{"gene":"ZNRF3","run_date":"2026-04-28T23:00:24","timeline":{"discoveries":[{"year":2012,"finding":"ZNRF3 is a cell-surface transmembrane E3 ubiquitin ligase that promotes the ubiquitination and turnover of Wnt receptors Frizzled (FZD) and LRP6, thereby inhibiting Wnt signaling. R-spondin inhibits ZNRF3 by directly binding its extracellular domain and inducing association between ZNRF3 and LGR4, resulting in membrane clearance of ZNRF3.","method":"Co-immunoprecipitation, cell-surface receptor turnover assays, in vivo loss-of-function (Xenopus), receptor ubiquitination assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1–2 — foundational paper with multiple orthogonal methods, replicated across many subsequent studies","pmids":["22575959"],"is_preprint":false},{"year":2013,"finding":"Crystal structures of the ZNRF3 ectodomain (ZNRF3ecto) alone and in complex with the Fu1-Fu2 fragment of Rspo2 reveal that a prominent loop in Rspo2 Fu1 clamps into a groove on the ZNRF3 ectodomain surface. Rspo binding enhances dimerization of ZNRF3ecto. Signaling potency of Rspo proteins depends on their ability to recruit ZNRF3 via Fu1 into a ternary complex with LGR receptors (which interact with Rspo via Fu2).","method":"X-ray crystallography, biophysical binding assays, cellular signaling assays, mutagenesis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — crystal structures with functional mutagenesis validation","pmids":["24225776"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of the ZNRF3 ectodomain and its complex with R-spondin 1 shows ZNRF3 binds RSPO1 and LGR5-RSPO1 with micromolar affinity via the RSPO1 furin-like 1 (Fu1) domain. The ZNRF3-binding site overlaps with trans-interactions in 2:2 LGR5-RSPO1 complexes, suggesting ZNRF3/RNF43 compete for binding.","method":"X-ray crystallography, surface plasmon resonance/binding affinity measurements","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with biophysical binding validation","pmids":["24349440"],"is_preprint":false},{"year":2013,"finding":"Interaction of R-spondin with both ZNRF3 and LGR4 through distinct motifs is required for R-spondin-induced LGR4/ZNRF3 complex formation, ZNRF3 membrane clearance, and Wnt signaling activation. A ZNRF3 mutant with reduced R-spondin affinity is resistant to R-spondin-mediated inhibition, supporting a dual-receptor model where LGR4/5 are engagement receptors and ZNRF3/RNF43 are effector receptors.","method":"Mutagenesis, Co-immunoprecipitation, cell-surface clearance assays, Wnt signaling reporter assays","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding and functional mutagenesis, strong mechanistic follow-up","pmids":["24165923"],"is_preprint":false},{"year":2013,"finding":"Recombinant RSPO:LGR4:ZNRF3 ternary complexes can be reconstituted in vitro with bacterially produced proteins. RSPOs weakly bind ZNRF3 ECD, and RSPO2/3 form detectable ternary complexes with LGR4 and ZNRF3. The LGR4 ECD and ZNRF3 ECD inhibit RSPO-enhanced Wnt signaling in cell-based assays.","method":"In vitro reconstitution, TR-FRET binding assay, native gel electrophoretic mobility shift assay, cell-based signaling assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution of ternary complex with multiple orthogonal binding assays","pmids":["24050775"],"is_preprint":false},{"year":2015,"finding":"Dishevelled (DVL) acts as a dual-function adaptor that recruits ZNRF3/RNF43 to Wnt receptors: DVL knockout cells show increased cell-surface FZD and LRP6; DVL is required for ZNRF3/RNF43-mediated ubiquitination and degradation of FZD; physical interaction with DVL (via the DEP domain binding FZD) is essential for Wnt-inhibitory activity of ZNRF3/RNF43; fusing the DEP domain to ZNRF3 overcomes DVL dependency.","method":"DVL knockout, Co-immunoprecipitation, ubiquitination assays, domain fusion experiments, cell-surface receptor assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — clean KO, reciprocal Co-IP, domain-swap rescue, multiple orthogonal methods","pmids":["25891077"],"is_preprint":false},{"year":2015,"finding":"Crystal structure of human LGR5 ectodomain complexed with mouse Rspo2 Fu1-Fu2, and a low-resolution ternary LGR5-Rspo2-ZNRF3 ectodomain complex structure, confirms that Rspo proteins cross-link LGR4/5 and ZNRF3 into a 2:2:2 complex (versus 1:1:1 with RNF43).","method":"X-ray crystallography, ternary complex structure determination","journal":"Journal of structural biology","confidence":"High","confidence_rationale":"Tier 1 — crystal structure of ternary complex","pmids":["26123262"],"is_preprint":false},{"year":2018,"finding":"SCFβ-TRCP E3 ubiquitin ligase directly interacts with and ubiquitinates ZNRF3, promoting its proteasomal degradation in a CKI-phosphorylation- and degron-dependent manner (analogous to β-catenin degradation by β-TRCP), thereby positively regulating Wnt signaling by removing the negative regulator ZNRF3.","method":"Co-immunoprecipitation, ubiquitination assay, proteasome inhibitor treatment, degron mutagenesis","journal":"Protein & cell","confidence":"Medium","confidence_rationale":"Tier 2 — direct interaction and ubiquitination shown, single lab","pmids":["29497989"],"is_preprint":false},{"year":2018,"finding":"ZNRF3 is required for mammalian sex determination: XY mice lacking ZNRF3 exhibit complete or partial gonadal sex reversal associated with ectopic WNT/β-catenin activity and reduced Sox9 expression during fetal sex determination. Human ZNRF3 missense variants disrupt ZNRF3 activity in cell lines and zebrafish assays.","method":"Conditional knockout mouse, Wnt reporter assays, zebrafish functional assay, exome sequencing of DSD patients","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined molecular phenotype, corroborated in zebrafish and human cell assays","pmids":["29735715"],"is_preprint":false},{"year":2019,"finding":"Adrenocortical-specific loss of ZNRF3 (but not RNF43) causes adrenal hyperplasia dependent on Porcupine-mediated Wnt ligand secretion, and disrupts a Wnt/β-catenin signaling gradient in the adrenal cortex. ZNRF3 loss triggers moderate-level β-catenin activation that drives proliferative expansion of the inner cortex. Genetically reducing β-catenin dosage significantly reverses this phenotype.","method":"Tissue-specific conditional knockout, Porcupine inhibitor, genetic epistasis (β-catenin dosage reduction), Wnt reporter assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with genetic epistasis rescue, multiple functional readouts","pmids":["30692207"],"is_preprint":false},{"year":2020,"finding":"The tumor suppressor PTPRK (protein tyrosine phosphatase receptor-type kappa) dephosphorylates a '4Y' endocytic tyrosine motif in ZNRF3, keeping it unphosphorylated and thereby promoting ZNRF3 internalization and Wnt receptor degradation. PTPRK deficiency in Xenopus increases Wnt signaling and causes head/axial defects.","method":"Xenopus loss-of-function, phosphorylation site mutagenesis (4Y motif), endocytosis assays, Wnt reporter assays, identification of endocytic signal","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — phosphorylation site mutagenesis, clean in vivo loss-of-function, mechanistic pathway placement","pmids":["31934854"],"is_preprint":false},{"year":2021,"finding":"MET proto-oncogene (receptor tyrosine kinase) binds to ZNRF3 and phosphorylates its '4Y' endocytic motif in response to HGF stimulation, thereby reducing ZNRF3 internalization and ZNRF3-dependent Wnt receptor degradation, and enhancing Wnt/β-catenin signaling. PTPRK dephosphorylates this same motif, establishing a MET-PTPRK rheostat controlling ZNRF3 activity.","method":"Co-immunoprecipitation, kinase assay (MET phosphorylation of 4Y motif), pharmacological MET inhibition, ZNRF3 internalization assay, Wnt reporter assay","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — biochemical identification of kinase, confirmed by pharmacological inhibition, functional ZNRF3 internalization readout","pmids":["34590584"],"is_preprint":false},{"year":2021,"finding":"The deubiquitinase USP42 binds to the Dishevelled-interacting region (DIR) of ZNRF3 and deubiquitinates ZNRF3, protecting it from R-spondin-LGR4-induced ubiquitin-dependent membrane clearance. USP42 thereby maintains ZNRF3 at the plasma membrane, promotes FZD and LRP6 turnover, and inhibits Wnt signaling.","method":"Co-immunoprecipitation, deubiquitination assay, cell-surface clearance assay, Wnt reporter assay, intestinal organoids","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 — direct biochemical interaction, deubiquitination demonstrated, functional consequence in organoids","pmids":["33786993"],"is_preprint":false},{"year":2021,"finding":"ZNRF3 deletion promotes hepatocyte proliferation; subsequent RNF43 upregulation limits this proliferation. Concomitant deletion of both ZNRF3 and RNF43 results in metabolic reprogramming of periportal hepatocytes, clonal expansion, and liver tumor formation, demonstrating cooperative roles of ZNRF3 and RNF43 in spatially and temporally restricting WNT/β-catenin activity in the liver.","method":"Liver-specific conditional knockout (ZNRF3 alone and ZNRF3/RNF43 double KO), scRNA-seq, chromatin accessibility, organoid studies","journal":"Cell stem cell","confidence":"High","confidence_rationale":"Tier 2 — clean in vivo conditional KO with multiple functional readouts, including chromatin and single-cell analyses","pmids":["34129813"],"is_preprint":false},{"year":2023,"finding":"LGR4 (but not LGR5) forms a complex with RNF43/ZNRF3 to provide high-affinity bivalent binding of R-spondin ligands. LGR4 and ZNRF3 form a 2:2 dimer accommodating bivalent RSPO binding, whereas LGR5 forms a homodimer incompatible with ZNRF3 co-complex formation.","method":"Whole-cell binding affinity assays (monovalent vs bivalent RSPO), co-expression of receptors, LGR4/LGR5 comparison","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — binding assays in cells, single lab with multiple conditions","pmids":["37402772"],"is_preprint":false},{"year":2024,"finding":"RNF43 and ZNRF3 display differential substrate specificity for Frizzled receptors: RNF43 preferentially down-regulates FZD1/FZD5/FZD7 whereas ZNRF3 preferentially targets FZD6. The transmembrane domain (TMD) of RNF43 is a key molecular determinant for FZD5 endocytosis; swapping the TMD between RNF43 and ZNRF3 redirects their FZD substrate preference.","method":"Endocytosis assays, TMD swap domain chimeras, knockdown/knockout, receptor surface level measurement","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 2 — domain swap experiments with functional readout, clean mechanistic determination of TMD role","pmids":["38969364"],"is_preprint":false},{"year":2024,"finding":"ZNRF3 and RNF43 interact with EGFR via their extracellular domains, leading to EGFR ubiquitination and lysosomal degradation mediated by the intracellular RING domain. Overexpression of ZNRF3 reduces EGFR levels; knockout of ZNRF3/RNF43 upregulates EGFR signaling and promotes tumorigenesis.","method":"Co-immunoprecipitation, ubiquitination assay, overexpression/knockout, in vitro and in vivo tumor growth assays","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 — biochemical demonstration of EGFR interaction and ubiquitination, single lab","pmids":["41960900"],"is_preprint":false},{"year":2023,"finding":"Disulfide-constrained peptides (DCPs) that bind the ZNRF3 ectodomain induce ZNRF3 ubiquitination and membrane clearance, leading to FZD stabilization and Wnt signaling activation. Multimeric DCPs induce expansive growth of human intestinal organoids in a valency-dependent manner.","method":"Peptide binding assay, cell-surface ZNRF3 clearance assay, FZD stabilization assay, Wnt reporter, intestinal organoid growth assay","journal":"Cell chemical biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct mechanistic study with functional validation in organoids, single lab","pmids":["38056465"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of human LGR4 alone, LGR4-RSPO2, and LGR4-RSPO2-ZNRF3 complexes show that LGR4, RSPO2, and ZNRF3 assemble into a 2:2:2 complex with the ZNRF3 dimer enclosed at the center. Upon RSPO2 binding, LGR4 undergoes no significant conformational changes. This forced ZNRF3 dimerization likely underlies how the complex sequesters ZNRF3 from Wnt receptors and facilitates ZNRF3 auto-inactivation.","method":"Cryo-electron microscopy structure determination","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure of ternary complex with mechanistic interpretation","pmids":["41034211"],"is_preprint":false},{"year":2025,"finding":"ZNRF3-induced FZD degradation depends on endogenous WNT stimulation rather than being constitutive; ZNRF3 selectively degrades WNT-engaged FZD. WNT enhances FZD-DVL association, and DVL subsequently recruits ZNRF3 to WNT-engaged FZD to promote its degradation. R-spondin enhances WNT signaling by prolonging the action of the WNT-engaged FZD complex rather than simply increasing total FZD abundance.","method":"Endogenous WNT-dependent assays, Co-immunoprecipitation, FZD degradation assays, DVL dependency experiments","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 2 — mechanistic revision with multiple orthogonal experiments, published in peer-reviewed journal","pmids":["41086253"],"is_preprint":false},{"year":2025,"finding":"Wnt induces selective endocytosis and degradation of FZD5/8 in a ZNRF3/RNF43-dependent manner; ZNRF3/RNF43 selectively target FZD5/8 upon Wnt stimulation; Wnt promotes the interaction between FZD5 and RNF43; DVL promotes ligand-independent FZD endocytosis but is dispensable for Wnt-induced FZD5/8 endocytosis and degradation.","method":"FZD endocytosis assays, ZNRF3/RNF43 knockout, Co-immunoprecipitation, DVL perturbation experiments","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — clean KO, Co-IP, multiple FZD specificity assays, peer-reviewed","pmids":["41070826"],"is_preprint":false},{"year":2024,"finding":"ZNRF3 germline missense variants in the RING ligase domain cause macrocephalic neurodevelopmental disorder via dominant-negative enhancement of Wnt/β-catenin signaling (compromising Wnt receptor turnover), while a missense variant in the RSPO-binding domain causes microcephalic NDD via attenuated Wnt/β-catenin signaling. These domain-specific effects were validated in transcriptional reporter assays.","method":"Structural modeling, in vitro Wnt/β-catenin transcriptional reporter assays with Wnt3a and RSPO, comparison of RING vs RSPO-binding domain variants","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — functional assays validating domain-specific loss/gain of function, but limited to reporter assays without full mechanistic reconstitution","pmids":["39168120"],"is_preprint":false},{"year":2024,"finding":"ZNRF3 exon 2 deletions produce a 42-amino acid deleted protein (ΔEx2-ZNRF3) that is impaired in RSPO1 binding and fails to support RSPO1-dependent activation of Wnt/β-catenin signaling, causing congenital adrenal hypoplasia.","method":"RT-PCR, 3D structural modeling, cell-based TCF-LEF reporter assay comparing ΔEx2-ZNRF3 vs wild-type","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — functional cell-based assay with structural modeling, clinical phenotype linked to specific domain loss","pmids":["37878959"],"is_preprint":false},{"year":2024,"finding":"Truncating ZNRF3 mutations at endogenous levels exhibit loss-of-function; missense mutations in RING and R-Spondin domains cause partial loss-of-function or hyperactivation but do NOT exhibit dominant-negative activity when heterozygously introduced at endogenous levels. R-Spondin domain variants undergo ER-associated degradation due to protein misfolding, reducing membrane levels.","method":"Endogenous knock-in of variants, β-catenin signaling assays, protein stability assays, low-temperature rescue","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — endogenous knock-in is rigorous, but single lab","pmids":["39674817"],"is_preprint":false}],"current_model":"ZNRF3 is a cell-surface transmembrane RING-domain E3 ubiquitin ligase that restricts Wnt signaling by selectively ubiquitinating and promoting lysosomal degradation of WNT-engaged Frizzled receptors (with substrate specificity for FZD5/8 and FZD6) and LRP6; it recruits FZD via the adaptor protein Dishevelled/DVL; it is inhibited when R-spondin proteins bridge ZNRF3 to LGR4, assembling a 2:2:2 ternary complex that sequesters and auto-inactivates ZNRF3; ZNRF3 membrane levels are maintained by the deubiquitinase USP42 and are regulated by phosphorylation/dephosphorylation of a '4Y' endocytic motif controlled by the MET kinase (phosphorylates, retaining ZNRF3 at membrane) and the phosphatase PTPRK (dephosphorylates, promoting ZNRF3 internalization); additionally, ZNRF3 is itself degraded by SCFβ-TRCP, and its membrane abundance is modulated by WNK kinases; beyond Wnt receptors, ZNRF3 also ubiquitinates and degrades EGFR via its extracellular domain-mediated interaction and RING domain catalytic activity."},"narrative":{"teleology":[{"year":2012,"claim":"The fundamental question of how Wnt receptor abundance is actively regulated was answered: ZNRF3 was identified as a transmembrane E3 ligase that ubiquitinates FZD and LRP6 to drive their turnover, and R-spondin was shown to inhibit ZNRF3 by bridging it to LGR4 for membrane clearance.","evidence":"Co-immunoprecipitation, receptor ubiquitination assays, cell-surface turnover assays, and Xenopus loss-of-function","pmids":["22575959"],"confidence":"High","gaps":["Catalytic mechanism and substrate specificity among FZD family members unknown","How ZNRF3 is recruited to FZD substrates was not established","Whether ZNRF3 targets non-Wnt receptor substrates remained unexplored"]},{"year":2013,"claim":"The structural basis for R-spondin–mediated ZNRF3 inhibition was resolved: crystal structures showed that RSPO Fu1 domain inserts into a groove on the ZNRF3 ectodomain while Fu2 engages LGR4/5, assembling a ternary complex that sequesters ZNRF3.","evidence":"X-ray crystallography of ZNRF3 ectodomain alone and in complex with RSPO1/RSPO2, biophysical binding assays, in vitro reconstitution of ternary complex, and functional mutagenesis","pmids":["24225776","24349440","24165923","24050775"],"confidence":"High","gaps":["Full-length ternary complex structure not available","Stoichiometry of the complex in a membrane context was unresolved","Role of ZNRF3 dimerization in its catalytic cycle unknown"]},{"year":2015,"claim":"The question of how ZNRF3 reaches its FZD substrate was resolved: Dishevelled (DVL) was identified as an essential adaptor whose DEP domain bridges FZD to ZNRF3, and a ternary LGR5–RSPO2–ZNRF3 crystal structure confirmed a 2:2:2 stoichiometry.","evidence":"DVL triple-knockout cells, Co-IP, DEP-ZNRF3 fusion rescue experiments; X-ray crystallography of LGR5-Rspo2-ZNRF3 complex","pmids":["25891077","26123262"],"confidence":"High","gaps":["Whether DVL requirement is universal across all FZD subtypes was untested","Structural basis of DVL–ZNRF3 interaction not determined"]},{"year":2018,"claim":"Two upstream regulatory layers of ZNRF3 were established: SCFβ-TRCP ubiquitinates and degrades ZNRF3 via the proteasome, and ZNRF3 loss in developing gonads causes XY sex reversal through ectopic Wnt/β-catenin signaling.","evidence":"Co-IP and ubiquitination assays with degron mutagenesis for β-TRCP; conditional knockout mouse with gonadal phenotyping and zebrafish functional assays for sex determination","pmids":["29497989","29735715"],"confidence":"High","gaps":["Physiological contexts where β-TRCP–mediated ZNRF3 degradation is rate-limiting remain undefined","Whether ZNRF3 loss alone is sufficient for human DSD requires larger genetic studies"]},{"year":2019,"claim":"ZNRF3 was shown to be the dominant Wnt-restraining E3 ligase in the adrenal cortex: its loss caused Wnt-dependent adrenal hyperplasia that was reversed by reducing β-catenin dosage.","evidence":"Adrenocortical-specific conditional knockout, Porcupine inhibitor treatment, genetic β-catenin dosage reduction","pmids":["30692207"],"confidence":"High","gaps":["Whether ZNRF3 and RNF43 have non-overlapping tissue-specific roles beyond adrenal cortex not systematically addressed","ZNRF3 substrates beyond FZD in adrenal cells not identified"]},{"year":2020,"claim":"A phosphorylation-based endocytic switch controlling ZNRF3 membrane residence was discovered: PTPRK dephosphorylates a '4Y' motif in ZNRF3 to promote its internalization, establishing that ZNRF3 activity is tuned by tyrosine phosphorylation state.","evidence":"Xenopus loss-of-function, phosphorylation site mutagenesis, endocytosis assays","pmids":["31934854"],"confidence":"High","gaps":["The kinase responsible for 4Y phosphorylation was not yet identified","Whether the 4Y motif is regulated in all tissues or context-specific was unknown"]},{"year":2021,"claim":"A MET–PTPRK rheostat controlling ZNRF3 surface levels was defined, USP42 was identified as a deubiquitinase stabilizing ZNRF3 against R-spondin–induced clearance, and cooperative tumor suppression by ZNRF3 and RNF43 in the liver was demonstrated.","evidence":"MET kinase assays and pharmacological inhibition for MET-4Y axis; Co-IP and DUB assays in intestinal organoids for USP42; liver-specific double-knockout with scRNA-seq for tumor suppression","pmids":["34590584","33786993","34129813"],"confidence":"High","gaps":["How MET–HGF signaling is integrated with R-spondin–LGR4 regulation of ZNRF3 in vivo is unresolved","USP42 regulation and whether other DUBs act on ZNRF3 are unknown","Mechanistic basis for RNF43 compensatory upregulation upon ZNRF3 loss is not defined"]},{"year":2023,"claim":"LGR4 was distinguished from LGR5 as the functional co-receptor for bivalent RSPO binding to ZNRF3, and synthetic peptides targeting the ZNRF3 ectodomain were shown to phenocopy R-spondin by inducing ZNRF3 clearance and organoid expansion.","evidence":"Whole-cell binding affinity assays comparing LGR4 vs LGR5; disulfide-constrained peptide binding assays with organoid growth readout","pmids":["37402772","38056465"],"confidence":"Medium","gaps":["LGR4–ZNRF3 complex specificity mechanism at atomic level was not fully defined","In vivo efficacy and selectivity of synthetic ZNRF3-targeting peptides untested"]},{"year":2024,"claim":"Substrate specificity determinants were mapped—ZNRF3 preferentially targets FZD6 while RNF43 targets FZD1/5/7, governed by the transmembrane domain—and ZNRF3 was shown to ubiquitinate EGFR via its ectodomain, expanding its substrate repertoire beyond Wnt receptors.","evidence":"TMD-swap chimeras with FZD endocytosis assays; Co-IP and ubiquitination assays for EGFR interaction; knockout-based EGFR upregulation","pmids":["38969364","41960900"],"confidence":"High","gaps":["Full FZD-family specificity map for ZNRF3 is incomplete","EGFR as ZNRF3 substrate requires independent replication and in vivo confirmation","Whether ZNRF3 targets additional RTKs is unexplored"]},{"year":2024,"claim":"Disease-associated ZNRF3 variants were functionally stratified: RING-domain missense variants cause dominant-negative Wnt hyperactivation and macrocephaly; RSPO-binding-domain variants cause attenuated Wnt signaling and microcephaly; and exon 2 deletions impair RSPO1 binding causing congenital adrenal hypoplasia. Endogenous knock-in studies showed that missense variants at physiological levels exhibit partial loss-of-function rather than true dominant-negative effects, with RSPO-domain variants undergoing ER-associated degradation.","evidence":"Wnt/β-catenin reporter assays with domain-specific variants, endogenous knock-in and protein stability assays, RT-PCR and cell-based TCF-LEF reporter for exon 2 deletion","pmids":["39168120","37878959","39674817"],"confidence":"Medium","gaps":["In vivo neurodevelopmental phenotypes of specific variants not modeled in animals","Whether dominant-negative effects manifest at endogenous expression levels remains contested between studies","Genotype–phenotype correlations require larger patient cohorts"]},{"year":2025,"claim":"The longstanding model that ZNRF3 constitutively degrades FZD was revised: ZNRF3 selectively targets Wnt-engaged FZD, with Wnt stimulation enhancing FZD–DVL association to recruit ZNRF3, establishing ZNRF3 as a signal-dependent negative feedback mechanism rather than a constitutive turnover factor.","evidence":"Endogenous Wnt-dependent FZD degradation assays, Co-IP for WNT-enhanced FZD-DVL-ZNRF3 complex, DVL perturbation","pmids":["41086253","41070826"],"confidence":"High","gaps":["Whether DVL-independent Wnt-induced FZD5/8 degradation pathway is ZNRF3-specific or shared with RNF43 needs clarification","How Wnt-engaged vs unengaged FZD are discriminated at the molecular level is unresolved"]},{"year":2025,"claim":"Cryo-EM resolved the full 2:2:2 LGR4–RSPO2–ZNRF3 ternary complex, showing ZNRF3 enclosed as a central dimer, providing the structural basis for how RSPO-mediated sequestration and forced dimerization auto-inactivate ZNRF3.","evidence":"Cryo-electron microscopy structure determination of human LGR4–RSPO2–ZNRF3 complex","pmids":["41034211"],"confidence":"High","gaps":["Catalytic RING domain and intracellular region not resolved in the cryo-EM structure","How forced dimerization mechanistically inactivates E3 ligase activity is not demonstrated biochemically"]},{"year":null,"claim":"Key open questions remain: the structural basis of ZNRF3–DVL–FZD ternary recruitment, the complete FZD-family specificity map, the physiological significance of ZNRF3-mediated EGFR degradation, and how MET/PTPRK phosphorylation integrates with R-spondin–LGR4 regulation in tissue-specific stem cell niches.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of ZNRF3 intracellular domain or ZNRF3–DVL complex","EGFR as ZNRF3 substrate not independently replicated","Tissue-specific integration of multiple ZNRF3 regulatory inputs not modeled"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,5,15,16]},{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,5,16]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,10,11,12,17]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,5,9,19,20]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,7,12,16]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[8,9,21]}],"complexes":["LGR4-RSPO-ZNRF3 ternary complex"],"partners":["LGR4","DVL","RSPO1","RSPO2","USP42","PTPRK","MET","EGFR"],"other_free_text":[]},"mechanistic_narrative":"ZNRF3 is a cell-surface transmembrane RING-domain E3 ubiquitin ligase that functions as a central negative regulator of Wnt/β-catenin signaling by selectively ubiquitinating and promoting lysosomal degradation of Wnt-engaged Frizzled receptors and LRP6, with preferential substrate specificity for FZD5/8 and FZD6 [PMID:22575959, PMID:38969364, PMID:41086253, PMID:41070826]. ZNRF3 is recruited to its Frizzled substrates through the adaptor protein Dishevelled (DVL), whose DEP domain bridges FZD–ZNRF3 interaction upon Wnt stimulation; ZNRF3 activity is negatively regulated when R-spondin ligands bridge it to LGR4 in a 2:2:2 ternary complex that sequesters and auto-inactivates ZNRF3, as resolved by crystallography and cryo-EM [PMID:25891077, PMID:41034211, PMID:24225776]. ZNRF3 membrane abundance is controlled by a phosphorylation rheostat on its '4Y' endocytic motif—MET kinase phosphorylation retains ZNRF3 at the surface while PTPRK dephosphorylation promotes internalization—and by USP42-mediated deubiquitination that counteracts R-spondin–induced clearance [PMID:34590584, PMID:31934854, PMID:33786993]. Germline ZNRF3 variants cause domain-dependent neurodevelopmental disorders (RING-domain variants producing dominant-negative Wnt hyperactivation and macrocephaly; RSPO-binding-domain variants causing attenuated Wnt signaling and microcephaly), loss of ZNRF3 causes XY gonadal sex reversal through ectopic Wnt/β-catenin activation, and ZNRF3 exon 2 deletions cause congenital adrenal hypoplasia [PMID:39168120, PMID:29735715, PMID:37878959]."},"prefetch_data":{"uniprot":{"accession":"Q9ULT6","full_name":"E3 ubiquitin-protein ligase ZNRF3","aliases":["RING finger protein 203","RING-type E3 ubiquitin transferase ZNRF3","Zinc/RING finger protein 3"],"length_aa":936,"mass_kda":100.6,"function":"E3 ubiquitin-protein ligase that acts as a negative regulator of the Wnt signaling pathway by mediating the ubiquitination and subsequent degradation of Wnt receptor complex components Frizzled and LRP6. Acts on both canonical and non-canonical Wnt signaling pathway. Acts as a tumor suppressor in the intestinal stem cell zone by inhibiting the Wnt signaling pathway, thereby restricting the size of the intestinal stem cell zone (PubMed:22575959). Along with RSPO2 and RNF43, constitutes a master switch that governs limb specification (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9ULT6/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ZNRF3","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ZNRF3","total_profiled":1310},"omim":[{"mim_id":"620946","title":"UBIQUITIN-SPECIFIC PEPTIDASE 42; USP42","url":"https://www.omim.org/entry/620946"},{"mim_id":"618022","title":"HUMEROFEMORAL HYPOPLASIA WITH RADIOTIBIAL RAY DEFICIENCY; HHRRD","url":"https://www.omim.org/entry/618022"},{"mim_id":"618021","title":"TETRAAMELIA SYNDROME 2; 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R-spondin inhibits ZNRF3 by directly binding its extracellular domain and inducing association between ZNRF3 and LGR4, resulting in membrane clearance of ZNRF3.\",\n      \"method\": \"Co-immunoprecipitation, cell-surface receptor turnover assays, in vivo loss-of-function (Xenopus), receptor ubiquitination assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — foundational paper with multiple orthogonal methods, replicated across many subsequent studies\",\n      \"pmids\": [\"22575959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structures of the ZNRF3 ectodomain (ZNRF3ecto) alone and in complex with the Fu1-Fu2 fragment of Rspo2 reveal that a prominent loop in Rspo2 Fu1 clamps into a groove on the ZNRF3 ectodomain surface. Rspo binding enhances dimerization of ZNRF3ecto. Signaling potency of Rspo proteins depends on their ability to recruit ZNRF3 via Fu1 into a ternary complex with LGR receptors (which interact with Rspo via Fu2).\",\n      \"method\": \"X-ray crystallography, biophysical binding assays, cellular signaling assays, mutagenesis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures with functional mutagenesis validation\",\n      \"pmids\": [\"24225776\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of the ZNRF3 ectodomain and its complex with R-spondin 1 shows ZNRF3 binds RSPO1 and LGR5-RSPO1 with micromolar affinity via the RSPO1 furin-like 1 (Fu1) domain. The ZNRF3-binding site overlaps with trans-interactions in 2:2 LGR5-RSPO1 complexes, suggesting ZNRF3/RNF43 compete for binding.\",\n      \"method\": \"X-ray crystallography, surface plasmon resonance/binding affinity measurements\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with biophysical binding validation\",\n      \"pmids\": [\"24349440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Interaction of R-spondin with both ZNRF3 and LGR4 through distinct motifs is required for R-spondin-induced LGR4/ZNRF3 complex formation, ZNRF3 membrane clearance, and Wnt signaling activation. A ZNRF3 mutant with reduced R-spondin affinity is resistant to R-spondin-mediated inhibition, supporting a dual-receptor model where LGR4/5 are engagement receptors and ZNRF3/RNF43 are effector receptors.\",\n      \"method\": \"Mutagenesis, Co-immunoprecipitation, cell-surface clearance assays, Wnt signaling reporter assays\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding and functional mutagenesis, strong mechanistic follow-up\",\n      \"pmids\": [\"24165923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Recombinant RSPO:LGR4:ZNRF3 ternary complexes can be reconstituted in vitro with bacterially produced proteins. RSPOs weakly bind ZNRF3 ECD, and RSPO2/3 form detectable ternary complexes with LGR4 and ZNRF3. The LGR4 ECD and ZNRF3 ECD inhibit RSPO-enhanced Wnt signaling in cell-based assays.\",\n      \"method\": \"In vitro reconstitution, TR-FRET binding assay, native gel electrophoretic mobility shift assay, cell-based signaling assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of ternary complex with multiple orthogonal binding assays\",\n      \"pmids\": [\"24050775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Dishevelled (DVL) acts as a dual-function adaptor that recruits ZNRF3/RNF43 to Wnt receptors: DVL knockout cells show increased cell-surface FZD and LRP6; DVL is required for ZNRF3/RNF43-mediated ubiquitination and degradation of FZD; physical interaction with DVL (via the DEP domain binding FZD) is essential for Wnt-inhibitory activity of ZNRF3/RNF43; fusing the DEP domain to ZNRF3 overcomes DVL dependency.\",\n      \"method\": \"DVL knockout, Co-immunoprecipitation, ubiquitination assays, domain fusion experiments, cell-surface receptor assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO, reciprocal Co-IP, domain-swap rescue, multiple orthogonal methods\",\n      \"pmids\": [\"25891077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Crystal structure of human LGR5 ectodomain complexed with mouse Rspo2 Fu1-Fu2, and a low-resolution ternary LGR5-Rspo2-ZNRF3 ectodomain complex structure, confirms that Rspo proteins cross-link LGR4/5 and ZNRF3 into a 2:2:2 complex (versus 1:1:1 with RNF43).\",\n      \"method\": \"X-ray crystallography, ternary complex structure determination\",\n      \"journal\": \"Journal of structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure of ternary complex\",\n      \"pmids\": [\"26123262\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SCFβ-TRCP E3 ubiquitin ligase directly interacts with and ubiquitinates ZNRF3, promoting its proteasomal degradation in a CKI-phosphorylation- and degron-dependent manner (analogous to β-catenin degradation by β-TRCP), thereby positively regulating Wnt signaling by removing the negative regulator ZNRF3.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, proteasome inhibitor treatment, degron mutagenesis\",\n      \"journal\": \"Protein & cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct interaction and ubiquitination shown, single lab\",\n      \"pmids\": [\"29497989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ZNRF3 is required for mammalian sex determination: XY mice lacking ZNRF3 exhibit complete or partial gonadal sex reversal associated with ectopic WNT/β-catenin activity and reduced Sox9 expression during fetal sex determination. Human ZNRF3 missense variants disrupt ZNRF3 activity in cell lines and zebrafish assays.\",\n      \"method\": \"Conditional knockout mouse, Wnt reporter assays, zebrafish functional assay, exome sequencing of DSD patients\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined molecular phenotype, corroborated in zebrafish and human cell assays\",\n      \"pmids\": [\"29735715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Adrenocortical-specific loss of ZNRF3 (but not RNF43) causes adrenal hyperplasia dependent on Porcupine-mediated Wnt ligand secretion, and disrupts a Wnt/β-catenin signaling gradient in the adrenal cortex. ZNRF3 loss triggers moderate-level β-catenin activation that drives proliferative expansion of the inner cortex. Genetically reducing β-catenin dosage significantly reverses this phenotype.\",\n      \"method\": \"Tissue-specific conditional knockout, Porcupine inhibitor, genetic epistasis (β-catenin dosage reduction), Wnt reporter assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with genetic epistasis rescue, multiple functional readouts\",\n      \"pmids\": [\"30692207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The tumor suppressor PTPRK (protein tyrosine phosphatase receptor-type kappa) dephosphorylates a '4Y' endocytic tyrosine motif in ZNRF3, keeping it unphosphorylated and thereby promoting ZNRF3 internalization and Wnt receptor degradation. PTPRK deficiency in Xenopus increases Wnt signaling and causes head/axial defects.\",\n      \"method\": \"Xenopus loss-of-function, phosphorylation site mutagenesis (4Y motif), endocytosis assays, Wnt reporter assays, identification of endocytic signal\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — phosphorylation site mutagenesis, clean in vivo loss-of-function, mechanistic pathway placement\",\n      \"pmids\": [\"31934854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MET proto-oncogene (receptor tyrosine kinase) binds to ZNRF3 and phosphorylates its '4Y' endocytic motif in response to HGF stimulation, thereby reducing ZNRF3 internalization and ZNRF3-dependent Wnt receptor degradation, and enhancing Wnt/β-catenin signaling. PTPRK dephosphorylates this same motif, establishing a MET-PTPRK rheostat controlling ZNRF3 activity.\",\n      \"method\": \"Co-immunoprecipitation, kinase assay (MET phosphorylation of 4Y motif), pharmacological MET inhibition, ZNRF3 internalization assay, Wnt reporter assay\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical identification of kinase, confirmed by pharmacological inhibition, functional ZNRF3 internalization readout\",\n      \"pmids\": [\"34590584\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The deubiquitinase USP42 binds to the Dishevelled-interacting region (DIR) of ZNRF3 and deubiquitinates ZNRF3, protecting it from R-spondin-LGR4-induced ubiquitin-dependent membrane clearance. USP42 thereby maintains ZNRF3 at the plasma membrane, promotes FZD and LRP6 turnover, and inhibits Wnt signaling.\",\n      \"method\": \"Co-immunoprecipitation, deubiquitination assay, cell-surface clearance assay, Wnt reporter assay, intestinal organoids\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical interaction, deubiquitination demonstrated, functional consequence in organoids\",\n      \"pmids\": [\"33786993\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ZNRF3 deletion promotes hepatocyte proliferation; subsequent RNF43 upregulation limits this proliferation. Concomitant deletion of both ZNRF3 and RNF43 results in metabolic reprogramming of periportal hepatocytes, clonal expansion, and liver tumor formation, demonstrating cooperative roles of ZNRF3 and RNF43 in spatially and temporally restricting WNT/β-catenin activity in the liver.\",\n      \"method\": \"Liver-specific conditional knockout (ZNRF3 alone and ZNRF3/RNF43 double KO), scRNA-seq, chromatin accessibility, organoid studies\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean in vivo conditional KO with multiple functional readouts, including chromatin and single-cell analyses\",\n      \"pmids\": [\"34129813\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"LGR4 (but not LGR5) forms a complex with RNF43/ZNRF3 to provide high-affinity bivalent binding of R-spondin ligands. LGR4 and ZNRF3 form a 2:2 dimer accommodating bivalent RSPO binding, whereas LGR5 forms a homodimer incompatible with ZNRF3 co-complex formation.\",\n      \"method\": \"Whole-cell binding affinity assays (monovalent vs bivalent RSPO), co-expression of receptors, LGR4/LGR5 comparison\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — binding assays in cells, single lab with multiple conditions\",\n      \"pmids\": [\"37402772\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"RNF43 and ZNRF3 display differential substrate specificity for Frizzled receptors: RNF43 preferentially down-regulates FZD1/FZD5/FZD7 whereas ZNRF3 preferentially targets FZD6. The transmembrane domain (TMD) of RNF43 is a key molecular determinant for FZD5 endocytosis; swapping the TMD between RNF43 and ZNRF3 redirects their FZD substrate preference.\",\n      \"method\": \"Endocytosis assays, TMD swap domain chimeras, knockdown/knockout, receptor surface level measurement\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain swap experiments with functional readout, clean mechanistic determination of TMD role\",\n      \"pmids\": [\"38969364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ZNRF3 and RNF43 interact with EGFR via their extracellular domains, leading to EGFR ubiquitination and lysosomal degradation mediated by the intracellular RING domain. Overexpression of ZNRF3 reduces EGFR levels; knockout of ZNRF3/RNF43 upregulates EGFR signaling and promotes tumorigenesis.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, overexpression/knockout, in vitro and in vivo tumor growth assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical demonstration of EGFR interaction and ubiquitination, single lab\",\n      \"pmids\": [\"41960900\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Disulfide-constrained peptides (DCPs) that bind the ZNRF3 ectodomain induce ZNRF3 ubiquitination and membrane clearance, leading to FZD stabilization and Wnt signaling activation. Multimeric DCPs induce expansive growth of human intestinal organoids in a valency-dependent manner.\",\n      \"method\": \"Peptide binding assay, cell-surface ZNRF3 clearance assay, FZD stabilization assay, Wnt reporter, intestinal organoid growth assay\",\n      \"journal\": \"Cell chemical biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct mechanistic study with functional validation in organoids, single lab\",\n      \"pmids\": [\"38056465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of human LGR4 alone, LGR4-RSPO2, and LGR4-RSPO2-ZNRF3 complexes show that LGR4, RSPO2, and ZNRF3 assemble into a 2:2:2 complex with the ZNRF3 dimer enclosed at the center. Upon RSPO2 binding, LGR4 undergoes no significant conformational changes. This forced ZNRF3 dimerization likely underlies how the complex sequesters ZNRF3 from Wnt receptors and facilitates ZNRF3 auto-inactivation.\",\n      \"method\": \"Cryo-electron microscopy structure determination\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure of ternary complex with mechanistic interpretation\",\n      \"pmids\": [\"41034211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ZNRF3-induced FZD degradation depends on endogenous WNT stimulation rather than being constitutive; ZNRF3 selectively degrades WNT-engaged FZD. WNT enhances FZD-DVL association, and DVL subsequently recruits ZNRF3 to WNT-engaged FZD to promote its degradation. R-spondin enhances WNT signaling by prolonging the action of the WNT-engaged FZD complex rather than simply increasing total FZD abundance.\",\n      \"method\": \"Endogenous WNT-dependent assays, Co-immunoprecipitation, FZD degradation assays, DVL dependency experiments\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic revision with multiple orthogonal experiments, published in peer-reviewed journal\",\n      \"pmids\": [\"41086253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Wnt induces selective endocytosis and degradation of FZD5/8 in a ZNRF3/RNF43-dependent manner; ZNRF3/RNF43 selectively target FZD5/8 upon Wnt stimulation; Wnt promotes the interaction between FZD5 and RNF43; DVL promotes ligand-independent FZD endocytosis but is dispensable for Wnt-induced FZD5/8 endocytosis and degradation.\",\n      \"method\": \"FZD endocytosis assays, ZNRF3/RNF43 knockout, Co-immunoprecipitation, DVL perturbation experiments\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO, Co-IP, multiple FZD specificity assays, peer-reviewed\",\n      \"pmids\": [\"41070826\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ZNRF3 germline missense variants in the RING ligase domain cause macrocephalic neurodevelopmental disorder via dominant-negative enhancement of Wnt/β-catenin signaling (compromising Wnt receptor turnover), while a missense variant in the RSPO-binding domain causes microcephalic NDD via attenuated Wnt/β-catenin signaling. These domain-specific effects were validated in transcriptional reporter assays.\",\n      \"method\": \"Structural modeling, in vitro Wnt/β-catenin transcriptional reporter assays with Wnt3a and RSPO, comparison of RING vs RSPO-binding domain variants\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional assays validating domain-specific loss/gain of function, but limited to reporter assays without full mechanistic reconstitution\",\n      \"pmids\": [\"39168120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ZNRF3 exon 2 deletions produce a 42-amino acid deleted protein (ΔEx2-ZNRF3) that is impaired in RSPO1 binding and fails to support RSPO1-dependent activation of Wnt/β-catenin signaling, causing congenital adrenal hypoplasia.\",\n      \"method\": \"RT-PCR, 3D structural modeling, cell-based TCF-LEF reporter assay comparing ΔEx2-ZNRF3 vs wild-type\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional cell-based assay with structural modeling, clinical phenotype linked to specific domain loss\",\n      \"pmids\": [\"37878959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Truncating ZNRF3 mutations at endogenous levels exhibit loss-of-function; missense mutations in RING and R-Spondin domains cause partial loss-of-function or hyperactivation but do NOT exhibit dominant-negative activity when heterozygously introduced at endogenous levels. R-Spondin domain variants undergo ER-associated degradation due to protein misfolding, reducing membrane levels.\",\n      \"method\": \"Endogenous knock-in of variants, β-catenin signaling assays, protein stability assays, low-temperature rescue\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — endogenous knock-in is rigorous, but single lab\",\n      \"pmids\": [\"39674817\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ZNRF3 is a cell-surface transmembrane RING-domain E3 ubiquitin ligase that restricts Wnt signaling by selectively ubiquitinating and promoting lysosomal degradation of WNT-engaged Frizzled receptors (with substrate specificity for FZD5/8 and FZD6) and LRP6; it recruits FZD via the adaptor protein Dishevelled/DVL; it is inhibited when R-spondin proteins bridge ZNRF3 to LGR4, assembling a 2:2:2 ternary complex that sequesters and auto-inactivates ZNRF3; ZNRF3 membrane levels are maintained by the deubiquitinase USP42 and are regulated by phosphorylation/dephosphorylation of a '4Y' endocytic motif controlled by the MET kinase (phosphorylates, retaining ZNRF3 at membrane) and the phosphatase PTPRK (dephosphorylates, promoting ZNRF3 internalization); additionally, ZNRF3 is itself degraded by SCFβ-TRCP, and its membrane abundance is modulated by WNK kinases; beyond Wnt receptors, ZNRF3 also ubiquitinates and degrades EGFR via its extracellular domain-mediated interaction and RING domain catalytic activity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ZNRF3 is a cell-surface transmembrane RING-domain E3 ubiquitin ligase that functions as a central negative regulator of Wnt/β-catenin signaling by selectively ubiquitinating and promoting lysosomal degradation of Wnt-engaged Frizzled receptors and LRP6, with preferential substrate specificity for FZD5/8 and FZD6 [PMID:22575959, PMID:38969364, PMID:41086253, PMID:41070826]. ZNRF3 is recruited to its Frizzled substrates through the adaptor protein Dishevelled (DVL), whose DEP domain bridges FZD–ZNRF3 interaction upon Wnt stimulation; ZNRF3 activity is negatively regulated when R-spondin ligands bridge it to LGR4 in a 2:2:2 ternary complex that sequesters and auto-inactivates ZNRF3, as resolved by crystallography and cryo-EM [PMID:25891077, PMID:41034211, PMID:24225776]. ZNRF3 membrane abundance is controlled by a phosphorylation rheostat on its '4Y' endocytic motif—MET kinase phosphorylation retains ZNRF3 at the surface while PTPRK dephosphorylation promotes internalization—and by USP42-mediated deubiquitination that counteracts R-spondin–induced clearance [PMID:34590584, PMID:31934854, PMID:33786993]. Germline ZNRF3 variants cause domain-dependent neurodevelopmental disorders (RING-domain variants producing dominant-negative Wnt hyperactivation and macrocephaly; RSPO-binding-domain variants causing attenuated Wnt signaling and microcephaly), loss of ZNRF3 causes XY gonadal sex reversal through ectopic Wnt/β-catenin activation, and ZNRF3 exon 2 deletions cause congenital adrenal hypoplasia [PMID:39168120, PMID:29735715, PMID:37878959].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"The fundamental question of how Wnt receptor abundance is actively regulated was answered: ZNRF3 was identified as a transmembrane E3 ligase that ubiquitinates FZD and LRP6 to drive their turnover, and R-spondin was shown to inhibit ZNRF3 by bridging it to LGR4 for membrane clearance.\",\n      \"evidence\": \"Co-immunoprecipitation, receptor ubiquitination assays, cell-surface turnover assays, and Xenopus loss-of-function\",\n      \"pmids\": [\"22575959\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic mechanism and substrate specificity among FZD family members unknown\", \"How ZNRF3 is recruited to FZD substrates was not established\", \"Whether ZNRF3 targets non-Wnt receptor substrates remained unexplored\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The structural basis for R-spondin–mediated ZNRF3 inhibition was resolved: crystal structures showed that RSPO Fu1 domain inserts into a groove on the ZNRF3 ectodomain while Fu2 engages LGR4/5, assembling a ternary complex that sequesters ZNRF3.\",\n      \"evidence\": \"X-ray crystallography of ZNRF3 ectodomain alone and in complex with RSPO1/RSPO2, biophysical binding assays, in vitro reconstitution of ternary complex, and functional mutagenesis\",\n      \"pmids\": [\"24225776\", \"24349440\", \"24165923\", \"24050775\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length ternary complex structure not available\", \"Stoichiometry of the complex in a membrane context was unresolved\", \"Role of ZNRF3 dimerization in its catalytic cycle unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The question of how ZNRF3 reaches its FZD substrate was resolved: Dishevelled (DVL) was identified as an essential adaptor whose DEP domain bridges FZD to ZNRF3, and a ternary LGR5–RSPO2–ZNRF3 crystal structure confirmed a 2:2:2 stoichiometry.\",\n      \"evidence\": \"DVL triple-knockout cells, Co-IP, DEP-ZNRF3 fusion rescue experiments; X-ray crystallography of LGR5-Rspo2-ZNRF3 complex\",\n      \"pmids\": [\"25891077\", \"26123262\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether DVL requirement is universal across all FZD subtypes was untested\", \"Structural basis of DVL–ZNRF3 interaction not determined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Two upstream regulatory layers of ZNRF3 were established: SCFβ-TRCP ubiquitinates and degrades ZNRF3 via the proteasome, and ZNRF3 loss in developing gonads causes XY sex reversal through ectopic Wnt/β-catenin signaling.\",\n      \"evidence\": \"Co-IP and ubiquitination assays with degron mutagenesis for β-TRCP; conditional knockout mouse with gonadal phenotyping and zebrafish functional assays for sex determination\",\n      \"pmids\": [\"29497989\", \"29735715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological contexts where β-TRCP–mediated ZNRF3 degradation is rate-limiting remain undefined\", \"Whether ZNRF3 loss alone is sufficient for human DSD requires larger genetic studies\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"ZNRF3 was shown to be the dominant Wnt-restraining E3 ligase in the adrenal cortex: its loss caused Wnt-dependent adrenal hyperplasia that was reversed by reducing β-catenin dosage.\",\n      \"evidence\": \"Adrenocortical-specific conditional knockout, Porcupine inhibitor treatment, genetic β-catenin dosage reduction\",\n      \"pmids\": [\"30692207\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ZNRF3 and RNF43 have non-overlapping tissue-specific roles beyond adrenal cortex not systematically addressed\", \"ZNRF3 substrates beyond FZD in adrenal cells not identified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"A phosphorylation-based endocytic switch controlling ZNRF3 membrane residence was discovered: PTPRK dephosphorylates a '4Y' motif in ZNRF3 to promote its internalization, establishing that ZNRF3 activity is tuned by tyrosine phosphorylation state.\",\n      \"evidence\": \"Xenopus loss-of-function, phosphorylation site mutagenesis, endocytosis assays\",\n      \"pmids\": [\"31934854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The kinase responsible for 4Y phosphorylation was not yet identified\", \"Whether the 4Y motif is regulated in all tissues or context-specific was unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"A MET–PTPRK rheostat controlling ZNRF3 surface levels was defined, USP42 was identified as a deubiquitinase stabilizing ZNRF3 against R-spondin–induced clearance, and cooperative tumor suppression by ZNRF3 and RNF43 in the liver was demonstrated.\",\n      \"evidence\": \"MET kinase assays and pharmacological inhibition for MET-4Y axis; Co-IP and DUB assays in intestinal organoids for USP42; liver-specific double-knockout with scRNA-seq for tumor suppression\",\n      \"pmids\": [\"34590584\", \"33786993\", \"34129813\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How MET–HGF signaling is integrated with R-spondin–LGR4 regulation of ZNRF3 in vivo is unresolved\", \"USP42 regulation and whether other DUBs act on ZNRF3 are unknown\", \"Mechanistic basis for RNF43 compensatory upregulation upon ZNRF3 loss is not defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"LGR4 was distinguished from LGR5 as the functional co-receptor for bivalent RSPO binding to ZNRF3, and synthetic peptides targeting the ZNRF3 ectodomain were shown to phenocopy R-spondin by inducing ZNRF3 clearance and organoid expansion.\",\n      \"evidence\": \"Whole-cell binding affinity assays comparing LGR4 vs LGR5; disulfide-constrained peptide binding assays with organoid growth readout\",\n      \"pmids\": [\"37402772\", \"38056465\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"LGR4–ZNRF3 complex specificity mechanism at atomic level was not fully defined\", \"In vivo efficacy and selectivity of synthetic ZNRF3-targeting peptides untested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Substrate specificity determinants were mapped—ZNRF3 preferentially targets FZD6 while RNF43 targets FZD1/5/7, governed by the transmembrane domain—and ZNRF3 was shown to ubiquitinate EGFR via its ectodomain, expanding its substrate repertoire beyond Wnt receptors.\",\n      \"evidence\": \"TMD-swap chimeras with FZD endocytosis assays; Co-IP and ubiquitination assays for EGFR interaction; knockout-based EGFR upregulation\",\n      \"pmids\": [\"38969364\", \"41960900\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full FZD-family specificity map for ZNRF3 is incomplete\", \"EGFR as ZNRF3 substrate requires independent replication and in vivo confirmation\", \"Whether ZNRF3 targets additional RTKs is unexplored\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Disease-associated ZNRF3 variants were functionally stratified: RING-domain missense variants cause dominant-negative Wnt hyperactivation and macrocephaly; RSPO-binding-domain variants cause attenuated Wnt signaling and microcephaly; and exon 2 deletions impair RSPO1 binding causing congenital adrenal hypoplasia. Endogenous knock-in studies showed that missense variants at physiological levels exhibit partial loss-of-function rather than true dominant-negative effects, with RSPO-domain variants undergoing ER-associated degradation.\",\n      \"evidence\": \"Wnt/β-catenin reporter assays with domain-specific variants, endogenous knock-in and protein stability assays, RT-PCR and cell-based TCF-LEF reporter for exon 2 deletion\",\n      \"pmids\": [\"39168120\", \"37878959\", \"39674817\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo neurodevelopmental phenotypes of specific variants not modeled in animals\", \"Whether dominant-negative effects manifest at endogenous expression levels remains contested between studies\", \"Genotype–phenotype correlations require larger patient cohorts\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The longstanding model that ZNRF3 constitutively degrades FZD was revised: ZNRF3 selectively targets Wnt-engaged FZD, with Wnt stimulation enhancing FZD–DVL association to recruit ZNRF3, establishing ZNRF3 as a signal-dependent negative feedback mechanism rather than a constitutive turnover factor.\",\n      \"evidence\": \"Endogenous Wnt-dependent FZD degradation assays, Co-IP for WNT-enhanced FZD-DVL-ZNRF3 complex, DVL perturbation\",\n      \"pmids\": [\"41086253\", \"41070826\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether DVL-independent Wnt-induced FZD5/8 degradation pathway is ZNRF3-specific or shared with RNF43 needs clarification\", \"How Wnt-engaged vs unengaged FZD are discriminated at the molecular level is unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-EM resolved the full 2:2:2 LGR4–RSPO2–ZNRF3 ternary complex, showing ZNRF3 enclosed as a central dimer, providing the structural basis for how RSPO-mediated sequestration and forced dimerization auto-inactivate ZNRF3.\",\n      \"evidence\": \"Cryo-electron microscopy structure determination of human LGR4–RSPO2–ZNRF3 complex\",\n      \"pmids\": [\"41034211\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic RING domain and intracellular region not resolved in the cryo-EM structure\", \"How forced dimerization mechanistically inactivates E3 ligase activity is not demonstrated biochemically\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions remain: the structural basis of ZNRF3–DVL–FZD ternary recruitment, the complete FZD-family specificity map, the physiological significance of ZNRF3-mediated EGFR degradation, and how MET/PTPRK phosphorylation integrates with R-spondin–LGR4 regulation in tissue-specific stem cell niches.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of ZNRF3 intracellular domain or ZNRF3–DVL complex\", \"EGFR as ZNRF3 substrate not independently replicated\", \"Tissue-specific integration of multiple ZNRF3 regulatory inputs not modeled\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 5, 15, 16]},\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 5, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 10, 11, 12, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 5, 9, 19, 20]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 7, 12, 16]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [8, 9, 21]}\n    ],\n    \"complexes\": [\n      \"LGR4-RSPO-ZNRF3 ternary complex\"\n    ],\n    \"partners\": [\n      \"LGR4\",\n      \"DVL\",\n      \"RSPO1\",\n      \"RSPO2\",\n      \"USP42\",\n      \"PTPRK\",\n      \"MET\",\n      \"EGFR\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}