ACVR2B

ACVR2B is a type II receptor of the TGF-β superfamily that serves as a shared, ligand-competitive hub transducing signals from activins, myostatin, and select BMPs to determine the balance of downstream SMAD activation (PMID:26047946, PMID:22911153). Its extracellular domain alone is sufficient to bind and neutralize myostatin, and soluble ACVR2B-Fc traps act as broad ligand antagonists (PMID:22911153). Activin A binds ACVR2B and, in combination with type I receptors such as ALK2, antagonizes BMP-6 and BMP-9 signaling, so that competitive ligand occupancy of ACVR2B sets the BMP-versus-activin signaling balance (PMID:26047946); this balance is itself tunable through exosomal miRNA control of receptor levels that shifts signaling toward BMPR2-elicited SMAD1/5/9 phosphorylation during osteoblast differentiation (PMID:33838528). ACVR2B forms stable homomeric complexes enhanced by Activin A and constitutively assembles ligand-independent heterocomplexes with the FOP-causing ALK2-R206H mutant, enabling its aberrant SMAD1/5/8 activation; ACVR2B receptor dosage dose-dependently restricts this hyperactive ACVR1-R206H signaling (PMID:38334613, PMID:41279820). In development, ACVR2B is the primary type II receptor transducing BMP signaling in the gastrula and is required for posterior neural crest patterning (PMID:41279820, PMID:15977175). Physiologically, ACVR2B acts redundantly with ACVR2A to drive activin-dependent FSH production in gonadotropes, with double loss causing FSH deficiency, hypogonadism and sterility (PMID:32270195), and it mediates pathogenic signaling in skeletal and joint tissue, transducing BMP3-dependent suppression of osteoblast differentiation (PMID:22074949) and activin A-driven, NOX4-amplified catabolic SMAD2/3 signaling in osteoarthritic cartilage (PMID:36950748).

1. 2005 Medium

   Established a developmental role for ACVR2B distinct from its paralog by showing it patterns a specific embryonic territory.

   Evidence Morpholino depletion of acvr2b in zebrafish with phenotyping of neural crest-derived structures

   PMID:15977175

   Open questions at the time

   • Did not identify the ligand driving posterior neural crest patterning
   • Morpholino specificity not corroborated by genetic mutant
   • Downstream SMAD branch not resolved
2. 2011 Medium

   Identified ACVR2B as the receptor through which BMP3 inhibits skeletal progenitor commitment, assigning a defined ligand-receptor function in bone.

   Evidence siRNA knockdown and BMP3 overexpression in primary bone marrow stromal cells with colony-forming assays

   PMID:22074949

   Open questions at the time

   • Type I receptor partner not defined
   • SMAD branch mediating suppression not identified
   • In vivo relevance not tested
3. 2012 Medium

   Demonstrated that the ACVR2B extracellular domain alone is sufficient to bind and neutralize myostatin, defining the structural basis for ligand-trap antagonism.

   Evidence Yeast-expressed, N-glycosylated Acvr2b-ECD tested in CAGA-luciferase MSTN inhibition assay

   PMID:22911153

   Open questions at the time

   • Single in vitro reconstitution assay
   • Binding affinity and stoichiometry not quantified
   • Used fish ortholog ECD
4. 2015 Medium

   Showed ACVR2B is a competitive shared receptor for both activin and select BMPs, establishing that ligand occupancy sets the signaling balance.

   Evidence Cell-based ligand competition assays in myeloma lines with characterized receptor expression

   PMID:26047946

   Open questions at the time

   • Competition shown functionally, not by direct binding measurement
   • Did not establish in vivo balance
   • Type I partner contribution not fully dissected
5. 2018 Low

   Linked chromatin state at the ACVR2B promoter to receptor transcription and downstream myogenic gene regulation.

   Evidence ChIP for H3K56ac at ACVR2B promoter and Smad3 binding at Myf5/MyoD in spermidine-treated satellite cells

   PMID:29224337

   Open questions at the time

   • Single ChIP assay with no functional link to phenotype established
   • Causality between promoter acetylation and ACVR2B output not demonstrated
   • Effect on receptor protein level not measured
6. 2019 Medium

   Defined a therapeutic context: ligand blockade of ACVR2B dampens SMAD2 signaling and protects the heart from ischemic injury.

   Evidence ACVR2B-Fc treatment in mouse cardiac ischemia-reperfusion model with cardiomyocyte hypoxia assay and pSMAD2 readout

   PMID:30765322

   Open questions at the time

   • Specific ligand(s) blocked not pinpointed
   • On-target receptor contribution vs other ACVR2B ligands not separated
   • Single lab
7. 2020 High

   Resolved the physiological requirement for ACVR2B in reproductive endocrinology and its redundancy with ACVR2A.

   Evidence Gonadotrope-specific conditional single and double knockout of Acvr2a/Acvr2b in mice with FSH and fertility phenotyping

   PMID:32270195

   Open questions at the time

   • Type I receptor partner in gonadotropes not defined
   • Mechanism of partial vs complete deficiency not molecularly dissected
8. 2021 Medium

   Showed ACVR2B receptor levels are tunable by exosomal miRNAs that shift the Bmpr2/Acvr2b competitive balance toward osteogenic SMAD1/5/9 output.

   Evidence miRNA microarray, siRNA and miRNA transfection with Smad1/5/9 phosphorylation readout in vitro

   PMID:33838528

   Open questions at the time

   • Direct miRNA-ACVR2B target binding not validated
   • In vivo relevance not tested
   • Quantitative shift in receptor balance not measured
9. 2023 Medium

   Identified a pathogenic signaling amplifier on ACVR2B in joint disease, where NOX4 binds the receptor complex to boost catabolic SMAD2/3 signaling.

   Evidence Transgenic/knockout mouse OA models, shRNA knockdown of ACVR2B, and protein-protein interaction studies

   PMID:36950748

   Open questions at the time

   • Structural basis of NOX4 binding to the C-terminal site not resolved
   • Whether amplification is ACVR2B-specific vs shared with ACVR2A unclear
   • Single lab
10. 2024 High

    Provided the biophysical basis for ACVR2B's role in FOP by showing it homodimerizes and forms ligand-independent complexes with ALK2-R206H to drive constitutive SMAD1/5/8 signaling.

    Evidence FRAP-based receptor interaction quantification with R206H mutagenesis and pSMAD1/5/8 / BRE-Luc readouts

    PMID:38334613

    Open questions at the time

    • Structural detail of the homomeric interface not resolved
    • Why ACVR2B but not ACVR2A homodimerizes ligand-independently not explained mechanistically
11. 2024 Medium

    Demonstrated translational utility of ACVR2B-Fc traps delivered by engineered cells to suppress aberrant BMP/Activin signaling and heterotopic ossification in FOP.

    Evidence iPSC-derived MSC-secreted ACVR2B-Fc in patient iMSC signaling assays and transplantation into ACVR1-R206H FOP mice

    PMID:38500216

    Open questions at the time

    • Durability and dosing not established
    • Specific ligand contributions to ossification not separated
    • Single lab
12. 2025 Medium

    Established ACVR2B as the dominant type II receptor for BMP signaling in early vertebrate patterning and as a dose-dependent restrictor of FOP mutant signaling.

    Evidence Complete maternal-zygotic genetic depletion of all acvr2a/acvr2b genes in zebrafish with BMP readouts and ACVR1-R206H epistasis (preprint)

    PMID:41279820

    Open questions at the time

    • Preprint, not yet peer-reviewed
    • Mammalian early-embryo dependence not confirmed
    • Mechanism of dosage restriction of R206H not molecularly defined

• How ACVR2B's homodimerization interface and ligand-selective complex assembly are structurally encoded, and how these determine the activin-versus-BMP signaling balance across tissues, remain unresolved.
• No atomic structure of ACVR2B homomeric or heteromeric complexes in the corpus
• Tissue-specific type I partner choice not systematically mapped
• Direct kinase substrate-level mechanism not characterized in the timeline