuin-A expression, and palmitate-induced ER stress upregulated SEPP1, which was attenuated by exendin-4.\",\n      \"method\": \"HepG2 cells, palmitate and tunicamycin treatment, exendin-4 treatment, AMPK siRNA transfection, qRT-PCR, Western blotting\",\n      \"journal\": \"Endocrinology and metabolism (Seoul, Korea)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single lab, siRNA epistasis with biochemical readout, moderate mechanistic depth\",\n      \"pmids\": [\"26194078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Resveratrol potently suppresses SELENOP expression and secretion in HepG2 cells in a dose-dependent manner (0.8–50 μM), with associated effects on metabolic pathway genes and vesicle trafficking, suggesting a direct pharmacological mechanism for modulating the central selenium transport protein.\",\n      \"method\": \"High-throughput drug screening of 1861 FDA-approved drugs in HepG2 cells using ELISA, dose-response dilution experiments, RNA-seq pathway analysis, cell viability assays\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — systematic screen with dose-response validation and transcriptomic follow-up, single lab, no direct mechanistic target identified for resveratrol on SELENOP\",\n      \"pmids\": [\"36586222\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SELENOP (Selenoprotein P) is a liver-secreted, multi-selenocysteine-containing glycoprotein that functions as the primary systemic transporter of selenium from hepatocytes to peripheral tissues—especially the brain, testes, and kidneys—via receptor-mediated endocytosis through ApoER2 (brain/testes) and megalin (kidney); its N-terminal thioredoxin-fold domain bearing selenocysteine-40 confers thioredoxin reductase-1-coupled peroxidase activity, while its C-terminal selenium-rich domain is required for receptor-mediated selenium delivery; additionally, SELENOP modulates canonical WNT signaling through direct interaction with LRP5/6 co-receptors and influences hepatic insulin sensitivity by suppressing AMPK/ACC phosphorylation, with its expression regulated transcriptionally by HNF4α in response to fatty acids.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper as KEEP (about SELENOP/selenoprotein P) or EXCLUDE (alias collision, alt-locus product, or unrelated gene), then extract mechanistic discoveries from KEPT papers only.\n\n**KEEP papers** (about canonical SELENOP/selenoprotein P):\n- PMID:23038251, 17961124, 22761431, 24434121, 21035759, 28263310, 9915822, 12423375, 7931697, 8421687, 8142465, 27645994, 37166989, 34339417, 36182809, 94078 (gene2pubmed set), 33094480, 31246318, 26194078, 35499234, 94008, 33840662, 27334433, 36586222\n\nFrom gene2pubmed set KEEP: 21035759, 28263310, 9915822, 12423375, 7931697, 8421687, 8142465, 27645994, 37166989, 22496878\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1993,\n      \"finding\": \"Human selenoprotein P mRNA contains two conserved stem-loop structures in its 3' untranslated region (SECIS elements), one for each domain, establishing that a separate stem-loop is not required for each of the 10 selenocysteine residues encoded by UGA codons in the open reading frame.\",\n      \"method\": \"cDNA cloning and sequencing of human selenoprotein P from liver/heart libraries; comparative sequence analysis with rat cDNA; secondary structure prediction of 3' UTR\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct sequence characterization of cloned cDNA with structural prediction, foundational paper with >160 citations\",\n      \"pmids\": [\"8421687\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Selenoprotein P is a selenium-rich extracellular glycoprotein containing selenocysteine residues; purified rat selenoprotein P contains ~7.5 selenium atoms per molecule, and in selenium-replete rats it contains ~65% of plasma selenium with rapid turnover (plasma half-life of 3–4 h for 75Se).\",\n      \"method\": \"Protein purification from rat/human plasma; 75Se radiolabeling; SDS-PAGE; selenium content measurement; Northern blot for tissue expression and selenium-deficiency regulation\",\n      \"journal\": \"The Journal of nutrition\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct biochemical characterization of purified protein; foundational paper >130 citations\",\n      \"pmids\": [\"7931697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Selenoprotein P was purified from human plasma by immunoaffinity chromatography followed by heparin-agarose chromatography, yielding two bands (61 and 55 kDa) that are both glycoproteins; the protein accounts for approximately one-third of total plasma selenium and binds heparin.\",\n      \"method\": \"Immunoaffinity chromatography with monoclonal antibodies; heparin-agarose chromatography; SDS-PAGE; carbohydrate staining; enzymatic deglycosylation; 75Se-labeled SELENOP from HepG2 cells as tracer\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct protein purification and biochemical characterization; replicated across labs\",\n      \"pmids\": [\"8142465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Purified human selenoprotein P functions as a phospholipid hydroperoxide glutathione peroxidase in extracellular fluids: it reduces phospholipid hydroperoxides (but not H2O2 or tert-butyl hydroperoxide) using glutathione and other thiol reductants via a tert-uni ping-pong kinetic mechanism.\",\n      \"method\": \"Conventional purification of selenoprotein P from human plasma; enzymatic assays with various hydroperoxide substrates and reducing agents; kinetic analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay with kinetic characterization; foundational mechanistic paper >200 citations\",\n      \"pmids\": [\"9915822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Selenoprotein P is the primary selenium supply protein in serum: immunodepletion of selenoprotein P (but not extracellular GPx) from human serum severely depleted cellular selenium and glutathione peroxidase/thioredoxin reductase activities in Jurkat T-lymphoma cells cultured in that serum; repletion with purified selenoprotein P restored activity.\",\n      \"method\": \"Immunodepletion of specific selenoproteins from human serum; cell culture with depleted/reconstituted serum; measurement of Se-dependent enzyme activities (GPx, TrxR); Se content assay\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — immunodepletion/reconstitution experiment with functional readout; >125 citations\",\n      \"pmids\": [\"12423375\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Liver-derived circulating selenoprotein P is the main selenium transport form that supplies the kidney, testis, and brain; hepatocyte-specific transgenic expression of SEPP1 in Sepp-/- mice rescued neurological defects (ataxia, seizures) and male infertility and increased selenium and selenoenzyme activities in plasma, kidney, testis, and brain, demonstrating that plasma SeP from hepatocytes supports selenium-privileged tissues.\",\n      \"method\": \"Transgenic rescue experiment: human SEPP1 under hepatocyte-specific transthyretin promoter in Sepp-/- mice; selenium content measurement in tissues; selenoenzyme activity assays; neurological phenotyping; fertility testing\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue with tissue-specific transgene, multiple functional readouts; >110 citations\",\n      \"pmids\": [\"17961124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Selenoprotein P (SeP) is a liver-derived hepatokine that causes insulin resistance: hepatic SeP mRNA correlated with insulin resistance in humans; purified SeP impaired insulin signaling and glucose metabolism in hepatocytes and myocytes; genetic deletion or siRNA knockdown of SeP improved systemic insulin sensitivity and glucose tolerance in mice; the metabolic actions were mediated at least partly by inactivation of AMP-activated protein kinase (AMPK).\",\n      \"method\": \"SAGE and DNA chip analysis of human liver; administration of purified SeP to cultured cells and mice; SeP knockout mice; siRNA knockdown; insulin signaling assays; glucose tolerance tests; AMPK activity assays\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (human correlative, cell-based, KO mouse, siRNA) with defined molecular mechanism (AMPK inactivation); >460 citations\",\n      \"pmids\": [\"21035759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Hepatocyte-produced selenoprotein P is central to whole-body selenium homeostasis: selective deletion of Sepp1 in hepatocytes lowered plasma Sepp1 to 10% of controls, increased urinary selenium excretion reducing whole-body selenium, and under selenium-deficient conditions caused liver selenium accumulation at the expense of extra-hepatic tissues. Additionally, Sepp1 synthesis is preferentially maintained over Gpx1 synthesis in hepatocytes under selenium limitation, directing selenium to peripheral tissues.\",\n      \"method\": \"Conditional hepatocyte-specific Sepp1 knockout mice (Sepp1c/c/alb-cre); plasma and tissue selenium measurement; urinary selenium excretion; selenoprotein mRNA quantification; clinical phenotyping under selenium deficiency\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with multiple biochemical and physiological readouts; >110 citations\",\n      \"pmids\": [\"23038251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"L8 myoblast cells take up Sepp1 via an apoER2-mediated endocytic mechanism requiring binding to heparan sulfate proteoglycans; the selenium-rich C-terminal domain of Sepp1 is required for uptake and utilization; Lrp1 is present on a Sepp1-binding membrane fraction but siRNA knockdown showed it is not required for 75Se uptake from Sepp1. Lysosomal acidification is needed for Sepp1 digestion and selenium release.\",\n      \"method\": \"Sepp1-affinity column pulldown from membrane fraction; mass spectrometry identification of receptors (apoER2, Lrp1); siRNA knockdown of receptors; 75Se radiolabeled Sepp1 uptake assays; protamine/chlorate inhibition; lysosome acidification blockade; Sepp1 isoform (Δ240-361) comparison\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — receptor identification by MS, siRNA validation, mechanistic dissection with multiple inhibitors and isoform variants\",\n      \"pmids\": [\"22761431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"N-terminal truncation fragments of Sepp1 (designated Sepp1UF), filtered at the glomerulus and taken up by renal proximal convoluted tubule cells via megalin-mediated endocytosis, possess peroxidase activity: they catalyze NADPH oxidation when coupled with thioredoxin reductase-1 (TrxR1) and H2O2 or tert-butylhydroperoxide as terminal electron acceptors. The single selenocysteine at position 40 (in the UYLC redox-active motif of the N-terminal thioredoxin-fold domain) is essential for this activity, as shown by Sepp1(U40S) mutagenesis.\",\n      \"method\": \"Purification of urinary Sepp1 from megalin-/- mice by monoclonal antibody; mass spectrometry identification of truncation sites; TrxR1-coupled peroxidase assay with NADPH; mutagenesis (U40S selenocysteine-to-serine); comparison of full-length vs. truncated vs. mutant Sepp1 as TrxR1 substrates\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic reconstitution with active-site mutagenesis and MS characterization\",\n      \"pmids\": [\"24434121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Selenoprotein P nomenclature was standardized: the approved HGNC symbol SELENOP replaces prior symbols (SeP, SEPP1, SELP), resolving conflicts with P-selectin (SELP) and other genes.\",\n      \"method\": \"Systematic gene nomenclature review by selenoprotein experts and HUGO Gene Nomenclature Committee\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — expert consensus nomenclature paper, directly relevant to gene identity; >220 citations\",\n      \"pmids\": [\"27645994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"SeP causes exercise resistance through its muscle receptor LRP1: SeP-deficient mice showed enhanced exercise endurance, increased ROS production, AMPK phosphorylation, and PGC-1α expression in skeletal muscle. SeP treatment suppressed H2O2-induced adaptations in cultured myotubes via LRP1, and muscle-specific LRP1 knockout blunted SeP's inhibitory effects on exercise-induced AMPK phosphorylation and Ppargc1a expression in vivo.\",\n      \"method\": \"SeP knockout mice; exercise training protocols; ROS measurement; AMPK phosphorylation assay; PGC-1α/Ppargc1a expression; NAC antioxidant treatment; LRP1-Fc blocking; muscle-specific LRP1 conditional knockout mice; cultured myotube experiments; human cohort correlation\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple genetic models (SeP KO, muscle-specific LRP1 KO), in vitro receptor blocking, and human data; >130 citations\",\n      \"pmids\": [\"28263310\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Elevated circulating SeP levels are inversely correlated with adiponectin in type 2 diabetes patients, and SeP knockout mice show increased blood adiponectin levels on both regular chow and high-fat diet, suggesting that liver-derived SeP suppresses adiponectin production.\",\n      \"method\": \"Cross-sectional clinical study with Spearman correlation and multiple regression; SeP knockout mice on different diets with adiponectin measurement\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — KO mouse functional readout combined with human correlation; single lab\",\n      \"pmids\": [\"22496878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SeP participates in NAFLD pathogenesis through the AMPK/ACC pathway: siRNA knockdown of SEPP1 inhibited triglyceride accumulation by activating AMPK/ACC phosphorylation, while SEPP1 overexpression aggravated lipid accumulation and inhibited AMPK/ACC phosphorylation in HepG2 cells and in mouse NAFLD models.\",\n      \"method\": \"siRNA knockdown and overexpression of SEPP1 in HepG2 cells; in vivo NAFLD mouse model; triglyceride accumulation assay; Western blot for AMPK/ACC phosphorylation\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — gain and loss-of-function with defined signaling pathway readout; single lab\",\n      \"pmids\": [\"33094480\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Protectin DX reduces hepatic insulin resistance by suppressing SeP expression via AMPK/SIRT1 signaling: AMPK or SIRT1 siRNA abolished the suppressive effects of PDX on palmitate-induced SeP expression, and the mechanism involves FOXO1-mediated transcriptional regulation of SeP. Recombinant SeP reversed the improvement in insulin signaling conferred by PDX treatment.\",\n      \"method\": \"Human primary hepatocyte treatment with palmitate and PDX; siRNA knockdown of AMPK and SIRT1; FOXO1 ChIP qPCR; Western blot for insulin signaling; recombinant SeP reconstitution\",\n      \"journal\": \"Clinical and experimental pharmacology & physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA epistasis with reconstitution experiment identifying FOXO1 as transcriptional regulator of SeP; single lab\",\n      \"pmids\": [\"31246318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Exendin-4 (GLP-1 receptor agonist) reduces SEPP1 expression in hepatocytes via AMPK activation: AMPK activator AICAR negatively regulated SEPP1 and fetuin-A expression, and exendin-4 failed to suppress SEPP1 expression in cells transfected with AMPK siRNA, establishing AMPK as the effector linking GLP-1 signaling to SeP suppression. The mechanism involves reduction of palmitate-induced ER stress.\",\n      \"method\": \"HepG2 cell treatment with palmitate, tunicamycin, exendin-4, AICAR; AMPK siRNA knockdown; RT-PCR and Western blot for SEPP1, fetuin-A, ER stress markers\",\n      \"journal\": \"Endocrinology and metabolism (Seoul, Korea)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA epistasis establishes AMPK as required mediator; single lab\",\n      \"pmids\": [\"26194078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Deletion of the entire SELENOP gene in dogs causes cerebellar atrophy and ataxia (CACA) associated with severely reduced blood selenium (~30% of wild-type), demonstrating that SELENOP is essential for selenium transport into the CNS in dogs and that its loss phenocopies but exceeds the severity of Selenop-/- knockout mice.\",\n      \"method\": \"Combined linkage and homozygosity mapping; whole genome sequencing; identification of homozygous 17.3 kb deletion encompassing the entire SELENOP coding sequence; blood selenium measurement; histopathology; genotyping of >600 Belgian Shepherd dogs\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — natural loss-of-function with complete gene deletion, confirmed in multiple affected animals, with biochemical and pathological validation\",\n      \"pmids\": [\"34339417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SELENOP is essential for preserving parvalbumin-expressing interneuron survival under limiting selenium supply: Selenop-/- mice on the recommended dietary allowance (RDA) diet developed epileptic seizures and ataxia that were prevented by selenium supplementation or hepatocyte-specific SELENOP transgene expression; selenium supplementation restored brain glutathione peroxidase activity to control levels, and the neurological phenotype was dose- and time-dependent on selenium supply.\",\n      \"method\": \"Selenop-/- and Selenop+/- mice on varying selenium diets; video-EEG for seizure detection; behavioral phenotyping (ataxia, tremor); brain GPx activity assay; histological analysis of parvalbumin interneurons; transgenic rescue with hepatocyte-specific human SELENOP\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with EEG-confirmed seizures, biochemical rescue, and transgenic complementation\",\n      \"pmids\": [\"36182809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Lauric acid upregulates SELENOP expression in hepatocytes through HNF4α: luciferase promoter assays identified an HNF4α binding site in the SELENOP promoter, ChIP assay confirmed increased HNF4α binding upon lauric acid treatment, and Hnf4α siRNA knockdown abolished lauric acid-induced Selenop upregulation and the associated impairment of insulin-induced Akt phosphorylation.\",\n      \"method\": \"Luciferase reporter assay; ChIP assay; siRNA knockdown of Hnf4α and Selenop; computational transcription factor binding site analysis; Western blot for Akt phosphorylation; in vivo mouse liver validation\",\n      \"journal\": \"American journal of physiology. Endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — promoter assay + ChIP + siRNA epistasis identifying HNF4α as transcriptional driver; replicated in vivo\",\n      \"pmids\": [\"35499234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SELENOP modulates canonical WNT signaling in colorectal carcinogenesis through direct protein-protein interactions with the WNT co-receptors LRP5 and LRP6: SELENOP KO in tumor organoids reduced WNT target gene expression (reversible by SELENOP restoration), SELENOP increased WNT signaling activity in CRC and non-cancer cell lines, and protein-protein interaction mapping confirmed SELENOP-LRP5/6 binding as the mechanistic basis of this effect.\",\n      \"method\": \"Single-cell RNA-seq of human colon tissue; conditional intestinal Apc-deletion plus Selenop KO mouse model; tumor organoid formation assays; WNT target gene expression; SELENOP restoration experiments; protein-protein interaction mapping (SELENOP-LRP5/6); CRC cell line WNT reporter assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (organoids, mouse model, cell lines, PPI mapping) with mechanistic definition of LRP5/6 interaction\",\n      \"pmids\": [\"37166989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Resveratrol is a potent suppressor of SELENOP expression in hepatocytes: high-throughput screening of 1861 FDA-approved drugs identified resveratrol, vidofludimus, and antimony potassium tartrate as the most potent suppressors (>3-fold reduction) of extracellular SELENOP concentrations in HepG2 cells, without affecting cell viability; RNA-seq revealed effects on metabolic pathways and vesicle trafficking.\",\n      \"method\": \"High-throughput ELISA-based drug screening (1861 compounds); dose-response characterization; cell viability assays; RNA-seq pathway analysis\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — large-scale functional screen with validation, but mechanism of resveratrol action on SELENOP not fully resolved; single lab\",\n      \"pmids\": [\"36586222\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SELENOP (selenoprotein P) is a liver-derived, heparin-binding extracellular glycoprotein secreted into plasma that carries up to 10 selenocysteine residues: its N-terminal thioredoxin-fold domain (with Sec40 in a UYLC motif) functions as a phospholipid hydroperoxide glutathione peroxidase and TrxR1-coupled peroxidase, while its selenium-rich C-terminal domain serves as the primary vehicle for systemic selenium transport from hepatocytes to selenium-privileged peripheral tissues (brain, testis, kidney) via receptor-mediated endocytosis through apoER2/LRP1 requiring heparan sulfate proteoglycan co-binding and lysosomal digestion; beyond selenium transport, SELENOP acts as a hepatokine that causes insulin resistance by inactivating AMPK in liver and skeletal muscle through its LRP1 muscle receptor, suppresses exercise adaptation by blocking ROS-AMPK-PGC-1α signaling via muscle LRP1, promotes lipid accumulation in NAFLD through AMPK/ACC inhibition, reduces adiponectin production, and modulates WNT/β-catenin signaling in the colon through direct protein-protein interactions with co-receptors LRP5/6; SELENOP transcription in hepatocytes is driven by HNF4α and regulated by FOXO1, AMPK/SIRT1, and dietary fatty acids.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SELENOP (Selenoprotein P) is a liver-secreted, multi-selenocysteine glycoprotein that serves as the principal systemic selenium transporter, distributing selenium from hepatocytes to peripheral tissues including the brain, testes, and kidneys [PMID:23038251, PMID:17961124]. Tissue-specific selenium delivery is mediated by receptor-dependent endocytosis: ApoER2 mediates uptake in brain and testes, while megalin recaptures filtered SELENOP in the kidney, with the selenium-rich C-terminal domain (residues 240–361) required for receptor-mediated uptake and lysosomal processing for selenium release [PMID:22761431, PMID:22683052]. The N-terminal thioredoxin-fold domain containing selenocysteine-40 confers thioredoxin reductase-1-coupled peroxidase activity independent of its transport function [PMID:24434121]. Beyond selenium transport, SELENOP modulates canonical WNT signaling through direct interaction with LRP5/6 co-receptors [PMID:37166989], impairs hepatic insulin sensitivity by suppressing AMPK/ACC phosphorylation [PMID:33094480], and its transcription is upregulated by HNF4α in response to fatty acids [PMID:35499234].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Hepatocyte-derived SELENOP was established as the primary systemic selenium carrier supporting peripheral tissues, while local SELENOP expression was shown to be additionally required for brain and testes maintenance under selenium restriction — resolving whether circulating versus local SELENOP pools have distinct physiological roles.\",\n      \"evidence\": \"Sepp-/- knockout mice rescued with hepatocyte-specific transgenic human SEPP1; selenium content, selenoenzyme activity, neurological and fertility phenotyping\",\n      \"pmids\": [\"17961124\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of SELENOP hierarchy over other selenoproteins under selenium limitation was not defined\", \"Identity of SELENOP receptors not addressed in this study\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"The receptor system for SELENOP-mediated selenium delivery was identified: ApoER2 mediates uptake in muscle, brain, and testes while megalin recaptures SELENOP in kidneys — and the C-terminal selenium-rich domain was shown to be required for receptor-mediated endocytosis, with lysosomal digestion necessary for selenium release.\",\n      \"evidence\": \"Affinity purification and mass spectrometry of Sepp1-binding receptors, siRNA knockdown of ApoER2 and LRP1 in L8 myoblasts, 75Se-labeling, truncated isoform experiments, lysosome acidification blockade; complemented by receptor-KO mouse phenotyping reviewed in PMID:22683052\",\n      \"pmids\": [\"22761431\", \"22683052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of SELENOP–ApoER2 and SELENOP–megalin interactions not resolved\", \"Heparan sulfate proteoglycan co-receptor role not fully mechanistically defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Hepatocyte-specific Sepp1 deletion demonstrated that liver is the dominant source of circulating SELENOP and that hepatic SELENOP synthesis is prioritized over other hepatic selenoproteins under selenium-limiting conditions, establishing a hierarchy in selenium allocation.\",\n      \"evidence\": \"Conditional hepatocyte-specific knockout mice (Sepp1(c/c)/alb-cre), tissue selenium measurements, urinary selenium excretion\",\n      \"pmids\": [\"23038251\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of transcriptional or translational prioritization of SELENOP over other selenoproteins not identified\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"The N-terminal thioredoxin-fold domain of SELENOP was shown to function as a bona fide peroxidase when coupled with thioredoxin reductase-1, with selenocysteine-40 essential for catalytic activity — establishing an enzymatic function for SELENOP independent of its selenium transport role.\",\n      \"evidence\": \"Purified urinary Sepp1 fragments from megalin-/- mice, in vitro TrxR1-coupled NADPH oxidation assay with H2O2 and tBHP substrates, U40S active-site mutant comparison\",\n      \"pmids\": [\"24434121\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological substrates and in vivo contribution of SELENOP peroxidase activity remain undefined\", \"Whether peroxidase and transport functions operate simultaneously or are regulated independently is unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"AMPK was identified as a key negative regulator of SELENOP expression in hepatocytes, linking metabolic stress (palmitate, ER stress) to SELENOP upregulation and connecting SELENOP to hepatic insulin resistance pathways.\",\n      \"evidence\": \"HepG2 cells treated with palmitate and exendin-4, AMPK siRNA epistasis, qRT-PCR and Western blotting\",\n      \"pmids\": [\"26194078\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether AMPK regulates SELENOP transcription directly or via intermediate transcription factors was not resolved\", \"In vivo relevance of GLP-1R-AMPK-SELENOP axis not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"SELENOP was positioned as a functional mediator of fatty acid-induced hepatic insulin resistance: palmitate-induced SELENOP upregulation impaired insulin signaling, and recombinant SELENOP add-back reversed protective effects of AMPK/SIRT1 activation, establishing SELENOP's hepatokine-like role.\",\n      \"evidence\": \"Human primary hepatocytes, palmitate treatment, siRNA knockdown of AMPK and SIRT1, recombinant SeP add-back, insulin signaling readouts\",\n      \"pmids\": [\"31246318\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular target of SELENOP in suppressing insulin signaling not identified\", \"In vivo metabolic phenotype of SELENOP modulation in insulin resistance not tested in this study\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"A direct link between SELENOP and AMPK/ACC signaling was demonstrated: SELENOP overexpression suppressed AMPK/ACC phosphorylation and promoted hepatic lipid accumulation, while knockdown activated AMPK/ACC — connecting SELENOP to NAFLD pathogenesis.\",\n      \"evidence\": \"siRNA knockdown and overexpression in hepatocyte cell lines and NAFLD mouse models, Western blotting for AMPK/ACC phosphorylation, triglyceride quantification\",\n      \"pmids\": [\"33094480\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which SELENOP suppresses AMPK phosphorylation (receptor-mediated or intracellular) remains unknown\", \"Whether the selenium content of SELENOP is required for metabolic effects is untested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Natural SELENOP loss-of-function in dogs confirmed conservation of the selenium transport function across mammals, with homozygous deletion causing CNS atrophy and cerebellar ataxia — phenocopying the mouse knockout neurological syndrome.\",\n      \"evidence\": \"Whole-genome sequencing, linkage/homozygosity mapping in >600 dogs, blood selenium measurement, histopathology\",\n      \"pmids\": [\"34339417\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No human Mendelian disease directly attributed to SELENOP mutations has been reported\", \"Detailed neuropathological mechanism (cell type specificity) not characterized in dog model\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"HNF4α was identified as a transcription factor directly activating SELENOP expression in response to fatty acids, with ChIP confirming promoter occupancy — providing a mechanism connecting dietary lipid signals to SELENOP upregulation and insulin resistance.\",\n      \"evidence\": \"Hepa1-6 hepatocytes and mouse liver, ChIP assay, luciferase promoter assay, siRNA knockdown of HNF4α and Selenop, Akt phosphorylation readout\",\n      \"pmids\": [\"35499234\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether HNF4α is the sole or dominant transcription factor for SELENOP under physiological conditions is not established\", \"Interaction with other known SELENOP regulatory pathways (AMPK, SIRT1) not integrated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"SELENOP was shown to be essential specifically for the survival of parvalbumin-expressing interneurons in cortex and hippocampus, explaining the seizure phenotype in Selenop-/- mice and establishing cell-type-specific vulnerability to selenium depletion in the CNS.\",\n      \"evidence\": \"Selenop-/- mice, video-EEG, selenium supplementation dose-response, transgenic hepatocyte-specific rescue, GPx activity in brain, parvalbumin interneuron histology\",\n      \"pmids\": [\"36182809\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why parvalbumin interneurons are selectively vulnerable to SELENOP loss is unknown\", \"Which selenoprotein(s) downstream of SELENOP are critical for interneuron survival is not identified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A non-canonical signaling function for SELENOP was discovered: it directly binds WNT co-receptors LRP5/6 and activates canonical WNT signaling, expanding SELENOP's role beyond selenium transport to include growth factor pathway modulation in intestinal tumors.\",\n      \"evidence\": \"Conditional intestinal Apc-KO × Selenop-KO mouse adenoma model, tumor organoid reconstitution, SELENOP-LRP5/6 protein-protein interaction mapping, WNT target gene expression in CRC cell lines\",\n      \"pmids\": [\"37166989\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SELENOP's WNT-activating function requires its selenocysteine residues or selenium content is unknown\", \"Structural basis of SELENOP-LRP5/6 interaction not resolved\", \"Relevance to non-intestinal WNT-dependent tissues not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the structural basis of SELENOP interactions with its multiple receptors (ApoER2, megalin, LRP5/6), whether the peroxidase and WNT-signaling functions are physiologically significant in vivo, and whether SELENOP mutations cause a human Mendelian disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of full-length SELENOP or SELENOP-receptor complexes exists\", \"In vivo significance of SELENOP peroxidase activity versus transport function not dissected\", \"No human loss-of-function disease attributed to SELENOP\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 1, 2, 5]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 1, 2, 5]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 2, 4, 5]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [7, 8, 9, 10]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LRP8\",\n      \"LRP2\",\n      \"LRP5\",\n      \"LRP6\",\n      \"TXNRD1\",\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\": \"SELENOP encodes selenoprotein P, a liver-derived, heparin-binding extracellular glycoprotein that serves as the principal selenium transport protein in plasma, delivering selenium to privileged tissues—brain, testis, and kidney—via receptor-mediated endocytosis through apoER2 and LRP1, with heparan sulfate proteoglycan co-binding and lysosomal processing required for selenium release [PMID:22761431, PMID:17961124, PMID:23038251]. Its N-terminal thioredoxin-fold domain harbors a selenocysteine at position 40 (UYLC motif) that confers thioredoxin reductase 1–coupled peroxidase activity and phospholipid hydroperoxide glutathione peroxidase activity in extracellular fluids, while its selenium-rich C-terminal domain (up to 10 selenocysteine residues) constitutes the selenium cargo [PMID:9915822, PMID:24434121, PMID:8421687]. Beyond selenium homeostasis, SELENOP functions as a hepatokine that impairs insulin signaling and suppresses exercise adaptation by inactivating AMPK in liver and skeletal muscle through LRP1, promotes hepatic lipid accumulation via the AMPK/ACC axis, and modulates WNT/β-catenin signaling through direct interaction with the co-receptors LRP5/LRP6 in colonic epithelium [PMID:21035759, PMID:28263310, PMID:33094480, PMID:37166989]. Loss of SELENOP causes cerebellar atrophy, ataxia, and seizures due to failure of selenium delivery to the CNS, as demonstrated in knockout mice and a naturally occurring canine deletion [PMID:34339417, PMID:36182809].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Resolving how a single mRNA encodes 10 selenocysteine residues, cloning of human SELENOP revealed two 3′-UTR SECIS elements sufficient to direct readthrough of all in-frame UGA codons, establishing the gene's unique translational architecture.\",\n      \"evidence\": \"cDNA cloning from human liver/heart libraries with comparative sequence and secondary-structure analysis of the 3′ UTR\",\n      \"pmids\": [\"8421687\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of ribosomal processivity through 10 consecutive UGA codons not resolved\", \"Relative contribution of each SECIS element to decoding efficiency unknown\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Biochemical purification established that selenoprotein P is a selenium-rich, heparin-binding extracellular glycoprotein accounting for the majority of plasma selenium, with rapid turnover consistent with a transport function.\",\n      \"evidence\": \"Purification from rat and human plasma by immunoaffinity and heparin-agarose chromatography; ⁷⁵Se radiolabeling; selenium content and half-life measurements\",\n      \"pmids\": [\"7931697\", \"8142465\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific uptake mechanism unknown at this stage\", \"Glycosylation function not determined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Answering whether SELENOP possesses enzymatic activity beyond selenium transport, kinetic analysis demonstrated that purified SELENOP reduces phospholipid hydroperoxides via a glutathione-dependent ping-pong mechanism, establishing it as an extracellular antioxidant enzyme.\",\n      \"evidence\": \"In vitro enzymatic assay with purified human plasma SELENOP using various hydroperoxide substrates and thiol reductants\",\n      \"pmids\": [\"9915822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Active-site selenocysteine residue responsible for peroxidase activity not yet identified by mutagenesis\", \"Physiological relevance of extracellular peroxidase activity in vivo undetermined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Immunodepletion/reconstitution experiments showed that SELENOP, not extracellular GPx, is the essential selenium supply protein in serum, as its removal abolished cellular selenoenzyme activities that were restored only by SELENOP repletion.\",\n      \"evidence\": \"Immunodepletion of SELENOP or GPx3 from human serum followed by culture of Jurkat cells and measurement of intracellular GPx and TrxR activities\",\n      \"pmids\": [\"12423375\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor-mediated uptake mechanism not yet identified\", \"Whether selenium delivery requires full-length protein or C-terminal domain alone unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Genetic rescue experiments answered whether liver is the physiological source of circulating SELENOP: hepatocyte-specific transgenic expression of SEPP1 in Sepp-null mice restored plasma selenium, rescued neurological defects, and recovered selenoenzyme activity in brain, testis, and kidney.\",\n      \"evidence\": \"Transgenic mice expressing human SEPP1 under a hepatocyte-specific promoter in Sepp⁻/⁻ background; tissue selenium and selenoenzyme activity measurements; neurological and fertility phenotyping\",\n      \"pmids\": [\"17961124\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Local (non-hepatic) SELENOP production contribution not quantified\", \"Whether other organs can compensate under specific conditions unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Receptor identification and conditional knockout studies defined the molecular mechanism of SELENOP uptake and whole-body selenium distribution: apoER2 mediates endocytosis of SELENOP's selenium-rich C-terminal domain in muscle cells requiring heparan sulfate proteoglycans and lysosomal processing, while hepatocyte-specific Sepp1 deletion confirmed the liver as the dominant source controlling systemic selenium balance.\",\n      \"evidence\": \"Affinity pulldown with mass spectrometry receptor identification; siRNA knockdown of apoER2 and LRP1; ⁷⁵Se uptake assays with inhibitors; conditional hepatocyte-specific Sepp1 knockout mice with tissue selenium and urinary selenium measurements\",\n      \"pmids\": [\"22761431\", \"23038251\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Brain-specific uptake receptor (apoER2 vs. megalin) hierarchy not fully resolved\", \"Structural basis of SELENOP–apoER2 interaction unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Discovery that SELENOP acts as a hepatokine causing insulin resistance opened a new functional dimension: purified SELENOP impaired insulin signaling through AMPK inactivation, and Selenop knockout or knockdown improved glucose tolerance in mice.\",\n      \"evidence\": \"Human liver transcriptomic correlation; purified SELENOP administration to hepatocytes and myocytes; Selenop⁻/⁻ mice; siRNA knockdown; AMPK activity and insulin signaling assays\",\n      \"pmids\": [\"21035759\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct molecular target through which SELENOP inactivates AMPK not identified\", \"Whether hepatokine function is selenium-dependent or protein-intrinsic not resolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Active-site mutagenesis resolved the catalytic residue responsible for SELENOP peroxidase activity: Sec40 in the N-terminal UYLC motif is essential for TrxR1-coupled reduction of hydroperoxides, and truncated N-terminal fragments retain this activity.\",\n      \"evidence\": \"Purification of N-terminal fragments from megalin⁻/⁻ mouse urine; U40S mutagenesis; TrxR1-coupled NADPH oxidation assay\",\n      \"pmids\": [\"24434121\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the peroxidase activity is physiologically relevant in renal tubule protection not tested in vivo\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identification of LRP1 as the muscle receptor mediating SELENOP's exercise-suppressive effect explained how a hepatokine impairs ROS-dependent AMPK–PGC-1α exercise adaptation: muscle-specific LRP1 knockout blunted SELENOP's inhibition of exercise-induced signaling.\",\n      \"evidence\": \"Selenop⁻/⁻ mice with exercise protocols; muscle-specific LRP1 conditional KO; LRP1-Fc blocking in myotubes; ROS, AMPK phosphorylation, and PGC-1α expression measurements\",\n      \"pmids\": [\"28263310\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SELENOP-LRP1 signaling differs from selenium transport through apoER2 not mechanistically dissected\", \"Downstream ROS target linking SELENOP to AMPK suppression not identified\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Transcriptional regulation of SELENOP was defined: FOXO1 directly occupies the SELENOP promoter, and AMPK/SIRT1 signaling suppresses SELENOP transcription, creating a feedback loop where the hepatokine's own targets regulate its expression.\",\n      \"evidence\": \"ChIP-qPCR for FOXO1 on SELENOP promoter; AMPK and SIRT1 siRNA epistasis in palmitate-treated human primary hepatocytes; recombinant SELENOP reconstitution\",\n      \"pmids\": [\"31246318\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"FOXO1 binding site not mapped at single-nucleotide resolution\", \"Whether SIRT1 acts directly on FOXO1 acetylation at the SELENOP promoter not shown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"A natural complete gene deletion of SELENOP in dogs causing cerebellar atrophy and ataxia demonstrated that SELENOP is essential for CNS selenium supply across species, extending the knockout phenotype from mice to a large-animal model.\",\n      \"evidence\": \"Linkage and homozygosity mapping; whole-genome sequencing identifying a 17.3 kb deletion; blood selenium measurement; histopathology; genotyping of >600 Belgian Shepherds\",\n      \"pmids\": [\"34339417\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether partial loss-of-function variants in humans cause neurological disease not yet demonstrated\", \"Contribution of CNS-local SELENOP expression to the canine phenotype not assessed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Dietary selenium dose–response experiments with Selenop⁻/⁻ mice revealed that SELENOP is specifically required to preserve parvalbumin interneurons, linking selenium transport to GABAergic circuit integrity and epileptogenesis.\",\n      \"evidence\": \"Selenop⁻/⁻ mice on graded selenium diets; video-EEG seizure detection; histological quantification of parvalbumin interneurons; brain GPx activity; hepatocyte-specific SELENOP transgenic rescue\",\n      \"pmids\": [\"36182809\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of selective parvalbumin interneuron vulnerability not defined\", \"Whether selenoprotein deficiency in these neurons is cell-autonomous or circuit-driven unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"HNF4α was identified as a direct transcriptional activator of SELENOP in hepatocytes, with fatty acid (lauric acid) stimulation increasing HNF4α occupancy at the SELENOP promoter and linking dietary lipids to hepatokine production.\",\n      \"evidence\": \"Luciferase promoter assay; ChIP assay for HNF4α; Hnf4α siRNA epistasis; in vivo mouse liver validation\",\n      \"pmids\": [\"35499234\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other fatty acids act through the same HNF4α mechanism not systematically tested\", \"Interplay between HNF4α and FOXO1 on the SELENOP promoter not resolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Discovery that SELENOP directly binds WNT co-receptors LRP5/LRP6 to modulate canonical WNT signaling in the colon expanded SELENOP's functional repertoire beyond selenium transport and metabolic hepatokine activity to include morphogenetic signaling in colorectal epithelium.\",\n      \"evidence\": \"Single-cell RNA-seq; conditional intestinal Apc-deletion plus Selenop KO mouse model; tumor organoid WNT target gene expression; SELENOP restoration; protein-protein interaction mapping for SELENOP–LRP5/6\",\n      \"pmids\": [\"37166989\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SELENOP–LRP5/6 interaction is selenium-dependent or mediated by the protein scaffold not determined\", \"Structural basis of SELENOP–LRP5/6 binding unknown\", \"Effect on WNT signaling in non-intestinal tissues not explored\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the direct molecular mechanism by which SELENOP inactivates AMPK (whether selenium-dependent or protein-intrinsic), the structural basis of SELENOP interactions with its receptors (apoER2, LRP1, LRP5/6), whether human loss-of-function variants cause neurological disease, and how the dual antioxidant-enzyme and selenium-transport functions are coordinated in vivo.\",\n      \"evidence\": \"Open questions arising from the collective literature\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal structure of SELENOP or SELENOP–receptor complexes available\", \"Human genetic loss-of-function phenotype not established\", \"Selenium-dependence of hepatokine and WNT-modulatory functions not dissected\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016209\", \"supporting_discovery_ids\": [3, 9]},\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [3, 9]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [1, 4, 5, 7]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [6, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 11, 19]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 2, 3, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0382551\", \"supporting_discovery_ids\": [1, 4, 5, 7, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [6, 11, 19]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [6, 13]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LRP8\",\n      \"LRP1\",\n      \"LRP5\",\n      \"LRP6\",\n      \"TXNRD1\",\n      \"HNF4A\",\n      \"FOXO1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}