{"gene":"ACVR1B","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2002,"finding":"SB-431542 selectively inhibits ALK4 (ACVR1B), ALK5, and ALK7 kinase activity but not other ALK family members (ALK1, ALK2, ALK3, ALK6), blocking activin and TGF-β signaling without affecting BMP, ERK, JNK, or p38 MAPK pathways.","method":"In vitro kinase inhibition assays, cell-based reporter assays, signaling pathway analysis","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinase assay with comprehensive selectivity panel, replicated across multiple labs","pmids":["12065756"],"is_preprint":false},{"year":2001,"finding":"ALK4 (ACVR1B) acts as a type I receptor for Nodal signaling in collaboration with ActRIIB; both receptors can independently bind Xnr1, and Cripto interacts directly with both Xnr1 and ALK4 to enhance Nodal responsiveness. A dominant-negative ALK4 blocks all mesoderm-inducing ligands including Nodal, Xnr1, Xnr2, Xnr4, and Activin.","method":"Receptor reconstitution experiments, dominant-negative/constitutively active receptor assays in Xenopus embryos, co-immunoprecipitation","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1-2 — reconstitution + functional assays in Xenopus, biochemical binding, replicated by multiple labs","pmids":["11485994"],"is_preprint":false},{"year":1998,"finding":"ActRIB (ACVR1B) is required for egg cylinder organization and gastrulation in mice; ActRIB-/- embryos show disorganized epiblast and extraembryonic ectoderm and arrest before gastrulation. Chimera analysis revealed ActRIB functions in both epiblast and extraembryonic cells to mediate signals required for primitive streak formation.","method":"Gene targeting (knockout), chimera analysis, embryo histology","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout with defined developmental phenotype, chimera analysis for cell-autonomous function, foundational study","pmids":["9512518"],"is_preprint":false},{"year":2001,"finding":"ACVR1B harbors novel somatic mutations in pancreatic carcinoma, establishing it as a mutated tumor-suppressor gene in this cancer type.","method":"Direct sequencing of pancreatic cancer specimens, gene structure characterization","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — direct sequencing identifying somatic mutations, single study","pmids":["11248065"],"is_preprint":false},{"year":2002,"finding":"Cripto-1 binds directly to ALK4 on mammalian epithelial cell surfaces (shown by FACS and co-immunoprecipitation), and Cripto-1 can phosphorylate Smad2 only in the presence of both Nodal and ALK4, establishing Cripto-1 as a co-receptor that bridges Nodal to ALK4 for Smad signaling.","method":"Phage display library screening, co-immunoprecipitation, FACS binding, Smad2 phosphorylation assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP plus functional Smad assay, multiple orthogonal methods","pmids":["11909953"],"is_preprint":false},{"year":2004,"finding":"SB-505124 selectively inhibits ALK4, ALK5, and ALK7 kinase activity, blocking downstream Smad2/3 phosphorylation and TGF-β-induced MAPK activation without affecting ALK1, ALK2, ALK3, or ALK6 signaling; it is 3-5x more potent than SB-431542.","method":"In vitro kinase assays, Smad2/3 reporter assays, cell-based signaling pathway analysis","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinase assays with selectivity panel, functional cell assays","pmids":["14978253"],"is_preprint":false},{"year":2004,"finding":"An activin-A mutant (M108A) retains wild-type affinity for ActRII but cannot form a cross-linked complex with ALK4, demonstrating that the fingertip/wrist region of activin contacts ALK4 directly and identifying M108A as a type II receptor antagonist that blocks activin and myostatin but not TGF-β signaling.","method":"Site-directed mutagenesis, cross-linking assays with 125I-activin-A, cell-based signaling assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution + mutagenesis + cross-linking binding assay","pmids":["15123686"],"is_preprint":false},{"year":2003,"finding":"Five hydrophobic residues on the ALK4 extracellular domain (Leu40, Ile70, Val73, Leu75, Pro77) constitute the activin-binding surface; mutation of these residues to alanine substantially reduces ALK4-trunc dominant-negative activity and activin-A cross-linking in the presence of ActRII.","method":"Extracellular domain mutagenesis, dominant-negative activity assay, 125I-activin cross-linking","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with biochemical binding validation and functional assay","pmids":["12665502"],"is_preprint":false},{"year":2000,"finding":"Truncated ALK4 isoforms (Alk4-2, 4-3, 4-4) expressed in human pituitary tumors function as dominant-negative receptors: they form complexes with type II activin receptors (shown by co-immunoprecipitation) but are not phosphorylated, blocking activin-induced transcription and the antiproliferative effect of activin.","method":"Co-immunoprecipitation, reporter assay, stable transfection with growth inhibition assay","journal":"Molecular endocrinology","confidence":"High","confidence_rationale":"Tier 2 — co-IP plus multiple functional readouts (transcription, proliferation, cell cycle)","pmids":["11117535"],"is_preprint":false},{"year":1997,"finding":"Constitutively active ALK4 (ALK4*) induces dose-dependent dorso-anterior mesodermal and endodermal markers in Xenopus animal caps, including a secondary axis when injected ventrally, while constitutively active ALK2 (ALK2*) induces only ventral mesodermal markers and antagonizes ALK4* signaling, revealing distinct and interfering signaling pathways for the two activin type I receptors.","method":"Constitutively active and dominant-negative receptor injection in Xenopus embryos, animal cap assay, gene expression analysis","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with constitutively active/dominant-negative receptors in vertebrate embryos, multiple dose-response readouts","pmids":["9367435"],"is_preprint":false},{"year":1999,"finding":"The loop between kinase subdomains IV and V (7 amino acids) in ALK4 mediates its strong dorsal gene-inducing specificity; transferring this loop from ALK4* to ALK2* enables the chimeric receptor to induce dorsal-specific gene expression in Xenopus.","method":"Chimeric receptor construction, Xenopus animal cap and embryo assay, gene expression analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — chimeric receptor domain-swap experiment, single lab","pmids":["10075688"],"is_preprint":false},{"year":2007,"finding":"miR-24 targets the 3'-UTR of ALK4 (ACVR1B) mRNA to reduce ALK4 protein levels, thereby interfering with activin-induced Smad2 phosphorylation and inhibiting erythroid differentiation in K562 cells and CD34+ hematopoietic progenitors.","method":"Reporter assay (3'-UTR luciferase), Western blot, ectopic miR-24 expression, erythroid colony assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — reporter assay validating direct miRNA targeting plus functional downstream signaling and differentiation assays","pmids":["17906079"],"is_preprint":false},{"year":2009,"finding":"miR-210 directly targets the AcvR1b (ALK4) 3'-UTR (validated by reporter assay), and its overexpression promotes BMP-4-induced osteoblastic differentiation of ST2 stromal cells by suppressing TGF-β/activin signaling through ALK4 inhibition.","method":"Reporter assay, sense/antisense miR-210 transfection, SB431542 pharmacological inhibition, osteoblastic differentiation assay","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — direct reporter assay plus functional differentiation phenotype, single lab","pmids":["19520079"],"is_preprint":false},{"year":2006,"finding":"ALK4 mediates activin A and B signaling to stimulate FSHβ transcription in gonadotrope cells via Smad3 phosphorylation; ALK7 selectively potentiates activin B (but not activin A) effects on Fshb promoter activity, and both constitutively active ALK4 and ALK7 stimulate Smad2/3 phosphorylation.","method":"RT-PCR, transfection of wild-type, kinase-dead, and constitutively active receptor constructs, reporter assay, Western blot, shRNA depletion","journal":"Reproductive biology and endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple receptor constructs with functional readouts, single lab","pmids":["17040568"],"is_preprint":false},{"year":2004,"finding":"ALK4 co-immunoprecipitates with multiple TGF-β ligands (AVg Activin-Vg1 chimera, Xnr1, derriere, and endogenous Vg1) in Xenopus embryo homogenates, and constitutively active ALK4 expressed on the right side causes left-right organ reversals; dominant-negative ALK4 on the left side also disrupts organ situs, establishing a left-side requirement for ALK4 signaling in LR axis determination.","method":"Co-immunoprecipitation from embryo homogenates, dominant-negative and constitutively active receptor microinjection in Xenopus, Pitx2 expression analysis","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal co-IP and genetic loss/gain-of-function in vertebrate embryo, single lab","pmids":["15063168"],"is_preprint":false},{"year":2010,"finding":"Conditional epithelial-specific deletion of Acvr1b (K14-Cre) in mice causes disruption of hair follicle cycling and severe progressive hair loss, and persistent proliferation of skin epithelial cells, establishing a cell-autonomous role for ACVR1B in hair follicle development and cycling.","method":"Conditional knockout mouse (Cre-lox), histological analysis, postnatal phenotype characterization","journal":"The Journal of investigative dermatology","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with specific cellular phenotype, genetic demonstration of requirement","pmids":["21191412"],"is_preprint":false},{"year":2015,"finding":"Activin A induces human trophoblast cell invasion through ALK4-SMAD2/3-SMAD4 signaling, which upregulates SNAIL and subsequently MMP2; siRNA knockdown of ALK4, SMAD2/3, SMAD4, or SNAIL each abolished activin A-induced MMP2 expression and invasion.","method":"siRNA knockdown, Western blot, Matrigel-coated transwell invasion assay, pharmacological inhibition (SB431542)","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — multiple siRNA targets with consistent functional readout, replicated in primary and immortalized cells","pmids":["26305619"],"is_preprint":false},{"year":2013,"finding":"Pharmacological inhibition of ALK4 and ALK7 (with SB431542) in fetal mouse gonads reveals that Activin/NODAL receptors ALK4 and ALK7 are required for promoting differentiation of male germ cells and their entry into mitotic arrest, while ALK4/5/7 signaling is required for testis cord formation during sex determination.","method":"FACS-purified gonadal cells, SB431542 pharmacological inhibition ex vivo, histological and gene expression analysis","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological inhibition with specific cellular phenotype, single lab","pmids":["23342175"],"is_preprint":false},{"year":2009,"finding":"The Cripto CFC domain interacts directly with ALK4 extracellular domain (Kd in μM range by SPR); NMR structural analysis shows residues H120 and W124 are externally exposed and molecular docking identifies the protein-protein interface consistent with prior mutagenesis, with no binding detected to ActRIIB.","method":"NMR structural analysis, surface plasmon resonance binding, molecular docking, synthetic CFC domain peptide","journal":"Journal of peptide science","confidence":"Medium","confidence_rationale":"Tier 1-2 — NMR + SPR binding assay, single lab","pmids":["19035567"],"is_preprint":false},{"year":2010,"finding":"Activin A induces neuronal differentiation and survival in SK-N-SH neuroblastoma cells through ALK4 via a SMAD-independent mechanism: ALK4 activation does not induce SMAD2/3 phosphorylation, SMAD2/3-SMAD4 interaction, nuclear SMAD2/3 accumulation, or SMAD2/3 promoter binding, yet activates TGF-β target genes and promotes neurite outgrowth.","method":"siRNA knockdown, Western blot, co-immunoprecipitation, ChIP, neurite outgrowth assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods establishing SMAD-independent signaling, single lab","pmids":["20226172"],"is_preprint":false},{"year":2017,"finding":"Activin A/ALK4 signaling activates Smad2/3 in atrial fibroblasts to drive atrial fibrosis; ALK4-haplodeficient mice show reduced atrial fibroblast activation, reduced atrial fibrosis, and reduced atrial fibrillation vulnerability after angiotensin II stimulation.","method":"ALK4 haplodeficient knockout mice, Ang-II stimulation model, electrophysiological studies, histology, Western blot","journal":"Basic research in cardiology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic loss-of-function with defined fibrotic and electrophysiological phenotype, single lab","pmids":["28639003"],"is_preprint":false},{"year":2020,"finding":"Activin A increases integrin β1 expression in human trophoblasts through ALK4-SMAD2/3-SMAD4 signaling; knockdown of ALK4 or SMAD4 abolishes activin A-induced integrin β1 upregulation, and integrin β1 knockdown reduces activin A-stimulated invasion.","method":"siRNA knockdown, Western blot, Matrigel invasion assay, SB431542 pharmacological inhibition","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 — multiple siRNA targets with functional invasion readout, consistent across immortalized and primary cells","pmids":["33230889"],"is_preprint":false},{"year":2022,"finding":"ALK4 (ACVR1B) cooperates with dopamine D1 receptor signaling to regulate FosB alternative splicing in nucleus accumbens medium spiny neurons: concurrent D1 and ALK4 activation induces PCBP1 interaction with Smad3, nuclear translocation, and binding to FosB exon-4/intron-4 sequences to generate ΔFosB mRNA; ALK4 ablation in MSNs impairs ΔFosB induction and cocaine behavioral sensitization.","method":"Co-immunoprecipitation, nuclear translocation assay, conditional ALK4 KO in MSNs, behavioral sensitization assay, RNA binding protein analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — co-IP, conditional KO, behavioral phenotype, multiple orthogonal methods, novel mechanism","pmids":["35730718"],"is_preprint":false},{"year":2022,"finding":"Simultaneous muscle-specific knockout of Tgfbr1 and Acvr1b in mice induces substantial myofibre hypertrophy via increased Akt and p70S6K phosphorylation and reduced E3 ligase expression; combined knockout also increases satellite cell numbers and improves regeneration after cardiotoxin injury, while single receptor knockouts do not produce these effects.","method":"Double conditional knockout mouse, histology, Western blot (Akt/p70S6K phosphorylation), cardiotoxin injury model, gene expression analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis via double KO demonstrating synergy, multiple readouts including signaling pathway analysis","pmids":["35323108"],"is_preprint":false},{"year":2021,"finding":"ActRIIB:ALK4-Fc, a heterodimeric fusion of ALK4 and ActRIIB extracellular domains, has a distinct ligand-binding profile compared to ActRIIB-Fc homodimer: it sequesters ActRIIB ligands that inhibit muscle growth but does not trap BMP9 (shown by SPR), improving muscle mass, function, and neuromuscular junction abnormalities in murine DMD, ALS, and disuse atrophy models.","method":"Surface plasmon resonance, retinal explant vascular assay, murine disease models, muscle mass/function measurements","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1-2 — SPR ligand profiling + multiple in vivo models with functional readouts","pmids":["33586684"],"is_preprint":false},{"year":2020,"finding":"ALK4 signaling in oligodendrocytes mediates Activin A-induced white matter remyelination after ischemic stroke; AAV-based ACVR1B shRNA (with Olig2 promoter) reversed rmActivin A-induced increases in mature oligodendrocyte number, myelin protein levels, and neurological function recovery in MCAO/R mice.","method":"AAV-shRNA oligodendrocyte-specific knockdown, MCAO/R mouse model, immunohistochemistry, Western blot, neurological scoring","journal":"Experimental neurology","confidence":"Medium","confidence_rationale":"Tier 2 — cell-type-specific genetic knockdown with functional neurological and histological readouts, single lab","pmids":["33345977"],"is_preprint":false},{"year":2020,"finding":"ALK4 (ACVR1B) signaling promotes cadmium-induced death of renal proximal tubular cells via Smad3 phosphorylation and Akt pathway activation; siRNA knockdown of ALK4 or Smad3, or Smad3 inhibitor SIS3, attenuated cadmium-induced cell death.","method":"siRNA knockdown, pharmacological inhibition (SB431542, SB505124, SIS3), Western blot, cell viability assay","journal":"Cell death and differentiation","confidence":"Medium","confidence_rationale":"Tier 2 — multiple genetic and pharmacological interventions, single lab","pmids":["30804470"],"is_preprint":false},{"year":2016,"finding":"ALK4 mediates activin A-stimulated aromatase (P450arom) expression and estradiol secretion in endometrial stromal cells via the ALK4-Smad pathway; pretreatment with ALK4 inhibitor or Smad4 siRNA partially abrogated these effects.","method":"Pharmacological inhibition (SB431542), Smad4 siRNA, RT-PCR, ELISA","journal":"BioMed research international","confidence":"Low","confidence_rationale":"Tier 3 — pharmacological inhibition and single siRNA target, single lab, partial abrogation","pmids":["27833918"],"is_preprint":false},{"year":2019,"finding":"ALK4-SMAD2/3-SMAD4 signaling mediates activin A-induced suppression of PTX3 in human granulosa-lutein cells; siRNA knockdown of ALK4, SMAD2, SMAD3, or SMAD4 each reversed activin A-induced PTX3 suppression, and ChIP analysis showed phosphorylated SMADs bind to the PTX3 promoter.","method":"siRNA knockdown, Western blot, RT-qPCR, ChIP assay, pharmacological inhibition (SB431542)","journal":"Molecular and cellular endocrinology","confidence":"High","confidence_rationale":"Tier 2 — multiple siRNA targets with functional readout plus ChIP mechanistic evidence, consistent with pathway model","pmids":["31185247"],"is_preprint":false},{"year":2020,"finding":"ALK4-SMAD3-SMAD4 (but not SMAD2) mediates activin A-induced reduction in betaglycan shedding in endometriotic cells; this pathway is distinct from SMAD2 and involves ALK4-dependent suppression of soluble betaglycan release, as shown by ALK4 siRNA and the SMAD3 inhibitor SIS3.","method":"siRNA knockdown, SMAD3 inhibitor (SIS3), ELISA, RT-qPCR, Western blot","journal":"Biomolecules","confidence":"Medium","confidence_rationale":"Tier 2 — multiple molecular interventions establishing SMAD3 specificity over SMAD2, single lab","pmids":["36551177"],"is_preprint":false},{"year":2022,"finding":"ALK4 is preferentially expressed in adipocyte precursors where it suppresses differentiation by repressing CEBPα and PPARγ expression, promoting proliferation and adipose tissue expansion; ALK4 deletion induces premature adipocyte differentiation rescued by CEBPα knockdown.","method":"Conditional knockout, CEBPα/PPARγ expression analysis, CEBPα knockdown rescue, adipocyte differentiation assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — conditional KO with mechanistic rescue, single lab","pmids":["36403856"],"is_preprint":false},{"year":2020,"finding":"ALK4 signaling in cortical somatostatin interneurons regulates their specification: activin-mediated ALK4 signaling induces Smad2 interaction with transcription factor SATB1 and promotes SATB1 nuclear translocation and repositioning at the somatostatin gene promoter; mice lacking ALK4 in GABAergic MGE neurons show deficits in somatostatin interneuron subpopulations.","method":"Conditional knockout (GABAergic neuron-specific), co-immunoprecipitation (Smad2-SATB1), nuclear translocation assay, ChIP (SATB1 at somatostatin promoter), immunohistochemistry","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — conditional KO plus co-IP and ChIP demonstrating molecular mechanism, multiple orthogonal methods","pmids":["31676717"],"is_preprint":false},{"year":2022,"finding":"Activin A promotes integrin β3 expression in human trophoblasts via ALK4-SMAD4 signaling; knockdown of ALK4 or SMAD4 abolished activin A-induced integrin β3 upregulation, and integrin β3 knockdown reduced activin A-promoted invasion.","method":"siRNA knockdown, Western blot, RT-PCR, Matrigel invasion assay, SB431542 inhibition","journal":"Placenta","confidence":"Medium","confidence_rationale":"Tier 2 — consistent across cell lines with multiple siRNA targets, single lab","pmids":["36244196"],"is_preprint":false},{"year":2025,"finding":"Cripto-1 acts as a molecular bridge connecting Nodal (bound to type II receptor ActRIIB) to type I receptor ALK4, forming a unique ternary complex for SMAD2/3 activation; AlphaFold3 modeling, SPR-based interaction analysis, domain-specific antibodies, and functional studies in NTERA-2 cells confirmed that Cripto-1 uses distinct structural domains to bind Nodal and ALK4 separately, contrasting with canonical TGF-β signaling.","method":"AlphaFold3 structural modeling, surface plasmon resonance, domain-specific anti-Cripto-1 antibodies, functional SMAD2/3 activation assay","journal":"Protein science","confidence":"High","confidence_rationale":"Tier 1 — structural modeling validated by SPR and functional assays, multiple orthogonal methods","pmids":["39840816"],"is_preprint":false},{"year":2025,"finding":"ALK4 loss enhances canonical TGF-β signaling by upregulating MGAT5 and galectin-3, which stabilize TGF-β receptors at the cell surface through β1,6 N-acetylglucosaminyltransferase V-mediated N-linked glycosylation; depleting MGAT5 or inhibiting N-glycosylation suppresses ALK4-loss-induced TGF-β signaling and cancer progression in breast and pancreatic cancer models.","method":"In vitro migration/invasion assays, in vivo cancer models, glycosylation analysis, MGAT5/galectin-3 depletion, receptor surface stability assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple in vitro and in vivo models with mechanistic rescue via MGAT5 depletion and glycosylation inhibition","pmids":["41408046"],"is_preprint":false},{"year":2021,"finding":"A missense variant p.M345I in ACVR1B reduces transactivation of NODAL-responsive reporters (AR3-Luc, CAGA12-Luc, SBE4-Luc) and decreases SMAD2/3 phosphorylation, indicating the mutation impairs ACVR1B kinase signaling function and is associated with congenital heart disease.","method":"Reporter assay, Western blot (SMAD2/3 phosphorylation), 3D protein modeling","journal":"Experimental cell research","confidence":"Low","confidence_rationale":"Tier 3 — single variant functional assay, single lab, no rescue experiment","pmids":["34666056"],"is_preprint":false},{"year":2024,"finding":"Loss of Acvr1b in the presence of oncogenic Kras accelerates formation of pancreatic intraepithelial neoplasia and IPMN-like precancerous lesions from both acinar and ductal cells, establishing ACVR1B as a tumor suppressor in pancreatic exocrine cells; the effect is context-dependent on cell of origin.","method":"Cell-type-specific conditional knockout (Ptf1aCreER or Sox9CreER combined with Acvr1b flox/flox and KrasLSL-G12D), MRI, immunohistochemistry, histopathology","journal":"Cellular and molecular gastroenterology and hepatology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis in two cell-type-specific mouse models with defined histopathological outcomes","pmids":["39111635"],"is_preprint":false},{"year":2025,"finding":"In a mouse model of SLE-associated pulmonary hypertension, Activin A signals via ALK4 in Th17 cells to induce IL-17 secretion, and ALK4 activation in pulmonary microvascular endothelial cells induces endothelial-mesenchymal transition via CTGF upregulation dependent on synergistic pSmad2 and pSTAT3 increases; ALK4 overexpression in Th17 cells worsens SLE-PH in mice while CD4+ T cell depletion alleviates it.","method":"CyTOF, Th17-PMEC co-culture, ALK4 overexpression, mouse SLE-PH model, CD4+ T cell depletion, ALK4 inhibitor (vactosertib/TEW-7197), Western blot","journal":"Arthritis & rheumatology","confidence":"Medium","confidence_rationale":"Tier 2 — co-culture system, genetic overexpression, pharmacological inhibition, in vivo model, single lab","pmids":["40395196"],"is_preprint":false}],"current_model":"ACVR1B (ALK4) is a type I serine/threonine kinase receptor of the TGF-β superfamily that forms heteromeric complexes with type II receptors (ActRII/ActRIIB) to transduce signals from activins, Nodal, and related ligands primarily via SMAD2/3-SMAD4 phosphorylation; its extracellular domain directly binds activin at a defined hydrophobic interface (Leu40, Ile70, Val73, Leu75, Pro77), with Cripto-1 acting as a co-receptor that bridges Nodal to ALK4; ALK4 is required for gastrulation, mesoderm induction, hair follicle cycling, somatostatin interneuron specification, and male germ cell differentiation, and functions as a tumor suppressor in the pancreas—where truncated dominant-negative isoforms block its antiproliferative signaling—while also regulating muscle atrophy/hypertrophy, adipocyte differentiation, erythropoiesis, trophoblast invasion (via SNAIL-MMP2 and integrin pathways), and neuronal ΔFosB alternative splicing through convergent dopamine/ALK4-PCBP1-Smad3 signaling."},"narrative":{"teleology":[{"year":1997,"claim":"Constitutively active ALK4 was shown to induce dorso-anterior mesoderm and endoderm in Xenopus, establishing that ALK4 signals in a pathway distinct from ALK2 and mediates activin-like dorsal patterning.","evidence":"Constitutively active/dominant-negative receptor injection in Xenopus animal caps with dose-response gene expression readouts","pmids":["9367435"],"confidence":"High","gaps":["Endogenous ligand identity not resolved","Downstream transcriptional targets not identified","Mechanism of ALK2 antagonism unclear"]},{"year":1998,"claim":"Genetic knockout demonstrated that ALK4 is indispensable for gastrulation: null embryos arrest before primitive streak formation, and chimera analysis established that ALK4 functions cell-autonomously in both epiblast and extraembryonic ectoderm.","evidence":"Gene targeting (knockout) and chimera analysis in mice with embryo histology","pmids":["9512518"],"confidence":"High","gaps":["Specific ligand(s) acting through ALK4 in vivo not identified","Downstream signaling pathway in gastrulation not characterized"]},{"year":1999,"claim":"Domain-swap experiments identified a 7-amino-acid loop between kinase subdomains IV and V as the structural determinant of ALK4's dorsal mesoderm-inducing specificity, distinguishing it from ALK2.","evidence":"Chimeric ALK4/ALK2 receptor construction with Xenopus animal cap assays","pmids":["10075688"],"confidence":"Medium","gaps":["Crystal structure of kinase domain not available","Specific SMAD or non-SMAD effectors engaged by this loop not determined"]},{"year":2000,"claim":"Truncated ALK4 isoforms found in pituitary tumors were shown to act as dominant-negative receptors that complex with type II receptors but cannot be phosphorylated, blocking activin's antiproliferative effect — the first evidence for ALK4 loss-of-function in human neoplasia.","evidence":"Co-immunoprecipitation, reporter assay, and growth inhibition assay in stably transfected cells","pmids":["11117535"],"confidence":"High","gaps":["Prevalence of truncated isoforms across tumor types unknown","In vivo tumor-promoting role not demonstrated"]},{"year":2001,"claim":"Two concurrent advances defined ALK4 as the obligate type I receptor for Nodal signaling and identified somatic ACVR1B mutations in pancreatic cancer, establishing its dual role in developmental signaling and tumor suppression.","evidence":"Receptor reconstitution and dominant-negative assays in Xenopus embryos with co-IP (Nodal); direct sequencing of pancreatic cancer specimens (mutations)","pmids":["11485994","11248065"],"confidence":"High","gaps":["Cripto-ALK4 binding interface not structurally resolved","Functional consequence of specific pancreatic cancer mutations not tested"]},{"year":2002,"claim":"SB-431542 was characterized as a selective inhibitor of ALK4/ALK5/ALK7 kinase activity, providing a pharmacological tool that discriminates activin/TGF-β signaling from BMP and MAPK pathways, and Cripto-1 was shown to directly bind ALK4 on mammalian cell surfaces to enable Nodal-dependent SMAD2 phosphorylation.","evidence":"In vitro kinase inhibition assays with selectivity panel; FACS binding, co-IP, and SMAD2 phosphorylation assay for Cripto-ALK4 interaction","pmids":["12065756","11909953"],"confidence":"High","gaps":["Co-crystal structure of inhibitor-ALK4 not available","Cripto binding site on ALK4 not mapped at residue level"]},{"year":2003,"claim":"Mutagenesis of the ALK4 extracellular domain identified five hydrophobic residues (Leu40, Ile70, Val73, Leu75, Pro77) as the activin-binding surface, providing the first residue-level map of ligand-receptor contact.","evidence":"Extracellular domain mutagenesis with 125I-activin cross-linking and dominant-negative activity assay","pmids":["12665502"],"confidence":"High","gaps":["Crystal structure of the ternary activin-ActRII-ALK4 complex not solved","Whether these residues also contact Nodal or other ligands not tested"]},{"year":2004,"claim":"Activin's fingertip/wrist region was shown to contact ALK4 directly via an M108A mutant that retains type II receptor binding but loses ALK4 recruitment, and ALK4 was demonstrated to participate in left-right axis determination in Xenopus through signaling downstream of multiple Nodal-related ligands.","evidence":"Site-directed mutagenesis with cross-linking (activin-ALK4); co-IP from embryo homogenates with dominant-negative/constitutively active receptor injection for L-R axis","pmids":["15123686","15063168"],"confidence":"High","gaps":["Left-right phenotype not validated in mammalian models","Structural basis of M108A disruption not determined at atomic level"]},{"year":2007,"claim":"miR-24 was identified as a post-transcriptional regulator of ALK4, demonstrating that ALK4 protein levels and consequently activin-SMAD2 signaling in erythroid differentiation are subject to microRNA control.","evidence":"3'-UTR luciferase reporter assay, ectopic miR-24 expression, erythroid colony assay in K562 and CD34+ cells","pmids":["17906079"],"confidence":"High","gaps":["Physiological relevance during normal hematopoiesis not established","Whether other miRNAs cooperate in ALK4 regulation not explored"]},{"year":2009,"claim":"NMR and SPR analyses structurally characterized the Cripto CFC domain–ALK4 interaction, identifying exposed residues H120 and W124 on Cripto as key contact points, confirming the direct and specific nature of the co-receptor–receptor interaction.","evidence":"NMR structure of CFC domain, SPR binding kinetics, molecular docking","pmids":["19035567"],"confidence":"Medium","gaps":["Co-crystal structure of CFC-ALK4 complex not obtained","Affinity measured in µM range — physiological relevance of weak interaction debated"]},{"year":2010,"claim":"Conditional epithelial-specific deletion of Acvr1b revealed a cell-autonomous requirement for ALK4 in hair follicle cycling; separately, ALK4 was found to promote neuronal differentiation via a SMAD-independent mechanism in neuroblastoma cells.","evidence":"K14-Cre conditional KO in mice with histological phenotyping (hair follicle); siRNA knockdown, ChIP, and neurite outgrowth assay in SK-N-SH cells (SMAD-independent)","pmids":["21191412","20226172"],"confidence":"High","gaps":["Identity of SMAD-independent downstream effectors not determined","Hair follicle phenotype mechanism (which SMAD, which target genes) not resolved"]},{"year":2015,"claim":"The full activin A→ALK4→SMAD2/3→SMAD4→SNAIL→MMP2 signaling cascade was delineated in trophoblast invasion, providing a complete pathway from receptor to effector protease.","evidence":"Sequential siRNA knockdown of ALK4, SMAD2/3, SMAD4, SNAIL each abolished activin-induced MMP2 and invasion in Matrigel assay","pmids":["26305619"],"confidence":"High","gaps":["Whether this cascade operates in vivo during implantation not tested","Contribution of other type I receptors (ALK7) not excluded"]},{"year":2020,"claim":"Multiple studies expanded ALK4's tissue-specific roles: ALK4 mediates activin A-induced integrin β1 upregulation in trophoblasts, white matter remyelination via oligodendrocyte maturation after ischemic stroke, cadmium-induced renal cell death via Smad3/Akt, and somatostatin interneuron specification through Smad2-SATB1 interaction at the somatostatin promoter.","evidence":"siRNA/shRNA knockdown and conditional KO with invasion assays, MCAO/R mouse model, cell viability assays, and ChIP respectively","pmids":["33230889","33345977","30804470","31676717"],"confidence":"High","gaps":["Whether oligodendrocyte remyelination role is ALK4-specific or shared with ALK5/ALK7 not resolved","In vivo validation of cadmium toxicity pathway lacking"]},{"year":2022,"claim":"Three distinct functional arenas for ALK4 were established: cooperation with D1 receptor signaling to regulate ΔFosB splicing in striatal neurons via PCBP1-Smad3; redundancy with TGF-βRI (ALK5) in suppressing muscle hypertrophy; and suppression of adipocyte differentiation via CEBPα/PPARγ repression in precursor cells.","evidence":"Conditional KO in MSNs with behavioral assays; double Tgfbr1/Acvr1b muscle-specific KO with Akt/p70S6K signaling; adipocyte precursor conditional KO with CEBPα rescue","pmids":["35730718","35323108","36403856"],"confidence":"High","gaps":["PCBP1-Smad3 splicing mechanism not reconstituted in vitro","Whether ALK4 and ALK5 engage identical SMAD complexes in muscle not determined","Upstream activin ligand identity in adipose tissue not confirmed"]},{"year":2024,"claim":"Cell-type-specific conditional knockout combined with oncogenic Kras formally established ACVR1B as a pancreatic tumor suppressor: Acvr1b loss accelerated precancerous lesion formation from both acinar and ductal cells, resolving two decades of correlative evidence from sequencing studies.","evidence":"Ptf1aCreER and Sox9CreER conditional KO with KrasLSL-G12D, MRI, and histopathology in mice","pmids":["39111635"],"confidence":"High","gaps":["Mechanism by which ALK4 loss cooperates with KRAS not molecularly defined","Whether SMAD2 or SMAD3 mediates the tumor-suppressive arm not resolved"]},{"year":2025,"claim":"Structural modeling and SPR confirmed that Cripto-1 uses distinct domains to simultaneously bind Nodal and ALK4, forming a ternary bridge complex unique among TGF-β family signaling modes; separately, ALK4 loss was found to enhance canonical TGF-β signaling by upregulating MGAT5/galectin-3-mediated glycosylation that stabilizes TGF-β receptors at the cell surface.","evidence":"AlphaFold3 modeling validated by SPR and functional SMAD assays (Cripto); MGAT5/galectin-3 depletion and glycosylation inhibition in breast/pancreatic cancer models (TGF-β crosstalk)","pmids":["39840816","41408046"],"confidence":"High","gaps":["Experimental co-crystal structure of Cripto-Nodal-ALK4-ActRIIB quaternary complex not yet solved","Whether MGAT5-mediated TGF-β receptor stabilization drives tumor progression in patient tumors not confirmed","Therapeutic potential of targeting MGAT5 in ACVR1B-mutant cancers not tested"]},{"year":null,"claim":"Key unresolved questions include the full atomic structure of the activin/Nodal–type II receptor–ALK4–Cripto signaling complex, the identity of SMAD-independent effectors in neurons, the mechanism by which ALK4 loss cooperates with oncogenic KRAS in pancreatic tumorigenesis, and whether ALK4's diverse tissue-specific roles use shared or distinct downstream transcriptional programs.","evidence":"","pmids":[],"confidence":"High","gaps":["No experimental co-crystal structure of the full signaling complex","SMAD-independent neuronal effectors unidentified","Transcriptional program specificity across tissues not systematically compared"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,5,9,10,22,28]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[8,34]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[1,4,33]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,18,34]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,4,5,9,16,22,28,33]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[2,9,14,31]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,8,36,34]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[13,28,30]}],"complexes":["Activin-ActRII/ActRIIB-ALK4 receptor complex","Nodal-Cripto-ALK4-ActRIIB signaling complex"],"partners":["ACVR2A","ACVR2B","TDGF1","SMAD2","SMAD3","SMAD4","TGFBR1","PCBP1"],"other_free_text":[]},"mechanistic_narrative":"ACVR1B (ALK4) is a type I serine/threonine kinase receptor of the TGF-β superfamily that transduces activin, Nodal, and related ligand signals primarily through SMAD2/3-SMAD4 phosphorylation, governing embryonic patterning, tissue homeostasis, and cell fate decisions across diverse lineages. ALK4 forms heteromeric complexes with type II receptors (ActRII/ActRIIB) and engages ligands through a hydrophobic interface on its extracellular domain (Leu40, Ile70, Val73, Leu75, Pro77), while the co-receptor Cripto-1 bridges Nodal to ALK4 via its CFC domain to enable non-canonical ligand recruitment [PMID:12665502, PMID:39840816, PMID:11909953]. ALK4 is essential for gastrulation, left-right axis determination, hair follicle cycling, somatostatin interneuron specification, adipocyte precursor maintenance, and muscle mass regulation, and it functions as a tumor suppressor in the pancreas where its loss cooperates with oncogenic KRAS to accelerate neoplasia [PMID:9512518, PMID:21191412, PMID:31676717, PMID:36403856, PMID:35323108, PMID:39111635]. Beyond canonical SMAD signaling, ALK4 operates through SMAD-independent pathways in neuronal differentiation and cooperates with dopamine D1 receptor signaling to regulate ΔFosB alternative splicing via PCBP1-Smad3 nuclear translocation in striatal neurons [PMID:20226172, PMID:35730718]."},"prefetch_data":{"uniprot":{"accession":"P36896","full_name":"Activin receptor type-1B","aliases":["Activin receptor type IB","ACTR-IB","Activin receptor-like kinase 4","ALK-4","Serine/threonine-protein kinase receptor R2","SKR2"],"length_aa":505,"mass_kda":56.8,"function":"Transmembrane serine/threonine kinase activin type-1 receptor forming an activin receptor complex with activin receptor type-2 (ACVR2A or ACVR2B). Transduces the activin signal from the cell surface to the cytoplasm and is thus regulating a many physiological and pathological processes including neuronal differentiation and neuronal survival, hair follicle development and cycling, FSH production by the pituitary gland, wound healing, extracellular matrix production, immunosuppression and carcinogenesis. Activin is also thought to have a paracrine or autocrine role in follicular development in the ovary. Within the receptor complex, type-2 receptors (ACVR2A and/or ACVR2B) act as a primary activin receptors whereas the type-1 receptors like ACVR1B act as downstream transducers of activin signals. Activin binds to type-2 receptor at the plasma membrane and activates its serine-threonine kinase. The activated receptor type-2 then phosphorylates and activates the type-1 receptor such as ACVR1B. Once activated, the type-1 receptor binds and phosphorylates the SMAD proteins SMAD2 and SMAD3, on serine residues of the C-terminal tail. Soon after their association with the activin receptor and subsequent phosphorylation, SMAD2 and SMAD3 are released into the cytoplasm where they interact with the common partner SMAD4. This SMAD complex translocates into the nucleus where it mediates activin-induced transcription. Inhibitory SMAD7, which is recruited to ACVR1B through FKBP1A, can prevent the association of SMAD2 and SMAD3 with the activin receptor complex, thereby blocking the activin signal. Activin signal transduction is also antagonized by the binding to the receptor of inhibin-B via the IGSF1 inhibin coreceptor. ACVR1B also phosphorylates TDP2","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P36896/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ACVR1B","classification":"Not Classified","n_dependent_lines":22,"n_total_lines":1208,"dependency_fraction":0.018211920529801324},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ACVR1B","total_profiled":1310},"omim":[{"mim_id":"620847","title":"BONE MORPHOGENETIC PROTEIN 8A; BMP8A","url":"https://www.omim.org/entry/620847"},{"mim_id":"619560","title":"MICRO RNA 135B; MIR135B","url":"https://www.omim.org/entry/619560"},{"mim_id":"613992","title":"PROTEIN PHOSPHATASE 2, REGULATORY SUBUNIT B, DELTA; PPP2R2D","url":"https://www.omim.org/entry/613992"},{"mim_id":"613347","title":"PANCREATIC CANCER, SUSCEPTIBILITY TO, 2","url":"https://www.omim.org/entry/613347"},{"mim_id":"609705","title":"MICRO RNA 24-1; MIR24-1","url":"https://www.omim.org/entry/609705"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ACVR1B"},"hgnc":{"alias_symbol":["ALK4","SKR2","ActRIB"],"prev_symbol":["ACVRLK4"]},"alphafold":{"accession":"P36896","domains":[{"cath_id":"2.10.60.10","chopping":"31-95","consensus_level":"high","plddt":84.0892,"start":31,"end":95},{"cath_id":"3.30.200.20","chopping":"187-284","consensus_level":"medium","plddt":86.9721,"start":187,"end":284},{"cath_id":"1.10.510.10","chopping":"288-505","consensus_level":"medium","plddt":94.7679,"start":288,"end":505}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P36896","model_url":"https://alphafold.ebi.ac.uk/files/AF-P36896-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P36896-F1-predicted_aligned_error_v6.png","plddt_mean":83.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ACVR1B","jax_strain_url":"https://www.jax.org/strain/search?query=ACVR1B"},"sequence":{"accession":"P36896","fasta_url":"https://rest.uniprot.org/uniprotkb/P36896.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P36896/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P36896"}},"corpus_meta":[{"pmid":"12065756","id":"PMC_12065756","title":"SB-431542 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Japanese journal of hygiene","url":"https://pubmed.ncbi.nlm.nih.gov/33342936","citation_count":1,"is_preprint":false},{"pmid":"35920165","id":"PMC_35920165","title":"The activin receptor ligand trap ActRIIB:ALK4-Fc ameliorates cardiomyopathy induced by neuromuscular disease and diabetes.","date":"2022","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/35920165","citation_count":1,"is_preprint":false},{"pmid":"39544188","id":"PMC_39544188","title":"Genetic insights into endurance athlete status: A meta-analysis of ACVR1B, AGT, FTO, IL-6, and NRF2 gene polymorphisms.","date":"2024","source":"Journal of biological methods","url":"https://pubmed.ncbi.nlm.nih.gov/39544188","citation_count":1,"is_preprint":false},{"pmid":"40395196","id":"PMC_40395196","title":"Activin A-Activated ALK4 Induces Pathogenic Th17-Involved Endothelial-Mesenchymal Transition in Systemic Lupus Erythematosus-Associated Pulmonary Arterial Hypertension.","date":"2025","source":"Arthritis & rheumatology (Hoboken, N.J.)","url":"https://pubmed.ncbi.nlm.nih.gov/40395196","citation_count":1,"is_preprint":false},{"pmid":"37462480","id":"PMC_37462480","title":"[Up-regulated expression of miR-576 inhibits ALK4 expression, regulates JAK/STAT signaling pathway and promotes proliferation and migration of prostatic cancer cells].","date":"2022","source":"Zhonghua nan ke xue = National journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/37462480","citation_count":1,"is_preprint":false},{"pmid":"41408046","id":"PMC_41408046","title":"Loss of ALK4 promotes cancer progression through regulating TGF-β receptor N-glycosylation.","date":"2025","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/41408046","citation_count":0,"is_preprint":false},{"pmid":"40494204","id":"PMC_40494204","title":"Activin A exacerbates neonatal necrotizing enterocolitis via ALK4-mediated apoptosis and barrier disruption.","date":"2025","source":"International immunopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/40494204","citation_count":0,"is_preprint":false},{"pmid":"41278748","id":"PMC_41278748","title":"Targeting the Alk4 pathway protects against age-related bone loss.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/41278748","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.01.23.25321014","title":"Activin A-activated ALK4 induces pathogenic Th17 involvement in endothelial–mesenchymal transition in systemic lupus erythematosus-associated pulmonary arterial hypertension","date":"2025-01-25","source":"bioRxiv","url":"https://doi.org/10.1101/2025.01.23.25321014","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.02.07.637115","title":"CryoEM structure of ALK2:BMP6 reveals distinct mechanisms that allow ALK2 to interact with both BMP and Activin ligands","date":"2025-02-08","source":"bioRxiv","url":"https://doi.org/10.1101/2025.02.07.637115","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":41070,"output_tokens":8921,"usd":0.128513},"stage2":{"model":"claude-opus-4-6","input_tokens":12857,"output_tokens":4376,"usd":0.260528},"total_usd":0.389041,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"SB-431542 selectively inhibits ALK4 (ACVR1B), ALK5, and ALK7 kinase activity but not other ALK family members (ALK1, ALK2, ALK3, ALK6), blocking activin and TGF-β signaling without affecting BMP, ERK, JNK, or p38 MAPK pathways.\",\n      \"method\": \"In vitro kinase inhibition assays, cell-based reporter assays, signaling pathway analysis\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assay with comprehensive selectivity panel, replicated across multiple labs\",\n      \"pmids\": [\"12065756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"ALK4 (ACVR1B) acts as a type I receptor for Nodal signaling in collaboration with ActRIIB; both receptors can independently bind Xnr1, and Cripto interacts directly with both Xnr1 and ALK4 to enhance Nodal responsiveness. A dominant-negative ALK4 blocks all mesoderm-inducing ligands including Nodal, Xnr1, Xnr2, Xnr4, and Activin.\",\n      \"method\": \"Receptor reconstitution experiments, dominant-negative/constitutively active receptor assays in Xenopus embryos, co-immunoprecipitation\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution + functional assays in Xenopus, biochemical binding, replicated by multiple labs\",\n      \"pmids\": [\"11485994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"ActRIB (ACVR1B) is required for egg cylinder organization and gastrulation in mice; ActRIB-/- embryos show disorganized epiblast and extraembryonic ectoderm and arrest before gastrulation. Chimera analysis revealed ActRIB functions in both epiblast and extraembryonic cells to mediate signals required for primitive streak formation.\",\n      \"method\": \"Gene targeting (knockout), chimera analysis, embryo histology\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout with defined developmental phenotype, chimera analysis for cell-autonomous function, foundational study\",\n      \"pmids\": [\"9512518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"ACVR1B harbors novel somatic mutations in pancreatic carcinoma, establishing it as a mutated tumor-suppressor gene in this cancer type.\",\n      \"method\": \"Direct sequencing of pancreatic cancer specimens, gene structure characterization\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct sequencing identifying somatic mutations, single study\",\n      \"pmids\": [\"11248065\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Cripto-1 binds directly to ALK4 on mammalian epithelial cell surfaces (shown by FACS and co-immunoprecipitation), and Cripto-1 can phosphorylate Smad2 only in the presence of both Nodal and ALK4, establishing Cripto-1 as a co-receptor that bridges Nodal to ALK4 for Smad signaling.\",\n      \"method\": \"Phage display library screening, co-immunoprecipitation, FACS binding, Smad2 phosphorylation assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus functional Smad assay, multiple orthogonal methods\",\n      \"pmids\": [\"11909953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"SB-505124 selectively inhibits ALK4, ALK5, and ALK7 kinase activity, blocking downstream Smad2/3 phosphorylation and TGF-β-induced MAPK activation without affecting ALK1, ALK2, ALK3, or ALK6 signaling; it is 3-5x more potent than SB-431542.\",\n      \"method\": \"In vitro kinase assays, Smad2/3 reporter assays, cell-based signaling pathway analysis\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assays with selectivity panel, functional cell assays\",\n      \"pmids\": [\"14978253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"An activin-A mutant (M108A) retains wild-type affinity for ActRII but cannot form a cross-linked complex with ALK4, demonstrating that the fingertip/wrist region of activin contacts ALK4 directly and identifying M108A as a type II receptor antagonist that blocks activin and myostatin but not TGF-β signaling.\",\n      \"method\": \"Site-directed mutagenesis, cross-linking assays with 125I-activin-A, cell-based signaling assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution + mutagenesis + cross-linking binding assay\",\n      \"pmids\": [\"15123686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Five hydrophobic residues on the ALK4 extracellular domain (Leu40, Ile70, Val73, Leu75, Pro77) constitute the activin-binding surface; mutation of these residues to alanine substantially reduces ALK4-trunc dominant-negative activity and activin-A cross-linking in the presence of ActRII.\",\n      \"method\": \"Extracellular domain mutagenesis, dominant-negative activity assay, 125I-activin cross-linking\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with biochemical binding validation and functional assay\",\n      \"pmids\": [\"12665502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Truncated ALK4 isoforms (Alk4-2, 4-3, 4-4) expressed in human pituitary tumors function as dominant-negative receptors: they form complexes with type II activin receptors (shown by co-immunoprecipitation) but are not phosphorylated, blocking activin-induced transcription and the antiproliferative effect of activin.\",\n      \"method\": \"Co-immunoprecipitation, reporter assay, stable transfection with growth inhibition assay\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — co-IP plus multiple functional readouts (transcription, proliferation, cell cycle)\",\n      \"pmids\": [\"11117535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Constitutively active ALK4 (ALK4*) induces dose-dependent dorso-anterior mesodermal and endodermal markers in Xenopus animal caps, including a secondary axis when injected ventrally, while constitutively active ALK2 (ALK2*) induces only ventral mesodermal markers and antagonizes ALK4* signaling, revealing distinct and interfering signaling pathways for the two activin type I receptors.\",\n      \"method\": \"Constitutively active and dominant-negative receptor injection in Xenopus embryos, animal cap assay, gene expression analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with constitutively active/dominant-negative receptors in vertebrate embryos, multiple dose-response readouts\",\n      \"pmids\": [\"9367435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The loop between kinase subdomains IV and V (7 amino acids) in ALK4 mediates its strong dorsal gene-inducing specificity; transferring this loop from ALK4* to ALK2* enables the chimeric receptor to induce dorsal-specific gene expression in Xenopus.\",\n      \"method\": \"Chimeric receptor construction, Xenopus animal cap and embryo assay, gene expression analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — chimeric receptor domain-swap experiment, single lab\",\n      \"pmids\": [\"10075688\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"miR-24 targets the 3'-UTR of ALK4 (ACVR1B) mRNA to reduce ALK4 protein levels, thereby interfering with activin-induced Smad2 phosphorylation and inhibiting erythroid differentiation in K562 cells and CD34+ hematopoietic progenitors.\",\n      \"method\": \"Reporter assay (3'-UTR luciferase), Western blot, ectopic miR-24 expression, erythroid colony assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay validating direct miRNA targeting plus functional downstream signaling and differentiation assays\",\n      \"pmids\": [\"17906079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"miR-210 directly targets the AcvR1b (ALK4) 3'-UTR (validated by reporter assay), and its overexpression promotes BMP-4-induced osteoblastic differentiation of ST2 stromal cells by suppressing TGF-β/activin signaling through ALK4 inhibition.\",\n      \"method\": \"Reporter assay, sense/antisense miR-210 transfection, SB431542 pharmacological inhibition, osteoblastic differentiation assay\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct reporter assay plus functional differentiation phenotype, single lab\",\n      \"pmids\": [\"19520079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"ALK4 mediates activin A and B signaling to stimulate FSHβ transcription in gonadotrope cells via Smad3 phosphorylation; ALK7 selectively potentiates activin B (but not activin A) effects on Fshb promoter activity, and both constitutively active ALK4 and ALK7 stimulate Smad2/3 phosphorylation.\",\n      \"method\": \"RT-PCR, transfection of wild-type, kinase-dead, and constitutively active receptor constructs, reporter assay, Western blot, shRNA depletion\",\n      \"journal\": \"Reproductive biology and endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple receptor constructs with functional readouts, single lab\",\n      \"pmids\": [\"17040568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"ALK4 co-immunoprecipitates with multiple TGF-β ligands (AVg Activin-Vg1 chimera, Xnr1, derriere, and endogenous Vg1) in Xenopus embryo homogenates, and constitutively active ALK4 expressed on the right side causes left-right organ reversals; dominant-negative ALK4 on the left side also disrupts organ situs, establishing a left-side requirement for ALK4 signaling in LR axis determination.\",\n      \"method\": \"Co-immunoprecipitation from embryo homogenates, dominant-negative and constitutively active receptor microinjection in Xenopus, Pitx2 expression analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP and genetic loss/gain-of-function in vertebrate embryo, single lab\",\n      \"pmids\": [\"15063168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Conditional epithelial-specific deletion of Acvr1b (K14-Cre) in mice causes disruption of hair follicle cycling and severe progressive hair loss, and persistent proliferation of skin epithelial cells, establishing a cell-autonomous role for ACVR1B in hair follicle development and cycling.\",\n      \"method\": \"Conditional knockout mouse (Cre-lox), histological analysis, postnatal phenotype characterization\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with specific cellular phenotype, genetic demonstration of requirement\",\n      \"pmids\": [\"21191412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Activin A induces human trophoblast cell invasion through ALK4-SMAD2/3-SMAD4 signaling, which upregulates SNAIL and subsequently MMP2; siRNA knockdown of ALK4, SMAD2/3, SMAD4, or SNAIL each abolished activin A-induced MMP2 expression and invasion.\",\n      \"method\": \"siRNA knockdown, Western blot, Matrigel-coated transwell invasion assay, pharmacological inhibition (SB431542)\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple siRNA targets with consistent functional readout, replicated in primary and immortalized cells\",\n      \"pmids\": [\"26305619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Pharmacological inhibition of ALK4 and ALK7 (with SB431542) in fetal mouse gonads reveals that Activin/NODAL receptors ALK4 and ALK7 are required for promoting differentiation of male germ cells and their entry into mitotic arrest, while ALK4/5/7 signaling is required for testis cord formation during sex determination.\",\n      \"method\": \"FACS-purified gonadal cells, SB431542 pharmacological inhibition ex vivo, histological and gene expression analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological inhibition with specific cellular phenotype, single lab\",\n      \"pmids\": [\"23342175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The Cripto CFC domain interacts directly with ALK4 extracellular domain (Kd in μM range by SPR); NMR structural analysis shows residues H120 and W124 are externally exposed and molecular docking identifies the protein-protein interface consistent with prior mutagenesis, with no binding detected to ActRIIB.\",\n      \"method\": \"NMR structural analysis, surface plasmon resonance binding, molecular docking, synthetic CFC domain peptide\",\n      \"journal\": \"Journal of peptide science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — NMR + SPR binding assay, single lab\",\n      \"pmids\": [\"19035567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Activin A induces neuronal differentiation and survival in SK-N-SH neuroblastoma cells through ALK4 via a SMAD-independent mechanism: ALK4 activation does not induce SMAD2/3 phosphorylation, SMAD2/3-SMAD4 interaction, nuclear SMAD2/3 accumulation, or SMAD2/3 promoter binding, yet activates TGF-β target genes and promotes neurite outgrowth.\",\n      \"method\": \"siRNA knockdown, Western blot, co-immunoprecipitation, ChIP, neurite outgrowth assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods establishing SMAD-independent signaling, single lab\",\n      \"pmids\": [\"20226172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Activin A/ALK4 signaling activates Smad2/3 in atrial fibroblasts to drive atrial fibrosis; ALK4-haplodeficient mice show reduced atrial fibroblast activation, reduced atrial fibrosis, and reduced atrial fibrillation vulnerability after angiotensin II stimulation.\",\n      \"method\": \"ALK4 haplodeficient knockout mice, Ang-II stimulation model, electrophysiological studies, histology, Western blot\",\n      \"journal\": \"Basic research in cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with defined fibrotic and electrophysiological phenotype, single lab\",\n      \"pmids\": [\"28639003\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Activin A increases integrin β1 expression in human trophoblasts through ALK4-SMAD2/3-SMAD4 signaling; knockdown of ALK4 or SMAD4 abolishes activin A-induced integrin β1 upregulation, and integrin β1 knockdown reduces activin A-stimulated invasion.\",\n      \"method\": \"siRNA knockdown, Western blot, Matrigel invasion assay, SB431542 pharmacological inhibition\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple siRNA targets with functional invasion readout, consistent across immortalized and primary cells\",\n      \"pmids\": [\"33230889\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ALK4 (ACVR1B) cooperates with dopamine D1 receptor signaling to regulate FosB alternative splicing in nucleus accumbens medium spiny neurons: concurrent D1 and ALK4 activation induces PCBP1 interaction with Smad3, nuclear translocation, and binding to FosB exon-4/intron-4 sequences to generate ΔFosB mRNA; ALK4 ablation in MSNs impairs ΔFosB induction and cocaine behavioral sensitization.\",\n      \"method\": \"Co-immunoprecipitation, nuclear translocation assay, conditional ALK4 KO in MSNs, behavioral sensitization assay, RNA binding protein analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — co-IP, conditional KO, behavioral phenotype, multiple orthogonal methods, novel mechanism\",\n      \"pmids\": [\"35730718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Simultaneous muscle-specific knockout of Tgfbr1 and Acvr1b in mice induces substantial myofibre hypertrophy via increased Akt and p70S6K phosphorylation and reduced E3 ligase expression; combined knockout also increases satellite cell numbers and improves regeneration after cardiotoxin injury, while single receptor knockouts do not produce these effects.\",\n      \"method\": \"Double conditional knockout mouse, histology, Western blot (Akt/p70S6K phosphorylation), cardiotoxin injury model, gene expression analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via double KO demonstrating synergy, multiple readouts including signaling pathway analysis\",\n      \"pmids\": [\"35323108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ActRIIB:ALK4-Fc, a heterodimeric fusion of ALK4 and ActRIIB extracellular domains, has a distinct ligand-binding profile compared to ActRIIB-Fc homodimer: it sequesters ActRIIB ligands that inhibit muscle growth but does not trap BMP9 (shown by SPR), improving muscle mass, function, and neuromuscular junction abnormalities in murine DMD, ALS, and disuse atrophy models.\",\n      \"method\": \"Surface plasmon resonance, retinal explant vascular assay, murine disease models, muscle mass/function measurements\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — SPR ligand profiling + multiple in vivo models with functional readouts\",\n      \"pmids\": [\"33586684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ALK4 signaling in oligodendrocytes mediates Activin A-induced white matter remyelination after ischemic stroke; AAV-based ACVR1B shRNA (with Olig2 promoter) reversed rmActivin A-induced increases in mature oligodendrocyte number, myelin protein levels, and neurological function recovery in MCAO/R mice.\",\n      \"method\": \"AAV-shRNA oligodendrocyte-specific knockdown, MCAO/R mouse model, immunohistochemistry, Western blot, neurological scoring\",\n      \"journal\": \"Experimental neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific genetic knockdown with functional neurological and histological readouts, single lab\",\n      \"pmids\": [\"33345977\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ALK4 (ACVR1B) signaling promotes cadmium-induced death of renal proximal tubular cells via Smad3 phosphorylation and Akt pathway activation; siRNA knockdown of ALK4 or Smad3, or Smad3 inhibitor SIS3, attenuated cadmium-induced cell death.\",\n      \"method\": \"siRNA knockdown, pharmacological inhibition (SB431542, SB505124, SIS3), Western blot, cell viability assay\",\n      \"journal\": \"Cell death and differentiation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic and pharmacological interventions, single lab\",\n      \"pmids\": [\"30804470\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ALK4 mediates activin A-stimulated aromatase (P450arom) expression and estradiol secretion in endometrial stromal cells via the ALK4-Smad pathway; pretreatment with ALK4 inhibitor or Smad4 siRNA partially abrogated these effects.\",\n      \"method\": \"Pharmacological inhibition (SB431542), Smad4 siRNA, RT-PCR, ELISA\",\n      \"journal\": \"BioMed research international\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — pharmacological inhibition and single siRNA target, single lab, partial abrogation\",\n      \"pmids\": [\"27833918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"ALK4-SMAD2/3-SMAD4 signaling mediates activin A-induced suppression of PTX3 in human granulosa-lutein cells; siRNA knockdown of ALK4, SMAD2, SMAD3, or SMAD4 each reversed activin A-induced PTX3 suppression, and ChIP analysis showed phosphorylated SMADs bind to the PTX3 promoter.\",\n      \"method\": \"siRNA knockdown, Western blot, RT-qPCR, ChIP assay, pharmacological inhibition (SB431542)\",\n      \"journal\": \"Molecular and cellular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple siRNA targets with functional readout plus ChIP mechanistic evidence, consistent with pathway model\",\n      \"pmids\": [\"31185247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ALK4-SMAD3-SMAD4 (but not SMAD2) mediates activin A-induced reduction in betaglycan shedding in endometriotic cells; this pathway is distinct from SMAD2 and involves ALK4-dependent suppression of soluble betaglycan release, as shown by ALK4 siRNA and the SMAD3 inhibitor SIS3.\",\n      \"method\": \"siRNA knockdown, SMAD3 inhibitor (SIS3), ELISA, RT-qPCR, Western blot\",\n      \"journal\": \"Biomolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple molecular interventions establishing SMAD3 specificity over SMAD2, single lab\",\n      \"pmids\": [\"36551177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"ALK4 is preferentially expressed in adipocyte precursors where it suppresses differentiation by repressing CEBPα and PPARγ expression, promoting proliferation and adipose tissue expansion; ALK4 deletion induces premature adipocyte differentiation rescued by CEBPα knockdown.\",\n      \"method\": \"Conditional knockout, CEBPα/PPARγ expression analysis, CEBPα knockdown rescue, adipocyte differentiation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with mechanistic rescue, single lab\",\n      \"pmids\": [\"36403856\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ALK4 signaling in cortical somatostatin interneurons regulates their specification: activin-mediated ALK4 signaling induces Smad2 interaction with transcription factor SATB1 and promotes SATB1 nuclear translocation and repositioning at the somatostatin gene promoter; mice lacking ALK4 in GABAergic MGE neurons show deficits in somatostatin interneuron subpopulations.\",\n      \"method\": \"Conditional knockout (GABAergic neuron-specific), co-immunoprecipitation (Smad2-SATB1), nuclear translocation assay, ChIP (SATB1 at somatostatin promoter), immunohistochemistry\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO plus co-IP and ChIP demonstrating molecular mechanism, multiple orthogonal methods\",\n      \"pmids\": [\"31676717\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Activin A promotes integrin β3 expression in human trophoblasts via ALK4-SMAD4 signaling; knockdown of ALK4 or SMAD4 abolished activin A-induced integrin β3 upregulation, and integrin β3 knockdown reduced activin A-promoted invasion.\",\n      \"method\": \"siRNA knockdown, Western blot, RT-PCR, Matrigel invasion assay, SB431542 inhibition\",\n      \"journal\": \"Placenta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — consistent across cell lines with multiple siRNA targets, single lab\",\n      \"pmids\": [\"36244196\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cripto-1 acts as a molecular bridge connecting Nodal (bound to type II receptor ActRIIB) to type I receptor ALK4, forming a unique ternary complex for SMAD2/3 activation; AlphaFold3 modeling, SPR-based interaction analysis, domain-specific antibodies, and functional studies in NTERA-2 cells confirmed that Cripto-1 uses distinct structural domains to bind Nodal and ALK4 separately, contrasting with canonical TGF-β signaling.\",\n      \"method\": \"AlphaFold3 structural modeling, surface plasmon resonance, domain-specific anti-Cripto-1 antibodies, functional SMAD2/3 activation assay\",\n      \"journal\": \"Protein science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural modeling validated by SPR and functional assays, multiple orthogonal methods\",\n      \"pmids\": [\"39840816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ALK4 loss enhances canonical TGF-β signaling by upregulating MGAT5 and galectin-3, which stabilize TGF-β receptors at the cell surface through β1,6 N-acetylglucosaminyltransferase V-mediated N-linked glycosylation; depleting MGAT5 or inhibiting N-glycosylation suppresses ALK4-loss-induced TGF-β signaling and cancer progression in breast and pancreatic cancer models.\",\n      \"method\": \"In vitro migration/invasion assays, in vivo cancer models, glycosylation analysis, MGAT5/galectin-3 depletion, receptor surface stability assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple in vitro and in vivo models with mechanistic rescue via MGAT5 depletion and glycosylation inhibition\",\n      \"pmids\": [\"41408046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A missense variant p.M345I in ACVR1B reduces transactivation of NODAL-responsive reporters (AR3-Luc, CAGA12-Luc, SBE4-Luc) and decreases SMAD2/3 phosphorylation, indicating the mutation impairs ACVR1B kinase signaling function and is associated with congenital heart disease.\",\n      \"method\": \"Reporter assay, Western blot (SMAD2/3 phosphorylation), 3D protein modeling\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single variant functional assay, single lab, no rescue experiment\",\n      \"pmids\": [\"34666056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Loss of Acvr1b in the presence of oncogenic Kras accelerates formation of pancreatic intraepithelial neoplasia and IPMN-like precancerous lesions from both acinar and ductal cells, establishing ACVR1B as a tumor suppressor in pancreatic exocrine cells; the effect is context-dependent on cell of origin.\",\n      \"method\": \"Cell-type-specific conditional knockout (Ptf1aCreER or Sox9CreER combined with Acvr1b flox/flox and KrasLSL-G12D), MRI, immunohistochemistry, histopathology\",\n      \"journal\": \"Cellular and molecular gastroenterology and hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in two cell-type-specific mouse models with defined histopathological outcomes\",\n      \"pmids\": [\"39111635\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In a mouse model of SLE-associated pulmonary hypertension, Activin A signals via ALK4 in Th17 cells to induce IL-17 secretion, and ALK4 activation in pulmonary microvascular endothelial cells induces endothelial-mesenchymal transition via CTGF upregulation dependent on synergistic pSmad2 and pSTAT3 increases; ALK4 overexpression in Th17 cells worsens SLE-PH in mice while CD4+ T cell depletion alleviates it.\",\n      \"method\": \"CyTOF, Th17-PMEC co-culture, ALK4 overexpression, mouse SLE-PH model, CD4+ T cell depletion, ALK4 inhibitor (vactosertib/TEW-7197), Western blot\",\n      \"journal\": \"Arthritis & rheumatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-culture system, genetic overexpression, pharmacological inhibition, in vivo model, single lab\",\n      \"pmids\": [\"40395196\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACVR1B (ALK4) is a type I serine/threonine kinase receptor of the TGF-β superfamily that forms heteromeric complexes with type II receptors (ActRII/ActRIIB) to transduce signals from activins, Nodal, and related ligands primarily via SMAD2/3-SMAD4 phosphorylation; its extracellular domain directly binds activin at a defined hydrophobic interface (Leu40, Ile70, Val73, Leu75, Pro77), with Cripto-1 acting as a co-receptor that bridges Nodal to ALK4; ALK4 is required for gastrulation, mesoderm induction, hair follicle cycling, somatostatin interneuron specification, and male germ cell differentiation, and functions as a tumor suppressor in the pancreas—where truncated dominant-negative isoforms block its antiproliferative signaling—while also regulating muscle atrophy/hypertrophy, adipocyte differentiation, erythropoiesis, trophoblast invasion (via SNAIL-MMP2 and integrin pathways), and neuronal ΔFosB alternative splicing through convergent dopamine/ALK4-PCBP1-Smad3 signaling.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ACVR1B (ALK4) is a type I serine/threonine kinase receptor of the TGF-β superfamily that transduces activin, Nodal, and related ligand signals primarily through SMAD2/3-SMAD4 phosphorylation, governing embryonic patterning, tissue homeostasis, and cell fate decisions across diverse lineages. ALK4 forms heteromeric complexes with type II receptors (ActRII/ActRIIB) and engages ligands through a hydrophobic interface on its extracellular domain (Leu40, Ile70, Val73, Leu75, Pro77), while the co-receptor Cripto-1 bridges Nodal to ALK4 via its CFC domain to enable non-canonical ligand recruitment [PMID:12665502, PMID:39840816, PMID:11909953]. ALK4 is essential for gastrulation, left-right axis determination, hair follicle cycling, somatostatin interneuron specification, adipocyte precursor maintenance, and muscle mass regulation, and it functions as a tumor suppressor in the pancreas where its loss cooperates with oncogenic KRAS to accelerate neoplasia [PMID:9512518, PMID:21191412, PMID:31676717, PMID:36403856, PMID:35323108, PMID:39111635]. Beyond canonical SMAD signaling, ALK4 operates through SMAD-independent pathways in neuronal differentiation and cooperates with dopamine D1 receptor signaling to regulate ΔFosB alternative splicing via PCBP1-Smad3 nuclear translocation in striatal neurons [PMID:20226172, PMID:35730718].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Constitutively active ALK4 was shown to induce dorso-anterior mesoderm and endoderm in Xenopus, establishing that ALK4 signals in a pathway distinct from ALK2 and mediates activin-like dorsal patterning.\",\n      \"evidence\": \"Constitutively active/dominant-negative receptor injection in Xenopus animal caps with dose-response gene expression readouts\",\n      \"pmids\": [\"9367435\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous ligand identity not resolved\", \"Downstream transcriptional targets not identified\", \"Mechanism of ALK2 antagonism unclear\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Genetic knockout demonstrated that ALK4 is indispensable for gastrulation: null embryos arrest before primitive streak formation, and chimera analysis established that ALK4 functions cell-autonomously in both epiblast and extraembryonic ectoderm.\",\n      \"evidence\": \"Gene targeting (knockout) and chimera analysis in mice with embryo histology\",\n      \"pmids\": [\"9512518\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific ligand(s) acting through ALK4 in vivo not identified\", \"Downstream signaling pathway in gastrulation not characterized\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Domain-swap experiments identified a 7-amino-acid loop between kinase subdomains IV and V as the structural determinant of ALK4's dorsal mesoderm-inducing specificity, distinguishing it from ALK2.\",\n      \"evidence\": \"Chimeric ALK4/ALK2 receptor construction with Xenopus animal cap assays\",\n      \"pmids\": [\"10075688\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Crystal structure of kinase domain not available\", \"Specific SMAD or non-SMAD effectors engaged by this loop not determined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Truncated ALK4 isoforms found in pituitary tumors were shown to act as dominant-negative receptors that complex with type II receptors but cannot be phosphorylated, blocking activin's antiproliferative effect — the first evidence for ALK4 loss-of-function in human neoplasia.\",\n      \"evidence\": \"Co-immunoprecipitation, reporter assay, and growth inhibition assay in stably transfected cells\",\n      \"pmids\": [\"11117535\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Prevalence of truncated isoforms across tumor types unknown\", \"In vivo tumor-promoting role not demonstrated\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Two concurrent advances defined ALK4 as the obligate type I receptor for Nodal signaling and identified somatic ACVR1B mutations in pancreatic cancer, establishing its dual role in developmental signaling and tumor suppression.\",\n      \"evidence\": \"Receptor reconstitution and dominant-negative assays in Xenopus embryos with co-IP (Nodal); direct sequencing of pancreatic cancer specimens (mutations)\",\n      \"pmids\": [\"11485994\", \"11248065\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cripto-ALK4 binding interface not structurally resolved\", \"Functional consequence of specific pancreatic cancer mutations not tested\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"SB-431542 was characterized as a selective inhibitor of ALK4/ALK5/ALK7 kinase activity, providing a pharmacological tool that discriminates activin/TGF-β signaling from BMP and MAPK pathways, and Cripto-1 was shown to directly bind ALK4 on mammalian cell surfaces to enable Nodal-dependent SMAD2 phosphorylation.\",\n      \"evidence\": \"In vitro kinase inhibition assays with selectivity panel; FACS binding, co-IP, and SMAD2 phosphorylation assay for Cripto-ALK4 interaction\",\n      \"pmids\": [\"12065756\", \"11909953\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Co-crystal structure of inhibitor-ALK4 not available\", \"Cripto binding site on ALK4 not mapped at residue level\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Mutagenesis of the ALK4 extracellular domain identified five hydrophobic residues (Leu40, Ile70, Val73, Leu75, Pro77) as the activin-binding surface, providing the first residue-level map of ligand-receptor contact.\",\n      \"evidence\": \"Extracellular domain mutagenesis with 125I-activin cross-linking and dominant-negative activity assay\",\n      \"pmids\": [\"12665502\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Crystal structure of the ternary activin-ActRII-ALK4 complex not solved\", \"Whether these residues also contact Nodal or other ligands not tested\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Activin's fingertip/wrist region was shown to contact ALK4 directly via an M108A mutant that retains type II receptor binding but loses ALK4 recruitment, and ALK4 was demonstrated to participate in left-right axis determination in Xenopus through signaling downstream of multiple Nodal-related ligands.\",\n      \"evidence\": \"Site-directed mutagenesis with cross-linking (activin-ALK4); co-IP from embryo homogenates with dominant-negative/constitutively active receptor injection for L-R axis\",\n      \"pmids\": [\"15123686\", \"15063168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Left-right phenotype not validated in mammalian models\", \"Structural basis of M108A disruption not determined at atomic level\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"miR-24 was identified as a post-transcriptional regulator of ALK4, demonstrating that ALK4 protein levels and consequently activin-SMAD2 signaling in erythroid differentiation are subject to microRNA control.\",\n      \"evidence\": \"3'-UTR luciferase reporter assay, ectopic miR-24 expression, erythroid colony assay in K562 and CD34+ cells\",\n      \"pmids\": [\"17906079\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance during normal hematopoiesis not established\", \"Whether other miRNAs cooperate in ALK4 regulation not explored\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"NMR and SPR analyses structurally characterized the Cripto CFC domain–ALK4 interaction, identifying exposed residues H120 and W124 on Cripto as key contact points, confirming the direct and specific nature of the co-receptor–receptor interaction.\",\n      \"evidence\": \"NMR structure of CFC domain, SPR binding kinetics, molecular docking\",\n      \"pmids\": [\"19035567\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Co-crystal structure of CFC-ALK4 complex not obtained\", \"Affinity measured in µM range — physiological relevance of weak interaction debated\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Conditional epithelial-specific deletion of Acvr1b revealed a cell-autonomous requirement for ALK4 in hair follicle cycling; separately, ALK4 was found to promote neuronal differentiation via a SMAD-independent mechanism in neuroblastoma cells.\",\n      \"evidence\": \"K14-Cre conditional KO in mice with histological phenotyping (hair follicle); siRNA knockdown, ChIP, and neurite outgrowth assay in SK-N-SH cells (SMAD-independent)\",\n      \"pmids\": [\"21191412\", \"20226172\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of SMAD-independent downstream effectors not determined\", \"Hair follicle phenotype mechanism (which SMAD, which target genes) not resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The full activin A→ALK4→SMAD2/3→SMAD4→SNAIL→MMP2 signaling cascade was delineated in trophoblast invasion, providing a complete pathway from receptor to effector protease.\",\n      \"evidence\": \"Sequential siRNA knockdown of ALK4, SMAD2/3, SMAD4, SNAIL each abolished activin-induced MMP2 and invasion in Matrigel assay\",\n      \"pmids\": [\"26305619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this cascade operates in vivo during implantation not tested\", \"Contribution of other type I receptors (ALK7) not excluded\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Multiple studies expanded ALK4's tissue-specific roles: ALK4 mediates activin A-induced integrin β1 upregulation in trophoblasts, white matter remyelination via oligodendrocyte maturation after ischemic stroke, cadmium-induced renal cell death via Smad3/Akt, and somatostatin interneuron specification through Smad2-SATB1 interaction at the somatostatin promoter.\",\n      \"evidence\": \"siRNA/shRNA knockdown and conditional KO with invasion assays, MCAO/R mouse model, cell viability assays, and ChIP respectively\",\n      \"pmids\": [\"33230889\", \"33345977\", \"30804470\", \"31676717\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether oligodendrocyte remyelination role is ALK4-specific or shared with ALK5/ALK7 not resolved\", \"In vivo validation of cadmium toxicity pathway lacking\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Three distinct functional arenas for ALK4 were established: cooperation with D1 receptor signaling to regulate ΔFosB splicing in striatal neurons via PCBP1-Smad3; redundancy with TGF-βRI (ALK5) in suppressing muscle hypertrophy; and suppression of adipocyte differentiation via CEBPα/PPARγ repression in precursor cells.\",\n      \"evidence\": \"Conditional KO in MSNs with behavioral assays; double Tgfbr1/Acvr1b muscle-specific KO with Akt/p70S6K signaling; adipocyte precursor conditional KO with CEBPα rescue\",\n      \"pmids\": [\"35730718\", \"35323108\", \"36403856\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"PCBP1-Smad3 splicing mechanism not reconstituted in vitro\", \"Whether ALK4 and ALK5 engage identical SMAD complexes in muscle not determined\", \"Upstream activin ligand identity in adipose tissue not confirmed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Cell-type-specific conditional knockout combined with oncogenic Kras formally established ACVR1B as a pancreatic tumor suppressor: Acvr1b loss accelerated precancerous lesion formation from both acinar and ductal cells, resolving two decades of correlative evidence from sequencing studies.\",\n      \"evidence\": \"Ptf1aCreER and Sox9CreER conditional KO with KrasLSL-G12D, MRI, and histopathology in mice\",\n      \"pmids\": [\"39111635\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which ALK4 loss cooperates with KRAS not molecularly defined\", \"Whether SMAD2 or SMAD3 mediates the tumor-suppressive arm not resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Structural modeling and SPR confirmed that Cripto-1 uses distinct domains to simultaneously bind Nodal and ALK4, forming a ternary bridge complex unique among TGF-β family signaling modes; separately, ALK4 loss was found to enhance canonical TGF-β signaling by upregulating MGAT5/galectin-3-mediated glycosylation that stabilizes TGF-β receptors at the cell surface.\",\n      \"evidence\": \"AlphaFold3 modeling validated by SPR and functional SMAD assays (Cripto); MGAT5/galectin-3 depletion and glycosylation inhibition in breast/pancreatic cancer models (TGF-β crosstalk)\",\n      \"pmids\": [\"39840816\", \"41408046\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Experimental co-crystal structure of Cripto-Nodal-ALK4-ActRIIB quaternary complex not yet solved\", \"Whether MGAT5-mediated TGF-β receptor stabilization drives tumor progression in patient tumors not confirmed\", \"Therapeutic potential of targeting MGAT5 in ACVR1B-mutant cancers not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full atomic structure of the activin/Nodal–type II receptor–ALK4–Cripto signaling complex, the identity of SMAD-independent effectors in neurons, the mechanism by which ALK4 loss cooperates with oncogenic KRAS in pancreatic tumorigenesis, and whether ALK4's diverse tissue-specific roles use shared or distinct downstream transcriptional programs.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental co-crystal structure of the full signaling complex\", \"SMAD-independent neuronal effectors unidentified\", \"Transcriptional program specificity across tissues not systematically compared\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 5, 9, 10, 22, 28]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [8, 34]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [1, 4, 33]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 18, 34]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 4, 5, 9, 16, 22, 28, 33]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [2, 9, 14, 31]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 8, 36, 34]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [13, 28, 30]}\n    ],\n    \"complexes\": [\n      \"Activin-ActRII/ActRIIB-ALK4 receptor complex\",\n      \"Nodal-Cripto-ALK4-ActRIIB signaling complex\"\n    ],\n    \"partners\": [\n      \"ACVR2A\",\n      \"ACVR2B\",\n      \"TDGF1\",\n      \"SMAD2\",\n      \"SMAD3\",\n      \"SMAD4\",\n      \"TGFBR1\",\n      \"PCBP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}