{"gene":"FST","run_date":"2026-04-28T17:46:04","timeline":{"discoveries":[{"year":1988,"finding":"The human follistatin (FST) gene encodes a single-chain precursor of 344 or 317 amino acids (via alternative splicing). The mature protein contains four contiguous domains encoded by separate exons, three of which are highly similar to each other and to human EGF and pancreatic secretory trypsin inhibitor, suggesting exon shuffling during evolution.","method":"cDNA library screening, genomic library analysis, sequencing, Southern blot, structural domain analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — direct sequencing and genomic organization with structural validation","pmids":["3380788"],"is_preprint":false},{"year":1991,"finding":"Follistatin binds to both activin and inhibin through the common beta-subunit. Activin-A has two binding sites for follistatin, whereas inhibin-A has only one binding site, as determined by double-ligand blotting.","method":"Double-ligand blotting technique","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1 — direct binding assay with defined stoichiometry, foundational paper with >270 citations","pmids":["2036994"],"is_preprint":false},{"year":1994,"finding":"Human placenta, fetal membranes (amnion, chorion), and decidua express follistatin mRNA and protein. Follistatin is localized in syncytial cells and secreted by cultured placental cells. Exogenous follistatin completely reverses activin A-induced hCG and progesterone release from placental cells, demonstrating a functional paracrine role in regulating placental hormone production.","method":"RT-PCR, immunohistochemistry, RIA of culture medium, functional cell culture assay with recombinant follistatin","journal":"The Journal of Clinical Endocrinology and Metabolism","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (RT-PCR, IHC, secretion assay, functional reversal) in single study","pmids":["8288705"],"is_preprint":false},{"year":1998,"finding":"The majority of follistatin's biological functions are mediated through its high-affinity binding to activin, neutralizing activin's effects. This activin-follistatin interplay constitutes a powerful regulatory mechanism in multiple tissues including the gonad, pituitary, pregnancy membranes, vasculature, and liver.","method":"Biochemical characterization, binding studies, functional assays (review synthesizing experimental work)","journal":"Frontiers in Neuroendocrinology","confidence":"High","confidence_rationale":"Tier 2 — synthesis of replicated experimental findings across multiple labs","pmids":["9799587"],"is_preprint":false},{"year":2002,"finding":"Activin A induces transcription of both FLRG and follistatin (FST) at the mRNA and protein level through Smad proteins, establishing a negative feedback loop: activin activates Smad-dependent FST transcription, and the secreted follistatin then neutralizes activin signaling.","method":"Transcriptional reporter assay, RT-PCR, Western blot, Smad pathway analysis","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods demonstrating Smad-mediated transcriptional feedback loop","pmids":["11948405"],"is_preprint":false},{"year":2003,"finding":"Follistatin modulates activin A-induced erythropoiesis in human primary CD34+ cells. Both follistatin and FLRG downmodulate the effects of activin A and BMP2 on erythroid maturation, indicating FST acts as a negative regulator of TGF-beta family member signaling during human erythropoiesis.","method":"In vitro erythroid differentiation of CD34+ primary cells, functional assay with follistatin protein","journal":"Experimental Cell Research","confidence":"Medium","confidence_rationale":"Tier 2 — clean functional assay in primary human cells, single lab","pmids":["12531697"],"is_preprint":false},{"year":2005,"finding":"Crystal structure of the follistatin:activin A complex reveals that two follistatin molecules encircle activin, burying one-third of its residues and blocking both type I and type II receptor binding sites. The follistatin N-terminal domain has an unexpected fold that mimics a universal type I receptor motif, directly occupying the type I receptor binding site on activin. The structure also explains how follistatin:BMP:type I receptor ternary complexes can form.","method":"X-ray crystallography (crystal structure determination)","journal":"Developmental Cell","confidence":"High","confidence_rationale":"Tier 1 — atomic resolution crystal structure with functional implications validated by structural analysis; >230 citations","pmids":["16198295"],"is_preprint":false},{"year":2006,"finding":"Three FST isoforms (FST288, FST303, FST315) differ in cell-surface binding activity (FST288 > FST303 > FST315), which determines their biological activity in inhibiting endogenous activin. Cell-surface proteoglycan binding is the primary determinant of differential isoform activity: FST288 suppresses whereas FST315 enhances activin-dependent TT cell proliferation. FST isoforms also differ from FSTL3 in ligand specificity: FST isoforms bind myostatin and some BMPs, whereas FSTL3 does not bind BMPs 2 and 4. Membrane anchoring of FSTL3 dramatically increases its endogenous activin inhibitory activity.","method":"Recombinant protein production, competitive binding assays, in vitro bioassay (TT cell proliferation), kinetic binding analysis (activin-binding affinities), membrane-anchoring construct experiments","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1–2 — reconstituted binding assays with multiple orthogonal functional approaches in single rigorous study","pmids":["16627583"],"is_preprint":false},{"year":2007,"finding":"A follistatin-derived myostatin inhibitor (FS I-I) selectively inhibits myostatin (Kd ~46.8 nM) with ~1000-fold reduced affinity for activin (Kd ~64.3 μM), compared to full-length follistatin which binds activin with Kd ~1.72 nM and myostatin with Kd ~12.3 nM. Transgenic expression of FS I-I in skeletal muscle increases muscle mass and strength via both hyperplasia and hypertrophy, and ameliorates pathology in mdx dystrophic mice.","method":"In vitro binding assays (Kd determination), transgenic mouse generation, muscle morphometry, functional strength testing, mdx mouse crossing","journal":"FASEB Journal","confidence":"High","confidence_rationale":"Tier 1–2 — quantitative binding assays combined with in vivo transgenic rescue experiments","pmids":["17893249"],"is_preprint":false},{"year":2008,"finding":"FoxL2 and Smad3 coordinately regulate follistatin gene transcription. Activin induces follistatin transcription primarily through Smad3 at an intronic Smad-binding element (SBE1). FoxL2 binds an adjacent forkhead-binding element (FKHB) and directly associates with Smad3 (via Smad3's MH2 domain and FoxL2's intact forkhead domain) to drive SBE1-mediated transcription. Endogenous FoxL2 and Smad3 are recruited to the intronic enhancer by ChIP assay.","method":"Chromatin immunoprecipitation (ChIP), promoter reporter assay, shRNA knockdown, co-immunoprecipitation, site-directed mutagenesis of binding elements, proteomics approach for partner identification","journal":"The Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal approaches (ChIP, CoIP, mutagenesis, KD) in single rigorous study","pmids":["19106105"],"is_preprint":false},{"year":2008,"finding":"Integrin alpha6beta4 signaling induces demethylation of the FST promoter, leading to upregulation of FST mRNA expression. This epigenetic regulation is confirmed by bisulfite sequencing, and FST upregulation contributes to the motile/invasive phenotype conferred by alpha6beta4 in breast carcinoma cells.","method":"Affymetrix GeneChip expression profiling, siRNA, promoter analysis, ChIP, bisulfite sequencing, DNA methyltransferase inhibitor experiments","journal":"The Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods showing epigenetic regulation of FST, but FST is one of several genes studied","pmids":["19011242"],"is_preprint":false},{"year":2010,"finding":"Plasma follistatin increases markedly (~7-fold peak) during exercise and recovery. No net release was detected from the exercising limb (arterial-venous difference). In mice, swimming exercise increased both plasma follistatin and hepatic follistatin mRNA and protein, identifying the liver as the major source of circulating follistatin. IL-6 and epinephrine did not drive hepatic follistatin secretion.","method":"Arterial-venous difference measurements in humans, quantitative PCR, Western blotting in mouse tissues, cytokine/epinephrine infusion experiments","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (AV difference, tissue mRNA/protein, pharmacological) across human and mouse, replicated finding","pmids":["21068158"],"is_preprint":false},{"year":2014,"finding":"Overexpressed ERRβ induces FST-mediated apoptosis in breast cancer cells and enhances E-cadherin expression via upregulation of FST. ERRβ drives FST transcription through β-catenin/TCF4 recruitment to the FST promoter (identified by ChIP cloning). ERRβ-mediated BCAS2 upregulation inhibits FST transcription by downregulating β-catenin/TCF4 recruitment, constituting a regulatory circuit. FST overexpression also restricts MMP7 expression.","method":"ChIP cloning, gel supershift assay, co-immunoprecipitation, confocal microscopy, Western blotting, quantitative RT-PCR, functional apoptosis assays","journal":"British Journal of Cancer","confidence":"Medium","confidence_rationale":"Tier 2–3 — ChIP and CoIP with functional readouts, single lab","pmids":["24667650"],"is_preprint":false},{"year":2015,"finding":"The liver is the major contributor to circulating follistatin in humans, both at rest and during exercise, as demonstrated by arterial-to-venous differences across the splanchnic bed. Glucagon increases and insulin inhibits follistatin secretion from the liver both in vivo and in vitro, mediated via the secondary messenger cAMP in hepatocytes. Short-term follistatin treatment reduced glucagon secretion from islets of Langerhans, and long-term treatment prevented apoptosis and induced proliferation of rat β cells.","method":"Arterial-to-venous difference measurements (splanchnic bed), cultured hepatocyte experiments, cAMP pathway analysis, islet of Langerhans functional assays","journal":"The Journal of Clinical Endocrinology and Metabolism","confidence":"High","confidence_rationale":"Tier 2 — definitive organ-level AV difference in humans combined with mechanistic in vitro hepatocyte experiments","pmids":["26652766"],"is_preprint":false},{"year":2017,"finding":"Crystal structures of the GDF11:FS288 complex, apo-GDF8, and apo-GDF11 reveal that despite high similarity, GDF11 has unique structural properties in the type I receptor binding site that confer greater SMAD2/3 signaling potency than GDF8. Follistatin-288 (FST288 isoform) forms a complex with GDF11 analogous to the activin:follistatin complex, neutralizing this TGF-beta ligand.","method":"X-ray crystallography (crystal structure determination), SMAD2/3 signaling assays, receptor binding assays, site-directed mutagenesis (GDF8 with GDF11 residues)","journal":"BMC Biology","confidence":"High","confidence_rationale":"Tier 1 — crystal structures with functional validation by mutagenesis and signaling assays","pmids":["28257634"],"is_preprint":false},{"year":2018,"finding":"Follistatin (FST) functions as a pathological hepatokine during hepatic insulin resistance. Hepatic FoxO1 inactivation normalizes elevated FST levels in insulin receptor substrate 1/2 double knockout (LDKO) mice. Knockdown of hepatic FST in LDKO mice restored glucose tolerance, white adipose tissue insulin signaling, and insulin-mediated suppression of hepatic glucose production. Conversely, overexpression of FST in healthy mouse liver impaired glucose homeostasis. Serum FST decreased in parallel with HbA1c in obese diabetic patients following gastric bypass surgery.","method":"Hepatic knockdown (shRNA/siRNA), transgenic mouse models (LDKO, LTKO), glucose tolerance tests, insulin signaling assays, hepatokine identification, human clinical correlation","journal":"Nature Medicine","confidence":"High","confidence_rationale":"Tier 2 — gain- and loss-of-function in vivo experiments with multiple metabolic readouts, replicated across genetic and dietary models","pmids":["29867232"],"is_preprint":false}],"current_model":"Follistatin (FST) is a secreted glycoprotein that functions primarily as a high-affinity activin-binding and neutralizing protein: two FST molecules encircle activin to block both type I and type II receptor binding sites (crystal structure established), with the N-terminal domain mimicking a type I receptor motif; FST also binds myostatin, inhibin (via the beta-subunit), and select BMPs with isoform-specific affinity determined by differential cell-surface proteoglycan binding (FST288 > FST303 > FST315); activin induces FST transcription via Smad3 and FoxL2 at an intronic enhancer, creating a negative feedback loop; the liver is the principal source of circulating FST, regulated by the glucagon-to-insulin ratio via cAMP, and excess hepatic FST acts as a pathological hepatokine impairing glucose homeostasis during insulin resistance."},"narrative":{"teleology":[{"year":1988,"claim":"Cloning of the FST gene revealed the modular exon-encoded domain architecture and two alternative splice forms, establishing the structural framework for understanding FST's multi-domain binding properties.","evidence":"cDNA and genomic library screening with sequencing and Southern blot in human tissues","pmids":["3380788"],"confidence":"High","gaps":["Post-translational processing (glycosylation, proteolytic maturation) not resolved","Binding partners not yet identified"]},{"year":1991,"claim":"Demonstration that FST binds both activin and inhibin via their shared β-subunit, with activin providing two FST-binding sites versus one for inhibin, established the stoichiometric basis for FST's preferential activin neutralization.","evidence":"Double-ligand blotting with purified proteins","pmids":["2036994"],"confidence":"High","gaps":["Binding affinities not quantified","Structural basis of the 2:1 stoichiometry unknown"]},{"year":1994,"claim":"Functional paracrine activity was demonstrated in human placenta, where FST completely reversed activin A-induced hormone secretion, extending FST's role beyond the pituitary-gonadal axis to reproductive tissues.","evidence":"RT-PCR, immunohistochemistry, and functional reversal assays in cultured placental cells","pmids":["8288705"],"confidence":"High","gaps":["In vivo placental relevance not confirmed by genetic models","Other tissue-specific paracrine roles unexplored"]},{"year":2002,"claim":"Discovery that activin induces FST transcription through Smad proteins established a direct negative feedback loop in which the target (FST) neutralizes its own inducer (activin).","evidence":"Transcriptional reporter assays, RT-PCR, and Western blot with Smad pathway analysis","pmids":["11948405"],"confidence":"High","gaps":["Specific Smad isoform and cis-element mediating the response not mapped","Feedback kinetics not quantified"]},{"year":2005,"claim":"The crystal structure of the FST:activin A complex resolved the long-standing question of how FST neutralizes activin: two FST molecules wrap around activin, and the N-terminal domain adopts a type I receptor-mimicking fold that occludes the type I binding site.","evidence":"X-ray crystallography of the 2:1 FST:activin complex","pmids":["16198295"],"confidence":"High","gaps":["Dynamics of complex formation in solution not characterized","Structural basis for differential BMP versus activin affinity not fully resolved"]},{"year":2006,"claim":"Systematic comparison of the three FST isoforms showed that differential cell-surface proteoglycan binding (FST288 > FST303 > FST315) is the primary determinant of their distinct bioactivities, and that FST isoforms bind myostatin and select BMPs whereas the related protein FSTL3 does not.","evidence":"Recombinant protein competitive binding assays, kinetic analysis, and TT cell proliferation bioassay","pmids":["16627583"],"confidence":"High","gaps":["Proteoglycan identity mediating surface retention not defined","In vivo isoform-specific functions not dissected genetically"]},{"year":2007,"claim":"Engineering of a follistatin-derived myostatin-selective inhibitor (FS I-I) demonstrated that FST's myostatin-binding capacity can be uncoupled from activin binding, and that selective myostatin inhibition increases muscle mass and ameliorates muscular dystrophy in mdx mice.","evidence":"Quantitative in vitro binding assays and transgenic mouse muscle phenotyping including mdx cross","pmids":["17893249"],"confidence":"High","gaps":["Long-term safety and off-target effects of FS I-I in vivo unknown","Mechanism of hyperplasia versus hypertrophy contribution not resolved"]},{"year":2008,"claim":"Identification of FoxL2 as a Smad3 co-factor at an intronic enhancer of the FST gene refined the transcriptional feedback model, showing that activin-driven FST expression requires cooperative binding of Smad3 and FoxL2.","evidence":"ChIP, co-immunoprecipitation, site-directed mutagenesis, and shRNA knockdown in ovarian granulosa-like cells","pmids":["19106105"],"confidence":"High","gaps":["Whether this enhancer module operates in non-gonadal tissues is unknown","Epigenetic regulation of this element not addressed"]},{"year":2010,"claim":"Exercise physiology studies identified the liver—not exercising skeletal muscle—as the principal source of the marked rise in circulating FST during exercise, reframing FST as a hepatokine.","evidence":"Arterial-venous difference measurements in exercising humans, hepatic mRNA/protein quantification in exercising mice","pmids":["21068158"],"confidence":"High","gaps":["Hepatic signaling pathway driving exercise-induced FST release not identified","Contribution of other organs (e.g., adipose) not excluded"]},{"year":2015,"claim":"Splanchnic arteriovenous measurements in humans confirmed the liver as the dominant source of circulating FST, and mechanistic studies showed glucagon stimulates and insulin inhibits hepatic FST secretion via cAMP, linking FST to metabolic regulation.","evidence":"Arteriovenous difference across splanchnic bed in humans, cAMP-pathway analysis in cultured hepatocytes, islet functional assays","pmids":["26652766"],"confidence":"High","gaps":["Direct cAMP-responsive element on FST promoter not mapped in hepatocytes","Autocrine/paracrine effects of FST on hepatocytes themselves unclear"]},{"year":2017,"claim":"Crystal structure of the GDF11:FST288 complex demonstrated that FST neutralizes GDF11 by a mechanism analogous to activin, broadening the structural understanding of FST's multi-ligand antagonism within the TGF-β family.","evidence":"X-ray crystallography of GDF11:FST288 complex with functional signaling and mutagenesis validation","pmids":["28257634"],"confidence":"High","gaps":["In vivo relevance of FST:GDF11 antagonism versus FST:myostatin antagonism not dissected","Affinity hierarchy across all TGF-β ligands not fully quantified"]},{"year":2018,"claim":"Gain- and loss-of-function experiments in mouse liver established FST as a pathological hepatokine: elevated hepatic FST during insulin resistance impairs systemic glucose homeostasis and adipose insulin signaling, effects reversed by hepatic FST knockdown.","evidence":"Hepatic shRNA knockdown and overexpression in LDKO and wild-type mice, GTT/ITT, human bariatric surgery cohort correlation","pmids":["29867232"],"confidence":"High","gaps":["Target tissue receptor or signaling mechanism through which circulating FST impairs glucose metabolism is unknown","Whether FST's metabolic effect is activin-dependent or involves a distinct receptor pathway is unresolved"]},{"year":null,"claim":"The receptor or signaling mechanism by which circulating FST exerts its metabolic (hepatokine) effects on peripheral tissues remains unidentified, and isoform-specific in vivo functions have not been resolved by conditional genetic models.","evidence":"","pmids":[],"confidence":"High","gaps":["No receptor for FST's metabolic actions identified","Conditional isoform-specific knockouts not reported","Structural basis for isoform-specific proteoglycan interaction unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,6,7,8,14]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[1,3,7,8]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[6,7,14]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[2,3,7,11,13,15]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,3,4,6,7]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[13,15]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[2,5,8]}],"complexes":[],"partners":["INHBA","MSTN","GDF11","SMAD3","FOXL2","BMP2"],"other_free_text":[]},"mechanistic_narrative":"Follistatin (FST) is a secreted glycoprotein that functions as a high-affinity antagonist of activin, myostatin, and select TGF-β family ligands, thereby regulating diverse processes including reproductive physiology, erythropoiesis, muscle mass, and metabolic homeostasis. Two FST molecules encircle activin, burying one-third of its surface and blocking both type I and type II receptor binding sites; the FST N-terminal domain adopts a fold that mimics the type I receptor motif to occupy the corresponding activin epitope [PMID:16198295]. Three splice isoforms (FST288, FST303, FST315) differ in cell-surface proteoglycan binding affinity, which determines their local versus systemic activin-neutralizing potency, while activin itself induces FST transcription through Smad3 and FoxL2 at an intronic enhancer, forming a negative feedback loop [PMID:16627583, PMID:19106105]. The liver is the principal source of circulating FST, regulated by the glucagon-to-insulin ratio via cAMP; excess hepatic FST acts as a pathological hepatokine that impairs glucose homeostasis during insulin resistance [PMID:26652766, PMID:29867232]."},"prefetch_data":{"uniprot":{"accession":"P19883","full_name":"Follistatin","aliases":["Activin-binding protein"],"length_aa":344,"mass_kda":38.0,"function":"Multifunctional regulatory protein whose primary function is to antagonize members of the transforming growth factor beta (TGF-beta) superfamily including activin, myostatin, GDF11 or bone morphogenetic proteins (BMPs) (PubMed:11279126, PubMed:16482217, PubMed:18535106). Mechanistically, binds to these ligands in the extracellular space, blocking their type II receptor-binding site to inhibit downstream signaling (PubMed:16482217). Plays an essential role in muscle fiber formation and growth both by preventing the repressive effects of myostatin and through SMAD3/AKT/mTOR signaling independently of myostatin (By similarity). Also promotes neural differentiation by antagonizing the action BMP4 (By similarity). Acts as a specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH) by sequestering activin A/INHBA (PubMed:11279126). On the other hand, translocates into the nucleus where it down-regulates rRNA synthesis and ribosome biogenesis to maintain cellular energy homeostasis by binding to rDNA","subcellular_location":"Secreted; Nucleus, nucleolus","url":"https://www.uniprot.org/uniprotkb/P19883/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FST","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FST","total_profiled":1310},"omim":[{"mim_id":"616634","title":"SUSHI, NIDOGEN, AND EGF-LIKE DOMAINS PROTEIN 1; SNED1","url":"https://www.omim.org/entry/616634"},{"mim_id":"612849","title":"UBIQUITIN-SPECIFIC PEPTIDASE 46; USP46","url":"https://www.omim.org/entry/612849"},{"mim_id":"612031","title":"INHIBIN, BETA E; INHBE","url":"https://www.omim.org/entry/612031"},{"mim_id":"611755","title":"LEBER CONGENITAL AMAUROSIS 10; LCA10","url":"https://www.omim.org/entry/611755"},{"mim_id":"610142","title":"CENTROSOMAL PROTEIN, 290-KD; CEP290","url":"https://www.omim.org/entry/610142"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"liver","ntpm":66.8}],"url":"https://www.proteinatlas.org/search/FST"},"hgnc":{"alias_symbol":["FS"],"prev_symbol":[]},"alphafold":{"accession":"P19883","domains":[{"cath_id":"3.90.290.10","chopping":"28-119","consensus_level":"medium","plddt":93.9335,"start":28,"end":119},{"cath_id":"3.30.60.30","chopping":"130-189","consensus_level":"high","plddt":97.7512,"start":130,"end":189},{"cath_id":"3.30.60.30","chopping":"201-239","consensus_level":"medium","plddt":97.5651,"start":201,"end":239},{"cath_id":"3.30.60.30","chopping":"243-317","consensus_level":"medium","plddt":93.7996,"start":243,"end":317}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P19883","model_url":"https://alphafold.ebi.ac.uk/files/AF-P19883-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P19883-F1-predicted_aligned_error_v6.png","plddt_mean":88.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FST","jax_strain_url":"https://www.jax.org/strain/search?query=FST"},"sequence":{"accession":"P19883","fasta_url":"https://rest.uniprot.org/uniprotkb/P19883.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P19883/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P19883"}},"corpus_meta":[{"pmid":"10098262","id":"PMC_10098262","title":"Indirect measures of gene flow and migration: FST not equal to 1/(4Nm + 1).","date":"1999","source":"Heredity","url":"https://pubmed.ncbi.nlm.nih.gov/10098262","citation_count":699,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21429174","id":"PMC_21429174","title":"Comparison of F(ST) outlier tests for SNP loci under selection.","date":"2011","source":"Molecular ecology resources","url":"https://pubmed.ncbi.nlm.nih.gov/21429174","citation_count":305,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"11752264","id":"PMC_11752264","title":"FLAGdb/FST: a database of mapped flanking insertion sites (FSTs) of Arabidopsis thaliana T-DNA transformants.","date":"2002","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/11752264","citation_count":196,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"21375616","id":"PMC_21375616","title":"G'ST and D do not replace FST.","date":"2011","source":"Molecular ecology","url":"https://pubmed.ncbi.nlm.nih.gov/21375616","citation_count":175,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"28305014","id":"PMC_28305014","title":"fs(1)K10, a germline-dependent female sterile mutation causing abnormal chorion morphology inDrosophila melanogaster.","date":"1978","source":"Wilhelm Roux's archives of developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/28305014","citation_count":116,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"20705896","id":"PMC_20705896","title":"Rapid developmental maturation of neocortical FS cell intrinsic excitability.","date":"2010","source":"Cerebral cortex (New York, N.Y. : 1991)","url":"https://pubmed.ncbi.nlm.nih.gov/20705896","citation_count":103,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"3007240","id":"PMC_3007240","title":"Genetic and molecular analysis of fs(1)h, a maternal effect homeotic gene in Drosophila.","date":"1986","source":"Developmental 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signaling","url":"https://pubmed.ncbi.nlm.nih.gov/21900206","citation_count":258,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"3380788","id":"PMC_3380788","title":"Primary structure of the human follistatin precursor and its genomic organization.","date":"1988","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/3380788","citation_count":241,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16198295","id":"PMC_16198295","title":"The structure of the follistatin:activin complex reveals antagonism of both type I and type II receptor binding.","date":"2005","source":"Developmental cell","url":"https://pubmed.ncbi.nlm.nih.gov/16198295","citation_count":233,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"9799587","id":"PMC_9799587","title":"Follistatin: a multifunctional regulatory protein.","date":"1998","source":"Frontiers in 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follistatin increases skeletal muscle mass and ameliorates dystrophic pathology in mdx mice.","date":"2007","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/17893249","citation_count":156,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"21068158","id":"PMC_21068158","title":"Exercise induces a marked increase in plasma follistatin: evidence that follistatin is a contraction-induced hepatokine.","date":"2010","source":"Endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/21068158","citation_count":151,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"19011242","id":"PMC_19011242","title":"Integrin alpha6beta4 controls the expression of genes associated with cell motility, invasion, and metastasis, including S100A4/metastasin.","date":"2008","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19011242","citation_count":131,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"34591612","id":"PMC_34591612","title":"A protein interaction landscape of breast cancer.","date":"2021","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/34591612","citation_count":111,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20734064","id":"PMC_20734064","title":"A large-scale candidate gene association study of age at menarche and age at natural menopause.","date":"2010","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/20734064","citation_count":106,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26652766","id":"PMC_26652766","title":"Circulating Follistatin Is Liver-Derived and Regulated by the Glucagon-to-Insulin Ratio.","date":"2015","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/26652766","citation_count":105,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"30021884","id":"PMC_30021884","title":"Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.","date":"2018","source":"Molecular & cellular proteomics : MCP","url":"https://pubmed.ncbi.nlm.nih.gov/30021884","citation_count":101,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"11232039","id":"PMC_11232039","title":"Evidence for association of polycystic ovary syndrome in caucasian women with a marker at the insulin receptor gene locus.","date":"2001","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/11232039","citation_count":100,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"28257634","id":"PMC_28257634","title":"Structural basis for potency differences between GDF8 and GDF11.","date":"2017","source":"BMC biology","url":"https://pubmed.ncbi.nlm.nih.gov/28257634","citation_count":95,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"19106105","id":"PMC_19106105","title":"FoxL2 and Smad3 coordinately regulate follistatin gene transcription.","date":"2008","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19106105","citation_count":94,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15340161","id":"PMC_15340161","title":"Signal peptide prediction based on analysis of experimentally verified cleavage sites.","date":"2004","source":"Protein science : a publication of the Protein Society","url":"https://pubmed.ncbi.nlm.nih.gov/15340161","citation_count":92,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12531697","id":"PMC_12531697","title":"Regulation of human erythropoiesis by activin A, BMP2, and BMP4, members of the TGFbeta family.","date":"2003","source":"Experimental cell research","url":"https://pubmed.ncbi.nlm.nih.gov/12531697","citation_count":90,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"29867232","id":"PMC_29867232","title":"Inactivating hepatic follistatin alleviates hyperglycemia.","date":"2018","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/29867232","citation_count":83,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"12720540","id":"PMC_12720540","title":"Serum concentrations of activin and follistatin are elevated and run in parallel in patients with septicemia.","date":"2003","source":"European journal of endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/12720540","citation_count":82,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"11948405","id":"PMC_11948405","title":"Transcription activation of FLRG and follistatin by activin A, through Smad proteins, participates in a negative feedback loop to modulate activin A function.","date":"2002","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/11948405","citation_count":79,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"8288705","id":"PMC_8288705","title":"Local production and action of follistatin in human placenta.","date":"1994","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/8288705","citation_count":70,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":45433,"output_tokens":751,"usd":0.073782},"stage2":{"model":"claude-opus-4-6","input_tokens":3610,"output_tokens":660,"usd":0.051825},"total_usd":0.416036,"stage1_batch_id":"msgbatch_0118RScg3WGaGkaZRtWZbtce","stage2_batch_id":"msgbatch_01Cpfzvtbj42aqp5oYZ5Te2s","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":58890,"output_tokens":4979,"usd":0.125677},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":7792,"output_tokens":2835,"usd":0.164752}},"stage1_raw":"Looking at the corpus, I need to identify which papers are about human/mammalian FST (Follistatin). The vast majority of papers in this corpus are about population genetics FST statistics, PCDD/Fs (dioxins/furans), femtosecond (fs) laser physics, or other unrelated genes/organisms. Only a small number of papers are relevant to the human FST gene (Follistatin).\n\nLet me identify relevant papers:\n- PMID 24667650: ERRβ signaling through FST and BCAS2 in breast cancer cells\n- PMID 23265961: FST promoter methylation in PCOS (mostly expression/association, no mechanism)\n- PMID 28384189: FST SNPs and wool quality in sheep (association study)\n\nThe Drosophila papers (fs(1)K10, fs(1)Ya, fs(1)h, fs(1)Yb) are about female sterile genes in Drosophila that are NOT orthologs of human FST (Follistatin) — they are unrelated Drosophila female sterile mutations that happen to share \"fs\" in their names.\n\nFor human FST (Follistatin), only PMID 24667650 provides mechanistic information with direct experiments (ChIP, CoIP, western blotting, confocal microscopy).\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2014,\n      \"finding\": \"ERRβ directly transcriptionally activates FST in breast cancer cells, as demonstrated by chromatin immunoprecipitation (ChIP) cloning of ERRβ transcriptional targets, identifying FST as a downstream target gene. Overexpressed ERRβ induced FST-mediated apoptosis, and FST upregulation enhanced E-cadherin expression. Additionally, ERRβ-mediated upregulation of BCAS2 was shown to inhibit FST transcription through downregulation of β-catenin/TCF4 recruitment to the FST promoter, establishing a regulatory circuit in which FST expression is controlled by competing ERRβ and BCAS2 signals.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP) cloning, gel supershift assays, co-immunoprecipitation, confocal microscopy, western blotting, quantitative real-time PCR\",\n      \"journal\": \"British journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (ChIP, CoIP, functional overexpression assays) in a single study from one lab\",\n      \"pmids\": [\"24667650\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FST (Follistatin) is transcriptionally activated by ERRβ in breast cancer cells, where FST promotes apoptosis and E-cadherin expression; this pro-apoptotic FST signaling is counteracted by ERRβ-induced BCAS2, which suppresses FST transcription by reducing β-catenin/TCF4 recruitment to the FST promoter.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification of retrieved corpus (100 papers):**\n- Papers about FST (human follistatin gene): PMID 24667650 (ERRβ/FST in breast cancer), PMID 23265961 (FST methylation in PCOS), PMID 28384189 (FST SNPs in sheep - ortholog, KEEP for localization finding)\n- Papers about population genetics FST statistic: EXCLUDE (alias collision - FST as fixation index)\n- Papers about Drosophila fs(1)* genes: EXCLUDE (symbol collision - different genes)\n- Papers about PCDD/Fs (dioxins/furans): EXCLUDE (abbreviation collision)\n- Papers about fs laser pulses/femtosecond: EXCLUDE\n- Papers about Fst bacterial toxin: EXCLUDE (different gene/organism)\n- Papers about forced swim test (FST): EXCLUDE\n- Papers about FST database (Arabidopsis): EXCLUDE\n- Misc unrelated: EXCLUDE\n\n**Classification of gene2pubmed papers (30 papers):**\n- PMID 2036994: Follistatin binds activin/inhibin — KEEP\n- PMID 16198295: Follistatin:activin complex structure — KEEP\n- PMID 9799587: Follistatin multifunctional review — KEEP (mechanistic content)\n- PMID 3380788: Human follistatin primary structure — KEEP\n- PMID 16627583: FST isoforms biological activity — KEEP\n- PMID 17893249: Follistatin-derived myostatin inhibitor — KEEP\n- PMID 21068158: Exercise/follistatin hepatokine — KEEP\n- PMID 26652766: Circulating follistatin liver-derived — KEEP\n- PMID 19106105: FoxL2/Smad3 regulate FST transcription — KEEP\n- PMID 29867232: Hepatic follistatin/hyperglycemia — KEEP\n- PMID 12720540: Activin/follistatin in septicemia — KEEP (limited mechanism)\n- PMID 11948405: Activin induces FST via Smad/feedback loop — KEEP\n- PMID 8288705: Follistatin in placenta — KEEP\n- PMID 12531697: Follistatin modulates erythropoiesis — KEEP\n- PMID 28257634: GDF8/GDF11:FS288 structure — KEEP\n- PMID 10411917: FST linkage to PCOS — borderline (genetic linkage, limited mechanism) — EXCLUDE\n- PMID 19011242: Integrin alpha6beta4 regulates FST expression — KEEP\n- PMID 21900845: Exercise/myostatin/follistatin mRNA — expression data — EXCLUDE\n- PMID 12477932, 15489334: MGC cDNA projects — EXCLUDE\n- PMID 32296183, 33961781, 34591612, 21900206, 30021884: Interactome maps — EXCLUDE (no FST-specific mechanism)\n- PMID 21873635: GO propagation — EXCLUDE\n- PMID 19460752: shRNA screen — EXCLUDE\n- PMID 20734064: GWAS menarche/menopause — EXCLUDE\n- PMID 11232039: PCOS association — EXCLUDE\n- PMID 15340161: Signal peptide prediction — EXCLUDE\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1988,\n      \"finding\": \"The human follistatin (FST) gene encodes a single-chain precursor of 344 or 317 amino acids (via alternative splicing). The mature protein contains four contiguous domains encoded by separate exons, three of which are highly similar to each other and to human EGF and pancreatic secretory trypsin inhibitor, suggesting exon shuffling during evolution.\",\n      \"method\": \"cDNA library screening, genomic library analysis, sequencing, Southern blot, structural domain analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct sequencing and genomic organization with structural validation\",\n      \"pmids\": [\"3380788\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Follistatin binds to both activin and inhibin through the common beta-subunit. Activin-A has two binding sites for follistatin, whereas inhibin-A has only one binding site, as determined by double-ligand blotting.\",\n      \"method\": \"Double-ligand blotting technique\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding assay with defined stoichiometry, foundational paper with >270 citations\",\n      \"pmids\": [\"2036994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Human placenta, fetal membranes (amnion, chorion), and decidua express follistatin mRNA and protein. Follistatin is localized in syncytial cells and secreted by cultured placental cells. Exogenous follistatin completely reverses activin A-induced hCG and progesterone release from placental cells, demonstrating a functional paracrine role in regulating placental hormone production.\",\n      \"method\": \"RT-PCR, immunohistochemistry, RIA of culture medium, functional cell culture assay with recombinant follistatin\",\n      \"journal\": \"The Journal of Clinical Endocrinology and Metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (RT-PCR, IHC, secretion assay, functional reversal) in single study\",\n      \"pmids\": [\"8288705\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The majority of follistatin's biological functions are mediated through its high-affinity binding to activin, neutralizing activin's effects. This activin-follistatin interplay constitutes a powerful regulatory mechanism in multiple tissues including the gonad, pituitary, pregnancy membranes, vasculature, and liver.\",\n      \"method\": \"Biochemical characterization, binding studies, functional assays (review synthesizing experimental work)\",\n      \"journal\": \"Frontiers in Neuroendocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — synthesis of replicated experimental findings across multiple labs\",\n      \"pmids\": [\"9799587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Activin A induces transcription of both FLRG and follistatin (FST) at the mRNA and protein level through Smad proteins, establishing a negative feedback loop: activin activates Smad-dependent FST transcription, and the secreted follistatin then neutralizes activin signaling.\",\n      \"method\": \"Transcriptional reporter assay, RT-PCR, Western blot, Smad pathway analysis\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods demonstrating Smad-mediated transcriptional feedback loop\",\n      \"pmids\": [\"11948405\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Follistatin modulates activin A-induced erythropoiesis in human primary CD34+ cells. Both follistatin and FLRG downmodulate the effects of activin A and BMP2 on erythroid maturation, indicating FST acts as a negative regulator of TGF-beta family member signaling during human erythropoiesis.\",\n      \"method\": \"In vitro erythroid differentiation of CD34+ primary cells, functional assay with follistatin protein\",\n      \"journal\": \"Experimental Cell Research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean functional assay in primary human cells, single lab\",\n      \"pmids\": [\"12531697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Crystal structure of the follistatin:activin A complex reveals that two follistatin molecules encircle activin, burying one-third of its residues and blocking both type I and type II receptor binding sites. The follistatin N-terminal domain has an unexpected fold that mimics a universal type I receptor motif, directly occupying the type I receptor binding site on activin. The structure also explains how follistatin:BMP:type I receptor ternary complexes can form.\",\n      \"method\": \"X-ray crystallography (crystal structure determination)\",\n      \"journal\": \"Developmental Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — atomic resolution crystal structure with functional implications validated by structural analysis; >230 citations\",\n      \"pmids\": [\"16198295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Three FST isoforms (FST288, FST303, FST315) differ in cell-surface binding activity (FST288 > FST303 > FST315), which determines their biological activity in inhibiting endogenous activin. Cell-surface proteoglycan binding is the primary determinant of differential isoform activity: FST288 suppresses whereas FST315 enhances activin-dependent TT cell proliferation. FST isoforms also differ from FSTL3 in ligand specificity: FST isoforms bind myostatin and some BMPs, whereas FSTL3 does not bind BMPs 2 and 4. Membrane anchoring of FSTL3 dramatically increases its endogenous activin inhibitory activity.\",\n      \"method\": \"Recombinant protein production, competitive binding assays, in vitro bioassay (TT cell proliferation), kinetic binding analysis (activin-binding affinities), membrane-anchoring construct experiments\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — reconstituted binding assays with multiple orthogonal functional approaches in single rigorous study\",\n      \"pmids\": [\"16627583\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"A follistatin-derived myostatin inhibitor (FS I-I) selectively inhibits myostatin (Kd ~46.8 nM) with ~1000-fold reduced affinity for activin (Kd ~64.3 μM), compared to full-length follistatin which binds activin with Kd ~1.72 nM and myostatin with Kd ~12.3 nM. Transgenic expression of FS I-I in skeletal muscle increases muscle mass and strength via both hyperplasia and hypertrophy, and ameliorates pathology in mdx dystrophic mice.\",\n      \"method\": \"In vitro binding assays (Kd determination), transgenic mouse generation, muscle morphometry, functional strength testing, mdx mouse crossing\",\n      \"journal\": \"FASEB Journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — quantitative binding assays combined with in vivo transgenic rescue experiments\",\n      \"pmids\": [\"17893249\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FoxL2 and Smad3 coordinately regulate follistatin gene transcription. Activin induces follistatin transcription primarily through Smad3 at an intronic Smad-binding element (SBE1). FoxL2 binds an adjacent forkhead-binding element (FKHB) and directly associates with Smad3 (via Smad3's MH2 domain and FoxL2's intact forkhead domain) to drive SBE1-mediated transcription. Endogenous FoxL2 and Smad3 are recruited to the intronic enhancer by ChIP assay.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), promoter reporter assay, shRNA knockdown, co-immunoprecipitation, site-directed mutagenesis of binding elements, proteomics approach for partner identification\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal approaches (ChIP, CoIP, mutagenesis, KD) in single rigorous study\",\n      \"pmids\": [\"19106105\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Integrin alpha6beta4 signaling induces demethylation of the FST promoter, leading to upregulation of FST mRNA expression. This epigenetic regulation is confirmed by bisulfite sequencing, and FST upregulation contributes to the motile/invasive phenotype conferred by alpha6beta4 in breast carcinoma cells.\",\n      \"method\": \"Affymetrix GeneChip expression profiling, siRNA, promoter analysis, ChIP, bisulfite sequencing, DNA methyltransferase inhibitor experiments\",\n      \"journal\": \"The Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods showing epigenetic regulation of FST, but FST is one of several genes studied\",\n      \"pmids\": [\"19011242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Plasma follistatin increases markedly (~7-fold peak) during exercise and recovery. No net release was detected from the exercising limb (arterial-venous difference). In mice, swimming exercise increased both plasma follistatin and hepatic follistatin mRNA and protein, identifying the liver as the major source of circulating follistatin. IL-6 and epinephrine did not drive hepatic follistatin secretion.\",\n      \"method\": \"Arterial-venous difference measurements in humans, quantitative PCR, Western blotting in mouse tissues, cytokine/epinephrine infusion experiments\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (AV difference, tissue mRNA/protein, pharmacological) across human and mouse, replicated finding\",\n      \"pmids\": [\"21068158\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Overexpressed ERRβ induces FST-mediated apoptosis in breast cancer cells and enhances E-cadherin expression via upregulation of FST. ERRβ drives FST transcription through β-catenin/TCF4 recruitment to the FST promoter (identified by ChIP cloning). ERRβ-mediated BCAS2 upregulation inhibits FST transcription by downregulating β-catenin/TCF4 recruitment, constituting a regulatory circuit. FST overexpression also restricts MMP7 expression.\",\n      \"method\": \"ChIP cloning, gel supershift assay, co-immunoprecipitation, confocal microscopy, Western blotting, quantitative RT-PCR, functional apoptosis assays\",\n      \"journal\": \"British Journal of Cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — ChIP and CoIP with functional readouts, single lab\",\n      \"pmids\": [\"24667650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The liver is the major contributor to circulating follistatin in humans, both at rest and during exercise, as demonstrated by arterial-to-venous differences across the splanchnic bed. Glucagon increases and insulin inhibits follistatin secretion from the liver both in vivo and in vitro, mediated via the secondary messenger cAMP in hepatocytes. Short-term follistatin treatment reduced glucagon secretion from islets of Langerhans, and long-term treatment prevented apoptosis and induced proliferation of rat β cells.\",\n      \"method\": \"Arterial-to-venous difference measurements (splanchnic bed), cultured hepatocyte experiments, cAMP pathway analysis, islet of Langerhans functional assays\",\n      \"journal\": \"The Journal of Clinical Endocrinology and Metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — definitive organ-level AV difference in humans combined with mechanistic in vitro hepatocyte experiments\",\n      \"pmids\": [\"26652766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Crystal structures of the GDF11:FS288 complex, apo-GDF8, and apo-GDF11 reveal that despite high similarity, GDF11 has unique structural properties in the type I receptor binding site that confer greater SMAD2/3 signaling potency than GDF8. Follistatin-288 (FST288 isoform) forms a complex with GDF11 analogous to the activin:follistatin complex, neutralizing this TGF-beta ligand.\",\n      \"method\": \"X-ray crystallography (crystal structure determination), SMAD2/3 signaling assays, receptor binding assays, site-directed mutagenesis (GDF8 with GDF11 residues)\",\n      \"journal\": \"BMC Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures with functional validation by mutagenesis and signaling assays\",\n      \"pmids\": [\"28257634\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Follistatin (FST) functions as a pathological hepatokine during hepatic insulin resistance. Hepatic FoxO1 inactivation normalizes elevated FST levels in insulin receptor substrate 1/2 double knockout (LDKO) mice. Knockdown of hepatic FST in LDKO mice restored glucose tolerance, white adipose tissue insulin signaling, and insulin-mediated suppression of hepatic glucose production. Conversely, overexpression of FST in healthy mouse liver impaired glucose homeostasis. Serum FST decreased in parallel with HbA1c in obese diabetic patients following gastric bypass surgery.\",\n      \"method\": \"Hepatic knockdown (shRNA/siRNA), transgenic mouse models (LDKO, LTKO), glucose tolerance tests, insulin signaling assays, hepatokine identification, human clinical correlation\",\n      \"journal\": \"Nature Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain- and loss-of-function in vivo experiments with multiple metabolic readouts, replicated across genetic and dietary models\",\n      \"pmids\": [\"29867232\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Follistatin (FST) is a secreted glycoprotein that functions primarily as a high-affinity activin-binding and neutralizing protein: two FST molecules encircle activin to block both type I and type II receptor binding sites (crystal structure established), with the N-terminal domain mimicking a type I receptor motif; FST also binds myostatin, inhibin (via the beta-subunit), and select BMPs with isoform-specific affinity determined by differential cell-surface proteoglycan binding (FST288 > FST303 > FST315); activin induces FST transcription via Smad3 and FoxL2 at an intronic enhancer, creating a negative feedback loop; the liver is the principal source of circulating FST, regulated by the glucagon-to-insulin ratio via cAMP, and excess hepatic FST acts as a pathological hepatokine impairing glucose homeostasis during insulin resistance.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FST (Follistatin) is transcriptionally activated by ERRβ in breast cancer cells, where it promotes apoptosis and upregulates E-cadherin expression [PMID:24667650]. FST transcription is negatively regulated by BCAS2, which suppresses β-catenin/TCF4 recruitment to the FST promoter, establishing a regulatory circuit that balances pro-apoptotic FST signaling against BCAS2-mediated survival cues [PMID:24667650].\",\n  \"teleology\": [\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification of FST as a direct ERRβ transcriptional target in breast cancer cells established a mechanism by which ERRβ induces apoptosis and E-cadherin expression through FST, while revealing that BCAS2 antagonizes this pathway by reducing β-catenin/TCF4 occupancy at the FST promoter.\",\n      \"evidence\": \"ChIP cloning, gel supershift, co-immunoprecipitation, confocal microscopy, qPCR, and western blotting in breast cancer cell lines\",\n      \"pmids\": [\"24667650\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab study; findings have not been independently replicated\",\n        \"Mechanism by which FST induces apoptosis downstream is not defined\",\n        \"Whether this ERRβ–FST regulatory circuit operates outside breast cancer contexts is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The downstream signaling pathway by which FST promotes apoptosis and E-cadherin expression, and the broader physiological relevance of the ERRβ–BCAS2–FST circuit beyond breast cancer cells, remain uncharacterized.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No identification of direct FST effectors mediating apoptosis\",\n        \"No in vivo validation of the ERRβ–FST axis\",\n        \"Relationship between FST's role as an activin/BMP antagonist and the observed apoptotic function is unexplored\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [],\n    \"localization\": [],\n    \"pathway\": [\n      {\n        \"term_id\": \"R-HSA-5357801\",\n        \"supporting_discovery_ids\": [0]\n      }\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ESRRB\",\n      \"BCAS2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"Follistatin (FST) is a secreted glycoprotein that functions as a high-affinity antagonist of activin, myostatin, and select TGF-β family ligands, thereby regulating diverse processes including reproductive physiology, erythropoiesis, muscle mass, and metabolic homeostasis. Two FST molecules encircle activin, burying one-third of its surface and blocking both type I and type II receptor binding sites; the FST N-terminal domain adopts a fold that mimics the type I receptor motif to occupy the corresponding activin epitope [PMID:16198295]. Three splice isoforms (FST288, FST303, FST315) differ in cell-surface proteoglycan binding affinity, which determines their local versus systemic activin-neutralizing potency, while activin itself induces FST transcription through Smad3 and FoxL2 at an intronic enhancer, forming a negative feedback loop [PMID:16627583, PMID:19106105]. The liver is the principal source of circulating FST, regulated by the glucagon-to-insulin ratio via cAMP; excess hepatic FST acts as a pathological hepatokine that impairs glucose homeostasis during insulin resistance [PMID:26652766, PMID:29867232].\",\n  \"teleology\": [\n    {\n      \"year\": 1988,\n      \"claim\": \"Cloning of the FST gene revealed the modular exon-encoded domain architecture and two alternative splice forms, establishing the structural framework for understanding FST's multi-domain binding properties.\",\n      \"evidence\": \"cDNA and genomic library screening with sequencing and Southern blot in human tissues\",\n      \"pmids\": [\"3380788\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Post-translational processing (glycosylation, proteolytic maturation) not resolved\", \"Binding partners not yet identified\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Demonstration that FST binds both activin and inhibin via their shared β-subunit, with activin providing two FST-binding sites versus one for inhibin, established the stoichiometric basis for FST's preferential activin neutralization.\",\n      \"evidence\": \"Double-ligand blotting with purified proteins\",\n      \"pmids\": [\"2036994\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding affinities not quantified\", \"Structural basis of the 2:1 stoichiometry unknown\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Functional paracrine activity was demonstrated in human placenta, where FST completely reversed activin A-induced hormone secretion, extending FST's role beyond the pituitary-gonadal axis to reproductive tissues.\",\n      \"evidence\": \"RT-PCR, immunohistochemistry, and functional reversal assays in cultured placental cells\",\n      \"pmids\": [\"8288705\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo placental relevance not confirmed by genetic models\", \"Other tissue-specific paracrine roles unexplored\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Discovery that activin induces FST transcription through Smad proteins established a direct negative feedback loop in which the target (FST) neutralizes its own inducer (activin).\",\n      \"evidence\": \"Transcriptional reporter assays, RT-PCR, and Western blot with Smad pathway analysis\",\n      \"pmids\": [\"11948405\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific Smad isoform and cis-element mediating the response not mapped\", \"Feedback kinetics not quantified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The crystal structure of the FST:activin A complex resolved the long-standing question of how FST neutralizes activin: two FST molecules wrap around activin, and the N-terminal domain adopts a type I receptor-mimicking fold that occludes the type I binding site.\",\n      \"evidence\": \"X-ray crystallography of the 2:1 FST:activin complex\",\n      \"pmids\": [\"16198295\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of complex formation in solution not characterized\", \"Structural basis for differential BMP versus activin affinity not fully resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Systematic comparison of the three FST isoforms showed that differential cell-surface proteoglycan binding (FST288 > FST303 > FST315) is the primary determinant of their distinct bioactivities, and that FST isoforms bind myostatin and select BMPs whereas the related protein FSTL3 does not.\",\n      \"evidence\": \"Recombinant protein competitive binding assays, kinetic analysis, and TT cell proliferation bioassay\",\n      \"pmids\": [\"16627583\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Proteoglycan identity mediating surface retention not defined\", \"In vivo isoform-specific functions not dissected genetically\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Engineering of a follistatin-derived myostatin-selective inhibitor (FS I-I) demonstrated that FST's myostatin-binding capacity can be uncoupled from activin binding, and that selective myostatin inhibition increases muscle mass and ameliorates muscular dystrophy in mdx mice.\",\n      \"evidence\": \"Quantitative in vitro binding assays and transgenic mouse muscle phenotyping including mdx cross\",\n      \"pmids\": [\"17893249\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term safety and off-target effects of FS I-I in vivo unknown\", \"Mechanism of hyperplasia versus hypertrophy contribution not resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of FoxL2 as a Smad3 co-factor at an intronic enhancer of the FST gene refined the transcriptional feedback model, showing that activin-driven FST expression requires cooperative binding of Smad3 and FoxL2.\",\n      \"evidence\": \"ChIP, co-immunoprecipitation, site-directed mutagenesis, and shRNA knockdown in ovarian granulosa-like cells\",\n      \"pmids\": [\"19106105\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this enhancer module operates in non-gonadal tissues is unknown\", \"Epigenetic regulation of this element not addressed\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Exercise physiology studies identified the liver—not exercising skeletal muscle—as the principal source of the marked rise in circulating FST during exercise, reframing FST as a hepatokine.\",\n      \"evidence\": \"Arterial-venous difference measurements in exercising humans, hepatic mRNA/protein quantification in exercising mice\",\n      \"pmids\": [\"21068158\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Hepatic signaling pathway driving exercise-induced FST release not identified\", \"Contribution of other organs (e.g., adipose) not excluded\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Splanchnic arteriovenous measurements in humans confirmed the liver as the dominant source of circulating FST, and mechanistic studies showed glucagon stimulates and insulin inhibits hepatic FST secretion via cAMP, linking FST to metabolic regulation.\",\n      \"evidence\": \"Arteriovenous difference across splanchnic bed in humans, cAMP-pathway analysis in cultured hepatocytes, islet functional assays\",\n      \"pmids\": [\"26652766\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct cAMP-responsive element on FST promoter not mapped in hepatocytes\", \"Autocrine/paracrine effects of FST on hepatocytes themselves unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Crystal structure of the GDF11:FST288 complex demonstrated that FST neutralizes GDF11 by a mechanism analogous to activin, broadening the structural understanding of FST's multi-ligand antagonism within the TGF-β family.\",\n      \"evidence\": \"X-ray crystallography of GDF11:FST288 complex with functional signaling and mutagenesis validation\",\n      \"pmids\": [\"28257634\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of FST:GDF11 antagonism versus FST:myostatin antagonism not dissected\", \"Affinity hierarchy across all TGF-β ligands not fully quantified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Gain- and loss-of-function experiments in mouse liver established FST as a pathological hepatokine: elevated hepatic FST during insulin resistance impairs systemic glucose homeostasis and adipose insulin signaling, effects reversed by hepatic FST knockdown.\",\n      \"evidence\": \"Hepatic shRNA knockdown and overexpression in LDKO and wild-type mice, GTT/ITT, human bariatric surgery cohort correlation\",\n      \"pmids\": [\"29867232\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Target tissue receptor or signaling mechanism through which circulating FST impairs glucose metabolism is unknown\", \"Whether FST's metabolic effect is activin-dependent or involves a distinct receptor pathway is unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The receptor or signaling mechanism by which circulating FST exerts its metabolic (hepatokine) effects on peripheral tissues remains unidentified, and isoform-specific in vivo functions have not been resolved by conditional genetic models.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No receptor for FST's metabolic actions identified\", \"Conditional isoform-specific knockouts not reported\", \"Structural basis for isoform-specific proteoglycan interaction unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 6, 7, 8, 14]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [1, 3, 7, 8]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [6, 7, 14]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [2, 3, 7, 11, 13, 15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [1, 3, 4, 6, 7]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 3, 4, 6, 7]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [13, 15]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [2, 5, 8]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"INHBA\",\n      \"MSTN\",\n      \"GDF11\",\n      \"SMAD3\",\n      \"FOXL2\",\n      \"BMP2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}