{"gene":"HJV","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2006,"finding":"Hemojuvelin (HJV) functions as a bone morphogenetic protein (BMP) coreceptor that enhances BMP signaling to upregulate hepcidin expression in hepatocytes; HJV mutants associated with hemochromatosis have impaired BMP signaling ability, and BMP-mediated hepcidin upregulation is blunted in Hfe2-/- hepatocytes.","method":"Cell-based BMP signaling assays, Hfe2-/- mouse hepatocytes, reporter assays, co-receptor functional studies","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (cell assays, knockout hepatocytes, mutagenesis), foundational paper with >800 citations","pmids":["16604073"],"is_preprint":false},{"year":2008,"finding":"The serine protease matriptase-2 (TMPRSS6) cleaves membrane hemojuvelin (HJV) on the plasma membrane, releasing an inactive soluble form and thereby inhibiting hepcidin activation; matriptase-2 lacking the serine protease domain shows no cleavage activity, and matriptase-2 interacts with HJV through its ectodomain.","method":"Cell-based cleavage assays, domain deletion mutants, co-immunoprecipitation, zebrafish functional studies","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 1–2 — biochemical cleavage assay with domain mutants, reciprocal co-IP, in vivo zebrafish validation, >400 citations","pmids":["18976966"],"is_preprint":false},{"year":2005,"finding":"Cell-associated (membrane) HJV positively regulates hepcidin mRNA expression independently of the IL-6 pathway, while recombinant soluble HJV suppresses hepcidin mRNA in primary human hepatocytes in a dose-dependent manner; soluble HJV release from cells is inhibited by increasing iron concentrations.","method":"siRNA knockdown of HJV, recombinant soluble HJV treatment of primary hepatocytes, iron-modulation experiments","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal approaches (siRNA KD, recombinant protein treatment, iron titration), >200 citations","pmids":["15998830"],"is_preprint":false},{"year":2005,"finding":"HJV is required for dietary iron sensing in the liver; Hjv-mutant mice exhibit severe iron overload and dramatically decreased hepcidin expression, while cytokine-induced inflammation regulates hepcidin through an Hjv-independent pathway.","method":"Hjv-mutant mouse model, hepcidin expression analysis, dietary iron sensing experiments","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — clean knockout mouse with defined phenotype and pathway placement, >300 citations","pmids":["16075058"],"is_preprint":false},{"year":2005,"finding":"HJV is a GPI-anchored protein that undergoes partial autocatalytic cleavage during intracellular processing and co-immunoprecipitates with neogenin but not with the closely related receptor DCC; the G320V hemochromatosis mutant does not co-immunoprecipitate with neogenin; HJV-induced iron accumulation in HEK293 cells requires neogenin.","method":"Stable transfection of HJV in HEK293 cells, co-immunoprecipitation, GPI-anchor characterization, iron accumulation assays (transferrin-55Fe)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP, functional iron assay, disease mutant comparison, >100 citations","pmids":["16103117"],"is_preprint":false},{"year":2008,"finding":"HJV selectively uses BMP-2, BMP-4, and BMP-6 as endogenous ligands in hepatoma-derived cells; HJV uses BMP type II receptors ActRIIA and BMPRII but not ActRIIB, and enhances ActRIIA utilization by BMP-2/4; HJV-induced BMP signaling and hepcidin expression are independent of neogenin overexpression or knockdown.","method":"In vitro BMP signaling assays, receptor expression studies, neogenin overexpression/siRNA knockdown","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple receptor combinations tested with orthogonal methods, >180 citations","pmids":["18326817"],"is_preprint":false},{"year":2012,"finding":"HFE, TfR2, and HJV form a multi-protein membrane complex on hepatocyte surfaces; HFE and TfR2 bind HJV non-competitively; HJV competes with TfR1 for binding to HFE; residues 120–139 of the TfR2 extracellular domain are required for binding both HFE and HJV.","method":"Glycerol gradient sedimentation, co-immunoprecipitation in transfected HuH7 cells, domain mapping","journal":"Journal of hepatology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP plus sedimentation assay, domain mapping, >140 citations","pmids":["22728873"],"is_preprint":false},{"year":2007,"finding":"In iron-deficient rats, HJV shedding into serum increases without changes in HFE2 mRNA or protein levels; holo-transferrin suppresses HJV shedding in C2C12 and HepG2 cells; neogenin knockdown suppresses HJV shedding while neogenin overexpression enhances it, indicating HJV membrane shedding is mediated by neogenin.","method":"Iron-deficient rat model, holo-transferrin treatment, siRNA knockdown and overexpression of neogenin, cell-based shedding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — in vivo and in vitro complementary evidence, siRNA knockdown and overexpression, >95 citations","pmids":["17331953"],"is_preprint":false},{"year":2010,"finding":"Neogenin inhibits HJV secretion and stabilizes membrane HJV; livers of neogenin-mutant mice exhibit iron overload, low hepcidin, and reduced BMP signaling; mutant hepatocytes show impaired BMP2-induced Smad1/5/8 phosphorylation and hepcidin expression.","method":"Neogenin mutant mouse model, hepatocyte culture BMP signaling assays, Smad phosphorylation analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — in vivo neogenin mutant mouse with defined hepcidin/BMP phenotype plus in vitro mechanistic studies, >100 citations","pmids":["20065295"],"is_preprint":false},{"year":2007,"finding":"Iron stimulates hepcidin expression in primary murine hepatocytes through a hemojuvelin/BMP2/4-dependent pathway; BMP9, although expressed in liver, does not interact with hemojuvelin and its pathway does not affect iron regulation of hepcidin.","method":"Primary murine hepatocyte culture with holotransferrin, BMP neutralization experiments, siRNA knockdown of hemojuvelin","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — primary cell culture with multiple BMP ligand tests and HJV knockdown, >200 citations","pmids":["17540841"],"is_preprint":false},{"year":2009,"finding":"The HJV-neogenin interaction is required for BMP4-induced hepcidin expression; knockdown of neogenin in HJV-expressing HepG2 cells decreases BMP4-induced hepcidin mRNA 16-fold; disruption of the HJV-neogenin interaction markedly suppresses hepcidin; in vivo blocking of HJV-neogenin interaction with soluble neogenin fragment suppresses hepatic hepcidin in mice.","method":"siRNA knockdown of neogenin in HJV-expressing HepG2 cells, in vivo injection of soluble neogenin fragment in mice, G99V mutant analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — cell-based siRNA and in vivo mouse experiment, >60 citations","pmids":["19564337"],"is_preprint":false},{"year":2012,"finding":"Neogenin interacts with matriptase-2 (MT2) as well as with HJV, forming a ternary complex at the plasma membrane that facilitates MT2 cleavage of HJV; neogenin knockdown with siRNA increases MT2 and HJV on the plasma membrane; MT2 cleavage of cell-surface HJV is coupled to a transition from high-mannose to complex oligosaccharides on HJV.","method":"Co-immunoprecipitation, siRNA knockdown, oligosaccharide analysis, cell-surface cleavage assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — co-IP of ternary complex, siRNA validation, glycan analysis as mechanistic read-out","pmids":["22893705"],"is_preprint":false},{"year":2010,"finding":"Matriptase-2 cleaves HJV at Arg288, producing a soluble HJV form with decreased ability to bind BMP6 that does not suppress BMP6-induced hepcidin expression, distinct from the proprotein convertase (furin) cleavage product which does suppress hepcidin.","method":"In vitro cleavage assay, BMP6 binding assay, hepcidin reporter assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — biochemical cleavage site mapping with functional BMP binding and hepcidin reporter assays","pmids":["20937842"],"is_preprint":false},{"year":2008,"finding":"HJV undergoes autocatalytic cleavage generating a disulfide-linked heterodimer; neogenin binds to both cleaved and uncleaved HJV; the HJV-neogenin binding site maps to the membrane-proximal fifth and sixth FNIII domains of neogenin; BMP-2 and neogenin can bind HJV simultaneously, raising the possibility of a multiprotein complex.","method":"Recombinant protein production, binding assays, domain mapping with neogenin fragments, BMP-2 competition assays","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstituted binding with domain mapping and simultaneous binding demonstration","pmids":["18335997"],"is_preprint":false},{"year":2006,"finding":"HJV (RGMc) undergoes complex biosynthesis in skeletal muscle: two GPI-anchored glycosylated forms are produced — a full-length species released into extracellular fluid and a disulfide-linked heterodimer of N- and C-terminal fragments at the membrane; the G320V disease mutant fails to generate the heterodimeric membrane isoform.","method":"Pulse-chase studies, cell-surface labeling, glycosylation analysis, disease mutant characterization","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — multiple biochemical assays defining biosynthetic pathway with disease mutant validation","pmids":["16868025"],"is_preprint":false},{"year":2007,"finding":"Defective plasma membrane targeting of HJV is a common pathogenetic mechanism in juvenile hemochromatosis; proteolytic processing (autocleavage) is required for membrane export; mutants F170S, W191C, and G320V are retained in the ER and fail to reach the cell surface; membrane HJV level is increased by iron in wild-type but not mutant mice.","method":"HeLa and HepG2 cell transfection, cell-surface labeling, immunoblot analysis of processing, iron supplementation experiments","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple mutants characterized with orthogonal methods, in vivo mouse validation","pmids":["17264300"],"is_preprint":false},{"year":2008,"finding":"HJV processing requires retrograde trafficking from the plasma membrane back to the Golgi; cellular HJV reaches the plasma membrane with high-mannose oligosaccharides (bypassing Golgi processing), while secreted HJV has complex oligosaccharides derived from plasma membrane HJV after retrograde trafficking; neogenin is required for HJV release/shedding but not for trafficking to the cell surface.","method":"Glycosylation analysis, neogenin knockdown, endocytosis inhibitors (dynamin-independent, cholesterol-dependent pathway), HepG2 cells","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — glycan analysis plus endocytosis inhibitor studies defining trafficking pathway","pmids":["19029439"],"is_preprint":false},{"year":2008,"finding":"Pro-protein convertases (PCs), including furin, cleave 50 kDa RGMc/HJV at a conserved PC recognition site to generate a 40 kDa species with truncated C-terminus; cell-impermeable PC inhibitor blocks extracellular 40 kDa HJV; iron loading reduces HJV release from the cell membrane and diminishes accumulation of the 40 kDa species.","method":"Furin in vitro cleavage assay, PC inhibitor experiments, site-directed mutagenesis of cleavage site, iron-loading cell culture","journal":"BMC biochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro furin cleavage, mutagenesis, cell-impermeable inhibitor validation","pmids":["18384687"],"is_preprint":false},{"year":2008,"finding":"Neogenin-mediated HJV shedding occurs after HJV traffics to the plasma membrane; neogenin knockdown suppresses HJV release without affecting plasma membrane trafficking; HJV endocytosis is dynamin-independent and cholesterol-dependent; HJV release is coupled to lysosomal degradation of neogenin.","method":"Neogenin knockdown, cholesterol depletion (filipin), lysosomal inhibitor experiments, HepG2 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — pharmacological and siRNA dissection of trafficking pathway with multiple readouts","pmids":["18445598"],"is_preprint":false},{"year":2009,"finding":"Two BMP-responsive elements (BMP-RE1 at -84/-79 and BMP-RE2 at -2255/-2250) in the hepcidin promoter are both critical for basal hepcidin expression and response to BMP-2, BMP-6, and HJV-mediated BMP signals; BMP-RE1 is specifically required for IL-6 response while both elements show additive effects for HJV/BMP signals.","method":"Hepcidin promoter reporter assays, BMP-RE mutagenesis, BMP-2/BMP-6/IL-6 stimulation experiments","journal":"Journal of molecular medicine","confidence":"High","confidence_rationale":"Tier 1–2 — promoter mutagenesis with multiple ligand stimulations defining downstream transcriptional mechanism","pmids":["19229506"],"is_preprint":false},{"year":2011,"finding":"Hepatic HJV, but not skeletal muscle HJV, is necessary and sufficient for hepcidin regulation and systemic iron homeostasis; liver-specific Hjv knockout causes iron overload comparable to ubiquitous Hjv-/- mice with marked hepcidin suppression, while muscle-specific Hjv knockout causes no iron phenotype.","method":"Tissue-specific conditional Hjv knockout mice (hepatocyte- and muscle-specific Cre), iron parameter analysis, hepcidin mRNA quantification","journal":"Hepatology","confidence":"High","confidence_rationale":"Tier 2 — clean conditional tissue-specific knockouts with defined phenotypes, replicated by independent group","pmids":["21748766","21493799"],"is_preprint":false},{"year":2015,"finding":"In skeletal muscle, HJV acts as a coreceptor for TGF-β receptor II (TβRII) and inhibits TGF-β1/Smad3 signaling; HJV directly interacts with TβRII on the muscle membrane, and loss of HJV increases TGF-β1, TβRII, and p-Smad2/3 without altering the BMP/p-Smad1/5/8 pathway in muscle; overexpression of Hjv rescues dystrophic and age-related muscle wasting.","method":"Co-immunoprecipitation, dual-luciferase reporter assay, RNAi, conditional Hjv knockout mice, mdx mice, Hjv overexpression vector in vivo","journal":"Journal of cachexia, sarcopenia and muscle","confidence":"High","confidence_rationale":"Tier 2 — co-IP, reporter assay, multiple mouse models, identifies distinct mechanism from hepatic BMP co-receptor function","pmids":["30884219"],"is_preprint":false},{"year":2021,"finding":"Hepatocyte neogenin is required for HJV-mediated hepcidin expression and iron homeostasis; hepatocyte-specific Neo1 knockout mice develop iron overload and reduced hepcidin; the Neo1L1046E mutant that cannot interact with HJV fails to rescue the phenotype; HjvA183R with reduced Neo1 interaction shows blunted hepcidin induction; Hjv binding triggers cleavage of the Neo1 cytoplasmic domain, leading to accumulation of truncated Neo1 on the plasma membrane where it acts as a scaffold for BMP signaling complex assembly.","method":"Hepatocyte-specific Neo1 conditional knockout mice, interaction-deficient mutants (Neo1L1046E, HjvA183R), rescue experiments, BMP signaling analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — conditional knockout with rescue using interaction-deficient mutants, defines molecular mechanism","pmids":["33824974"],"is_preprint":false},{"year":2016,"finding":"Neogenin co-immunoprecipitates with ALK3 (type I BMP receptor); a HJV mutation specifically reducing neogenin interaction dramatically attenuates BMP signaling and hepcidin mRNA when expressed in Hjv-/- mice; furin-cleavage site mutation in Hjv does not alter hepcidin stimulation in vivo; neogenin acts as a scaffold facilitating assembly of the HJV·BMP·BMP receptor complex.","method":"Co-immunoprecipitation of neogenin with ALK3, neogenin-interaction mutant HJV in Hjv-/- mice, furin cleavage site mutant in vivo","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — in vivo mouse rescue with interaction-deficient mutants plus co-IP defining complex","pmids":["27072365"],"is_preprint":false},{"year":2018,"finding":"Hjv is required for inflammatory induction of hepcidin and the acute hypoferremic response to LPS, FSL1, and E. coli infection; Hjv deficiency severely impairs BMP6/Smad signaling and abolishes synergism with the IL-6/Stat pathway; Hfe-/- mice show near-normal inflammatory hepcidin; double Hjv-/-Hfe-/- mice phenocopy Hjv-/- mice.","method":"Hjv-/- and Hfe-/- mouse models with LPS/FSL1/E. coli challenge, primary hepatocyte BMP signaling assays, Smad phosphorylation analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple mouse models with in vivo infection and in vitro mechanistic assays","pmids":["30213871"],"is_preprint":false},{"year":2017,"finding":"BMP6 and HJV can signal to hepcidin partially independently; loss of Bmp6 further represses Smad signaling and hepcidin in Hjv-/- mice, indicating BMP6 provides a residual signal in the absence of HJV; both BMP6 and HJV are required for full hepcidin induction, with synergy between BMP/SMAD and IL-6/STAT3 pathways.","method":"Hjv-/- × Bmp6-/- double knockout mice, Smad signaling analysis, LPS challenge, iron accumulation analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — double knockout epistasis experiment with signaling pathway analysis","pmids":["29021231"],"is_preprint":false},{"year":2015,"finding":"HFE and HJV operate in the same pathway for hepcidin regulation; combined Hfe-/- Hjv-/- double knockout mice phenocopy single Hjv-/- mice with indistinguishable hepcidin suppression and iron overload, indicating HFE function is epistatic to or converges with HJV for hepcidin and Smad signaling.","method":"Double Hfe-/- Hjv-/- knockout mouse generation and phenotypic analysis, Smad signaling, iron parameters","journal":"Journal of molecular medicine","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis via double knockout mouse model","pmids":["25609138"],"is_preprint":false},{"year":2015,"finding":"TMPRSS6 cleaves HJV at residues Arg121 and Arg326 in both full-length and heterodimeric HJV isoforms; Arg176 and Arg288 contribute to additional cleavage; molecular dynamics modeling of HJV structure identified TMPRSS6-insensitive arginines within the von Willebrand domain due to partial protein structure destabilization.","method":"Arginine-to-alanine mutagenesis, cell-based cleavage assays, N-terminal FLAG-tagged HJV analysis, in silico molecular dynamics simulation","journal":"Journal of cellular and molecular medicine","confidence":"High","confidence_rationale":"Tier 1–2 — systematic mutagenesis with functional assay plus structural modeling","pmids":["25704252"],"is_preprint":false},{"year":2008,"finding":"BMP-2 binds to single-chain RGMc/HJV species in biochemical assays and to cell-associated RGMc; disease mutants D172E and G320V show reduced BMP-2 binding; G99V cannot bind BMP-2; neogenin preferentially binds heterodimeric membrane RGMc and can interact with wild-type RGMc and G99V but not G320V or D172E on cells.","method":"Biochemical binding assays, cell-based binding assays, pulldown, disease mutant analysis","journal":"American journal of physiology. Cell physiology","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding assays for multiple mutants; single lab, moderate evidence","pmids":["18287331"],"is_preprint":false},{"year":2010,"finding":"HFE-mediated hepcidin induction requires hemojuvelin (Hjv); overexpression of Hfe in the liver does not require the HFE cytoplasmic domain to induce hepcidin but does require Hjv; Hfe transgene-driven hepcidin induction is greatly attenuated in Hjv-/- mice.","method":"Transgenic mouse overexpression of Hfe with deleted cytoplasmic domain, Hjv-/- × Hfe transgenic crosses, hepcidin expression analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis in mice with structure-function analysis of HFE domains","pmids":["20837779"],"is_preprint":false},{"year":2009,"finding":"Mice lacking both functional matriptase-2 and hemojuvelin exhibit low hepcidin (Hamp1) expression and high serum/liver iron, consistent with hemojuvelin being a major in vivo substrate for matriptase-2; double mutant mice also show lower cardiac iron than single Hjv-/- mice, suggesting cardioprotective effect of matriptase-2 loss.","method":"Double knockout (matriptase-2 protease domain × Hjv-/-) mouse generation and phenotypic analysis","journal":"British journal of haematology","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic epistasis in double knockout mice","pmids":["19751239"],"is_preprint":false},{"year":2012,"finding":"In haemojuvelin-knockout (Hjv-/-) mice, iron accumulates in the retina with age, causing morphological damage; HJV-deficient RPE cells have a hyperproliferative phenotype and up-regulated xCT; BMP6 cannot induce hepcidin expression in Hjv-/- RPE cells, confirming that retinal cells require HJV for BMP6-mediated hepcidin pathway activation.","method":"Hjv-/- mouse retinal histology, primary RPE cell culture, BMP6 stimulation and hepcidin promoter assays","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo knockout phenotype plus cell-based BMP signaling assays; single lab","pmids":["21943374"],"is_preprint":false}],"current_model":"HJV (hemojuvelin) is a GPI-anchored membrane protein that functions as a BMP coreceptor (for BMP-2, BMP-4, and BMP-6) in hepatocytes, enhancing BMP/SMAD signaling to transcriptionally upregulate hepcidin expression via BMP-responsive elements in the hepcidin promoter; HJV requires interaction with neogenin (which scaffolds assembly of the HJV·BMP·BMP receptor complex at the plasma membrane), can be cleaved and inactivated by the serine protease matriptase-2 (TMPRSS6) at Arg121/Arg326, and released as a soluble decoy form by furin/proprotein convertases; HFE and TfR2 physically complex with HJV to modulate this pathway; in skeletal muscle HJV additionally suppresses TGF-β1/Smad3 signaling by acting as a coreceptor for TGF-β receptor II, independent of its BMP co-receptor function in liver."},"narrative":{"teleology":[{"year":2005,"claim":"Establishing HJV as an essential positive regulator of hepcidin: HJV-mutant mice developed severe iron overload with dramatically reduced hepcidin, and membrane HJV positively regulated hepcidin mRNA in hepatocytes while soluble HJV acted as a suppressor, defining the opposing roles of membrane versus soluble forms.","evidence":"Hjv-mutant mouse model with hepcidin/iron analysis; siRNA knockdown and recombinant soluble HJV treatment of primary hepatocytes; GPI-anchor characterization and neogenin co-IP in HEK293 cells","pmids":["16075058","15998830","16103117"],"confidence":"High","gaps":["Signaling pathway downstream of HJV not yet identified","Mechanism of soluble HJV generation unknown","How iron modulates HJV shedding unclear"]},{"year":2006,"claim":"Identification of HJV as a BMP coreceptor resolved the signaling pathway: HJV enhanced BMP signaling to upregulate hepcidin, and hemochromatosis-associated HJV mutants were defective in BMP signaling, linking disease mechanism to impaired BMP co-receptor function.","evidence":"Cell-based BMP signaling assays, Hfe2-/- hepatocyte studies, reporter assays with disease mutants","pmids":["16604073"],"confidence":"High","gaps":["Specific BMP ligands utilized by HJV not defined","Role of BMP type II receptors not characterized","Downstream promoter elements mediating BMP/HJV signal unknown"]},{"year":2006,"claim":"Biochemical dissection of HJV biosynthesis revealed that HJV undergoes autocatalytic cleavage to generate a disulfide-linked heterodimer at the membrane, and the G320V disease mutant fails this processing step, providing a molecular explanation for loss of function.","evidence":"Pulse-chase, cell-surface labeling, and glycosylation analysis in skeletal muscle cells with disease mutant comparison","pmids":["16868025"],"confidence":"High","gaps":["Protease responsible for autocatalytic cleavage not identified","Whether processing is required for BMP co-receptor activity not tested"]},{"year":2007,"claim":"Neogenin was established as a regulator of HJV membrane dynamics: iron status controlled HJV shedding via neogenin, and iron stimulated hepcidin through a HJV/BMP2/4-dependent pathway in hepatocytes, while disease-causing HJV mutants showed defective plasma membrane targeting.","evidence":"Iron-deficient rat serum analysis; neogenin siRNA/overexpression in cell lines; holotransferrin treatment of primary hepatocytes; cell-surface labeling of HJV mutants","pmids":["17331953","17540841","17264300"],"confidence":"High","gaps":["Molecular mechanism by which neogenin mediates HJV shedding unknown","Whether neogenin is required for BMP signaling or only for trafficking unclear"]},{"year":2008,"claim":"Multiple studies defined the ligand and receptor specificity of HJV-mediated BMP signaling and its regulation by proteolytic processing: HJV selectively used BMP-2/4/6 with ActRIIA and BMPRII; neogenin bound HJV simultaneously with BMP-2; furin cleaved HJV at a conserved site to generate soluble forms; and HJV trafficking involved an unconventional retrograde pathway from plasma membrane to Golgi.","evidence":"In vitro BMP signaling with receptor combinations; recombinant protein binding/domain mapping; furin cleavage assays with mutagenesis; glycan analysis with endocytosis inhibitors","pmids":["18326817","18335997","18384687","19029439"],"confidence":"High","gaps":["Relative contributions of furin versus other proteases in vivo unclear","Structural basis of HJV-BMP interaction not resolved","Whether retrograde trafficking is functionally required for signaling not demonstrated"]},{"year":2008,"claim":"Matriptase-2 (TMPRSS6) was identified as the physiological protease that cleaves membrane HJV, providing the molecular basis for TMPRSS6 as a negative regulator of hepcidin—HJV cleavage by TMPRSS6 required its serine protease domain and was validated in zebrafish.","evidence":"Cell-based cleavage assays with domain deletion mutants, co-immunoprecipitation, zebrafish functional studies","pmids":["18976966"],"confidence":"High","gaps":["Exact cleavage sites not mapped","Whether TMPRSS6 cleavage product differs functionally from furin-generated soluble HJV unknown"]},{"year":2009,"claim":"The transcriptional output of HJV/BMP signaling was mapped to two BMP-responsive elements in the hepcidin promoter, and genetic epistasis confirmed HJV as the major in vivo substrate for matriptase-2.","evidence":"Hepcidin promoter-reporter mutagenesis with BMP/IL-6 stimulation; double knockout (TMPRSS6 × Hjv) mouse phenotyping","pmids":["19229506","19751239"],"confidence":"High","gaps":["Transcription factors binding BMP-REs not identified","Whether additional hepcidin regulatory elements exist unknown"]},{"year":2009,"claim":"The HJV-neogenin interaction was shown to be functionally required for BMP4-induced hepcidin expression both in vitro and in vivo, establishing neogenin as an obligate partner rather than merely a trafficking factor.","evidence":"Neogenin siRNA in HJV-expressing HepG2 cells; in vivo injection of soluble neogenin fragment in mice","pmids":["19564337"],"confidence":"High","gaps":["Molecular mechanism by which neogenin promotes BMP signaling not defined","Whether neogenin interacts directly with BMP receptors untested"]},{"year":2010,"claim":"Neogenin-mutant mice confirmed the in vivo requirement for neogenin in HJV-dependent hepcidin regulation, and matriptase-2 cleavage of HJV was mapped to Arg288, producing a soluble form with reduced BMP6 binding distinct from furin-generated soluble HJV.","evidence":"Neogenin-mutant mouse model with hepatocyte BMP/Smad analysis; in vitro cleavage site mapping with BMP6 binding and hepcidin reporter assays","pmids":["20065295","20937842"],"confidence":"High","gaps":["Additional TMPRSS6 cleavage sites not fully characterized","How neogenin stabilizes HJV at the membrane mechanistically unclear"]},{"year":2011,"claim":"Tissue-specific knockouts definitively showed that hepatic HJV, not muscle HJV, is necessary and sufficient for hepcidin regulation and systemic iron homeostasis, resolving debate about the tissue source of functional HJV.","evidence":"Hepatocyte-specific and muscle-specific conditional Hjv knockout mice with iron parameter and hepcidin analysis","pmids":["21748766","21493799"],"confidence":"High","gaps":["Function of muscle HJV unknown at this point","Whether HJV has roles in other tissues not tested"]},{"year":2012,"claim":"The iron-sensing complex was expanded: HFE, TfR2, and HJV form a multi-protein membrane complex, and neogenin was shown to scaffold a ternary complex with matriptase-2 and HJV that facilitates HJV cleavage at the cell surface.","evidence":"Glycerol gradient sedimentation and co-IP in HuH7 cells for HFE/TfR2/HJV; co-IP and siRNA for neogenin/MT2/HJV ternary complex","pmids":["22728873","22893705"],"confidence":"High","gaps":["Stoichiometry of the multi-protein complex unknown","How iron status remodels complex composition not defined","Structural basis of HFE-TfR2-HJV interactions unresolved"]},{"year":2015,"claim":"Genetic epistasis established that HFE operates in the same pathway as HJV, TMPRSS6 cleavage sites on HJV were comprehensively mapped to Arg121 and Arg326, and a distinct muscle function for HJV was discovered—acting as a TGF-β receptor II coreceptor to suppress TGF-β1/Smad3 signaling independently of BMP signaling.","evidence":"Double Hfe/Hjv knockout mice; systematic Arg-to-Ala mutagenesis with cleavage assays; co-IP of HJV with TβRII, conditional Hjv knockout and overexpression in muscle/mdx mice","pmids":["25609138","25704252","30884219"],"confidence":"High","gaps":["Whether muscle HJV-TGF-β interaction is relevant to other tissues unknown","Structural determinants of HJV selectivity for BMP versus TGF-β receptors unresolved"]},{"year":2016,"claim":"Neogenin was shown to co-immunoprecipitate with the type I BMP receptor ALK3, and a HJV mutant specifically defective in neogenin binding failed to activate hepcidin in vivo, establishing that neogenin scaffolds the HJV·BMP·BMP receptor signaling complex rather than merely stabilizing HJV.","evidence":"Co-IP of neogenin with ALK3; neogenin-interaction-deficient HJV mutant expressed in Hjv-/- mice; furin cleavage-site mutant showing normal hepcidin in vivo","pmids":["27072365"],"confidence":"High","gaps":["Direct structural evidence for the ternary scaffolding complex lacking","Whether neogenin enhances BMP receptor kinase activity or proximity not tested"]},{"year":2017,"claim":"Double knockout of Bmp6 and Hjv revealed that BMP6 retains residual signaling capacity without HJV, demonstrating that HJV amplifies rather than absolutely gates BMP6-to-hepcidin signaling, and that full hepcidin induction requires synergy between BMP/SMAD and IL-6/STAT3 pathways.","evidence":"Hjv-/- × Bmp6-/- double knockout mice with Smad signaling and LPS challenge","pmids":["29021231"],"confidence":"High","gaps":["Whether other BMP ligands compensate partially in double knockouts unknown","Quantitative contribution of each pathway branch not modeled"]},{"year":2018,"claim":"HJV was found to be required for inflammatory hepcidin induction, overturning earlier models that placed inflammatory and iron-sensing pathways as independent—Hjv deficiency abolished the synergism between BMP/Smad and IL-6/Stat signaling needed for the hypoferremic response to infection.","evidence":"Hjv-/- mice challenged with LPS, FSL1, and live E. coli; primary hepatocyte BMP/Smad assays","pmids":["30213871"],"confidence":"High","gaps":["Whether pharmacological BMP supplementation can rescue inflammatory hepcidin in Hjv-/- mice untested","Mechanism of synergy between SMAD and STAT3 at the hepcidin promoter not fully resolved"]},{"year":2021,"claim":"The molecular mechanism of neogenin scaffolding was elucidated: HJV binding triggers cleavage of the neogenin cytoplasmic domain, and the resulting truncated neogenin accumulates at the plasma membrane where it assembles the BMP signaling complex; interaction-deficient mutants of both neogenin and HJV failed to rescue hepcidin in conditional knockouts.","evidence":"Hepatocyte-specific Neo1 knockout mice; Neo1L1046E and HjvA183R interaction-deficient mutant rescue experiments; BMP signaling analysis","pmids":["33824974"],"confidence":"High","gaps":["Identity of protease cleaving neogenin cytoplasmic domain unknown","How truncated neogenin specifically promotes BMP receptor assembly not structurally defined"]},{"year":null,"claim":"Key unresolved questions include the atomic structure of the HJV·neogenin·BMP·BMP receptor signaling complex, the identity of the protease that cleaves neogenin upon HJV engagement, the mechanism by which iron status is transduced to regulate HJV membrane levels, and whether the muscle TGF-β coreceptor function of HJV has therapeutic relevance for sarcopenia or muscular dystrophy.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of the HJV signaling complex","Iron-sensing mechanism upstream of HJV membrane dynamics unresolved","Therapeutic exploitation of muscle HJV-TGF-β axis unexplored"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,5,21]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,6,23]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,14,15,16]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[2,7,17]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[16]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,5,9,19,21,25]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[3,20,24]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,15,26]}],"complexes":["HJV·neogenin·BMP·BMP receptor complex","HFE·TfR2·HJV complex","Neogenin·matriptase-2·HJV ternary complex"],"partners":["NEO1","TMPRSS6","HFE","TFR2","ALK3","ACVR2A","BMPR2","TGFBR2"],"other_free_text":[]},"mechanistic_narrative":"HJV (hemojuvelin) is a GPI-anchored BMP coreceptor that serves as the central upstream regulator of hepcidin transcription in hepatocytes, thereby governing systemic iron homeostasis. Membrane-bound HJV enhances signaling by BMP-2, BMP-4, and BMP-6 through type I and type II BMP receptors (including ActRIIA and BMPRII), activating the SMAD1/5/8 cascade that drives hepcidin promoter activity via two BMP-responsive elements; this function requires neogenin, which scaffolds the HJV·BMP·BMP receptor complex at the plasma membrane and undergoes cytoplasmic domain cleavage upon HJV binding [PMID:16604073, PMID:18326817, PMID:19229506, PMID:33824974]. HJV is negatively regulated by the serine protease matriptase-2 (TMPRSS6), which cleaves it at Arg121 and Arg326 to release inactive soluble fragments, and by furin/proprotein convertases, which generate a distinct soluble form that acts as a decoy to suppress hepcidin; HFE and TfR2 form a multi-protein complex with HJV that converges on the same SMAD signaling pathway [PMID:18976966, PMID:25704252, PMID:18384687, PMID:22728873, PMID:25609138]. Loss-of-function mutations in HJV cause juvenile hemochromatosis (type 2A) characterized by severe iron overload and near-absent hepcidin expression; in skeletal muscle, HJV additionally functions as a TGF-β receptor II coreceptor that suppresses TGF-β1/Smad3 signaling independently of its hepatic BMP coreceptor role [PMID:16075058, PMID:17264300, PMID:21748766, PMID:30884219]."},"prefetch_data":{"uniprot":{"accession":"Q6ZVN8","full_name":"Hemojuvelin","aliases":["Hemochromatosis type 2 protein","Hemojuvelin BMP coreceptor","RGM domain family member C"],"length_aa":426,"mass_kda":45.1,"function":"Acts as a bone morphogenetic protein (BMP) coreceptor (PubMed:18976966). Through enhancement of BMP signaling regulates hepcidin (HAMP) expression and regulates iron homeostasis (PubMed:18976966)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q6ZVN8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HJV","classification":"Not Classified","n_dependent_lines":260,"n_total_lines":1208,"dependency_fraction":0.2152317880794702},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/HJV","total_profiled":1310},"omim":[{"mim_id":"613609","title":"HOMEOSTATIC IRON REGULATOR; HFE","url":"https://www.omim.org/entry/613609"},{"mim_id":"609862","title":"TRANSMEMBRANE PROTEASE, SERINE 6; TMPRSS6","url":"https://www.omim.org/entry/609862"},{"mim_id":"608736","title":"SOLUTE CARRIER FAMILY 39 (ZINC TRANSPORTER), MEMBER 14; SLC39A14","url":"https://www.omim.org/entry/608736"},{"mim_id":"608374","title":"HEMOJUVELIN BMP CORECEPTOR; HJV","url":"https://www.omim.org/entry/608374"},{"mim_id":"606464","title":"HEPCIDIN ANTIMICROBIAL PEPTIDE; HAMP","url":"https://www.omim.org/entry/606464"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"liver","ntpm":223.7},{"tissue":"skeletal muscle","ntpm":553.9},{"tissue":"tongue","ntpm":271.9}],"url":"https://www.proteinatlas.org/search/HJV"},"hgnc":{"alias_symbol":["JH","HFE2A","RGMC","hemojuvelin","haemojuvelin"],"prev_symbol":["HFE2"]},"alphafold":{"accession":"Q6ZVN8","domains":[{"cath_id":"-","chopping":"38-51_77-128","consensus_level":"medium","plddt":90.2409,"start":38,"end":128},{"cath_id":"3.40.1000.10","chopping":"163-325","consensus_level":"high","plddt":93.9065,"start":163,"end":325}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6ZVN8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6ZVN8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6ZVN8-F1-predicted_aligned_error_v6.png","plddt_mean":81.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HJV","jax_strain_url":"https://www.jax.org/strain/search?query=HJV"},"sequence":{"accession":"Q6ZVN8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6ZVN8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6ZVN8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6ZVN8"}},"corpus_meta":[{"pmid":"3924412","id":"PMC_3924412","title":"Cloning 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Peptide JH-3 Effectively Kills Salmonella enterica Serovar Typhimurium Strain CVCC541 and Reduces Its Pathogenicity in Mice.","date":"2019","source":"Probiotics and antimicrobial proteins","url":"https://pubmed.ncbi.nlm.nih.gov/31001786","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":47656,"output_tokens":7622,"usd":0.128649},"stage2":{"model":"claude-opus-4-6","input_tokens":11405,"output_tokens":4486,"usd":0.253763},"total_usd":0.382412,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"Hemojuvelin (HJV) functions as a bone morphogenetic protein (BMP) coreceptor that enhances BMP signaling to upregulate hepcidin expression in hepatocytes; HJV mutants associated with hemochromatosis have impaired BMP signaling ability, and BMP-mediated hepcidin upregulation is blunted in Hfe2-/- hepatocytes.\",\n      \"method\": \"Cell-based BMP signaling assays, Hfe2-/- mouse hepatocytes, reporter assays, co-receptor functional studies\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (cell assays, knockout hepatocytes, mutagenesis), foundational paper with >800 citations\",\n      \"pmids\": [\"16604073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The serine protease matriptase-2 (TMPRSS6) cleaves membrane hemojuvelin (HJV) on the plasma membrane, releasing an inactive soluble form and thereby inhibiting hepcidin activation; matriptase-2 lacking the serine protease domain shows no cleavage activity, and matriptase-2 interacts with HJV through its ectodomain.\",\n      \"method\": \"Cell-based cleavage assays, domain deletion mutants, co-immunoprecipitation, zebrafish functional studies\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — biochemical cleavage assay with domain mutants, reciprocal co-IP, in vivo zebrafish validation, >400 citations\",\n      \"pmids\": [\"18976966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cell-associated (membrane) HJV positively regulates hepcidin mRNA expression independently of the IL-6 pathway, while recombinant soluble HJV suppresses hepcidin mRNA in primary human hepatocytes in a dose-dependent manner; soluble HJV release from cells is inhibited by increasing iron concentrations.\",\n      \"method\": \"siRNA knockdown of HJV, recombinant soluble HJV treatment of primary hepatocytes, iron-modulation experiments\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal approaches (siRNA KD, recombinant protein treatment, iron titration), >200 citations\",\n      \"pmids\": [\"15998830\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"HJV is required for dietary iron sensing in the liver; Hjv-mutant mice exhibit severe iron overload and dramatically decreased hepcidin expression, while cytokine-induced inflammation regulates hepcidin through an Hjv-independent pathway.\",\n      \"method\": \"Hjv-mutant mouse model, hepcidin expression analysis, dietary iron sensing experiments\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout mouse with defined phenotype and pathway placement, >300 citations\",\n      \"pmids\": [\"16075058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"HJV is a GPI-anchored protein that undergoes partial autocatalytic cleavage during intracellular processing and co-immunoprecipitates with neogenin but not with the closely related receptor DCC; the G320V hemochromatosis mutant does not co-immunoprecipitate with neogenin; HJV-induced iron accumulation in HEK293 cells requires neogenin.\",\n      \"method\": \"Stable transfection of HJV in HEK293 cells, co-immunoprecipitation, GPI-anchor characterization, iron accumulation assays (transferrin-55Fe)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP, functional iron assay, disease mutant comparison, >100 citations\",\n      \"pmids\": [\"16103117\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HJV selectively uses BMP-2, BMP-4, and BMP-6 as endogenous ligands in hepatoma-derived cells; HJV uses BMP type II receptors ActRIIA and BMPRII but not ActRIIB, and enhances ActRIIA utilization by BMP-2/4; HJV-induced BMP signaling and hepcidin expression are independent of neogenin overexpression or knockdown.\",\n      \"method\": \"In vitro BMP signaling assays, receptor expression studies, neogenin overexpression/siRNA knockdown\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple receptor combinations tested with orthogonal methods, >180 citations\",\n      \"pmids\": [\"18326817\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"HFE, TfR2, and HJV form a multi-protein membrane complex on hepatocyte surfaces; HFE and TfR2 bind HJV non-competitively; HJV competes with TfR1 for binding to HFE; residues 120–139 of the TfR2 extracellular domain are required for binding both HFE and HJV.\",\n      \"method\": \"Glycerol gradient sedimentation, co-immunoprecipitation in transfected HuH7 cells, domain mapping\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus sedimentation assay, domain mapping, >140 citations\",\n      \"pmids\": [\"22728873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"In iron-deficient rats, HJV shedding into serum increases without changes in HFE2 mRNA or protein levels; holo-transferrin suppresses HJV shedding in C2C12 and HepG2 cells; neogenin knockdown suppresses HJV shedding while neogenin overexpression enhances it, indicating HJV membrane shedding is mediated by neogenin.\",\n      \"method\": \"Iron-deficient rat model, holo-transferrin treatment, siRNA knockdown and overexpression of neogenin, cell-based shedding assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo and in vitro complementary evidence, siRNA knockdown and overexpression, >95 citations\",\n      \"pmids\": [\"17331953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Neogenin inhibits HJV secretion and stabilizes membrane HJV; livers of neogenin-mutant mice exhibit iron overload, low hepcidin, and reduced BMP signaling; mutant hepatocytes show impaired BMP2-induced Smad1/5/8 phosphorylation and hepcidin expression.\",\n      \"method\": \"Neogenin mutant mouse model, hepatocyte culture BMP signaling assays, Smad phosphorylation analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo neogenin mutant mouse with defined hepcidin/BMP phenotype plus in vitro mechanistic studies, >100 citations\",\n      \"pmids\": [\"20065295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Iron stimulates hepcidin expression in primary murine hepatocytes through a hemojuvelin/BMP2/4-dependent pathway; BMP9, although expressed in liver, does not interact with hemojuvelin and its pathway does not affect iron regulation of hepcidin.\",\n      \"method\": \"Primary murine hepatocyte culture with holotransferrin, BMP neutralization experiments, siRNA knockdown of hemojuvelin\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — primary cell culture with multiple BMP ligand tests and HJV knockdown, >200 citations\",\n      \"pmids\": [\"17540841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The HJV-neogenin interaction is required for BMP4-induced hepcidin expression; knockdown of neogenin in HJV-expressing HepG2 cells decreases BMP4-induced hepcidin mRNA 16-fold; disruption of the HJV-neogenin interaction markedly suppresses hepcidin; in vivo blocking of HJV-neogenin interaction with soluble neogenin fragment suppresses hepatic hepcidin in mice.\",\n      \"method\": \"siRNA knockdown of neogenin in HJV-expressing HepG2 cells, in vivo injection of soluble neogenin fragment in mice, G99V mutant analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-based siRNA and in vivo mouse experiment, >60 citations\",\n      \"pmids\": [\"19564337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Neogenin interacts with matriptase-2 (MT2) as well as with HJV, forming a ternary complex at the plasma membrane that facilitates MT2 cleavage of HJV; neogenin knockdown with siRNA increases MT2 and HJV on the plasma membrane; MT2 cleavage of cell-surface HJV is coupled to a transition from high-mannose to complex oligosaccharides on HJV.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, oligosaccharide analysis, cell-surface cleavage assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — co-IP of ternary complex, siRNA validation, glycan analysis as mechanistic read-out\",\n      \"pmids\": [\"22893705\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Matriptase-2 cleaves HJV at Arg288, producing a soluble HJV form with decreased ability to bind BMP6 that does not suppress BMP6-induced hepcidin expression, distinct from the proprotein convertase (furin) cleavage product which does suppress hepcidin.\",\n      \"method\": \"In vitro cleavage assay, BMP6 binding assay, hepcidin reporter assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical cleavage site mapping with functional BMP binding and hepcidin reporter assays\",\n      \"pmids\": [\"20937842\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HJV undergoes autocatalytic cleavage generating a disulfide-linked heterodimer; neogenin binds to both cleaved and uncleaved HJV; the HJV-neogenin binding site maps to the membrane-proximal fifth and sixth FNIII domains of neogenin; BMP-2 and neogenin can bind HJV simultaneously, raising the possibility of a multiprotein complex.\",\n      \"method\": \"Recombinant protein production, binding assays, domain mapping with neogenin fragments, BMP-2 competition assays\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstituted binding with domain mapping and simultaneous binding demonstration\",\n      \"pmids\": [\"18335997\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"HJV (RGMc) undergoes complex biosynthesis in skeletal muscle: two GPI-anchored glycosylated forms are produced — a full-length species released into extracellular fluid and a disulfide-linked heterodimer of N- and C-terminal fragments at the membrane; the G320V disease mutant fails to generate the heterodimeric membrane isoform.\",\n      \"method\": \"Pulse-chase studies, cell-surface labeling, glycosylation analysis, disease mutant characterization\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple biochemical assays defining biosynthetic pathway with disease mutant validation\",\n      \"pmids\": [\"16868025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Defective plasma membrane targeting of HJV is a common pathogenetic mechanism in juvenile hemochromatosis; proteolytic processing (autocleavage) is required for membrane export; mutants F170S, W191C, and G320V are retained in the ER and fail to reach the cell surface; membrane HJV level is increased by iron in wild-type but not mutant mice.\",\n      \"method\": \"HeLa and HepG2 cell transfection, cell-surface labeling, immunoblot analysis of processing, iron supplementation experiments\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mutants characterized with orthogonal methods, in vivo mouse validation\",\n      \"pmids\": [\"17264300\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HJV processing requires retrograde trafficking from the plasma membrane back to the Golgi; cellular HJV reaches the plasma membrane with high-mannose oligosaccharides (bypassing Golgi processing), while secreted HJV has complex oligosaccharides derived from plasma membrane HJV after retrograde trafficking; neogenin is required for HJV release/shedding but not for trafficking to the cell surface.\",\n      \"method\": \"Glycosylation analysis, neogenin knockdown, endocytosis inhibitors (dynamin-independent, cholesterol-dependent pathway), HepG2 cells\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — glycan analysis plus endocytosis inhibitor studies defining trafficking pathway\",\n      \"pmids\": [\"19029439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Pro-protein convertases (PCs), including furin, cleave 50 kDa RGMc/HJV at a conserved PC recognition site to generate a 40 kDa species with truncated C-terminus; cell-impermeable PC inhibitor blocks extracellular 40 kDa HJV; iron loading reduces HJV release from the cell membrane and diminishes accumulation of the 40 kDa species.\",\n      \"method\": \"Furin in vitro cleavage assay, PC inhibitor experiments, site-directed mutagenesis of cleavage site, iron-loading cell culture\",\n      \"journal\": \"BMC biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro furin cleavage, mutagenesis, cell-impermeable inhibitor validation\",\n      \"pmids\": [\"18384687\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Neogenin-mediated HJV shedding occurs after HJV traffics to the plasma membrane; neogenin knockdown suppresses HJV release without affecting plasma membrane trafficking; HJV endocytosis is dynamin-independent and cholesterol-dependent; HJV release is coupled to lysosomal degradation of neogenin.\",\n      \"method\": \"Neogenin knockdown, cholesterol depletion (filipin), lysosomal inhibitor experiments, HepG2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological and siRNA dissection of trafficking pathway with multiple readouts\",\n      \"pmids\": [\"18445598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Two BMP-responsive elements (BMP-RE1 at -84/-79 and BMP-RE2 at -2255/-2250) in the hepcidin promoter are both critical for basal hepcidin expression and response to BMP-2, BMP-6, and HJV-mediated BMP signals; BMP-RE1 is specifically required for IL-6 response while both elements show additive effects for HJV/BMP signals.\",\n      \"method\": \"Hepcidin promoter reporter assays, BMP-RE mutagenesis, BMP-2/BMP-6/IL-6 stimulation experiments\",\n      \"journal\": \"Journal of molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — promoter mutagenesis with multiple ligand stimulations defining downstream transcriptional mechanism\",\n      \"pmids\": [\"19229506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Hepatic HJV, but not skeletal muscle HJV, is necessary and sufficient for hepcidin regulation and systemic iron homeostasis; liver-specific Hjv knockout causes iron overload comparable to ubiquitous Hjv-/- mice with marked hepcidin suppression, while muscle-specific Hjv knockout causes no iron phenotype.\",\n      \"method\": \"Tissue-specific conditional Hjv knockout mice (hepatocyte- and muscle-specific Cre), iron parameter analysis, hepcidin mRNA quantification\",\n      \"journal\": \"Hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional tissue-specific knockouts with defined phenotypes, replicated by independent group\",\n      \"pmids\": [\"21748766\", \"21493799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In skeletal muscle, HJV acts as a coreceptor for TGF-β receptor II (TβRII) and inhibits TGF-β1/Smad3 signaling; HJV directly interacts with TβRII on the muscle membrane, and loss of HJV increases TGF-β1, TβRII, and p-Smad2/3 without altering the BMP/p-Smad1/5/8 pathway in muscle; overexpression of Hjv rescues dystrophic and age-related muscle wasting.\",\n      \"method\": \"Co-immunoprecipitation, dual-luciferase reporter assay, RNAi, conditional Hjv knockout mice, mdx mice, Hjv overexpression vector in vivo\",\n      \"journal\": \"Journal of cachexia, sarcopenia and muscle\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — co-IP, reporter assay, multiple mouse models, identifies distinct mechanism from hepatic BMP co-receptor function\",\n      \"pmids\": [\"30884219\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Hepatocyte neogenin is required for HJV-mediated hepcidin expression and iron homeostasis; hepatocyte-specific Neo1 knockout mice develop iron overload and reduced hepcidin; the Neo1L1046E mutant that cannot interact with HJV fails to rescue the phenotype; HjvA183R with reduced Neo1 interaction shows blunted hepcidin induction; Hjv binding triggers cleavage of the Neo1 cytoplasmic domain, leading to accumulation of truncated Neo1 on the plasma membrane where it acts as a scaffold for BMP signaling complex assembly.\",\n      \"method\": \"Hepatocyte-specific Neo1 conditional knockout mice, interaction-deficient mutants (Neo1L1046E, HjvA183R), rescue experiments, BMP signaling analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional knockout with rescue using interaction-deficient mutants, defines molecular mechanism\",\n      \"pmids\": [\"33824974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Neogenin co-immunoprecipitates with ALK3 (type I BMP receptor); a HJV mutation specifically reducing neogenin interaction dramatically attenuates BMP signaling and hepcidin mRNA when expressed in Hjv-/- mice; furin-cleavage site mutation in Hjv does not alter hepcidin stimulation in vivo; neogenin acts as a scaffold facilitating assembly of the HJV·BMP·BMP receptor complex.\",\n      \"method\": \"Co-immunoprecipitation of neogenin with ALK3, neogenin-interaction mutant HJV in Hjv-/- mice, furin cleavage site mutant in vivo\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo mouse rescue with interaction-deficient mutants plus co-IP defining complex\",\n      \"pmids\": [\"27072365\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Hjv is required for inflammatory induction of hepcidin and the acute hypoferremic response to LPS, FSL1, and E. coli infection; Hjv deficiency severely impairs BMP6/Smad signaling and abolishes synergism with the IL-6/Stat pathway; Hfe-/- mice show near-normal inflammatory hepcidin; double Hjv-/-Hfe-/- mice phenocopy Hjv-/- mice.\",\n      \"method\": \"Hjv-/- and Hfe-/- mouse models with LPS/FSL1/E. coli challenge, primary hepatocyte BMP signaling assays, Smad phosphorylation analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mouse models with in vivo infection and in vitro mechanistic assays\",\n      \"pmids\": [\"30213871\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"BMP6 and HJV can signal to hepcidin partially independently; loss of Bmp6 further represses Smad signaling and hepcidin in Hjv-/- mice, indicating BMP6 provides a residual signal in the absence of HJV; both BMP6 and HJV are required for full hepcidin induction, with synergy between BMP/SMAD and IL-6/STAT3 pathways.\",\n      \"method\": \"Hjv-/- × Bmp6-/- double knockout mice, Smad signaling analysis, LPS challenge, iron accumulation analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — double knockout epistasis experiment with signaling pathway analysis\",\n      \"pmids\": [\"29021231\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"HFE and HJV operate in the same pathway for hepcidin regulation; combined Hfe-/- Hjv-/- double knockout mice phenocopy single Hjv-/- mice with indistinguishable hepcidin suppression and iron overload, indicating HFE function is epistatic to or converges with HJV for hepcidin and Smad signaling.\",\n      \"method\": \"Double Hfe-/- Hjv-/- knockout mouse generation and phenotypic analysis, Smad signaling, iron parameters\",\n      \"journal\": \"Journal of molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via double knockout mouse model\",\n      \"pmids\": [\"25609138\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"TMPRSS6 cleaves HJV at residues Arg121 and Arg326 in both full-length and heterodimeric HJV isoforms; Arg176 and Arg288 contribute to additional cleavage; molecular dynamics modeling of HJV structure identified TMPRSS6-insensitive arginines within the von Willebrand domain due to partial protein structure destabilization.\",\n      \"method\": \"Arginine-to-alanine mutagenesis, cell-based cleavage assays, N-terminal FLAG-tagged HJV analysis, in silico molecular dynamics simulation\",\n      \"journal\": \"Journal of cellular and molecular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — systematic mutagenesis with functional assay plus structural modeling\",\n      \"pmids\": [\"25704252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"BMP-2 binds to single-chain RGMc/HJV species in biochemical assays and to cell-associated RGMc; disease mutants D172E and G320V show reduced BMP-2 binding; G99V cannot bind BMP-2; neogenin preferentially binds heterodimeric membrane RGMc and can interact with wild-type RGMc and G99V but not G320V or D172E on cells.\",\n      \"method\": \"Biochemical binding assays, cell-based binding assays, pulldown, disease mutant analysis\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assays for multiple mutants; single lab, moderate evidence\",\n      \"pmids\": [\"18287331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"HFE-mediated hepcidin induction requires hemojuvelin (Hjv); overexpression of Hfe in the liver does not require the HFE cytoplasmic domain to induce hepcidin but does require Hjv; Hfe transgene-driven hepcidin induction is greatly attenuated in Hjv-/- mice.\",\n      \"method\": \"Transgenic mouse overexpression of Hfe with deleted cytoplasmic domain, Hjv-/- × Hfe transgenic crosses, hepcidin expression analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in mice with structure-function analysis of HFE domains\",\n      \"pmids\": [\"20837779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mice lacking both functional matriptase-2 and hemojuvelin exhibit low hepcidin (Hamp1) expression and high serum/liver iron, consistent with hemojuvelin being a major in vivo substrate for matriptase-2; double mutant mice also show lower cardiac iron than single Hjv-/- mice, suggesting cardioprotective effect of matriptase-2 loss.\",\n      \"method\": \"Double knockout (matriptase-2 protease domain × Hjv-/-) mouse generation and phenotypic analysis\",\n      \"journal\": \"British journal of haematology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic epistasis in double knockout mice\",\n      \"pmids\": [\"19751239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In haemojuvelin-knockout (Hjv-/-) mice, iron accumulates in the retina with age, causing morphological damage; HJV-deficient RPE cells have a hyperproliferative phenotype and up-regulated xCT; BMP6 cannot induce hepcidin expression in Hjv-/- RPE cells, confirming that retinal cells require HJV for BMP6-mediated hepcidin pathway activation.\",\n      \"method\": \"Hjv-/- mouse retinal histology, primary RPE cell culture, BMP6 stimulation and hepcidin promoter assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo knockout phenotype plus cell-based BMP signaling assays; single lab\",\n      \"pmids\": [\"21943374\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HJV (hemojuvelin) is a GPI-anchored membrane protein that functions as a BMP coreceptor (for BMP-2, BMP-4, and BMP-6) in hepatocytes, enhancing BMP/SMAD signaling to transcriptionally upregulate hepcidin expression via BMP-responsive elements in the hepcidin promoter; HJV requires interaction with neogenin (which scaffolds assembly of the HJV·BMP·BMP receptor complex at the plasma membrane), can be cleaved and inactivated by the serine protease matriptase-2 (TMPRSS6) at Arg121/Arg326, and released as a soluble decoy form by furin/proprotein convertases; HFE and TfR2 physically complex with HJV to modulate this pathway; in skeletal muscle HJV additionally suppresses TGF-β1/Smad3 signaling by acting as a coreceptor for TGF-β receptor II, independent of its BMP co-receptor function in liver.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"HJV (hemojuvelin) is a GPI-anchored BMP coreceptor that serves as the central upstream regulator of hepcidin transcription in hepatocytes, thereby governing systemic iron homeostasis. Membrane-bound HJV enhances signaling by BMP-2, BMP-4, and BMP-6 through type I and type II BMP receptors (including ActRIIA and BMPRII), activating the SMAD1/5/8 cascade that drives hepcidin promoter activity via two BMP-responsive elements; this function requires neogenin, which scaffolds the HJV·BMP·BMP receptor complex at the plasma membrane and undergoes cytoplasmic domain cleavage upon HJV binding [PMID:16604073, PMID:18326817, PMID:19229506, PMID:33824974]. HJV is negatively regulated by the serine protease matriptase-2 (TMPRSS6), which cleaves it at Arg121 and Arg326 to release inactive soluble fragments, and by furin/proprotein convertases, which generate a distinct soluble form that acts as a decoy to suppress hepcidin; HFE and TfR2 form a multi-protein complex with HJV that converges on the same SMAD signaling pathway [PMID:18976966, PMID:25704252, PMID:18384687, PMID:22728873, PMID:25609138]. Loss-of-function mutations in HJV cause juvenile hemochromatosis (type 2A) characterized by severe iron overload and near-absent hepcidin expression; in skeletal muscle, HJV additionally functions as a TGF-β receptor II coreceptor that suppresses TGF-β1/Smad3 signaling independently of its hepatic BMP coreceptor role [PMID:16075058, PMID:17264300, PMID:21748766, PMID:30884219].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Establishing HJV as an essential positive regulator of hepcidin: HJV-mutant mice developed severe iron overload with dramatically reduced hepcidin, and membrane HJV positively regulated hepcidin mRNA in hepatocytes while soluble HJV acted as a suppressor, defining the opposing roles of membrane versus soluble forms.\",\n      \"evidence\": \"Hjv-mutant mouse model with hepcidin/iron analysis; siRNA knockdown and recombinant soluble HJV treatment of primary hepatocytes; GPI-anchor characterization and neogenin co-IP in HEK293 cells\",\n      \"pmids\": [\"16075058\", \"15998830\", \"16103117\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling pathway downstream of HJV not yet identified\", \"Mechanism of soluble HJV generation unknown\", \"How iron modulates HJV shedding unclear\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of HJV as a BMP coreceptor resolved the signaling pathway: HJV enhanced BMP signaling to upregulate hepcidin, and hemochromatosis-associated HJV mutants were defective in BMP signaling, linking disease mechanism to impaired BMP co-receptor function.\",\n      \"evidence\": \"Cell-based BMP signaling assays, Hfe2-/- hepatocyte studies, reporter assays with disease mutants\",\n      \"pmids\": [\"16604073\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific BMP ligands utilized by HJV not defined\", \"Role of BMP type II receptors not characterized\", \"Downstream promoter elements mediating BMP/HJV signal unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Biochemical dissection of HJV biosynthesis revealed that HJV undergoes autocatalytic cleavage to generate a disulfide-linked heterodimer at the membrane, and the G320V disease mutant fails this processing step, providing a molecular explanation for loss of function.\",\n      \"evidence\": \"Pulse-chase, cell-surface labeling, and glycosylation analysis in skeletal muscle cells with disease mutant comparison\",\n      \"pmids\": [\"16868025\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Protease responsible for autocatalytic cleavage not identified\", \"Whether processing is required for BMP co-receptor activity not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Neogenin was established as a regulator of HJV membrane dynamics: iron status controlled HJV shedding via neogenin, and iron stimulated hepcidin through a HJV/BMP2/4-dependent pathway in hepatocytes, while disease-causing HJV mutants showed defective plasma membrane targeting.\",\n      \"evidence\": \"Iron-deficient rat serum analysis; neogenin siRNA/overexpression in cell lines; holotransferrin treatment of primary hepatocytes; cell-surface labeling of HJV mutants\",\n      \"pmids\": [\"17331953\", \"17540841\", \"17264300\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which neogenin mediates HJV shedding unknown\", \"Whether neogenin is required for BMP signaling or only for trafficking unclear\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Multiple studies defined the ligand and receptor specificity of HJV-mediated BMP signaling and its regulation by proteolytic processing: HJV selectively used BMP-2/4/6 with ActRIIA and BMPRII; neogenin bound HJV simultaneously with BMP-2; furin cleaved HJV at a conserved site to generate soluble forms; and HJV trafficking involved an unconventional retrograde pathway from plasma membrane to Golgi.\",\n      \"evidence\": \"In vitro BMP signaling with receptor combinations; recombinant protein binding/domain mapping; furin cleavage assays with mutagenesis; glycan analysis with endocytosis inhibitors\",\n      \"pmids\": [\"18326817\", \"18335997\", \"18384687\", \"19029439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of furin versus other proteases in vivo unclear\", \"Structural basis of HJV-BMP interaction not resolved\", \"Whether retrograde trafficking is functionally required for signaling not demonstrated\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Matriptase-2 (TMPRSS6) was identified as the physiological protease that cleaves membrane HJV, providing the molecular basis for TMPRSS6 as a negative regulator of hepcidin—HJV cleavage by TMPRSS6 required its serine protease domain and was validated in zebrafish.\",\n      \"evidence\": \"Cell-based cleavage assays with domain deletion mutants, co-immunoprecipitation, zebrafish functional studies\",\n      \"pmids\": [\"18976966\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact cleavage sites not mapped\", \"Whether TMPRSS6 cleavage product differs functionally from furin-generated soluble HJV unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The transcriptional output of HJV/BMP signaling was mapped to two BMP-responsive elements in the hepcidin promoter, and genetic epistasis confirmed HJV as the major in vivo substrate for matriptase-2.\",\n      \"evidence\": \"Hepcidin promoter-reporter mutagenesis with BMP/IL-6 stimulation; double knockout (TMPRSS6 × Hjv) mouse phenotyping\",\n      \"pmids\": [\"19229506\", \"19751239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transcription factors binding BMP-REs not identified\", \"Whether additional hepcidin regulatory elements exist unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The HJV-neogenin interaction was shown to be functionally required for BMP4-induced hepcidin expression both in vitro and in vivo, establishing neogenin as an obligate partner rather than merely a trafficking factor.\",\n      \"evidence\": \"Neogenin siRNA in HJV-expressing HepG2 cells; in vivo injection of soluble neogenin fragment in mice\",\n      \"pmids\": [\"19564337\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which neogenin promotes BMP signaling not defined\", \"Whether neogenin interacts directly with BMP receptors untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Neogenin-mutant mice confirmed the in vivo requirement for neogenin in HJV-dependent hepcidin regulation, and matriptase-2 cleavage of HJV was mapped to Arg288, producing a soluble form with reduced BMP6 binding distinct from furin-generated soluble HJV.\",\n      \"evidence\": \"Neogenin-mutant mouse model with hepatocyte BMP/Smad analysis; in vitro cleavage site mapping with BMP6 binding and hepcidin reporter assays\",\n      \"pmids\": [\"20065295\", \"20937842\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Additional TMPRSS6 cleavage sites not fully characterized\", \"How neogenin stabilizes HJV at the membrane mechanistically unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Tissue-specific knockouts definitively showed that hepatic HJV, not muscle HJV, is necessary and sufficient for hepcidin regulation and systemic iron homeostasis, resolving debate about the tissue source of functional HJV.\",\n      \"evidence\": \"Hepatocyte-specific and muscle-specific conditional Hjv knockout mice with iron parameter and hepcidin analysis\",\n      \"pmids\": [\"21748766\", \"21493799\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Function of muscle HJV unknown at this point\", \"Whether HJV has roles in other tissues not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"The iron-sensing complex was expanded: HFE, TfR2, and HJV form a multi-protein membrane complex, and neogenin was shown to scaffold a ternary complex with matriptase-2 and HJV that facilitates HJV cleavage at the cell surface.\",\n      \"evidence\": \"Glycerol gradient sedimentation and co-IP in HuH7 cells for HFE/TfR2/HJV; co-IP and siRNA for neogenin/MT2/HJV ternary complex\",\n      \"pmids\": [\"22728873\", \"22893705\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of the multi-protein complex unknown\", \"How iron status remodels complex composition not defined\", \"Structural basis of HFE-TfR2-HJV interactions unresolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Genetic epistasis established that HFE operates in the same pathway as HJV, TMPRSS6 cleavage sites on HJV were comprehensively mapped to Arg121 and Arg326, and a distinct muscle function for HJV was discovered—acting as a TGF-β receptor II coreceptor to suppress TGF-β1/Smad3 signaling independently of BMP signaling.\",\n      \"evidence\": \"Double Hfe/Hjv knockout mice; systematic Arg-to-Ala mutagenesis with cleavage assays; co-IP of HJV with TβRII, conditional Hjv knockout and overexpression in muscle/mdx mice\",\n      \"pmids\": [\"25609138\", \"25704252\", \"30884219\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether muscle HJV-TGF-β interaction is relevant to other tissues unknown\", \"Structural determinants of HJV selectivity for BMP versus TGF-β receptors unresolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Neogenin was shown to co-immunoprecipitate with the type I BMP receptor ALK3, and a HJV mutant specifically defective in neogenin binding failed to activate hepcidin in vivo, establishing that neogenin scaffolds the HJV·BMP·BMP receptor signaling complex rather than merely stabilizing HJV.\",\n      \"evidence\": \"Co-IP of neogenin with ALK3; neogenin-interaction-deficient HJV mutant expressed in Hjv-/- mice; furin cleavage-site mutant showing normal hepcidin in vivo\",\n      \"pmids\": [\"27072365\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct structural evidence for the ternary scaffolding complex lacking\", \"Whether neogenin enhances BMP receptor kinase activity or proximity not tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Double knockout of Bmp6 and Hjv revealed that BMP6 retains residual signaling capacity without HJV, demonstrating that HJV amplifies rather than absolutely gates BMP6-to-hepcidin signaling, and that full hepcidin induction requires synergy between BMP/SMAD and IL-6/STAT3 pathways.\",\n      \"evidence\": \"Hjv-/- × Bmp6-/- double knockout mice with Smad signaling and LPS challenge\",\n      \"pmids\": [\"29021231\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other BMP ligands compensate partially in double knockouts unknown\", \"Quantitative contribution of each pathway branch not modeled\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"HJV was found to be required for inflammatory hepcidin induction, overturning earlier models that placed inflammatory and iron-sensing pathways as independent—Hjv deficiency abolished the synergism between BMP/Smad and IL-6/Stat signaling needed for the hypoferremic response to infection.\",\n      \"evidence\": \"Hjv-/- mice challenged with LPS, FSL1, and live E. coli; primary hepatocyte BMP/Smad assays\",\n      \"pmids\": [\"30213871\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether pharmacological BMP supplementation can rescue inflammatory hepcidin in Hjv-/- mice untested\", \"Mechanism of synergy between SMAD and STAT3 at the hepcidin promoter not fully resolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The molecular mechanism of neogenin scaffolding was elucidated: HJV binding triggers cleavage of the neogenin cytoplasmic domain, and the resulting truncated neogenin accumulates at the plasma membrane where it assembles the BMP signaling complex; interaction-deficient mutants of both neogenin and HJV failed to rescue hepcidin in conditional knockouts.\",\n      \"evidence\": \"Hepatocyte-specific Neo1 knockout mice; Neo1L1046E and HjvA183R interaction-deficient mutant rescue experiments; BMP signaling analysis\",\n      \"pmids\": [\"33824974\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of protease cleaving neogenin cytoplasmic domain unknown\", \"How truncated neogenin specifically promotes BMP receptor assembly not structurally defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the atomic structure of the HJV·neogenin·BMP·BMP receptor signaling complex, the identity of the protease that cleaves neogenin upon HJV engagement, the mechanism by which iron status is transduced to regulate HJV membrane levels, and whether the muscle TGF-β coreceptor function of HJV has therapeutic relevance for sarcopenia or muscular dystrophy.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of the HJV signaling complex\", \"Iron-sensing mechanism upstream of HJV membrane dynamics unresolved\", \"Therapeutic exploitation of muscle HJV-TGF-β axis unexplored\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 5, 21]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 6, 23]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 14, 15, 16]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [2, 7, 17]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 5, 9, 19, 21, 25]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [3, 20, 24]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 15, 26]}\n    ],\n    \"complexes\": [\n      \"HJV·neogenin·BMP·BMP receptor complex\",\n      \"HFE·TfR2·HJV complex\",\n      \"Neogenin·matriptase-2·HJV ternary complex\"\n    ],\n    \"partners\": [\n      \"NEO1\",\n      \"TMPRSS6\",\n      \"HFE\",\n      \"TFR2\",\n      \"ALK3\",\n      \"ACVR2A\",\n      \"BMPR2\",\n      \"TGFBR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}