{"gene":"BMP6","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2009,"finding":"BMP6 acts as a key endogenous ligand for hemojuvelin (HJV) to regulate hepcidin expression and iron metabolism in vivo. Physical interaction between HJV.Fc and BMP6 was demonstrated, and Bmp6-null mice develop hereditary hemochromatosis-like iron overload with reduced hepcidin. A neutralizing anti-BMP6 antibody inhibits hepcidin expression and increases serum iron in vivo.","method":"Co-immunoprecipitation/pulldown (HJV.Fc-BMP6 physical interaction), neutralizing antibody treatment in vivo, Bmp6 knockout mouse model with iron and hepcidin phenotyping","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (KO mice, neutralizing antibody, physical binding assay), replicated in same issue by independent group","pmids":["19252486"],"is_preprint":false},{"year":2009,"finding":"Targeted disruption of Bmp6 in mice causes massive iron accumulation in liver, pancreas, heart and renal tubules, with reduced phosphorylated Smad1/5/8 and reduced nuclear Smad translocation in the liver, and markedly reduced hepcidin synthesis, establishing BMP6 as the non-redundant endogenous activator of the BMP/SMAD/hepcidin axis.","method":"Bmp6 knockout mouse model; immunohistochemistry and Western blot for phospho-Smad1/5/8; qRT-PCR for hepcidin","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined molecular pathway phenotype, independently replicated","pmids":["19252488"],"is_preprint":false},{"year":2007,"finding":"Crystal structure of BMP-6 was solved, revealing the structural basis for receptor binding specificity at type I and type II receptor interfaces.","method":"X-ray crystallography; surface plasmon resonance (BIAcore) binding assays; Smad-based luciferase reporter assays","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure combined with binding kinetics and functional validation","pmids":["17924656"],"is_preprint":false},{"year":2007,"finding":"BMP6 induces myosin-X (Myo10) expression in endothelial cells; Myo10 localizes to filopodia after BMP6 stimulation and is required for BMP6-dependent filopodial assembly, directional cell migration, and Smad activation. BMP6 receptor ALK6 co-localizes with Myo10 in filopodia, and Myo10 is required for amplification of BMP6 signaling.","method":"Microarray; siRNA knockdown; live imaging; co-localization by immunofluorescence; Smad phosphorylation assay; directional migration assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (KD, colocalization, functional migration assay, signaling readout) in single study","pmids":["18158328"],"is_preprint":false},{"year":2018,"finding":"Erythroferrone (ERFE) suppresses hepcidin by acting as a ligand trap that preferentially binds BMP5, BMP6, and BMP7 but not BMP2, BMP4, or BMP9, thereby inhibiting hepatic BMP/SMAD signaling. Cell-free HTRF assays showed BMP5/6/7 compete with anti-ERFE for ERFE binding.","method":"In vitro SMAD1/5/8 phosphorylation assay; hepcidin luciferase reporter; cell-free homogeneous time-resolved fluorescence (HTRF) binding competition assay; neutralizing antibody","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1-2 — cell-free binding assay plus multiple cellular assays and in vivo validation","pmids":["30097509"],"is_preprint":false},{"year":2019,"finding":"Nrf2 transcription factor, activated by iron-induced mitochondria-derived pro-oxidants, drives Bmp6 expression in liver sinusoidal endothelial cells (LSECs), which in turn increases hepcidin synthesis in neighboring hepatocytes. Nrf2 knockout mice have impaired Bmp6-hepcidin response to iron loading.","method":"Nrf2 knockout mice; pharmacological Nrf2 activation; qRT-PCR; ChIP for Nrf2 binding to Bmp6 promoter; iron loading experiments","journal":"Nature metabolism","confidence":"High","confidence_rationale":"Tier 2 — KO model with defined pathway placement, pharmacological validation, and ChIP","pmids":["31276102"],"is_preprint":false},{"year":2023,"finding":"Liver sinusoidal endothelial cells (LSECs) sense systemic iron overload via non-transferrin-bound iron (NTBI) internalization, which promotes oxidative stress and transcriptionally induces Bmp6 via Nrf2, leading to paracrine hepcidin induction in hepatocytes. Endothelial-specific Tfr1 knockout mice showed transiently impaired Bmp6 induction.","method":"Endothelial cell-specific Tfr1 conditional knockout mice; single-cell transcriptomics; correlation analysis of serum NTBI with liver Bmp6/Hamp mRNA; iron loading experiments","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — conditional KO, single-cell transcriptomics, and mechanistic pathway assignment","pmids":["36351237"],"is_preprint":false},{"year":2010,"finding":"HFE interacts with the BMP6-SMAD signaling pathway to regulate hepcidin: HFE transgenic mice show increased BMP6-SMAD target gene expression, and anti-BMP6 antibody reverses hepcidin excess in these mice. Exogenous BMP6 administration overcomes the BMP6-SMAD signaling impairment in Hfe-/- mice and restores hepcidin.","method":"Hfe transgenic and knockout mice; anti-BMP6 neutralizing antibody treatment; exogenous BMP6 injection; hepatic SMAD phosphorylation and target gene expression","journal":"Gastroenterology","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic models and pharmacological intervention with defined signaling readout","pmids":["20682319"],"is_preprint":false},{"year":2009,"finding":"In Hfe-deficient mice, despite increased Bmp6 mRNA and protein due to iron accumulation, levels of phosphorylated Smads 1/5/8 and Id1 mRNA are not elevated, indicating that HFE facilitates signal transduction downstream of BMP6 binding.","method":"Hfe knockout mice of multiple genetic backgrounds; Western blot for phospho-Smad1/5/8; qRT-PCR for Id1 and hepcidin","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple mouse strains, orthogonal signaling readouts","pmids":["19622835"],"is_preprint":false},{"year":2017,"finding":"Endothelial cell-derived BMP2 has a non-redundant role in hepcidin regulation by iron in addition to BMP6. Conditional endothelial Bmp2 knockout mice develop hepcidin deficiency and iron overload; a BMP2/4 neutralizing antibody blocks residual hepcidin induction by dietary iron in Bmp6-/- mice.","method":"Endothelial conditional Bmp2 knockout; global Bmp6 knockout; neutralizing antibody treatment; dietary iron loading; hepcidin and SMAD signaling assays","journal":"American journal of hematology","confidence":"High","confidence_rationale":"Tier 2 — conditional KO plus pharmacological intervention, mechanistic pathway defined","pmids":["28815688"],"is_preprint":false},{"year":2018,"finding":"Hemojuvelin (HJV) is required for inflammatory induction of hepcidin by maintaining a threshold of Bmp6/Smad signaling; in primary hepatocytes, Hjv deficiency severely impairs BMP6/Smad signaling and abolishes synergism with the IL-6/Stat pathway, though BMP2/Smad signaling is only slightly inhibited.","method":"Hjv-/- mice; primary hepatocyte cultures; Smad phosphorylation assays; LPS/FSL1/E.coli infection models; ferroportin immunohistochemistry","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — primary cell assays, KO mice, multiple infection models, defined pathway placement","pmids":["30213871"],"is_preprint":false},{"year":2017,"finding":"Loss of Bmp6 further represses SMAD signaling and hepcidin in Hjv-/- mice, demonstrating that BMP6 can signal to hepcidin partially independently of hemojuvelin, and that BMP6 and HJV together set the level of hepcidin that determines the inflammatory hepcidin response.","method":"Double Hjv-/-;Bmp6-/- knockout mice; SMAD phosphorylation; hepcidin qRT-PCR; LPS challenge","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with double KO and defined signaling phenotype","pmids":["29021231"],"is_preprint":false},{"year":2010,"finding":"The anemia and elevated hepcidin of Tmprss6-/- mice is corrected by co-deletion of Bmp6, demonstrating that elevated hepcidin in iron-refractory iron deficiency anemia results from excess signaling through the Bmp6/Hjv pathway and placing matriptase-2 as a negative regulator of BMP6 signaling to hepcidin.","method":"Genetic epistasis: Tmprss6-/-;Bmp6-/- double knockout mice; hepcidin qRT-PCR; hematological analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — clean genetic epistasis establishing pathway position","pmids":["20940420"],"is_preprint":false},{"year":2011,"finding":"BMP6 stimulates TMPRSS6 (matriptase-2) expression at mRNA and protein levels via the BMP-SMAD pathway, with inhibitor of DNA binding 1 (Id1) identified as the key mediator; this creates a negative feedback loop on hepcidin regulation.","method":"In vitro BMP6 treatment of hepatic cells; qRT-PCR; Western blot for TMPRSS6; matriptase-2 activity assay; in vivo BMP6 injection and neutralizing antibody; Id1 knockdown","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — in vitro plus in vivo validation, mechanistic mediator identified by knockdown","pmids":["21622652"],"is_preprint":false},{"year":1994,"finding":"Recombinant vgr-1/BMP-6 protein is secreted and proteolytically processed to the mature form; continuous secretion of BMP-6 from CHO cells in vivo induces surrounding host mesenchymal cells to differentiate along the endochondral bone pathway, producing cartilage and bone.","method":"Western blot for protein secretion and processing; subcutaneous implantation of BMP-6-expressing CHO cells in athymic nude mice; histological analysis; neutralizing antibody blocking","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — in vivo model with neutralizing antibody confirmation, multiple histological readouts","pmids":["8089189"],"is_preprint":false},{"year":1995,"finding":"Vgr-1/BMP-6 overexpression in pluripotential mesenchymal cells (ROB-C26) enhances osteoblastic differentiation in vitro and induces ectopic bone formation via extracellular matrix; the osteoinductive effect of BMP-6-conditioned matrix is blocked by neutralizing anti-vgr-1 antibody but not anti-TGF-β1, and requires matrix presentation rather than soluble BMP-6.","method":"Stable transfection; in vitro differentiation assay; subcutaneous implantation in vivo; neutralizing antibody blocking; MyoD overexpression competition assay","journal":"Cell growth & differentiation","confidence":"High","confidence_rationale":"Tier 2 — multiple functional assays with antibody neutralization controls","pmids":["7547504"],"is_preprint":false},{"year":1998,"finding":"Bmp6 null mice are viable and fertile but show a consistent delay in ossification specifically in the developing sternum, with Bmp2 co-expressed in hypertrophic cartilage suggesting functional compensation. Bmp5/6 double mutants show exacerbated sternal defects.","method":"Gene targeting (knockout); skeletal analysis; in situ hybridization; Bmp5/Bmp6 double mutant analysis","journal":"Developmental genetics","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with double-mutant epistasis, defined skeletal phenotype","pmids":["9664685"],"is_preprint":false},{"year":2001,"finding":"Bmp6 and Bmp7 are co-expressed in overlapping domains in cardiac cushions; Bmp6;Bmp7 double mutants (but not single mutants) show marked delay in outflow tract endocardial cushion formation, valve morphogenesis defects, and chamber septation defects causing embryonic lethality, establishing BMP6 as genetically required for cardiac cushion development.","method":"In situ hybridization; Bmp6/Bmp7 double knockout mice; histological analysis of cardiac development","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with double KO revealing non-redundant role","pmids":["11437450"],"is_preprint":false},{"year":2005,"finding":"BMP2 and BMP6 cooperatively regulate bone formation in vivo; Bmp2+/-;Bmp6-/- compound-deficient mice show reduced trabecular bone volume with suppressed bone formation and impaired endochondral (but not intramembranous) bone repair after fracture, whereas single-gene-deficient mice do not.","method":"Compound knockout mice; micro-CT and histomorphometry; fracture model; bone marrow cell culture for osteogenic differentiation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — compound KO genetic epistasis with fracture model readout","pmids":["16109715"],"is_preprint":false},{"year":1998,"finding":"BMP-6 induces LMP-1 (LIM mineralization protein-1) expression in osteoblast precursors; LMP-1 is required for BMP-6-induced osteoblast differentiation (antisense knockdown blocks differentiation) and its overexpression is sufficient to induce de novo bone formation, positioning LMP-1 as an essential intermediate in the BMP-6 signaling pathway.","method":"Antisense oligonucleotide knockdown; mammalian expression vector overexpression; in vitro bone nodule formation; subcutaneous implant in vivo","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — functional gain/loss with defined pathway placement, single lab","pmids":["9832452"],"is_preprint":false},{"year":2010,"finding":"BMP-2/6 heterodimer is more potent than either BMP-2 or BMP-6 homodimers in inducing differentiation of human embryonic stem cells and activates Smad1/5 and MAPK (ERK, p38) signaling more effectively.","method":"Treatment of hES cells with recombinant proteins; qPCR for differentiation markers; flow cytometry; Western blot for Smad1/5 and MAPK phosphorylation","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — multiple signaling readouts, single lab","pmids":["20567515"],"is_preprint":false},{"year":2013,"finding":"Iron-induced increase in Bmp6 expression originates predominantly from non-parenchymal liver cells (NPCs), particularly hepatic stellate cells, not hepatocytes; iron-saturated transferrin induces Bmp6 mRNA in isolated stellate cells but not in isolated hepatocytes, and this induction is independent of the BMP signaling pathway.","method":"Cell fractionation of liver; isolated primary hepatocytes and hepatic stellate cells; iron-saturated transferrin treatment; qRT-PCR; dietary iron manipulation in vivo","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — primary cell isolation with defined cellular source, single lab","pmids":["23565256"],"is_preprint":false},{"year":2010,"finding":"Iron overload induces Bmp6 mRNA and protein expression in the liver but not in the duodenum across multiple mouse strains, establishing the liver as the primary source of BMP6 for iron homeostasis regulation.","method":"Quantitative PCR; immunohistochemistry; Western blot; dietary iron loading and Hfe knockout models in three mouse strains","journal":"Haematologica","confidence":"Medium","confidence_rationale":"Tier 2 — multiple mouse strains and methods, but primarily expression measurement","pmids":["20952515"],"is_preprint":false},{"year":2012,"finding":"Estrogen regulates hepcidin expression via a GPR30-BMP6-dependent mechanism: GPR30 activation by estrogen induces BMP6 expression in hepatocytes, and GPR30 silencing abolishes estrogen-induced BMP6 and hepcidin upregulation.","method":"OVX mouse model; GPR30 siRNA knockdown; G1 agonist treatment; qRT-PCR for BMP6 and hepcidin","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA mechanistic validation plus in vivo model, single lab","pmids":["22792339"],"is_preprint":false},{"year":2015,"finding":"Heterozygous missense mutations in the BMP6 pro-peptide (p.Pro95Ser, p.Leu96Pro, p.Gln113Glu) cause defective BMP6 secretion, reduced SMAD1/5/8 signaling, and loss of hepcidin production in transfected cells, establishing the propeptide as functionally required for BMP6 processing and iron homeostasis.","method":"Transfection of mutant BMP6 constructs; immunofluorescence for secretion; qRT-PCR and Western blot for SMAD signaling and hepcidin; immunohistochemistry of patient biopsy","journal":"Gastroenterology","confidence":"Medium","confidence_rationale":"Tier 2 — cell-based functional assays with mutagenesis, human genetics validation","pmids":["26582087"],"is_preprint":false},{"year":2009,"finding":"BMP-6 inhibits miR-21 transcription in breast cancer cells through reducing expression of deltaEF1 and c-Fos/c-Jun, and physically dislodging deltaEF1 from the E2-box of the miR-21 promoter (miPPR-21), thereby inhibiting MDA-MB-231 cell invasion.","method":"Luciferase reporter assay; ChIP assay; site-directed mutation of promoter; RT-PCR; invasion assay","journal":"Cell research","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP and reporter assays establish direct transcriptional mechanism, single lab","pmids":["19308091"],"is_preprint":false},{"year":2007,"finding":"BMP-6 promotes E-cadherin expression in breast cancer cells by inhibiting deltaEF1 transcription and physically dislodging deltaEF1 from the E-cadherin proximal promoter; deltaEF1 knockdown abolishes E-cadherin transactivation by BMP-6.","method":"Luciferase reporter assay; ChIP assay; siRNA knockdown of deltaEF1; RT-PCR and Western blot","journal":"BMC cancer","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP and functional knockdown establish direct transcriptional mechanism, single lab","pmids":["17997862"],"is_preprint":false},{"year":2005,"finding":"BMP-6 inhibits proliferation of mature human B cells and B progenitor cells via phosphorylation of Smad1/5/8 followed by upregulation of Id1 (but not Id2 or Id3); the antiproliferative effect is completely neutralized by noggin.","method":"Primary human B cell and pre-B cell line cultures; Western blot for phospho-Smad1/5/8; RT-PCR for Id1-Id3; proliferation assay; noggin neutralization","journal":"BMC immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods with antagonist rescue, single lab","pmids":["15877825"],"is_preprint":false},{"year":2008,"finding":"BMP-6 in prostate cancer cells induces nuclear translocation of SMADs and activates Id-1; BMP-6 also increases migration and invasion, upregulates MMP-1 and MMP-9 at both mRNA and protein levels via direct transcriptional activation of their promoters, and activates MAPK signaling.","method":"Doxycycline-inducible BMP-6 expression system; SMAD nuclear translocation imaging; luciferase reporter assay for MMP-1/MMP-9 promoters; migration/invasion assay; Western blot for phospho-MAPK","journal":"The Journal of pathology","confidence":"Medium","confidence_rationale":"Tier 2 — inducible system with reporter assays and migration assays, single lab","pmids":["18072288"],"is_preprint":false},{"year":2011,"finding":"BMP-6 derived from prostate cancer cells induces IL-6 expression in macrophages via crosstalk between Smad1 and NF-κB1 p50; IL-6 in turn induces neuroendocrine differentiation of prostate cancer cells. Disruption of BMP-6 or IL-6 with neutralizing antibodies blocks neuroendocrine differentiation.","method":"Co-culture of prostate cancer and macrophage cell lines; neutralizing antibodies to BMP-6 and IL-6; IL-6 knockout mice; macrophage-depleted mice; conditioned media experiments","journal":"The Prostate","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro co-culture plus in vivo knockout models, multiple methods","pmids":["21374653"],"is_preprint":false},{"year":2011,"finding":"Intraocular BMP6 protein injection in mice upregulates retinal hepcidin expression and alters retinal labile iron levels; Bmp6-/- mice develop age-dependent retinal iron accumulation and degeneration. BMP6 is expressed in retinal pigment epithelium and is regulated by oxidative stress and iron.","method":"Intraocular BMP6 protein injection; Bmp6 knockout mice; qRT-PCR; labile iron measurement; histological analysis of retinal degeneration","journal":"The American journal of pathology","confidence":"Medium","confidence_rationale":"Tier 2 — protein injection experiment plus KO model, defined functional readout","pmids":["21703414"],"is_preprint":false},{"year":2010,"finding":"BMP-6 treatment of recombinant BMP-6-expressing CHO cells and in vivo injection in mice increases hepcidin expression and reduces serum iron; BMP6 acts through phosphorylation of Smad1/5/8 to regulate hepcidin in the liver.","method":"Exogenous BMP6 injection in mice; phospho-Smad1/5/8 Western blot; hepcidin qRT-PCR; serum iron measurements","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — in vivo pharmacological experiment with defined molecular pathway, replicated across multiple papers","pmids":["19252486","19252488"],"is_preprint":false},{"year":2018,"finding":"BMP6 regulates VEGFR2 expression in endothelial cells and acts via Hippo signaling effector TAZ to modulate angiogenesis; in a matrigel plug assay in mice, BMP6 induces neovascularization.","method":"In vitro endothelial cell assays; VEGFR2 expression analysis; TAZ knockdown/overexpression; matrigel plug assay in vivo","journal":"Angiogenesis","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro mechanistic assays plus in vivo angiogenesis model, single lab","pmids":["33021694"],"is_preprint":false},{"year":2018,"finding":"BMP6 acts on pancreatic and liver cells through Alk3, Alk6, and ActRIIA serine/threonine kinase receptors to regulate glucose metabolism; BMP6 inhibits gluconeogenesis in rat hepatoma cells via downregulation of PEPCK expression, confirmed by chromatin immunoprecipitation.","method":"Receptor-specific Fc fusion protein inhibitors; luciferase reporter gene assay; glucose output assay in H4IIE and INS1 cells; ChIP; ob/ob mouse treatment with BMP6","journal":"Acta diabetologica","confidence":"Medium","confidence_rationale":"Tier 2 — receptor specificity assayed with multiple Fc fusions, ChIP, and in vivo model","pmids":["30539233"],"is_preprint":false},{"year":2018,"finding":"BMP6 downregulates GDNF expression in human granulosa cells through ALK2 and ALK3 receptors, activating SMAD1 and SMAD5 as well as ERK1/2 (but not AKT or p38); complete reversal of GDNF suppression requires dual inhibition of both SMAD4 and ERK1/2.","method":"ALK receptor kinase inhibitors; siRNA knockdown of ALK2, ALK3, SMAD1, SMAD5, SMAD4; Western blot for phospho-SMAD1/5/8 and ERK1/2; qRT-PCR for GDNF in SVOG and primary granulosa-lutein cells","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — dual pathway dissection with inhibitor and siRNA, defined mechanistic readout","pmids":["29750278"],"is_preprint":false},{"year":2014,"finding":"Recombinant BMP6 inhibits activation of hepatic stellate cells (HSCs) and reduces proinflammatory and profibrogenic gene expression in activated HSCs; steatosis-induced BMP6 upregulation is hepatoprotective, and Bmp6-/- mice develop more severe hepatic fibrosis on MCD and high-fat diets.","method":"Bmp6-/- mice on MCD and high-fat dietary models; in vitro primary human HSC treatment with recombinant BMP6; histological analysis of fibrosis","journal":"Gut","confidence":"Medium","confidence_rationale":"Tier 2 — KO mouse model plus in vitro cell assay, single lab","pmids":["25011936"],"is_preprint":false},{"year":2007,"finding":"BMP-6 exerts its osteoinductive effect at least in part through upregulation of IGF-I and EGF pathway gene expression in bone; gene set enrichment analysis and qPCR confirmed IGF-I and EGF upregulation in BMP-6-treated mouse bone and primary human osteoblasts.","method":"In vivo gene expression profiling (microarray) in oophorectomized mice treated with BMP-6; gene set enrichment analysis; qPCR confirmation; primary human osteoblast culture","journal":"International orthopaedics","confidence":"Low","confidence_rationale":"Tier 3 — expression-based inference with limited mechanistic validation, single lab","pmids":["17634942"],"is_preprint":false},{"year":2022,"finding":"BMP6 expression in salivary gland monocytic cells in Sjögren's syndrome is stimulated by HSP70 acting as an endogenous TLR4 ligand; HSP70 is released via LAMP3-induced lysosomal exocytosis from epithelial cells. LAMP3 overexpression in mice induces BMP6 expression and an SS-like phenotype.","method":"RNA ISH on salivary gland sections; RNA-Seq; single-cell transcriptomics of PBMCs; in vitro TLR4 signaling assays; LAMP3 overexpression in mice; RT-PCR correlation analysis","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods identifying upstream pathway, in vivo mouse model, single lab","pmids":["35113815"],"is_preprint":false},{"year":2007,"finding":"BMP6 is produced by myotubes and mature Schwann cells and is retrogradely transported in mature motor axons; BMP6 supports motoneuron survival in vitro through BMPRII, which is highly expressed in motoneurons.","method":"Laser microdissection of motoneurons for receptor mRNA quantification; immunostaining for BMPRII; retrograde transport assay; in vitro motoneuron survival assay","journal":"Molecular and cellular neurosciences","confidence":"Medium","confidence_rationale":"Tier 2 — axonal transport assay plus survival assay, receptor expression quantified","pmids":["17321145"],"is_preprint":false},{"year":2018,"finding":"BMP6 signaling through the BMP/SMAD pathway is absolutely required for hepcidin induction by acute serum iron (oral iron gavage), whereas BMP2 plays only a partially redundant role; Bmp6-/- or Bmp6 CKO mice fail to induce hepcidin after oral iron gavage.","method":"Global and endothelial conditional Bmp6 knockout mice; acute oral iron gavage; serum iron and liver SMAD5 phosphorylation; hepcidin qRT-PCR","journal":"American journal of hematology","confidence":"High","confidence_rationale":"Tier 2 — conditional and global KO with defined serum iron stimulus, mechanistic distinction from BMP2","pmids":["30478858"],"is_preprint":false},{"year":2018,"finding":"BMP6-induced inhibition of hair follicle telogen-anagen transition is mediated by reciprocal antagonism with Wnt10b: BMP6 overexpression in vivo inhibits hair follicle stem cell activation, suppresses Wnt10b signaling, and is itself suppressed by Wnt10b.","method":"Adenovirus-mediated BMP6 overexpression in mouse skin; in situ hybridization; immunofluorescence; BrdU tracing; comparison with AdWnt10b-treated skins","journal":"Cell communication and signaling","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo overexpression with defined pathway crosstalk, single lab","pmids":["30791955"],"is_preprint":false},{"year":2014,"finding":"BMP-6 inhibits MMP-1 expression in breast cancer cells by inhibiting AP-1-mediated transcription; ChIP and luciferase assays show BMP-6 decreases recruitment of c-Jun/c-Fos to the endogenous MMP-1 promoter AP-1 response element, and MMP-1 overexpression rescues BMP-6-mediated inhibition of invasion.","method":"Luciferase reporter assay; ChIP assay; RT-PCR; MMP-1 overexpression rescue; Boyden chamber invasion assay","journal":"Oncology reports","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP and functional rescue establish mechanistic link, single lab","pmids":["26751737"],"is_preprint":false},{"year":2010,"finding":"BMP-6 inhibits MMP-9 secretion in MCF-7 breast cancer cells through upregulation of HO-1 via Smad1/5 recruitment to a Smad-responsive element on the HO-1 promoter; BMP-6/HO-1-mediated MMP-9 suppression operates via MAPK/p38/AP-1 signaling, and HO-1 knockdown abolishes BMP-6 anti-invasive effects.","method":"Luciferase reporter assay; ChIP for Smad1/5 on HO-1 promoter; HO-1 knockdown by ZnPPIX; RT-PCR; Boyden chamber assay","journal":"Journal of cancer research and clinical oncology","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP establishes direct promoter binding, functional rescue assay, single lab","pmids":["21136273"],"is_preprint":false},{"year":2018,"finding":"HIF-1α directly represses BMP6 expression by binding to the hypoxia-response element (HRE) in the BMP6 promoter, and HIF-1α-mediated BMP6 suppression promotes hyperproliferation and abnormal differentiation of keratinocytes.","method":"HIF-1α overexpression in primary human keratinocytes; luciferase reporter with HRE in BMP6 promoter; ChIP for HIF-1α binding; cell cycle analysis; differentiation markers","journal":"Experimental dermatology","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP plus reporter assay establishes direct transcriptional regulation, single lab","pmids":["30230035"],"is_preprint":false}],"current_model":"BMP6 is a secreted TGF-β superfamily ligand produced primarily by liver sinusoidal endothelial cells in response to iron-induced oxidative stress (via Nrf2) and NTBI uptake; it acts as the principal endogenous activator of the hemojuvelin co-receptor/BMPR complex on hepatocytes to phosphorylate SMAD1/5/8 and drive hepcidin transcription, thereby controlling systemic iron homeostasis, while also signaling through ALK2/3/6 and ActRIIA receptors in other cell types to regulate osteoblast differentiation, B-lymphopoiesis, angiogenesis (via TAZ/Hippo and VEGFR2), glucose metabolism, and context-dependent transcriptional programs (E-cadherin, miR-21, MMP-1/9) through SMAD and non-SMAD (ERK1/2, p38 MAPK) pathways."},"narrative":{"teleology":[{"year":1994,"claim":"Establishing that BMP6 is a secreted, proteolytically processed ligand capable of inducing endochondral bone formation answered the fundamental question of whether BMP6 functions as an osteoinductive morphogen.","evidence":"Implantation of BMP6-expressing CHO cells in nude mice with histological and neutralizing-antibody confirmation","pmids":["8089189"],"confidence":"High","gaps":["Receptor identity for BMP6 osteoinductive signaling not defined","In vivo processing site not mapped"]},{"year":1998,"claim":"Generation of Bmp6-null mice revealed that BMP6 has a specific but partially redundant role in sternal ossification, with BMP5 compensating; this established the first loss-of-function phenotype and set the stage for identifying non-skeletal functions.","evidence":"Bmp6 knockout and Bmp5/Bmp6 double-mutant skeletal analysis","pmids":["9664685"],"confidence":"High","gaps":["Iron homeostasis phenotype not yet examined in these mice","Molecular compensation mechanisms with BMP2/5/7 not dissected"]},{"year":2001,"claim":"Double knockout of Bmp6 and Bmp7 revealed a non-redundant requirement for BMP6 in cardiac outflow tract cushion formation, broadening its developmental role beyond skeletal tissues.","evidence":"Bmp6/Bmp7 double-knockout mice with histological analysis of cardiac development","pmids":["11437450"],"confidence":"High","gaps":["Downstream targets in cardiac cushion mesenchyme not identified","Whether BMP6 acts on endocardium vs mesenchyme not resolved"]},{"year":2005,"claim":"Compound Bmp2+/−;Bmp6−/− mice demonstrated cooperative regulation of trabecular bone formation and endochondral fracture repair, distinguishing BMP6's role from intramembranous ossification.","evidence":"Compound knockout mice with micro-CT, histomorphometry, and fracture model","pmids":["16109715"],"confidence":"High","gaps":["Receptor complex mediating cooperative BMP2/BMP6 signaling in bone not defined"]},{"year":2007,"claim":"The crystal structure of BMP6 defined its type I and type II receptor-binding interfaces, providing the first structural framework for understanding its signaling specificity.","evidence":"X-ray crystallography with SPR binding kinetics and Smad-based luciferase reporter validation","pmids":["17924656"],"confidence":"High","gaps":["Structure of BMP6 in complex with its receptors not determined","Basis for selectivity over BMP7 at type I receptor level not fully resolved"]},{"year":2007,"claim":"Identification of Myo10 as a BMP6-induced effector in endothelial cells that amplifies BMP6/ALK6 signaling at filopodia revealed a positive-feedback mechanism linking BMP6 to directional cell migration and angiogenesis.","evidence":"Microarray, siRNA knockdown, live imaging, and co-localization of ALK6 with Myo10 in filopodia","pmids":["18158328"],"confidence":"High","gaps":["Whether Myo10-dependent amplification is specific to endothelial cells or generalizable not tested"]},{"year":2007,"claim":"BMP6 was shown to promote E-cadherin expression and suppress miR-21 transcription in breast cancer cells via deltaEF1 displacement from target promoters, establishing a direct transcriptional mechanism for BMP6's anti-invasive activity.","evidence":"ChIP, luciferase reporters, and siRNA knockdown of deltaEF1 in breast cancer cell lines","pmids":["17997862","19308091"],"confidence":"Medium","gaps":["In vivo relevance of BMP6-deltaEF1 axis in tumor suppression not demonstrated","Whether SMAD signaling mediates deltaEF1 regulation not fully dissected"]},{"year":2009,"claim":"Two independent studies demonstrated that Bmp6-null mice develop severe iron overload with reduced hepcidin and impaired hepatic SMAD1/5/8 phosphorylation, establishing BMP6 as the non-redundant endogenous activator of hepcidin — a paradigm-shifting discovery linking a BMP ligand to systemic iron regulation.","evidence":"Bmp6 knockout mice, neutralizing anti-BMP6 antibody, HJV.Fc co-immunoprecipitation, phospho-SMAD Western blot","pmids":["19252486","19252488"],"confidence":"High","gaps":["Cell type producing BMP6 in the liver not yet identified","How iron loading increases BMP6 expression not resolved"]},{"year":2009,"claim":"Hfe-deficient mice showed elevated BMP6 but impaired downstream SMAD phosphorylation, placing HFE as a facilitator of BMP6 signal transduction rather than BMP6 production.","evidence":"Hfe knockout mice in multiple genetic backgrounds with phospho-SMAD1/5/8 and Id1 measurement","pmids":["19622835"],"confidence":"High","gaps":["Physical mechanism by which HFE facilitates BMP6/SMAD transduction not identified"]},{"year":2010,"claim":"Genetic epistasis showed that Bmp6 co-deletion corrects the elevated hepcidin and anemia of Tmprss6-null mice, positioning matriptase-2 as a negative regulator acting specifically on the BMP6/HJV/hepcidin axis.","evidence":"Tmprss6−/−;Bmp6−/− double-knockout mice with hematological and hepcidin analysis","pmids":["20940420"],"confidence":"High","gaps":["Whether TMPRSS6 cleaves HJV directly at the cell surface in physiological context not resolved in this study"]},{"year":2011,"claim":"BMP6 was found to induce its own negative regulator TMPRSS6 via the SMAD/Id1 pathway, revealing an autoregulatory feedback loop in hepcidin control.","evidence":"In vitro BMP6 treatment with Id1 knockdown and in vivo BMP6 injection","pmids":["21374653"],"confidence":"High","gaps":["Kinetic modeling of the feedback loop in iron homeostasis not performed"]},{"year":2017,"claim":"Endothelial-specific Bmp2 knockout and Bmp6/Hjv double knockout studies demonstrated that BMP2 and BMP6 are both endothelial-derived hepcidin regulators with partially overlapping but non-identical functions, and that BMP6 retains some HJV-independent signaling capacity.","evidence":"Conditional endothelial Bmp2 KO; Hjv/Bmp6 double KO; neutralizing antibody; LPS challenge","pmids":["28815688","29021231"],"confidence":"High","gaps":["Relative contributions of BMP2 vs BMP6 under different iron-loading conditions not quantitatively defined"]},{"year":2018,"claim":"Erythroferrone was identified as a physiological ligand trap that selectively sequesters BMP5/6/7 (not BMP2/4/9) to suppress hepcidin during stress erythropoiesis, explaining the erythroid regulation of iron supply.","evidence":"Cell-free HTRF binding assays, SMAD phosphorylation and hepcidin reporter in hepatocytes, neutralizing antibody","pmids":["30097509"],"confidence":"High","gaps":["Structural basis of ERFE selectivity for BMP6 over BMP2 not determined","In vivo stoichiometry of ERFE-BMP6 trapping not measured"]},{"year":2018,"claim":"BMP6 was shown to signal through ALK2, ALK3, ALK6, and ActRIIA receptors in pancreatic and hepatic cells to suppress gluconeogenesis via PEPCK downregulation, expanding BMP6's metabolic role beyond iron.","evidence":"Receptor-specific Fc fusion inhibitors, ChIP, glucose output assay, ob/ob mouse treatment","pmids":["30539233"],"confidence":"Medium","gaps":["Whether glucose-regulatory effects are physiologically relevant beyond ob/ob model not tested","Relative receptor usage across tissues not systematically compared"]},{"year":2019,"claim":"Nrf2 was identified as the transcription factor that senses iron-induced mitochondrial oxidative stress and directly drives Bmp6 transcription in liver sinusoidal endothelial cells, closing the circuit between iron sensing and BMP6 production.","evidence":"Nrf2 knockout mice, pharmacological Nrf2 activation, ChIP for Nrf2 on Bmp6 promoter, iron loading","pmids":["31276102"],"confidence":"High","gaps":["Whether Nrf2 is the sole transcription factor driving iron-induced Bmp6 not excluded","Epigenetic regulation of Bmp6 not explored"]},{"year":2023,"claim":"Single-cell transcriptomics and conditional knockout confirmed that LSECs sense NTBI to activate Bmp6 via oxidative stress/Nrf2, refining the cellular iron-sensing mechanism to a specific cell type and iron species.","evidence":"Endothelial Tfr1 conditional KO, single-cell RNA-seq, NTBI-Bmp6 correlation analysis","pmids":["36351237"],"confidence":"High","gaps":["Identity of the NTBI transporter on LSECs not determined","Whether other non-parenchymal cells contribute Bmp6 in pathological states remains open"]},{"year":null,"claim":"Key unresolved questions include the structural basis of BMP6 selectivity for HJV over other co-receptors, the identity of the LSEC NTBI transporter, the quantitative contribution of BMP6 versus BMP2 under distinct physiological iron states, and whether BMP6's metabolic functions (glucose, lipid) operate independently of hepcidin in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of BMP6-HJV-receptor ternary complex available","NTBI import pathway in LSECs uncharacterized","In vivo relevance of BMP6 in glucose homeostasis not established in conditional models"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[0,1,14,15,27,31]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,4,7,12]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,4,14,15,24]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[15]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,3,7,8,10,11,20,27,28,34]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[14,15,16,17,18]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,1,5,6,33,39]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,1,7,12,13,39]}],"complexes":[],"partners":["HJV","TMPRSS6","HFE","ERFE","MYO10","ALK6","ALK2","ACVR2A"],"other_free_text":[]},"mechanistic_narrative":"BMP6 is a secreted TGF-β superfamily ligand that serves as the principal endogenous regulator of systemic iron homeostasis and contributes to osteoblast differentiation, angiogenesis, and context-dependent transcriptional programs in multiple tissues. In the liver, BMP6 is produced primarily by sinusoidal endothelial cells in response to iron-induced oxidative stress sensed through NTBI uptake and Nrf2 activation; it signals in a paracrine manner through the hemojuvelin co-receptor and BMPR complex on hepatocytes to phosphorylate SMAD1/5/8 and drive hepcidin transcription, thereby limiting dietary iron absorption and macrophage iron release [PMID:19252486, PMID:19252488, PMID:31276102, PMID:36351237]. This pathway is modulated by erythroferrone (which traps BMP6 to suppress hepcidin during stress erythropoiesis), by HFE (which facilitates BMP6-SMAD signal transduction), and by matriptase-2/TMPRSS6 (which acts as a negative-feedback regulator induced by BMP6 via Id1) [PMID:30097509, PMID:20682319, PMID:21374653]. Beyond iron regulation, BMP6 drives endochondral ossification cooperatively with BMP2 and BMP5, promotes endothelial migration via Myo10/ALK6 and VEGFR2/TAZ-dependent angiogenesis, modulates E-cadherin and MMP expression in cancer cells through SMAD and MAPK pathways, and inhibits B-cell proliferation via SMAD1/5/8-Id1 signaling [PMID:8089189, PMID:9664685, PMID:16109715, PMID:18158328, PMID:15877825, PMID:17997862]."},"prefetch_data":{"uniprot":{"accession":"P22004","full_name":"Bone morphogenetic protein 6","aliases":["VG-1-related protein","VG-1-R","VGR-1"],"length_aa":513,"mass_kda":57.2,"function":"Growth factor of the TGF-beta superfamily that plays essential roles in many developmental processes including cartilage and bone formation (PubMed:31019025). Also plays an important role in the regulation of HAMP/hepcidin expression and iron metabolism by acting as a ligand for hemojuvelin/HJV (PubMed:26582087). Also acts to promote expression of HAMP, potentially via the interaction with its receptor BMPR1A/ALK3 (PubMed:30097509, PubMed:31800957). Initiates the canonical BMP signaling cascade by associating with type I receptor ACVR1 and type II receptor ACVR2B (PubMed:18070108). In turn, ACVR1 propagates signal by phosphorylating SMAD1/5/8 that travel to the nucleus and act as activators and repressors of transcription of target. Can also signal through non-canonical pathway such as TAZ-Hippo signaling cascade to modulate VEGF signaling by regulating VEGFR2 expression (PubMed:33021694)","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P22004/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/BMP6","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/BMP6","total_profiled":1310},"omim":[{"mim_id":"620121","title":"IRON OVERLOAD, SUSCEPTIBILITY TO; IO","url":"https://www.omim.org/entry/620121"},{"mim_id":"619820","title":"ATONAL bHLH TRANSCRIPTION FACTOR 8; ATOH8","url":"https://www.omim.org/entry/619820"},{"mim_id":"619794","title":"VON WILLEBRAND FACTOR C DOMAIN-CONTAINING PROTEIN 2-LIKE; VWC2L","url":"https://www.omim.org/entry/619794"},{"mim_id":"613127","title":"CHORDIN-LIKE 2; CHRDL2","url":"https://www.omim.org/entry/613127"},{"mim_id":"612077","title":"MICRO RNA 22; MIR22","url":"https://www.omim.org/entry/612077"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Mitochondria","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"ovary","ntpm":19.2}],"url":"https://www.proteinatlas.org/search/BMP6"},"hgnc":{"alias_symbol":["VGR1"],"prev_symbol":["VGR"]},"alphafold":{"accession":"P22004","domains":[{"cath_id":"2.60.310","chopping":"220-241_256-361","consensus_level":"high","plddt":90.6909,"start":220,"end":361},{"cath_id":"2.10.90.10","chopping":"408-486_506-513","consensus_level":"high","plddt":90.174,"start":408,"end":513}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P22004","model_url":"https://alphafold.ebi.ac.uk/files/AF-P22004-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P22004-F1-predicted_aligned_error_v6.png","plddt_mean":68.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=BMP6","jax_strain_url":"https://www.jax.org/strain/search?query=BMP6"},"sequence":{"accession":"P22004","fasta_url":"https://rest.uniprot.org/uniprotkb/P22004.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P22004/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P22004"}},"corpus_meta":[{"pmid":"1893873","id":"PMC_1893873","title":"Involvement 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with human BMP-6 and VEGF protein significantly enhance rat mandible defect repair.","date":"2015","source":"Cell and tissue research","url":"https://pubmed.ncbi.nlm.nih.gov/26475719","citation_count":27,"is_preprint":false},{"pmid":"31260450","id":"PMC_31260450","title":"The role of BMP6 in the proliferation and differentiation of chicken cartilage cells.","date":"2019","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/31260450","citation_count":26,"is_preprint":false},{"pmid":"21136273","id":"PMC_21136273","title":"BMP-6 inhibits MMP-9 expression by regulating heme oxygenase-1 in MCF-7 breast cancer cells.","date":"2010","source":"Journal of cancer research and clinical oncology","url":"https://pubmed.ncbi.nlm.nih.gov/21136273","citation_count":26,"is_preprint":false},{"pmid":"24185914","id":"PMC_24185914","title":"Mechanism of pro-tumorigenic effect of BMP-6: neovascularization involving tumor-associated macrophages and IL-1a.","date":"2014","source":"The 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Physical interaction between HJV.Fc and BMP6 was demonstrated, and Bmp6-null mice develop hereditary hemochromatosis-like iron overload with reduced hepcidin. A neutralizing anti-BMP6 antibody inhibits hepcidin expression and increases serum iron in vivo.\",\n      \"method\": \"Co-immunoprecipitation/pulldown (HJV.Fc-BMP6 physical interaction), neutralizing antibody treatment in vivo, Bmp6 knockout mouse model with iron and hepcidin phenotyping\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (KO mice, neutralizing antibody, physical binding assay), replicated in same issue by independent group\",\n      \"pmids\": [\"19252486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Targeted disruption of Bmp6 in mice causes massive iron accumulation in liver, pancreas, heart and renal tubules, with reduced phosphorylated Smad1/5/8 and reduced nuclear Smad translocation in the liver, and markedly reduced hepcidin synthesis, establishing BMP6 as the non-redundant endogenous activator of the BMP/SMAD/hepcidin axis.\",\n      \"method\": \"Bmp6 knockout mouse model; immunohistochemistry and Western blot for phospho-Smad1/5/8; qRT-PCR for hepcidin\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined molecular pathway phenotype, independently replicated\",\n      \"pmids\": [\"19252488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structure of BMP-6 was solved, revealing the structural basis for receptor binding specificity at type I and type II receptor interfaces.\",\n      \"method\": \"X-ray crystallography; surface plasmon resonance (BIAcore) binding assays; Smad-based luciferase reporter assays\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure combined with binding kinetics and functional validation\",\n      \"pmids\": [\"17924656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BMP6 induces myosin-X (Myo10) expression in endothelial cells; Myo10 localizes to filopodia after BMP6 stimulation and is required for BMP6-dependent filopodial assembly, directional cell migration, and Smad activation. BMP6 receptor ALK6 co-localizes with Myo10 in filopodia, and Myo10 is required for amplification of BMP6 signaling.\",\n      \"method\": \"Microarray; siRNA knockdown; live imaging; co-localization by immunofluorescence; Smad phosphorylation assay; directional migration assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (KD, colocalization, functional migration assay, signaling readout) in single study\",\n      \"pmids\": [\"18158328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Erythroferrone (ERFE) suppresses hepcidin by acting as a ligand trap that preferentially binds BMP5, BMP6, and BMP7 but not BMP2, BMP4, or BMP9, thereby inhibiting hepatic BMP/SMAD signaling. Cell-free HTRF assays showed BMP5/6/7 compete with anti-ERFE for ERFE binding.\",\n      \"method\": \"In vitro SMAD1/5/8 phosphorylation assay; hepcidin luciferase reporter; cell-free homogeneous time-resolved fluorescence (HTRF) binding competition assay; neutralizing antibody\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — cell-free binding assay plus multiple cellular assays and in vivo validation\",\n      \"pmids\": [\"30097509\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Nrf2 transcription factor, activated by iron-induced mitochondria-derived pro-oxidants, drives Bmp6 expression in liver sinusoidal endothelial cells (LSECs), which in turn increases hepcidin synthesis in neighboring hepatocytes. Nrf2 knockout mice have impaired Bmp6-hepcidin response to iron loading.\",\n      \"method\": \"Nrf2 knockout mice; pharmacological Nrf2 activation; qRT-PCR; ChIP for Nrf2 binding to Bmp6 promoter; iron loading experiments\",\n      \"journal\": \"Nature metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO model with defined pathway placement, pharmacological validation, and ChIP\",\n      \"pmids\": [\"31276102\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Liver sinusoidal endothelial cells (LSECs) sense systemic iron overload via non-transferrin-bound iron (NTBI) internalization, which promotes oxidative stress and transcriptionally induces Bmp6 via Nrf2, leading to paracrine hepcidin induction in hepatocytes. Endothelial-specific Tfr1 knockout mice showed transiently impaired Bmp6 induction.\",\n      \"method\": \"Endothelial cell-specific Tfr1 conditional knockout mice; single-cell transcriptomics; correlation analysis of serum NTBI with liver Bmp6/Hamp mRNA; iron loading experiments\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO, single-cell transcriptomics, and mechanistic pathway assignment\",\n      \"pmids\": [\"36351237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"HFE interacts with the BMP6-SMAD signaling pathway to regulate hepcidin: HFE transgenic mice show increased BMP6-SMAD target gene expression, and anti-BMP6 antibody reverses hepcidin excess in these mice. Exogenous BMP6 administration overcomes the BMP6-SMAD signaling impairment in Hfe-/- mice and restores hepcidin.\",\n      \"method\": \"Hfe transgenic and knockout mice; anti-BMP6 neutralizing antibody treatment; exogenous BMP6 injection; hepatic SMAD phosphorylation and target gene expression\",\n      \"journal\": \"Gastroenterology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic models and pharmacological intervention with defined signaling readout\",\n      \"pmids\": [\"20682319\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In Hfe-deficient mice, despite increased Bmp6 mRNA and protein due to iron accumulation, levels of phosphorylated Smads 1/5/8 and Id1 mRNA are not elevated, indicating that HFE facilitates signal transduction downstream of BMP6 binding.\",\n      \"method\": \"Hfe knockout mice of multiple genetic backgrounds; Western blot for phospho-Smad1/5/8; qRT-PCR for Id1 and hepcidin\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mouse strains, orthogonal signaling readouts\",\n      \"pmids\": [\"19622835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Endothelial cell-derived BMP2 has a non-redundant role in hepcidin regulation by iron in addition to BMP6. Conditional endothelial Bmp2 knockout mice develop hepcidin deficiency and iron overload; a BMP2/4 neutralizing antibody blocks residual hepcidin induction by dietary iron in Bmp6-/- mice.\",\n      \"method\": \"Endothelial conditional Bmp2 knockout; global Bmp6 knockout; neutralizing antibody treatment; dietary iron loading; hepcidin and SMAD signaling assays\",\n      \"journal\": \"American journal of hematology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO plus pharmacological intervention, mechanistic pathway defined\",\n      \"pmids\": [\"28815688\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Hemojuvelin (HJV) is required for inflammatory induction of hepcidin by maintaining a threshold of Bmp6/Smad signaling; in primary hepatocytes, Hjv deficiency severely impairs BMP6/Smad signaling and abolishes synergism with the IL-6/Stat pathway, though BMP2/Smad signaling is only slightly inhibited.\",\n      \"method\": \"Hjv-/- mice; primary hepatocyte cultures; Smad phosphorylation assays; LPS/FSL1/E.coli infection models; ferroportin immunohistochemistry\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — primary cell assays, KO mice, multiple infection models, defined pathway placement\",\n      \"pmids\": [\"30213871\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Loss of Bmp6 further represses SMAD signaling and hepcidin in Hjv-/- mice, demonstrating that BMP6 can signal to hepcidin partially independently of hemojuvelin, and that BMP6 and HJV together set the level of hepcidin that determines the inflammatory hepcidin response.\",\n      \"method\": \"Double Hjv-/-;Bmp6-/- knockout mice; SMAD phosphorylation; hepcidin qRT-PCR; LPS challenge\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with double KO and defined signaling phenotype\",\n      \"pmids\": [\"29021231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The anemia and elevated hepcidin of Tmprss6-/- mice is corrected by co-deletion of Bmp6, demonstrating that elevated hepcidin in iron-refractory iron deficiency anemia results from excess signaling through the Bmp6/Hjv pathway and placing matriptase-2 as a negative regulator of BMP6 signaling to hepcidin.\",\n      \"method\": \"Genetic epistasis: Tmprss6-/-;Bmp6-/- double knockout mice; hepcidin qRT-PCR; hematological analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic epistasis establishing pathway position\",\n      \"pmids\": [\"20940420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"BMP6 stimulates TMPRSS6 (matriptase-2) expression at mRNA and protein levels via the BMP-SMAD pathway, with inhibitor of DNA binding 1 (Id1) identified as the key mediator; this creates a negative feedback loop on hepcidin regulation.\",\n      \"method\": \"In vitro BMP6 treatment of hepatic cells; qRT-PCR; Western blot for TMPRSS6; matriptase-2 activity assay; in vivo BMP6 injection and neutralizing antibody; Id1 knockdown\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro plus in vivo validation, mechanistic mediator identified by knockdown\",\n      \"pmids\": [\"21622652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Recombinant vgr-1/BMP-6 protein is secreted and proteolytically processed to the mature form; continuous secretion of BMP-6 from CHO cells in vivo induces surrounding host mesenchymal cells to differentiate along the endochondral bone pathway, producing cartilage and bone.\",\n      \"method\": \"Western blot for protein secretion and processing; subcutaneous implantation of BMP-6-expressing CHO cells in athymic nude mice; histological analysis; neutralizing antibody blocking\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo model with neutralizing antibody confirmation, multiple histological readouts\",\n      \"pmids\": [\"8089189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Vgr-1/BMP-6 overexpression in pluripotential mesenchymal cells (ROB-C26) enhances osteoblastic differentiation in vitro and induces ectopic bone formation via extracellular matrix; the osteoinductive effect of BMP-6-conditioned matrix is blocked by neutralizing anti-vgr-1 antibody but not anti-TGF-β1, and requires matrix presentation rather than soluble BMP-6.\",\n      \"method\": \"Stable transfection; in vitro differentiation assay; subcutaneous implantation in vivo; neutralizing antibody blocking; MyoD overexpression competition assay\",\n      \"journal\": \"Cell growth & differentiation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional assays with antibody neutralization controls\",\n      \"pmids\": [\"7547504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Bmp6 null mice are viable and fertile but show a consistent delay in ossification specifically in the developing sternum, with Bmp2 co-expressed in hypertrophic cartilage suggesting functional compensation. Bmp5/6 double mutants show exacerbated sternal defects.\",\n      \"method\": \"Gene targeting (knockout); skeletal analysis; in situ hybridization; Bmp5/Bmp6 double mutant analysis\",\n      \"journal\": \"Developmental genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with double-mutant epistasis, defined skeletal phenotype\",\n      \"pmids\": [\"9664685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Bmp6 and Bmp7 are co-expressed in overlapping domains in cardiac cushions; Bmp6;Bmp7 double mutants (but not single mutants) show marked delay in outflow tract endocardial cushion formation, valve morphogenesis defects, and chamber septation defects causing embryonic lethality, establishing BMP6 as genetically required for cardiac cushion development.\",\n      \"method\": \"In situ hybridization; Bmp6/Bmp7 double knockout mice; histological analysis of cardiac development\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with double KO revealing non-redundant role\",\n      \"pmids\": [\"11437450\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"BMP2 and BMP6 cooperatively regulate bone formation in vivo; Bmp2+/-;Bmp6-/- compound-deficient mice show reduced trabecular bone volume with suppressed bone formation and impaired endochondral (but not intramembranous) bone repair after fracture, whereas single-gene-deficient mice do not.\",\n      \"method\": \"Compound knockout mice; micro-CT and histomorphometry; fracture model; bone marrow cell culture for osteogenic differentiation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — compound KO genetic epistasis with fracture model readout\",\n      \"pmids\": [\"16109715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"BMP-6 induces LMP-1 (LIM mineralization protein-1) expression in osteoblast precursors; LMP-1 is required for BMP-6-induced osteoblast differentiation (antisense knockdown blocks differentiation) and its overexpression is sufficient to induce de novo bone formation, positioning LMP-1 as an essential intermediate in the BMP-6 signaling pathway.\",\n      \"method\": \"Antisense oligonucleotide knockdown; mammalian expression vector overexpression; in vitro bone nodule formation; subcutaneous implant in vivo\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional gain/loss with defined pathway placement, single lab\",\n      \"pmids\": [\"9832452\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"BMP-2/6 heterodimer is more potent than either BMP-2 or BMP-6 homodimers in inducing differentiation of human embryonic stem cells and activates Smad1/5 and MAPK (ERK, p38) signaling more effectively.\",\n      \"method\": \"Treatment of hES cells with recombinant proteins; qPCR for differentiation markers; flow cytometry; Western blot for Smad1/5 and MAPK phosphorylation\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling readouts, single lab\",\n      \"pmids\": [\"20567515\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Iron-induced increase in Bmp6 expression originates predominantly from non-parenchymal liver cells (NPCs), particularly hepatic stellate cells, not hepatocytes; iron-saturated transferrin induces Bmp6 mRNA in isolated stellate cells but not in isolated hepatocytes, and this induction is independent of the BMP signaling pathway.\",\n      \"method\": \"Cell fractionation of liver; isolated primary hepatocytes and hepatic stellate cells; iron-saturated transferrin treatment; qRT-PCR; dietary iron manipulation in vivo\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — primary cell isolation with defined cellular source, single lab\",\n      \"pmids\": [\"23565256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Iron overload induces Bmp6 mRNA and protein expression in the liver but not in the duodenum across multiple mouse strains, establishing the liver as the primary source of BMP6 for iron homeostasis regulation.\",\n      \"method\": \"Quantitative PCR; immunohistochemistry; Western blot; dietary iron loading and Hfe knockout models in three mouse strains\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple mouse strains and methods, but primarily expression measurement\",\n      \"pmids\": [\"20952515\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Estrogen regulates hepcidin expression via a GPR30-BMP6-dependent mechanism: GPR30 activation by estrogen induces BMP6 expression in hepatocytes, and GPR30 silencing abolishes estrogen-induced BMP6 and hepcidin upregulation.\",\n      \"method\": \"OVX mouse model; GPR30 siRNA knockdown; G1 agonist treatment; qRT-PCR for BMP6 and hepcidin\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA mechanistic validation plus in vivo model, single lab\",\n      \"pmids\": [\"22792339\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Heterozygous missense mutations in the BMP6 pro-peptide (p.Pro95Ser, p.Leu96Pro, p.Gln113Glu) cause defective BMP6 secretion, reduced SMAD1/5/8 signaling, and loss of hepcidin production in transfected cells, establishing the propeptide as functionally required for BMP6 processing and iron homeostasis.\",\n      \"method\": \"Transfection of mutant BMP6 constructs; immunofluorescence for secretion; qRT-PCR and Western blot for SMAD signaling and hepcidin; immunohistochemistry of patient biopsy\",\n      \"journal\": \"Gastroenterology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-based functional assays with mutagenesis, human genetics validation\",\n      \"pmids\": [\"26582087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"BMP-6 inhibits miR-21 transcription in breast cancer cells through reducing expression of deltaEF1 and c-Fos/c-Jun, and physically dislodging deltaEF1 from the E2-box of the miR-21 promoter (miPPR-21), thereby inhibiting MDA-MB-231 cell invasion.\",\n      \"method\": \"Luciferase reporter assay; ChIP assay; site-directed mutation of promoter; RT-PCR; invasion assay\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and reporter assays establish direct transcriptional mechanism, single lab\",\n      \"pmids\": [\"19308091\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BMP-6 promotes E-cadherin expression in breast cancer cells by inhibiting deltaEF1 transcription and physically dislodging deltaEF1 from the E-cadherin proximal promoter; deltaEF1 knockdown abolishes E-cadherin transactivation by BMP-6.\",\n      \"method\": \"Luciferase reporter assay; ChIP assay; siRNA knockdown of deltaEF1; RT-PCR and Western blot\",\n      \"journal\": \"BMC cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and functional knockdown establish direct transcriptional mechanism, single lab\",\n      \"pmids\": [\"17997862\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"BMP-6 inhibits proliferation of mature human B cells and B progenitor cells via phosphorylation of Smad1/5/8 followed by upregulation of Id1 (but not Id2 or Id3); the antiproliferative effect is completely neutralized by noggin.\",\n      \"method\": \"Primary human B cell and pre-B cell line cultures; Western blot for phospho-Smad1/5/8; RT-PCR for Id1-Id3; proliferation assay; noggin neutralization\",\n      \"journal\": \"BMC immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods with antagonist rescue, single lab\",\n      \"pmids\": [\"15877825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"BMP-6 in prostate cancer cells induces nuclear translocation of SMADs and activates Id-1; BMP-6 also increases migration and invasion, upregulates MMP-1 and MMP-9 at both mRNA and protein levels via direct transcriptional activation of their promoters, and activates MAPK signaling.\",\n      \"method\": \"Doxycycline-inducible BMP-6 expression system; SMAD nuclear translocation imaging; luciferase reporter assay for MMP-1/MMP-9 promoters; migration/invasion assay; Western blot for phospho-MAPK\",\n      \"journal\": \"The Journal of pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — inducible system with reporter assays and migration assays, single lab\",\n      \"pmids\": [\"18072288\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"BMP-6 derived from prostate cancer cells induces IL-6 expression in macrophages via crosstalk between Smad1 and NF-κB1 p50; IL-6 in turn induces neuroendocrine differentiation of prostate cancer cells. Disruption of BMP-6 or IL-6 with neutralizing antibodies blocks neuroendocrine differentiation.\",\n      \"method\": \"Co-culture of prostate cancer and macrophage cell lines; neutralizing antibodies to BMP-6 and IL-6; IL-6 knockout mice; macrophage-depleted mice; conditioned media experiments\",\n      \"journal\": \"The Prostate\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro co-culture plus in vivo knockout models, multiple methods\",\n      \"pmids\": [\"21374653\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Intraocular BMP6 protein injection in mice upregulates retinal hepcidin expression and alters retinal labile iron levels; Bmp6-/- mice develop age-dependent retinal iron accumulation and degeneration. BMP6 is expressed in retinal pigment epithelium and is regulated by oxidative stress and iron.\",\n      \"method\": \"Intraocular BMP6 protein injection; Bmp6 knockout mice; qRT-PCR; labile iron measurement; histological analysis of retinal degeneration\",\n      \"journal\": \"The American journal of pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — protein injection experiment plus KO model, defined functional readout\",\n      \"pmids\": [\"21703414\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"BMP-6 treatment of recombinant BMP-6-expressing CHO cells and in vivo injection in mice increases hepcidin expression and reduces serum iron; BMP6 acts through phosphorylation of Smad1/5/8 to regulate hepcidin in the liver.\",\n      \"method\": \"Exogenous BMP6 injection in mice; phospho-Smad1/5/8 Western blot; hepcidin qRT-PCR; serum iron measurements\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo pharmacological experiment with defined molecular pathway, replicated across multiple papers\",\n      \"pmids\": [\"19252486\", \"19252488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BMP6 regulates VEGFR2 expression in endothelial cells and acts via Hippo signaling effector TAZ to modulate angiogenesis; in a matrigel plug assay in mice, BMP6 induces neovascularization.\",\n      \"method\": \"In vitro endothelial cell assays; VEGFR2 expression analysis; TAZ knockdown/overexpression; matrigel plug assay in vivo\",\n      \"journal\": \"Angiogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro mechanistic assays plus in vivo angiogenesis model, single lab\",\n      \"pmids\": [\"33021694\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BMP6 acts on pancreatic and liver cells through Alk3, Alk6, and ActRIIA serine/threonine kinase receptors to regulate glucose metabolism; BMP6 inhibits gluconeogenesis in rat hepatoma cells via downregulation of PEPCK expression, confirmed by chromatin immunoprecipitation.\",\n      \"method\": \"Receptor-specific Fc fusion protein inhibitors; luciferase reporter gene assay; glucose output assay in H4IIE and INS1 cells; ChIP; ob/ob mouse treatment with BMP6\",\n      \"journal\": \"Acta diabetologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — receptor specificity assayed with multiple Fc fusions, ChIP, and in vivo model\",\n      \"pmids\": [\"30539233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BMP6 downregulates GDNF expression in human granulosa cells through ALK2 and ALK3 receptors, activating SMAD1 and SMAD5 as well as ERK1/2 (but not AKT or p38); complete reversal of GDNF suppression requires dual inhibition of both SMAD4 and ERK1/2.\",\n      \"method\": \"ALK receptor kinase inhibitors; siRNA knockdown of ALK2, ALK3, SMAD1, SMAD5, SMAD4; Western blot for phospho-SMAD1/5/8 and ERK1/2; qRT-PCR for GDNF in SVOG and primary granulosa-lutein cells\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — dual pathway dissection with inhibitor and siRNA, defined mechanistic readout\",\n      \"pmids\": [\"29750278\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Recombinant BMP6 inhibits activation of hepatic stellate cells (HSCs) and reduces proinflammatory and profibrogenic gene expression in activated HSCs; steatosis-induced BMP6 upregulation is hepatoprotective, and Bmp6-/- mice develop more severe hepatic fibrosis on MCD and high-fat diets.\",\n      \"method\": \"Bmp6-/- mice on MCD and high-fat dietary models; in vitro primary human HSC treatment with recombinant BMP6; histological analysis of fibrosis\",\n      \"journal\": \"Gut\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO mouse model plus in vitro cell assay, single lab\",\n      \"pmids\": [\"25011936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BMP-6 exerts its osteoinductive effect at least in part through upregulation of IGF-I and EGF pathway gene expression in bone; gene set enrichment analysis and qPCR confirmed IGF-I and EGF upregulation in BMP-6-treated mouse bone and primary human osteoblasts.\",\n      \"method\": \"In vivo gene expression profiling (microarray) in oophorectomized mice treated with BMP-6; gene set enrichment analysis; qPCR confirmation; primary human osteoblast culture\",\n      \"journal\": \"International orthopaedics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — expression-based inference with limited mechanistic validation, single lab\",\n      \"pmids\": [\"17634942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"BMP6 expression in salivary gland monocytic cells in Sjögren's syndrome is stimulated by HSP70 acting as an endogenous TLR4 ligand; HSP70 is released via LAMP3-induced lysosomal exocytosis from epithelial cells. LAMP3 overexpression in mice induces BMP6 expression and an SS-like phenotype.\",\n      \"method\": \"RNA ISH on salivary gland sections; RNA-Seq; single-cell transcriptomics of PBMCs; in vitro TLR4 signaling assays; LAMP3 overexpression in mice; RT-PCR correlation analysis\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods identifying upstream pathway, in vivo mouse model, single lab\",\n      \"pmids\": [\"35113815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"BMP6 is produced by myotubes and mature Schwann cells and is retrogradely transported in mature motor axons; BMP6 supports motoneuron survival in vitro through BMPRII, which is highly expressed in motoneurons.\",\n      \"method\": \"Laser microdissection of motoneurons for receptor mRNA quantification; immunostaining for BMPRII; retrograde transport assay; in vitro motoneuron survival assay\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — axonal transport assay plus survival assay, receptor expression quantified\",\n      \"pmids\": [\"17321145\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BMP6 signaling through the BMP/SMAD pathway is absolutely required for hepcidin induction by acute serum iron (oral iron gavage), whereas BMP2 plays only a partially redundant role; Bmp6-/- or Bmp6 CKO mice fail to induce hepcidin after oral iron gavage.\",\n      \"method\": \"Global and endothelial conditional Bmp6 knockout mice; acute oral iron gavage; serum iron and liver SMAD5 phosphorylation; hepcidin qRT-PCR\",\n      \"journal\": \"American journal of hematology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional and global KO with defined serum iron stimulus, mechanistic distinction from BMP2\",\n      \"pmids\": [\"30478858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BMP6-induced inhibition of hair follicle telogen-anagen transition is mediated by reciprocal antagonism with Wnt10b: BMP6 overexpression in vivo inhibits hair follicle stem cell activation, suppresses Wnt10b signaling, and is itself suppressed by Wnt10b.\",\n      \"method\": \"Adenovirus-mediated BMP6 overexpression in mouse skin; in situ hybridization; immunofluorescence; BrdU tracing; comparison with AdWnt10b-treated skins\",\n      \"journal\": \"Cell communication and signaling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo overexpression with defined pathway crosstalk, single lab\",\n      \"pmids\": [\"30791955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"BMP-6 inhibits MMP-1 expression in breast cancer cells by inhibiting AP-1-mediated transcription; ChIP and luciferase assays show BMP-6 decreases recruitment of c-Jun/c-Fos to the endogenous MMP-1 promoter AP-1 response element, and MMP-1 overexpression rescues BMP-6-mediated inhibition of invasion.\",\n      \"method\": \"Luciferase reporter assay; ChIP assay; RT-PCR; MMP-1 overexpression rescue; Boyden chamber invasion assay\",\n      \"journal\": \"Oncology reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP and functional rescue establish mechanistic link, single lab\",\n      \"pmids\": [\"26751737\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"BMP-6 inhibits MMP-9 secretion in MCF-7 breast cancer cells through upregulation of HO-1 via Smad1/5 recruitment to a Smad-responsive element on the HO-1 promoter; BMP-6/HO-1-mediated MMP-9 suppression operates via MAPK/p38/AP-1 signaling, and HO-1 knockdown abolishes BMP-6 anti-invasive effects.\",\n      \"method\": \"Luciferase reporter assay; ChIP for Smad1/5 on HO-1 promoter; HO-1 knockdown by ZnPPIX; RT-PCR; Boyden chamber assay\",\n      \"journal\": \"Journal of cancer research and clinical oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP establishes direct promoter binding, functional rescue assay, single lab\",\n      \"pmids\": [\"21136273\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"HIF-1α directly represses BMP6 expression by binding to the hypoxia-response element (HRE) in the BMP6 promoter, and HIF-1α-mediated BMP6 suppression promotes hyperproliferation and abnormal differentiation of keratinocytes.\",\n      \"method\": \"HIF-1α overexpression in primary human keratinocytes; luciferase reporter with HRE in BMP6 promoter; ChIP for HIF-1α binding; cell cycle analysis; differentiation markers\",\n      \"journal\": \"Experimental dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP plus reporter assay establishes direct transcriptional regulation, single lab\",\n      \"pmids\": [\"30230035\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"BMP6 is a secreted TGF-β superfamily ligand produced primarily by liver sinusoidal endothelial cells in response to iron-induced oxidative stress (via Nrf2) and NTBI uptake; it acts as the principal endogenous activator of the hemojuvelin co-receptor/BMPR complex on hepatocytes to phosphorylate SMAD1/5/8 and drive hepcidin transcription, thereby controlling systemic iron homeostasis, while also signaling through ALK2/3/6 and ActRIIA receptors in other cell types to regulate osteoblast differentiation, B-lymphopoiesis, angiogenesis (via TAZ/Hippo and VEGFR2), glucose metabolism, and context-dependent transcriptional programs (E-cadherin, miR-21, MMP-1/9) through SMAD and non-SMAD (ERK1/2, p38 MAPK) pathways.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"BMP6 is a secreted TGF-β superfamily ligand that serves as the principal endogenous regulator of systemic iron homeostasis and contributes to osteoblast differentiation, angiogenesis, and context-dependent transcriptional programs in multiple tissues. In the liver, BMP6 is produced primarily by sinusoidal endothelial cells in response to iron-induced oxidative stress sensed through NTBI uptake and Nrf2 activation; it signals in a paracrine manner through the hemojuvelin co-receptor and BMPR complex on hepatocytes to phosphorylate SMAD1/5/8 and drive hepcidin transcription, thereby limiting dietary iron absorption and macrophage iron release [PMID:19252486, PMID:19252488, PMID:31276102, PMID:36351237]. This pathway is modulated by erythroferrone (which traps BMP6 to suppress hepcidin during stress erythropoiesis), by HFE (which facilitates BMP6-SMAD signal transduction), and by matriptase-2/TMPRSS6 (which acts as a negative-feedback regulator induced by BMP6 via Id1) [PMID:30097509, PMID:20682319, PMID:21374653]. Beyond iron regulation, BMP6 drives endochondral ossification cooperatively with BMP2 and BMP5, promotes endothelial migration via Myo10/ALK6 and VEGFR2/TAZ-dependent angiogenesis, modulates E-cadherin and MMP expression in cancer cells through SMAD and MAPK pathways, and inhibits B-cell proliferation via SMAD1/5/8-Id1 signaling [PMID:8089189, PMID:9664685, PMID:16109715, PMID:18158328, PMID:15877825, PMID:17997862].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Establishing that BMP6 is a secreted, proteolytically processed ligand capable of inducing endochondral bone formation answered the fundamental question of whether BMP6 functions as an osteoinductive morphogen.\",\n      \"evidence\": \"Implantation of BMP6-expressing CHO cells in nude mice with histological and neutralizing-antibody confirmation\",\n      \"pmids\": [\"8089189\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor identity for BMP6 osteoinductive signaling not defined\", \"In vivo processing site not mapped\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Generation of Bmp6-null mice revealed that BMP6 has a specific but partially redundant role in sternal ossification, with BMP5 compensating; this established the first loss-of-function phenotype and set the stage for identifying non-skeletal functions.\",\n      \"evidence\": \"Bmp6 knockout and Bmp5/Bmp6 double-mutant skeletal analysis\",\n      \"pmids\": [\"9664685\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Iron homeostasis phenotype not yet examined in these mice\", \"Molecular compensation mechanisms with BMP2/5/7 not dissected\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Double knockout of Bmp6 and Bmp7 revealed a non-redundant requirement for BMP6 in cardiac outflow tract cushion formation, broadening its developmental role beyond skeletal tissues.\",\n      \"evidence\": \"Bmp6/Bmp7 double-knockout mice with histological analysis of cardiac development\",\n      \"pmids\": [\"11437450\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream targets in cardiac cushion mesenchyme not identified\", \"Whether BMP6 acts on endocardium vs mesenchyme not resolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Compound Bmp2+/−;Bmp6−/− mice demonstrated cooperative regulation of trabecular bone formation and endochondral fracture repair, distinguishing BMP6's role from intramembranous ossification.\",\n      \"evidence\": \"Compound knockout mice with micro-CT, histomorphometry, and fracture model\",\n      \"pmids\": [\"16109715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Receptor complex mediating cooperative BMP2/BMP6 signaling in bone not defined\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The crystal structure of BMP6 defined its type I and type II receptor-binding interfaces, providing the first structural framework for understanding its signaling specificity.\",\n      \"evidence\": \"X-ray crystallography with SPR binding kinetics and Smad-based luciferase reporter validation\",\n      \"pmids\": [\"17924656\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of BMP6 in complex with its receptors not determined\", \"Basis for selectivity over BMP7 at type I receptor level not fully resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identification of Myo10 as a BMP6-induced effector in endothelial cells that amplifies BMP6/ALK6 signaling at filopodia revealed a positive-feedback mechanism linking BMP6 to directional cell migration and angiogenesis.\",\n      \"evidence\": \"Microarray, siRNA knockdown, live imaging, and co-localization of ALK6 with Myo10 in filopodia\",\n      \"pmids\": [\"18158328\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Myo10-dependent amplification is specific to endothelial cells or generalizable not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"BMP6 was shown to promote E-cadherin expression and suppress miR-21 transcription in breast cancer cells via deltaEF1 displacement from target promoters, establishing a direct transcriptional mechanism for BMP6's anti-invasive activity.\",\n      \"evidence\": \"ChIP, luciferase reporters, and siRNA knockdown of deltaEF1 in breast cancer cell lines\",\n      \"pmids\": [\"17997862\", \"19308091\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of BMP6-deltaEF1 axis in tumor suppression not demonstrated\", \"Whether SMAD signaling mediates deltaEF1 regulation not fully dissected\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Two independent studies demonstrated that Bmp6-null mice develop severe iron overload with reduced hepcidin and impaired hepatic SMAD1/5/8 phosphorylation, establishing BMP6 as the non-redundant endogenous activator of hepcidin — a paradigm-shifting discovery linking a BMP ligand to systemic iron regulation.\",\n      \"evidence\": \"Bmp6 knockout mice, neutralizing anti-BMP6 antibody, HJV.Fc co-immunoprecipitation, phospho-SMAD Western blot\",\n      \"pmids\": [\"19252486\", \"19252488\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cell type producing BMP6 in the liver not yet identified\", \"How iron loading increases BMP6 expression not resolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Hfe-deficient mice showed elevated BMP6 but impaired downstream SMAD phosphorylation, placing HFE as a facilitator of BMP6 signal transduction rather than BMP6 production.\",\n      \"evidence\": \"Hfe knockout mice in multiple genetic backgrounds with phospho-SMAD1/5/8 and Id1 measurement\",\n      \"pmids\": [\"19622835\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physical mechanism by which HFE facilitates BMP6/SMAD transduction not identified\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Genetic epistasis showed that Bmp6 co-deletion corrects the elevated hepcidin and anemia of Tmprss6-null mice, positioning matriptase-2 as a negative regulator acting specifically on the BMP6/HJV/hepcidin axis.\",\n      \"evidence\": \"Tmprss6−/−;Bmp6−/− double-knockout mice with hematological and hepcidin analysis\",\n      \"pmids\": [\"20940420\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TMPRSS6 cleaves HJV directly at the cell surface in physiological context not resolved in this study\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"BMP6 was found to induce its own negative regulator TMPRSS6 via the SMAD/Id1 pathway, revealing an autoregulatory feedback loop in hepcidin control.\",\n      \"evidence\": \"In vitro BMP6 treatment with Id1 knockdown and in vivo BMP6 injection\",\n      \"pmids\": [\"21374653\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetic modeling of the feedback loop in iron homeostasis not performed\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Endothelial-specific Bmp2 knockout and Bmp6/Hjv double knockout studies demonstrated that BMP2 and BMP6 are both endothelial-derived hepcidin regulators with partially overlapping but non-identical functions, and that BMP6 retains some HJV-independent signaling capacity.\",\n      \"evidence\": \"Conditional endothelial Bmp2 KO; Hjv/Bmp6 double KO; neutralizing antibody; LPS challenge\",\n      \"pmids\": [\"28815688\", \"29021231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of BMP2 vs BMP6 under different iron-loading conditions not quantitatively defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Erythroferrone was identified as a physiological ligand trap that selectively sequesters BMP5/6/7 (not BMP2/4/9) to suppress hepcidin during stress erythropoiesis, explaining the erythroid regulation of iron supply.\",\n      \"evidence\": \"Cell-free HTRF binding assays, SMAD phosphorylation and hepcidin reporter in hepatocytes, neutralizing antibody\",\n      \"pmids\": [\"30097509\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of ERFE selectivity for BMP6 over BMP2 not determined\", \"In vivo stoichiometry of ERFE-BMP6 trapping not measured\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"BMP6 was shown to signal through ALK2, ALK3, ALK6, and ActRIIA receptors in pancreatic and hepatic cells to suppress gluconeogenesis via PEPCK downregulation, expanding BMP6's metabolic role beyond iron.\",\n      \"evidence\": \"Receptor-specific Fc fusion inhibitors, ChIP, glucose output assay, ob/ob mouse treatment\",\n      \"pmids\": [\"30539233\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether glucose-regulatory effects are physiologically relevant beyond ob/ob model not tested\", \"Relative receptor usage across tissues not systematically compared\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Nrf2 was identified as the transcription factor that senses iron-induced mitochondrial oxidative stress and directly drives Bmp6 transcription in liver sinusoidal endothelial cells, closing the circuit between iron sensing and BMP6 production.\",\n      \"evidence\": \"Nrf2 knockout mice, pharmacological Nrf2 activation, ChIP for Nrf2 on Bmp6 promoter, iron loading\",\n      \"pmids\": [\"31276102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Nrf2 is the sole transcription factor driving iron-induced Bmp6 not excluded\", \"Epigenetic regulation of Bmp6 not explored\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Single-cell transcriptomics and conditional knockout confirmed that LSECs sense NTBI to activate Bmp6 via oxidative stress/Nrf2, refining the cellular iron-sensing mechanism to a specific cell type and iron species.\",\n      \"evidence\": \"Endothelial Tfr1 conditional KO, single-cell RNA-seq, NTBI-Bmp6 correlation analysis\",\n      \"pmids\": [\"36351237\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the NTBI transporter on LSECs not determined\", \"Whether other non-parenchymal cells contribute Bmp6 in pathological states remains open\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of BMP6 selectivity for HJV over other co-receptors, the identity of the LSEC NTBI transporter, the quantitative contribution of BMP6 versus BMP2 under distinct physiological iron states, and whether BMP6's metabolic functions (glucose, lipid) operate independently of hepcidin in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of BMP6-HJV-receptor ternary complex available\", \"NTBI import pathway in LSECs uncharacterized\", \"In vivo relevance of BMP6 in glucose homeostasis not established in conditional models\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 1, 14, 15, 27, 31]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 4, 7, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 4, 14, 15, 24]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 3, 7, 8, 10, 11, 20, 27, 28, 34]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [14, 15, 16, 17, 18]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 5, 6, 33, 39]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 1, 7, 12, 13, 39]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"HJV\",\n      \"TMPRSS6\",\n      \"HFE\",\n      \"ERFE\",\n      \"MYO10\",\n      \"ALK6\",\n      \"ALK2\",\n      \"ACVR2A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}