{"gene":"LGR4","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":2011,"finding":"LGR4 and LGR5 bind R-spondins (RSPOs) with high affinity and mediate potentiation of Wnt/β-catenin signaling by enhancing Wnt-induced LRP6 phosphorylation; neither receptor couples to heterotrimeric G proteins or β-arrestin upon RSPO stimulation, indicating a unique signaling mechanism.","method":"Binding assays, LRP6 phosphorylation assays, G protein coupling assays, β-arrestin assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal biochemical assays, replicated across independent labs","pmids":["21693646"],"is_preprint":false},{"year":2011,"finding":"LGR4 and LGR5 bind R-spondins via their Furin domains and promote R-spondin-mediated Wnt/β-catenin and Wnt/PCP signaling; R-spondin-triggered β-catenin signaling requires Clathrin-mediated endocytosis, whereas Wnt3a-mediated signaling requires Caveolin-mediated endocytosis.","method":"Gain- and loss-of-function experiments in mammalian cells and Xenopus embryos, endocytosis inhibitor assays","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 — orthogonal gain/loss-of-function in two model systems, independent lab replication","pmids":["21909076"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of LGR4 extracellular domain (ECD) in complex with RSPO1 N-terminal fragment reveals that LGR4-ECD adopts a TLR horseshoe structure and uses its concave surface near the N-terminus to bind RSPO1; both FU-CRD1 and FU-CRD2 domains of RSPO1 contribute to LGR4 binding.","method":"X-ray crystallography, binding assays, mutagenesis","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional mutagenesis validation","pmids":["23756652"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of LGR4 ectodomain alone and in complex with Rspo1 shows an extended horseshoe LRR architecture that binds ligand furin-like repeats via its concave side through electrostatic and hydrophobic interactions.","method":"X-ray crystallography","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 — independent crystal structure determination confirming binding interface","pmids":["23891289"],"is_preprint":false},{"year":2014,"finding":"Crystal structure of LGR4-Rspo1 complex confirms concave surface as the sole binding site for R-spondins; all Rspo1-binding residues are conserved in LGR4-6, suggesting identical binding surface; LGR4-6 use a one-site binding model distinct from group A and C LGR members.","method":"X-ray crystallography, structural analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with comparative structural analysis","pmids":["25480784"],"is_preprint":false},{"year":2013,"finding":"R-spondin interacts with ZNRF3/RNF43 and LGR4 through distinct motifs; both LGR4 and ZNRF3 binding motifs are required for R-spondin-induced LGR4/ZNRF3 complex formation, membrane clearance of ZNRF3, and Wnt signaling activation; LGR4/5 serve as engagement receptors while ZNRF3/RNF43 are effector receptors.","method":"Mutagenesis, co-immunoprecipitation, ZNRF3 membrane clearance assay, Wnt signaling assay","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis of distinct binding motifs combined with functional assays","pmids":["24165923"],"is_preprint":false},{"year":2013,"finding":"RSPO-LGR4 signaling functions via the intracellular scaffold protein IQGAP1: LGR4 interacts with IQGAP1, which bridges RSPO-LGR4 to the Wnt signalosome through enhanced IQGAP1-DVL interaction following RSPO stimulation, promoting MEK1/2-mediated LRP5/6 phosphorylation for canonical Wnt and actin dynamics regulation for non-canonical Wnt signaling.","method":"Co-immunoprecipitation, pulldown, signaling assays, actin dynamics assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP with multiple orthogonal functional assays","pmids":["24639526"],"is_preprint":false},{"year":2012,"finding":"LGR4 is the cognate receptor of RSPO identified by unbiased siRNA screen; RSPO binds the extracellular domain of LGR4; LGR4 depletion abolishes RSPO-induced β-catenin signaling; overexpression of LGR5 can compensate for LGR4 loss; no coupling to heterotrimeric G proteins detected.","method":"siRNA screen, binding assay, β-catenin signaling assay, G protein coupling assay, crypt culture","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1-2 — unbiased screen confirmed by multiple orthogonal assays","pmids":["22815884"],"is_preprint":false},{"year":2013,"finding":"Recombinant RSPO proteins bind LGR4 LRR1-14 with nanomolar affinities (order RSPO4>RSPO2>RSPO3>RSPO1); ternary RSPO:LGR4:ZNRF3 complexes form for RSPO2 and RSPO3; stronger signaling potencies of RSPO2 and RSPO3 result from strong binding of both LGR4 and ZNRF3.","method":"TR-FRET binding assay, native gel electrophoretic mobility shift assay, cell-based signaling assay, in vitro reconstitution with bacterially expressed proteins","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted ternary complex in vitro with quantitative binding measurements","pmids":["24050775"],"is_preprint":false},{"year":2016,"finding":"LGR4 is a second receptor for RANKL that competes with RANK to bind RANKL; RANKL binding to LGR4 activates the Gαq and GSK3-β signaling pathway, suppressing NFATc1 expression and activity to negatively regulate osteoclastogenesis; LGR4 extracellular domain (ECD) binds RANKL and inhibits osteoclast differentiation in vivo.","method":"Co-immunoprecipitation, competition binding, Gαq signaling assays, osteoclast differentiation assays, conditional knockout mice, in vivo ECD treatment","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding established, multiple signaling assays, in vivo genetic and pharmacological validation","pmids":["27064449"],"is_preprint":false},{"year":2020,"finding":"Full-length LGR4 forms a tight complex with ZNRF3 and RNF43 even without RSPO, whereas LGR5 does not interact with either E3 ligase; the seven-transmembrane domain of LGR4 confers interaction with E3 ligases; LGR5 potentiates Wnt signaling by interacting with FZD and LRP6 to enhance LRP6 phosphorylation; LGR4 and LGR5 exist as dimers on the cell surface.","method":"Co-immunoprecipitation, proximity ligation assay, competition binding, time-resolved FRET, domain-swapping experiments","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including domain-swap mutagenesis distinguishing LGR4 vs LGR5 mechanisms","pmids":["33262293"],"is_preprint":false},{"year":2016,"finding":"The RSPO-LGR4/5-ZNRF3/RNF43 module controls hepatic Wnt/β-catenin-mediated metabolic liver zonation; liver-specific LGR4/5 loss disrupts Wnt/β-catenin signaling and zonation; pathway activation with recombinant RSPO1 expanded the hepatic Wnt/β-catenin gradient in a reversible and LGR4/5-dependent manner.","method":"Conditional knockout mice, RSPO1 recombinant protein treatment, reporter assays, histology","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 — clean genetic loss-of-function with defined molecular phenotype, rescue experiments","pmids":["27088858"],"is_preprint":false},{"year":2013,"finding":"Norrin (Norrie disease gene product) is a ligand for LGR4 that stimulates Wnt signaling mediated specifically by LGR4 (not LGR5/6) in mammalian cells; mutagenesis of norrin revealed distinct residues required for LGR4 signaling versus Frizzled4 signaling versus BMP antagonism.","method":"Cell-based Wnt signaling assay, binding studies, mutagenesis","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 — functional signaling assay with mutagenesis, single lab","pmids":["23444378"],"is_preprint":false},{"year":2022,"finding":"Nidogen-2 is a novel endogenous ligand of LGR4 (KD = 26.77 nM) that biased-activates LGR4-Gαq-PKCα-AMPKα1 signaling to counteract VSMC osteogenic transition and vascular calcification; LGR4 deficiency in VSMCs abolishes the protective effect of nidogen-2.","method":"Membrane protein interactome, co-immunoprecipitation, surface plasmon resonance, flow cytometric binding assay, G protein signaling assays, conditional knockout","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding quantified by SPR, G protein signaling pathway defined, in vitro and in vivo genetic validation","pmids":["36354004"],"is_preprint":false},{"year":2018,"finding":"Lgr4 promotes macrophage M2 polarization through Rspo/Lgr4/Erk/Stat3 signaling; macrophage-specific Lgr4 knockout reduces M2 tumor-associated macrophages and increases CD8+ T lymphocyte infiltration in the tumor microenvironment.","method":"Macrophage-conditional knockout mice (Lgr4fl/fl Lyz2cre/+), tumor models, signaling pathway analysis","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 2 — clean cell-type-specific KO with defined phenotype and pathway placement","pmids":["29967265"],"is_preprint":false},{"year":2020,"finding":"Lgr4 governs a pro-inflammatory program in macrophages via enhancing CREB-mediated c-Fos, Fosl1, and Fosb transactivation to activate AP-1 in inflammatory macrophages; macrophage-specific Lgr4 knockout improves cardiac repair after myocardial infarction.","method":"Macrophage-specific conditional knockout mice, transcriptomic profiling, AP-1 activity assays, CREB activation assays","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 2 — clean cell-type-specific KO with mechanistic pathway defined by transcriptomics and signaling assays","pmids":["32600176"],"is_preprint":false},{"year":2013,"finding":"Lgr4 ablation promotes white-to-brown fat transition partially through retinoblastoma 1 gene (Rb1) reduction; Lgr4 mutant mice show increased energy expenditure with brown-like adipocytes in WAT depots.","method":"Lgr4 mutant mice, brown adipocyte differentiation assays from stromal vascular fraction, Rb1 expression analysis","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 — genetic loss-of-function with defined molecular mechanism (Rb1 reduction) and cellular phenotype","pmids":["24212090"],"is_preprint":false},{"year":2008,"finding":"Gpr48/LGR4 regulates definitive erythropoiesis through the cAMP-PKA-CREB pathway, which controls ATF4 expression; Gpr48-/- fetal livers show decreased definitive erythroid precursors with reduced c-Myc and cyclin D1 expression.","method":"Knockout mice, real-time PCR, histology, blood smear assay, signaling pathway analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined downstream pathway, single lab","pmids":["18955481"],"is_preprint":false},{"year":2008,"finding":"GPR48/LGR4 mediates keratinocyte proliferation and migration during eyelid development through activation of EGFR signaling; GPR48 regulates EGFR phosphorylation, and GPR48 inactivation results in decreased EGFR phosphorylation and reduced F-actin formation.","method":"Knockout mice, in vitro scratch assay, Western blot, immunostaining, scanning electron microscopy","journal":"Investigative ophthalmology & visual science","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined signaling mechanism, single lab","pmids":["18487371"],"is_preprint":false},{"year":2010,"finding":"GPR48/LGR4 mediates keratinocyte proliferation via HB-EGF-mediated EGFR transactivation; HB-EGF is reduced in Gpr48-/- cell culture medium and its addition restores EGFR, ERK, and STAT3 phosphorylation as well as cell proliferation.","method":"EGFR inhibitor treatment, conditioned media immunodepletion, HB-EGF reconstitution, Western blot","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological and biochemical dissection of signaling mechanism, single lab","pmids":["20732323"],"is_preprint":false},{"year":2021,"finding":"LGR4 promotes cancer metastasis through a Wnt-independent axis: LGR4 interacts with EGFR and blocks EGFR ubiquitination and degradation, resulting in persistent EGFR activation; LGR4 mutants unable to potentiate Wnt signaling still promote cancer cell migration and invasion.","method":"Co-immunoprecipitation, EGFR ubiquitination assay, Wnt pathway inhibition (pharmacological and genetic), in vitro migration/invasion, in vivo xenograft metastasis","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 2 — domain mutants separating Wnt vs EGFR functions, multiple orthogonal assays including in vivo validation","pmids":["34099494"],"is_preprint":false},{"year":2013,"finding":"Lgr4 is required for Paneth cell differentiation and intestinal stem cell maintenance; LGR4-deficient crypts cultured ex vivo die rapidly with downregulation of stem cell markers and Wnt target genes; partial rescue achieved with LiCl but not Wnt agonists.","method":"Lgr4 gene inactivation, ex vivo crypt culture, histology, marker gene analysis","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined cellular and molecular phenotype, single lab","pmids":["21508962"],"is_preprint":false},{"year":2013,"finding":"Lgr4 regulates Wnt/β-catenin signaling in peritubular myoid cells (PMCs) to support spermatogenesis; Lgr4 loss attenuates Wnt/β-catenin signaling in PMCs; reactivation by crossing with Apc(min/+) or GSK3β inhibitor partially rescues the Lgr4-/- testicular phenotype.","method":"Conditional knockout mice, signaling analysis, genetic rescue (Apc(min/+) cross), pharmacological rescue (GSK3β inhibitor)","journal":"Development","confidence":"High","confidence_rationale":"Tier 2 — pathway placement via genetic epistasis with rescue, replicated by pharmacological approach","pmids":["23533175"],"is_preprint":false},{"year":2013,"finding":"Lgr4 protects against inflammatory bowel disease through Wnt/β-catenin signaling; Lgr4 deficiency reduces Paneth and stem cells; crossing with Apc(min/+) mice or GSK3β inhibitor treatment partially restores Paneth cell number and reduces DSS-induced mortality.","method":"Hypomorphic mouse model, DSS colitis, genetic rescue (Apc(min/+)), pharmacological rescue, histology","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and pharmacological epistasis, single lab","pmids":["23393138"],"is_preprint":false},{"year":2013,"finding":"Lgr4/Gpr48 negatively regulates TLR2/4-associated innate immunity by suppressing CD14 expression; Lgr4 deletion increases CD14 expression through decreased cAMP-response element-binding protein (CREB) activity, enhancing TLR2/4-associated cytokine production.","method":"Lgr4 knockout macrophages, TLR signaling assays, CREB activity assay, CD14 expression analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined signaling mechanism, single lab","pmids":["23589304"],"is_preprint":false},{"year":2014,"finding":"Lgr4 regulates corpus luteum maturation through WNT-mediated EGFR-ERK signaling; Lgr4-deficient granulosa-lutein cells show attenuated EGFR-ERK signaling; Wnt/β-catenin target MMP9 activates EGF-like factors to facilitate ERK activation and luteinization.","method":"Lgr4-/- mice, ovarian granulosa cell culture, EGFR-ERK signaling assays, MMP9 inhibitor, HB-EGF rescue","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined signaling mechanism and partial rescue, single lab","pmids":["24877628"],"is_preprint":false},{"year":2013,"finding":"RSPO-LGR4 signaling in myoblasts activates Wnt/β-catenin pathway to upregulate follistatin (Fst) expression via TCF/LEF1 binding sites in the Fst promoter and intron 1; LGR4 depletion severely disrupts myogenic differentiation and diminishes RSPO2 response.","method":"siRNA depletion, luciferase reporter assay with TCF/LEF1 site mutagenesis, C2C12 differentiation assay, in vivo β-catenin activation/inactivation","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct target gene identified with promoter mutagenesis and in vivo validation, single lab","pmids":["24344199"],"is_preprint":false},{"year":2013,"finding":"Lgr4 regulates mammary gland development and stem cell activity through the Wnt/β-catenin/Lef1 pathway, which controls Sox2 expression; Sox2 overexpression restores in vivo mammary regeneration potential of Lgr4-/- mammary stem cells.","method":"Lgr4-/- mice, mammosphere formation, chromatin immunoprecipitation, Wnt3a rescue, Sox2 overexpression rescue","journal":"Stem cells","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP identifies direct transcriptional target, rescue validates causal pathway, single lab","pmids":["23712846"],"is_preprint":false},{"year":2014,"finding":"LGR4 inactivation down-regulates histone demethylases Jmjd2a and Fbxl10 through the cAMP-CREB signaling pathway, contributing to AGR syndrome-like developmental defects (aniridia, polycystic kidney disease, genitourinary anomalies, mental retardation) in Lgr4-/- mice.","method":"Lgr4-/- mice, cAMP-CREB pathway analysis, gene expression in MEF cells and tissues","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with defined epigenetic signaling mechanism, single lab","pmids":["24519938"],"is_preprint":false},{"year":2015,"finding":"LGR4 and LGR5 promote formation of long actin-rich cytoneme-like membrane protrusions (>80 μm) through stabilization of nascent filopodia; LGR5-induced cytonemes serve as conduits for transit of myosin X (Myo10) and β-arrestin-2 (Arrb2).","method":"Live cell imaging, overexpression, fluorescent protein tagging of cargo proteins","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 — direct imaging of cytoneme formation and cargo transit, single lab","pmids":["25653388"],"is_preprint":false},{"year":2020,"finding":"LGR4 deficiency delays puberty by impairing Wnt/β-catenin signaling in GnRH neuron development; Lgr4-/- mice have fewer GnRH neurons; lgr4 knockdown in zebrafish prevents GnRH neuron formation and migration; Lgr4-mediated Wnt/β-catenin activation during GnRH neuron development confirmed by genetic lineage tracing.","method":"LGR4 mutant functional characterization, conditional KO mice, zebrafish morpholino knockdown, genetic lineage tracing with Wnt reporter","journal":"JCI insight","confidence":"Medium","confidence_rationale":"Tier 2 — multiple model organisms with lineage tracing, single lab","pmids":["32493844"],"is_preprint":false},{"year":2022,"finding":"RSPO2 and RANKL signal through LGR4 to promote osteoclastic premetastatic niche formation; LGR4 modulates DKK1 expression through Gαq and β-catenin signaling; DKK1 facilitates osteoclast precursor recruitment via suppression of LRP5 (not LRP6); soluble LGR4-ECD decoy receptor alleviates bone metastasis.","method":"GPCR ligand/agonist screening, signaling pathway analysis, conditional models, in vivo bone metastasis model, LGR4-ECD treatment","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 — unbiased receptor screen with pathway dissection and in vivo validation, single lab","pmids":["34847079"],"is_preprint":false},{"year":2019,"finding":"The miR-34-LGR4 axis regulates keratinocyte inflammatory response; miR-34a and miR-34c target LGR4 directly; LGR4 regulates GSK-3β-induced p65 serine 468 phosphorylation to modulate NF-κB signaling pathway activity; Lgr4 knockout mice show impaired wound closure with enhanced inflammation.","method":"miRNA mimic transfection, wound healing assay, Lgr4 KO mice, NF-κB phosphorylation analysis","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO and molecular mechanism validated, single lab","pmids":["31376385"],"is_preprint":false},{"year":2020,"finding":"RSPO-LGR4 signaling is required for H. pylori-induced gastric hyperplasia; LGR4 regulates Lgr5 expression and drives NF-κB activity in proliferative stem cells; Lgr4-driven NF-κB activation upon H. pylori exposure causes expansion of the gland base module and chemokine expression inducing neutrophil recruitment.","method":"Conditional knockout mice, reporter assays for NF-κB activity, H. pylori infection model","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 — conditional KO with defined NF-κB pathway mechanism, single lab","pmids":["35767364"],"is_preprint":false},{"year":2020,"finding":"Rspo1/Rspo3-LGR4 signaling inhibits hepatic cholesterol synthesis via the AMPKα-SREBP2 pathway; LGR4 knockdown increases hepatic cholesterol synthesis and decreases AMPKα phosphorylation; AMPKα activation/inhibition abolishes effects of LGR4 deficiency or RSPO treatment.","method":"Liver-specific knockdown mice, in vitro hepatocyte assays, AMPKα agonist/antagonist/shRNA, SREBP2 nuclear translocation assay","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and pharmacological epistasis defining pathway, single lab","pmids":["32926477"],"is_preprint":false},{"year":2018,"finding":"LGR4 activation by Rspo1 protects hepatocytes from TNFα-induced cell death by inhibiting NF-κB signaling; LGR4 knockdown increases nuclear translocation of NF-κB-p65; IKKβ overexpression attenuates Rspo1 protective effects; Rspo1 potentiates β-catenin stabilization.","method":"Liver-specific LGR4 knockout, hepatocyte culture, NF-κB pathway assays, IKKβ overexpression, β-catenin stabilization assay","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and biochemical approaches defining mechanism, single lab","pmids":["30406697"],"is_preprint":false},{"year":2016,"finding":"Lgr4 mediates skin carcinogenesis by regulating MEK/ERK and Wnt/β-catenin signaling; TPA activates MEK1, ERK1/2 and AP-1 in wild-type but not Lgr4-depleted epidermal cells; MEK1/ERK1/2 pathway activation lies upstream of Wnt/β-catenin in this context.","method":"Lgr4-/- mice, TPA-induced papilloma model, MEK/ERK and Wnt pathway inhibitors, Western blot","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO with pathway ordering via pathway inhibitors, single lab","pmids":["27693558"],"is_preprint":false},{"year":2016,"finding":"Aberrant LGR4 expression in multiple myeloma is driven by IL-6/STAT3 signaling, allowing myeloma cells to hijack R-spondins from osteoblasts in the bone marrow niche, resulting in Wnt co-receptor stabilization and increased sensitivity to Wnt ligands.","method":"Co-immunoprecipitation, reporter assays, IL-6/STAT3 inhibitors, R-spondin binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — upstream driver identified (IL-6/STAT3) and mechanism defined (receptor stabilization), single lab","pmids":["28028233"],"is_preprint":false},{"year":2024,"finding":"LGR4-dependent Wnt signaling transcriptionally upregulates SLC7A11 (a key ferroptosis inhibitor) to confer acquired drug resistance in colorectal cancer; LGR4-mAb inhibits LGR4-Wnt signaling and sensitizes drug-induced ferroptosis.","method":"Organoid drug resistance model, transcriptomic analysis, LGR4 monoclonal antibody, ferroptosis assays, SLC7A11 expression analysis","journal":"Nature cancer","confidence":"Medium","confidence_rationale":"Tier 2 — defined transcriptional target with functional validation, single lab","pmids":["38291304"],"is_preprint":false}],"current_model":"LGR4 is a leucine-rich repeat-containing GPCR that functions primarily as a high-affinity receptor for R-spondins (and also RANKL, norrin, and nidogen-2), potentiating Wnt/β-catenin signaling by forming RSPO-bridged complexes with the E3 ubiquitin ligases ZNRF3/RNF43 (via its transmembrane domain) to prevent Wnt receptor ubiquitination, while also recruiting the scaffold IQGAP1 to the Wnt signalosome to promote MEK1/2-mediated LRP5/6 phosphorylation; it additionally signals through Gαq-GSK3β (suppressing NFATc1/osteoclastogenesis), cAMP-PKA-CREB-ATF4 (erythropoiesis), NF-κB (innate immunity and hepatoprotection), ERK/STAT3 (macrophage polarization), and a Wnt-independent LGR4-EGFR axis (blocking EGFR ubiquitination to sustain EGFR activation in cancer metastasis), with LGR4's seven-transmembrane domain—distinct from LGR5—being uniquely required for constitutive interaction with ZNRF3/RNF43."},"narrative":{"teleology":[{"year":2008,"claim":"Before the R-spondin connection was established, LGR4 was shown to be an orphan GPCR that regulated definitive erythropoiesis through cAMP-PKA-CREB-ATF4 signaling and keratinocyte EGFR transactivation, establishing its coupling to classical GPCR second messengers and crosstalk with receptor tyrosine kinases.","evidence":"Lgr4 knockout mice with hematopoietic and eyelid developmental phenotypes, signaling pathway analysis","pmids":["18955481","18487371"],"confidence":"Medium","gaps":["Endogenous ligand unknown at the time","cAMP-PKA pathway coupling not confirmed with purified receptor reconstitution","EGFR transactivation mechanism not fully defined"]},{"year":2011,"claim":"The identification of R-spondins as high-affinity LGR4 ligands resolved the orphan receptor problem and established a new paradigm: LGR4 potentiates Wnt/β-catenin signaling not through classical G protein coupling but through a unique mechanism involving enhanced LRP6 phosphorylation.","evidence":"Binding assays, LRP6 phosphorylation, G protein and β-arrestin coupling assays, gain/loss-of-function in mammalian cells and Xenopus","pmids":["21693646","21909076","22815884"],"confidence":"High","gaps":["Downstream effector mechanism of Wnt potentiation not yet defined","Relative contributions of LGR4 vs LGR5 in vivo unclear"]},{"year":2013,"claim":"Crystal structures of the LGR4 ectodomain–RSPO1 complex revealed a TLR-like horseshoe architecture engaging R-spondin furin-like repeats on its concave surface, providing the atomic basis for ligand recognition and explaining conservation across LGR4-6.","evidence":"X-ray crystallography with functional mutagenesis from multiple independent groups","pmids":["23756652","23891289","25480784"],"confidence":"High","gaps":["No full-length receptor structure including transmembrane domain","Structural basis for ZNRF3/RNF43 interaction unknown"]},{"year":2013,"claim":"The mechanism of Wnt potentiation was resolved: R-spondin simultaneously binds LGR4 and ZNRF3/RNF43 through distinct motifs, forming a ternary complex that triggers membrane clearance of the E3 ligases and prevents Frizzled/LRP ubiquitination; intracellularly, LGR4 recruits IQGAP1 to the Wnt signalosome to drive MEK1/2-mediated LRP5/6 phosphorylation.","evidence":"Mutagenesis of R-spondin dual-binding motifs, co-immunoprecipitation of ternary complex, ZNRF3 clearance assays, IQGAP1 pulldown and signaling assays, in vitro reconstitution of ternary complexes","pmids":["24165923","24050775","24639526"],"confidence":"High","gaps":["Stoichiometry of ternary complex in native membranes unclear","Role of clathrin-mediated endocytosis in ternary complex turnover not fully resolved"]},{"year":2013,"claim":"LGR4-dependent Wnt/β-catenin signaling was shown to be essential for intestinal stem cell maintenance, Paneth cell differentiation, spermatogenesis, mammary gland development, and myogenic differentiation, establishing LGR4 as a master amplifier of Wnt signaling across multiple stem/progenitor cell compartments.","evidence":"Lgr4 knockout and conditional knockout mice, ex vivo crypt cultures, genetic epistasis with Apc(min/+), Sox2 and follistatin as downstream targets identified by ChIP and promoter analysis","pmids":["21508962","23533175","23712846","24344199"],"confidence":"Medium","gaps":["Relative contributions of LGR4 vs LGR5 in each tissue not fully delineated","Whether RSPO isoform selectivity differs across tissues not resolved"]},{"year":2016,"claim":"LGR4 was identified as a second receptor for RANKL that signals through Gαq-GSK3β to suppress NFATc1 and osteoclastogenesis, revealing a Wnt-independent G protein-coupled signaling axis and establishing LGR4 as a multi-ligand receptor.","evidence":"Co-immunoprecipitation, competition binding with RANK, Gαq signaling assays, conditional knockout mice, soluble LGR4-ECD treatment in vivo","pmids":["27064449"],"confidence":"High","gaps":["Structural basis for RANKL–LGR4 interaction unknown","Whether RANKL and RSPO signaling through LGR4 are mutually exclusive not tested"]},{"year":2016,"claim":"The RSPO-LGR4/5-ZNRF3/RNF43 module was demonstrated to control hepatic metabolic zonation by maintaining the Wnt/β-catenin gradient across the liver lobule, showing that the ternary complex mechanism operates in terminally differentiated tissue homeostasis, not only in stem cells.","evidence":"Liver-specific LGR4/5 conditional knockout, recombinant RSPO1 treatment reversibly expanding the Wnt zone","pmids":["27088858"],"confidence":"High","gaps":["Whether RSPO dosage titrates zone boundaries quantitatively not fully modeled","Relative contributions of LGR4 vs LGR5 in hepatocytes not separated"]},{"year":2018,"claim":"LGR4 was shown to regulate macrophage polarization: RSPO-LGR4-ERK-STAT3 signaling promotes M2 polarization of tumor-associated macrophages, while LGR4-CREB-driven AP-1 transactivation governs pro-inflammatory macrophage programs, demonstrating context-dependent immune signaling outputs.","evidence":"Macrophage-specific Lgr4 conditional knockout mice, tumor models, cardiac injury models, transcriptomics, AP-1 and CREB activity assays","pmids":["29967265","32600176"],"confidence":"High","gaps":["How the same receptor achieves opposing macrophage polarization outcomes (M2 vs pro-inflammatory) in different contexts not mechanistically reconciled","Whether RSPO is the relevant ligand in both contexts not confirmed"]},{"year":2020,"claim":"Domain-swap experiments revealed that the LGR4 seven-transmembrane domain constitutively interacts with ZNRF3/RNF43 even without R-spondin, distinguishing LGR4 mechanistically from LGR5, which instead enhances Wnt signaling by interacting with FZD-LRP6; both receptors exist as dimers.","evidence":"Co-immunoprecipitation, proximity ligation, time-resolved FRET, domain-swap chimeras between LGR4 and LGR5","pmids":["33262293"],"confidence":"High","gaps":["Structure of TM domain–ZNRF3 interface not determined","Functional significance of LGR4 dimerization not established"]},{"year":2021,"claim":"A Wnt-independent LGR4-EGFR axis was uncovered: LGR4 physically interacts with EGFR and blocks its ubiquitination, sustaining EGFR activation to drive cancer metastasis; LGR4 mutants defective in Wnt potentiation retain full pro-metastatic capacity, functionally separating Wnt and EGFR outputs of LGR4.","evidence":"Co-immunoprecipitation, EGFR ubiquitination assay, Wnt-signaling-dead LGR4 mutants, in vivo xenograft metastasis","pmids":["34099494"],"confidence":"High","gaps":["Which LGR4 domain mediates EGFR interaction unknown","Whether EGFR-protective function occurs in normal physiology or is cancer-specific not tested"]},{"year":2022,"claim":"Nidogen-2 was identified as a novel endogenous ligand of LGR4, engaging Gαq-PKCα-AMPKα1 biased signaling to counteract vascular smooth muscle cell osteogenic transition and calcification, expanding the ligand repertoire beyond R-spondins and RANKL.","evidence":"Membrane protein interactome screen, SPR binding (KD ~27 nM), G protein signaling assays, conditional knockout","pmids":["36354004"],"confidence":"High","gaps":["Whether nidogen-2 and RSPO compete for LGR4 binding unknown","Structural basis for nidogen-2–LGR4 recognition not determined"]},{"year":2024,"claim":"LGR4-Wnt signaling was shown to transcriptionally upregulate SLC7A11 to inhibit ferroptosis, conferring acquired drug resistance in colorectal cancer; a therapeutic LGR4 monoclonal antibody reversed this resistance by sensitizing tumors to ferroptosis.","evidence":"Organoid drug resistance model, transcriptomics, LGR4-mAb treatment, ferroptosis assays","pmids":["38291304"],"confidence":"Medium","gaps":["Whether SLC7A11 upregulation is a direct β-catenin/TCF target or indirect not fully dissected","In vivo efficacy of LGR4-mAb across tumor types not established"]},{"year":null,"claim":"Key unresolved questions include the full-length structure of LGR4 including its transmembrane domain in complex with ZNRF3/RNF43 or EGFR, how a single receptor achieves biased signaling through Gαq, cAMP, NF-κB, and β-catenin depending on ligand and cellular context, and whether the diverse ligands (R-spondins, RANKL, norrin, nidogen-2) compete for or cooperate through overlapping or distinct binding sites.","evidence":"","pmids":[],"confidence":"Low","gaps":["No full-length LGR4 structure available","Biased signaling mechanism across multiple ligands not reconciled","Functional interplay among four known ligand classes not systematically tested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,7,9,13]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5,10,20]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,5,10,29]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,5,6,9,10,13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[21,22,27,30]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[14,15,24,33]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[20,37,38]}],"complexes":["RSPO–LGR4–ZNRF3/RNF43 ternary complex","LGR4–IQGAP1–DVL signalosome"],"partners":["ZNRF3","RNF43","IQGAP1","EGFR","RSPO1","RSPO2","RANKL","NID2"],"other_free_text":[]},"mechanistic_narrative":"LGR4 is a leucine-rich repeat-containing G protein-coupled receptor that serves as a high-affinity receptor for R-spondins and potentiates Wnt/β-catenin signaling by forming ternary complexes with the E3 ubiquitin ligases ZNRF3/RNF43, driving their membrane clearance and thereby stabilizing Wnt receptors [PMID:21693646, PMID:24165923, PMID:33262293]. Structurally, the LGR4 extracellular domain adopts a horseshoe-shaped leucine-rich repeat fold that engages R-spondin furin-like domains on its concave surface, and its seven-transmembrane domain uniquely mediates constitutive interaction with ZNRF3/RNF43 independently of R-spondin [PMID:23756652, PMID:33262293]. Beyond canonical Wnt potentiation, LGR4 functions as a receptor for RANKL, norrin, and nidogen-2, engaging Gαq, cAMP-PKA-CREB, NF-κB, and ERK/STAT3 pathways to regulate osteoclastogenesis, erythropoiesis, innate immunity, macrophage polarization, and vascular calcification [PMID:27064449, PMID:18955481, PMID:29967265, PMID:36354004]. LGR4 also directly interacts with EGFR through a Wnt-independent mechanism that blocks EGFR ubiquitination and sustains EGFR signaling, promoting cancer cell metastasis [PMID:34099494]."},"prefetch_data":{"uniprot":{"accession":"Q9BXB1","full_name":"Leucine-rich repeat-containing G-protein coupled receptor 4","aliases":["G-protein coupled receptor 48"],"length_aa":951,"mass_kda":104.5,"function":"Receptor for R-spondins that potentiates the canonical Wnt signaling pathway and is involved in the formation of various organs. Upon binding to R-spondins (RSPO1, RSPO2, RSPO3 or RSPO4), associates with phosphorylated LRP6 and frizzled receptors that are activated by extracellular Wnt receptors, triggering the canonical Wnt signaling pathway to increase expression of target genes. In contrast to classical G-protein coupled receptors, does not activate heterotrimeric G-proteins to transduce the signal. Its function as activator of the Wnt signaling pathway is required for the development of various organs, including liver, kidney, intestine, bone, reproductive tract and eye. May also act as a receptor for norrin (NDP), such results however require additional confirmation in vivo. Required during spermatogenesis to activate the Wnt signaling pathway in peritubular myoid cells. Required for the maintenance of intestinal stem cells and Paneth cell differentiation in postnatal intestinal crypts. Acts as a regulator of bone formation and remodeling. Involved in kidney development; required for maintaining the ureteric bud in an undifferentiated state. Involved in the development of the anterior segment of the eye. Required during erythropoiesis. Also acts as a negative regulator of innate immunity by inhibiting TLR2/TLR4 associated pattern-recognition and pro-inflammatory cytokine production. Plays an important role in regulating the circadian rhythms of plasma lipids, partially through regulating the rhythmic expression of MTTP (By similarity). Required for proper development of GnRH neurons (gonadotropin-releasing hormone expressing neurons) that control the release of reproductive hormones from the pituitary gland (By similarity)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9BXB1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LGR4","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/LGR4","total_profiled":1310},"omim":[{"mim_id":"620946","title":"UBIQUITIN-SPECIFIC PEPTIDASE 42; USP42","url":"https://www.omim.org/entry/620946"},{"mim_id":"619613","title":"DELAYED PUBERTY, SELF-LIMITED; DPSL","url":"https://www.omim.org/entry/619613"},{"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; TETAMS2","url":"https://www.omim.org/entry/618021"},{"mim_id":"615311","title":"BONE MINERAL DENSITY QUANTITATIVE TRAIT LOCUS 17; BMND17","url":"https://www.omim.org/entry/615311"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Centrosome","reliability":"Approved"},{"location":"Centriolar satellite","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LGR4"},"hgnc":{"alias_symbol":[],"prev_symbol":["GPR48"]},"alphafold":{"accession":"Q9BXB1","domains":[{"cath_id":"1.20.1070.10","chopping":"540-731_745-819","consensus_level":"high","plddt":84.9439,"start":540,"end":819}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXB1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXB1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BXB1-F1-predicted_aligned_error_v6.png","plddt_mean":78.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LGR4","jax_strain_url":"https://www.jax.org/strain/search?query=LGR4"},"sequence":{"accession":"Q9BXB1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BXB1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BXB1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BXB1"}},"corpus_meta":[{"pmid":"21693646","id":"PMC_21693646","title":"R-spondins function as 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neither receptor couples to heterotrimeric G proteins or β-arrestin upon RSPO stimulation, indicating a unique signaling mechanism.\",\n      \"method\": \"Binding assays, LRP6 phosphorylation assays, G protein coupling assays, β-arrestin assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal biochemical assays, replicated across independent labs\",\n      \"pmids\": [\"21693646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"LGR4 and LGR5 bind R-spondins via their Furin domains and promote R-spondin-mediated Wnt/β-catenin and Wnt/PCP signaling; R-spondin-triggered β-catenin signaling requires Clathrin-mediated endocytosis, whereas Wnt3a-mediated signaling requires Caveolin-mediated endocytosis.\",\n      \"method\": \"Gain- and loss-of-function experiments in mammalian cells and Xenopus embryos, endocytosis inhibitor assays\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal gain/loss-of-function in two model systems, independent lab replication\",\n      \"pmids\": [\"21909076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of LGR4 extracellular domain (ECD) in complex with RSPO1 N-terminal fragment reveals that LGR4-ECD adopts a TLR horseshoe structure and uses its concave surface near the N-terminus to bind RSPO1; both FU-CRD1 and FU-CRD2 domains of RSPO1 contribute to LGR4 binding.\",\n      \"method\": \"X-ray crystallography, binding assays, mutagenesis\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional mutagenesis validation\",\n      \"pmids\": [\"23756652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of LGR4 ectodomain alone and in complex with Rspo1 shows an extended horseshoe LRR architecture that binds ligand furin-like repeats via its concave side through electrostatic and hydrophobic interactions.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — independent crystal structure determination confirming binding interface\",\n      \"pmids\": [\"23891289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Crystal structure of LGR4-Rspo1 complex confirms concave surface as the sole binding site for R-spondins; all Rspo1-binding residues are conserved in LGR4-6, suggesting identical binding surface; LGR4-6 use a one-site binding model distinct from group A and C LGR members.\",\n      \"method\": \"X-ray crystallography, structural analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with comparative structural analysis\",\n      \"pmids\": [\"25480784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"R-spondin interacts with ZNRF3/RNF43 and LGR4 through distinct motifs; both LGR4 and ZNRF3 binding motifs are required for R-spondin-induced LGR4/ZNRF3 complex formation, membrane clearance of ZNRF3, and Wnt signaling activation; LGR4/5 serve as engagement receptors while ZNRF3/RNF43 are effector receptors.\",\n      \"method\": \"Mutagenesis, co-immunoprecipitation, ZNRF3 membrane clearance assay, Wnt signaling assay\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis of distinct binding motifs combined with functional assays\",\n      \"pmids\": [\"24165923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"RSPO-LGR4 signaling functions via the intracellular scaffold protein IQGAP1: LGR4 interacts with IQGAP1, which bridges RSPO-LGR4 to the Wnt signalosome through enhanced IQGAP1-DVL interaction following RSPO stimulation, promoting MEK1/2-mediated LRP5/6 phosphorylation for canonical Wnt and actin dynamics regulation for non-canonical Wnt signaling.\",\n      \"method\": \"Co-immunoprecipitation, pulldown, signaling assays, actin dynamics assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with multiple orthogonal functional assays\",\n      \"pmids\": [\"24639526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"LGR4 is the cognate receptor of RSPO identified by unbiased siRNA screen; RSPO binds the extracellular domain of LGR4; LGR4 depletion abolishes RSPO-induced β-catenin signaling; overexpression of LGR5 can compensate for LGR4 loss; no coupling to heterotrimeric G proteins detected.\",\n      \"method\": \"siRNA screen, binding assay, β-catenin signaling assay, G protein coupling assay, crypt culture\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — unbiased screen confirmed by multiple orthogonal assays\",\n      \"pmids\": [\"22815884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Recombinant RSPO proteins bind LGR4 LRR1-14 with nanomolar affinities (order RSPO4>RSPO2>RSPO3>RSPO1); ternary RSPO:LGR4:ZNRF3 complexes form for RSPO2 and RSPO3; stronger signaling potencies of RSPO2 and RSPO3 result from strong binding of both LGR4 and ZNRF3.\",\n      \"method\": \"TR-FRET binding assay, native gel electrophoretic mobility shift assay, cell-based signaling assay, in vitro reconstitution with bacterially expressed proteins\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted ternary complex in vitro with quantitative binding measurements\",\n      \"pmids\": [\"24050775\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"LGR4 is a second receptor for RANKL that competes with RANK to bind RANKL; RANKL binding to LGR4 activates the Gαq and GSK3-β signaling pathway, suppressing NFATc1 expression and activity to negatively regulate osteoclastogenesis; LGR4 extracellular domain (ECD) binds RANKL and inhibits osteoclast differentiation in vivo.\",\n      \"method\": \"Co-immunoprecipitation, competition binding, Gαq signaling assays, osteoclast differentiation assays, conditional knockout mice, in vivo ECD treatment\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding established, multiple signaling assays, in vivo genetic and pharmacological validation\",\n      \"pmids\": [\"27064449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Full-length LGR4 forms a tight complex with ZNRF3 and RNF43 even without RSPO, whereas LGR5 does not interact with either E3 ligase; the seven-transmembrane domain of LGR4 confers interaction with E3 ligases; LGR5 potentiates Wnt signaling by interacting with FZD and LRP6 to enhance LRP6 phosphorylation; LGR4 and LGR5 exist as dimers on the cell surface.\",\n      \"method\": \"Co-immunoprecipitation, proximity ligation assay, competition binding, time-resolved FRET, domain-swapping experiments\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including domain-swap mutagenesis distinguishing LGR4 vs LGR5 mechanisms\",\n      \"pmids\": [\"33262293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The RSPO-LGR4/5-ZNRF3/RNF43 module controls hepatic Wnt/β-catenin-mediated metabolic liver zonation; liver-specific LGR4/5 loss disrupts Wnt/β-catenin signaling and zonation; pathway activation with recombinant RSPO1 expanded the hepatic Wnt/β-catenin gradient in a reversible and LGR4/5-dependent manner.\",\n      \"method\": \"Conditional knockout mice, RSPO1 recombinant protein treatment, reporter assays, histology\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic loss-of-function with defined molecular phenotype, rescue experiments\",\n      \"pmids\": [\"27088858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Norrin (Norrie disease gene product) is a ligand for LGR4 that stimulates Wnt signaling mediated specifically by LGR4 (not LGR5/6) in mammalian cells; mutagenesis of norrin revealed distinct residues required for LGR4 signaling versus Frizzled4 signaling versus BMP antagonism.\",\n      \"method\": \"Cell-based Wnt signaling assay, binding studies, mutagenesis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional signaling assay with mutagenesis, single lab\",\n      \"pmids\": [\"23444378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Nidogen-2 is a novel endogenous ligand of LGR4 (KD = 26.77 nM) that biased-activates LGR4-Gαq-PKCα-AMPKα1 signaling to counteract VSMC osteogenic transition and vascular calcification; LGR4 deficiency in VSMCs abolishes the protective effect of nidogen-2.\",\n      \"method\": \"Membrane protein interactome, co-immunoprecipitation, surface plasmon resonance, flow cytometric binding assay, G protein signaling assays, conditional knockout\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding quantified by SPR, G protein signaling pathway defined, in vitro and in vivo genetic validation\",\n      \"pmids\": [\"36354004\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Lgr4 promotes macrophage M2 polarization through Rspo/Lgr4/Erk/Stat3 signaling; macrophage-specific Lgr4 knockout reduces M2 tumor-associated macrophages and increases CD8+ T lymphocyte infiltration in the tumor microenvironment.\",\n      \"method\": \"Macrophage-conditional knockout mice (Lgr4fl/fl Lyz2cre/+), tumor models, signaling pathway analysis\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean cell-type-specific KO with defined phenotype and pathway placement\",\n      \"pmids\": [\"29967265\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Lgr4 governs a pro-inflammatory program in macrophages via enhancing CREB-mediated c-Fos, Fosl1, and Fosb transactivation to activate AP-1 in inflammatory macrophages; macrophage-specific Lgr4 knockout improves cardiac repair after myocardial infarction.\",\n      \"method\": \"Macrophage-specific conditional knockout mice, transcriptomic profiling, AP-1 activity assays, CREB activation assays\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean cell-type-specific KO with mechanistic pathway defined by transcriptomics and signaling assays\",\n      \"pmids\": [\"32600176\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4 ablation promotes white-to-brown fat transition partially through retinoblastoma 1 gene (Rb1) reduction; Lgr4 mutant mice show increased energy expenditure with brown-like adipocytes in WAT depots.\",\n      \"method\": \"Lgr4 mutant mice, brown adipocyte differentiation assays from stromal vascular fraction, Rb1 expression analysis\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with defined molecular mechanism (Rb1 reduction) and cellular phenotype\",\n      \"pmids\": [\"24212090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Gpr48/LGR4 regulates definitive erythropoiesis through the cAMP-PKA-CREB pathway, which controls ATF4 expression; Gpr48-/- fetal livers show decreased definitive erythroid precursors with reduced c-Myc and cyclin D1 expression.\",\n      \"method\": \"Knockout mice, real-time PCR, histology, blood smear assay, signaling pathway analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined downstream pathway, single lab\",\n      \"pmids\": [\"18955481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"GPR48/LGR4 mediates keratinocyte proliferation and migration during eyelid development through activation of EGFR signaling; GPR48 regulates EGFR phosphorylation, and GPR48 inactivation results in decreased EGFR phosphorylation and reduced F-actin formation.\",\n      \"method\": \"Knockout mice, in vitro scratch assay, Western blot, immunostaining, scanning electron microscopy\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined signaling mechanism, single lab\",\n      \"pmids\": [\"18487371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"GPR48/LGR4 mediates keratinocyte proliferation via HB-EGF-mediated EGFR transactivation; HB-EGF is reduced in Gpr48-/- cell culture medium and its addition restores EGFR, ERK, and STAT3 phosphorylation as well as cell proliferation.\",\n      \"method\": \"EGFR inhibitor treatment, conditioned media immunodepletion, HB-EGF reconstitution, Western blot\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological and biochemical dissection of signaling mechanism, single lab\",\n      \"pmids\": [\"20732323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"LGR4 promotes cancer metastasis through a Wnt-independent axis: LGR4 interacts with EGFR and blocks EGFR ubiquitination and degradation, resulting in persistent EGFR activation; LGR4 mutants unable to potentiate Wnt signaling still promote cancer cell migration and invasion.\",\n      \"method\": \"Co-immunoprecipitation, EGFR ubiquitination assay, Wnt pathway inhibition (pharmacological and genetic), in vitro migration/invasion, in vivo xenograft metastasis\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain mutants separating Wnt vs EGFR functions, multiple orthogonal assays including in vivo validation\",\n      \"pmids\": [\"34099494\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4 is required for Paneth cell differentiation and intestinal stem cell maintenance; LGR4-deficient crypts cultured ex vivo die rapidly with downregulation of stem cell markers and Wnt target genes; partial rescue achieved with LiCl but not Wnt agonists.\",\n      \"method\": \"Lgr4 gene inactivation, ex vivo crypt culture, histology, marker gene analysis\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined cellular and molecular phenotype, single lab\",\n      \"pmids\": [\"21508962\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4 regulates Wnt/β-catenin signaling in peritubular myoid cells (PMCs) to support spermatogenesis; Lgr4 loss attenuates Wnt/β-catenin signaling in PMCs; reactivation by crossing with Apc(min/+) or GSK3β inhibitor partially rescues the Lgr4-/- testicular phenotype.\",\n      \"method\": \"Conditional knockout mice, signaling analysis, genetic rescue (Apc(min/+) cross), pharmacological rescue (GSK3β inhibitor)\",\n      \"journal\": \"Development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pathway placement via genetic epistasis with rescue, replicated by pharmacological approach\",\n      \"pmids\": [\"23533175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4 protects against inflammatory bowel disease through Wnt/β-catenin signaling; Lgr4 deficiency reduces Paneth and stem cells; crossing with Apc(min/+) mice or GSK3β inhibitor treatment partially restores Paneth cell number and reduces DSS-induced mortality.\",\n      \"method\": \"Hypomorphic mouse model, DSS colitis, genetic rescue (Apc(min/+)), pharmacological rescue, histology\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological epistasis, single lab\",\n      \"pmids\": [\"23393138\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4/Gpr48 negatively regulates TLR2/4-associated innate immunity by suppressing CD14 expression; Lgr4 deletion increases CD14 expression through decreased cAMP-response element-binding protein (CREB) activity, enhancing TLR2/4-associated cytokine production.\",\n      \"method\": \"Lgr4 knockout macrophages, TLR signaling assays, CREB activity assay, CD14 expression analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined signaling mechanism, single lab\",\n      \"pmids\": [\"23589304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Lgr4 regulates corpus luteum maturation through WNT-mediated EGFR-ERK signaling; Lgr4-deficient granulosa-lutein cells show attenuated EGFR-ERK signaling; Wnt/β-catenin target MMP9 activates EGF-like factors to facilitate ERK activation and luteinization.\",\n      \"method\": \"Lgr4-/- mice, ovarian granulosa cell culture, EGFR-ERK signaling assays, MMP9 inhibitor, HB-EGF rescue\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined signaling mechanism and partial rescue, single lab\",\n      \"pmids\": [\"24877628\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"RSPO-LGR4 signaling in myoblasts activates Wnt/β-catenin pathway to upregulate follistatin (Fst) expression via TCF/LEF1 binding sites in the Fst promoter and intron 1; LGR4 depletion severely disrupts myogenic differentiation and diminishes RSPO2 response.\",\n      \"method\": \"siRNA depletion, luciferase reporter assay with TCF/LEF1 site mutagenesis, C2C12 differentiation assay, in vivo β-catenin activation/inactivation\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct target gene identified with promoter mutagenesis and in vivo validation, single lab\",\n      \"pmids\": [\"24344199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Lgr4 regulates mammary gland development and stem cell activity through the Wnt/β-catenin/Lef1 pathway, which controls Sox2 expression; Sox2 overexpression restores in vivo mammary regeneration potential of Lgr4-/- mammary stem cells.\",\n      \"method\": \"Lgr4-/- mice, mammosphere formation, chromatin immunoprecipitation, Wnt3a rescue, Sox2 overexpression rescue\",\n      \"journal\": \"Stem cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP identifies direct transcriptional target, rescue validates causal pathway, single lab\",\n      \"pmids\": [\"23712846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"LGR4 inactivation down-regulates histone demethylases Jmjd2a and Fbxl10 through the cAMP-CREB signaling pathway, contributing to AGR syndrome-like developmental defects (aniridia, polycystic kidney disease, genitourinary anomalies, mental retardation) in Lgr4-/- mice.\",\n      \"method\": \"Lgr4-/- mice, cAMP-CREB pathway analysis, gene expression in MEF cells and tissues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined epigenetic signaling mechanism, single lab\",\n      \"pmids\": [\"24519938\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"LGR4 and LGR5 promote formation of long actin-rich cytoneme-like membrane protrusions (>80 μm) through stabilization of nascent filopodia; LGR5-induced cytonemes serve as conduits for transit of myosin X (Myo10) and β-arrestin-2 (Arrb2).\",\n      \"method\": \"Live cell imaging, overexpression, fluorescent protein tagging of cargo proteins\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct imaging of cytoneme formation and cargo transit, single lab\",\n      \"pmids\": [\"25653388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"LGR4 deficiency delays puberty by impairing Wnt/β-catenin signaling in GnRH neuron development; Lgr4-/- mice have fewer GnRH neurons; lgr4 knockdown in zebrafish prevents GnRH neuron formation and migration; Lgr4-mediated Wnt/β-catenin activation during GnRH neuron development confirmed by genetic lineage tracing.\",\n      \"method\": \"LGR4 mutant functional characterization, conditional KO mice, zebrafish morpholino knockdown, genetic lineage tracing with Wnt reporter\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple model organisms with lineage tracing, single lab\",\n      \"pmids\": [\"32493844\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RSPO2 and RANKL signal through LGR4 to promote osteoclastic premetastatic niche formation; LGR4 modulates DKK1 expression through Gαq and β-catenin signaling; DKK1 facilitates osteoclast precursor recruitment via suppression of LRP5 (not LRP6); soluble LGR4-ECD decoy receptor alleviates bone metastasis.\",\n      \"method\": \"GPCR ligand/agonist screening, signaling pathway analysis, conditional models, in vivo bone metastasis model, LGR4-ECD treatment\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — unbiased receptor screen with pathway dissection and in vivo validation, single lab\",\n      \"pmids\": [\"34847079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The miR-34-LGR4 axis regulates keratinocyte inflammatory response; miR-34a and miR-34c target LGR4 directly; LGR4 regulates GSK-3β-induced p65 serine 468 phosphorylation to modulate NF-κB signaling pathway activity; Lgr4 knockout mice show impaired wound closure with enhanced inflammation.\",\n      \"method\": \"miRNA mimic transfection, wound healing assay, Lgr4 KO mice, NF-κB phosphorylation analysis\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO and molecular mechanism validated, single lab\",\n      \"pmids\": [\"31376385\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"RSPO-LGR4 signaling is required for H. pylori-induced gastric hyperplasia; LGR4 regulates Lgr5 expression and drives NF-κB activity in proliferative stem cells; Lgr4-driven NF-κB activation upon H. pylori exposure causes expansion of the gland base module and chemokine expression inducing neutrophil recruitment.\",\n      \"method\": \"Conditional knockout mice, reporter assays for NF-κB activity, H. pylori infection model\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with defined NF-κB pathway mechanism, single lab\",\n      \"pmids\": [\"35767364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Rspo1/Rspo3-LGR4 signaling inhibits hepatic cholesterol synthesis via the AMPKα-SREBP2 pathway; LGR4 knockdown increases hepatic cholesterol synthesis and decreases AMPKα phosphorylation; AMPKα activation/inhibition abolishes effects of LGR4 deficiency or RSPO treatment.\",\n      \"method\": \"Liver-specific knockdown mice, in vitro hepatocyte assays, AMPKα agonist/antagonist/shRNA, SREBP2 nuclear translocation assay\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological epistasis defining pathway, single lab\",\n      \"pmids\": [\"32926477\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"LGR4 activation by Rspo1 protects hepatocytes from TNFα-induced cell death by inhibiting NF-κB signaling; LGR4 knockdown increases nuclear translocation of NF-κB-p65; IKKβ overexpression attenuates Rspo1 protective effects; Rspo1 potentiates β-catenin stabilization.\",\n      \"method\": \"Liver-specific LGR4 knockout, hepatocyte culture, NF-κB pathway assays, IKKβ overexpression, β-catenin stabilization assay\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and biochemical approaches defining mechanism, single lab\",\n      \"pmids\": [\"30406697\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Lgr4 mediates skin carcinogenesis by regulating MEK/ERK and Wnt/β-catenin signaling; TPA activates MEK1, ERK1/2 and AP-1 in wild-type but not Lgr4-depleted epidermal cells; MEK1/ERK1/2 pathway activation lies upstream of Wnt/β-catenin in this context.\",\n      \"method\": \"Lgr4-/- mice, TPA-induced papilloma model, MEK/ERK and Wnt pathway inhibitors, Western blot\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with pathway ordering via pathway inhibitors, single lab\",\n      \"pmids\": [\"27693558\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Aberrant LGR4 expression in multiple myeloma is driven by IL-6/STAT3 signaling, allowing myeloma cells to hijack R-spondins from osteoblasts in the bone marrow niche, resulting in Wnt co-receptor stabilization and increased sensitivity to Wnt ligands.\",\n      \"method\": \"Co-immunoprecipitation, reporter assays, IL-6/STAT3 inhibitors, R-spondin binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — upstream driver identified (IL-6/STAT3) and mechanism defined (receptor stabilization), single lab\",\n      \"pmids\": [\"28028233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"LGR4-dependent Wnt signaling transcriptionally upregulates SLC7A11 (a key ferroptosis inhibitor) to confer acquired drug resistance in colorectal cancer; LGR4-mAb inhibits LGR4-Wnt signaling and sensitizes drug-induced ferroptosis.\",\n      \"method\": \"Organoid drug resistance model, transcriptomic analysis, LGR4 monoclonal antibody, ferroptosis assays, SLC7A11 expression analysis\",\n      \"journal\": \"Nature cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined transcriptional target with functional validation, single lab\",\n      \"pmids\": [\"38291304\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LGR4 is a leucine-rich repeat-containing GPCR that functions primarily as a high-affinity receptor for R-spondins (and also RANKL, norrin, and nidogen-2), potentiating Wnt/β-catenin signaling by forming RSPO-bridged complexes with the E3 ubiquitin ligases ZNRF3/RNF43 (via its transmembrane domain) to prevent Wnt receptor ubiquitination, while also recruiting the scaffold IQGAP1 to the Wnt signalosome to promote MEK1/2-mediated LRP5/6 phosphorylation; it additionally signals through Gαq-GSK3β (suppressing NFATc1/osteoclastogenesis), cAMP-PKA-CREB-ATF4 (erythropoiesis), NF-κB (innate immunity and hepatoprotection), ERK/STAT3 (macrophage polarization), and a Wnt-independent LGR4-EGFR axis (blocking EGFR ubiquitination to sustain EGFR activation in cancer metastasis), with LGR4's seven-transmembrane domain—distinct from LGR5—being uniquely required for constitutive interaction with ZNRF3/RNF43.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LGR4 is a leucine-rich repeat-containing G protein-coupled receptor that serves as a high-affinity receptor for R-spondins and potentiates Wnt/β-catenin signaling by forming ternary complexes with the E3 ubiquitin ligases ZNRF3/RNF43, driving their membrane clearance and thereby stabilizing Wnt receptors [PMID:21693646, PMID:24165923, PMID:33262293]. Structurally, the LGR4 extracellular domain adopts a horseshoe-shaped leucine-rich repeat fold that engages R-spondin furin-like domains on its concave surface, and its seven-transmembrane domain uniquely mediates constitutive interaction with ZNRF3/RNF43 independently of R-spondin [PMID:23756652, PMID:33262293]. Beyond canonical Wnt potentiation, LGR4 functions as a receptor for RANKL, norrin, and nidogen-2, engaging Gαq, cAMP-PKA-CREB, NF-κB, and ERK/STAT3 pathways to regulate osteoclastogenesis, erythropoiesis, innate immunity, macrophage polarization, and vascular calcification [PMID:27064449, PMID:18955481, PMID:29967265, PMID:36354004]. LGR4 also directly interacts with EGFR through a Wnt-independent mechanism that blocks EGFR ubiquitination and sustains EGFR signaling, promoting cancer cell metastasis [PMID:34099494].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Before the R-spondin connection was established, LGR4 was shown to be an orphan GPCR that regulated definitive erythropoiesis through cAMP-PKA-CREB-ATF4 signaling and keratinocyte EGFR transactivation, establishing its coupling to classical GPCR second messengers and crosstalk with receptor tyrosine kinases.\",\n      \"evidence\": \"Lgr4 knockout mice with hematopoietic and eyelid developmental phenotypes, signaling pathway analysis\",\n      \"pmids\": [\"18955481\", \"18487371\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous ligand unknown at the time\", \"cAMP-PKA pathway coupling not confirmed with purified receptor reconstitution\", \"EGFR transactivation mechanism not fully defined\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The identification of R-spondins as high-affinity LGR4 ligands resolved the orphan receptor problem and established a new paradigm: LGR4 potentiates Wnt/β-catenin signaling not through classical G protein coupling but through a unique mechanism involving enhanced LRP6 phosphorylation.\",\n      \"evidence\": \"Binding assays, LRP6 phosphorylation, G protein and β-arrestin coupling assays, gain/loss-of-function in mammalian cells and Xenopus\",\n      \"pmids\": [\"21693646\", \"21909076\", \"22815884\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effector mechanism of Wnt potentiation not yet defined\", \"Relative contributions of LGR4 vs LGR5 in vivo unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Crystal structures of the LGR4 ectodomain–RSPO1 complex revealed a TLR-like horseshoe architecture engaging R-spondin furin-like repeats on its concave surface, providing the atomic basis for ligand recognition and explaining conservation across LGR4-6.\",\n      \"evidence\": \"X-ray crystallography with functional mutagenesis from multiple independent groups\",\n      \"pmids\": [\"23756652\", \"23891289\", \"25480784\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No full-length receptor structure including transmembrane domain\", \"Structural basis for ZNRF3/RNF43 interaction unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The mechanism of Wnt potentiation was resolved: R-spondin simultaneously binds LGR4 and ZNRF3/RNF43 through distinct motifs, forming a ternary complex that triggers membrane clearance of the E3 ligases and prevents Frizzled/LRP ubiquitination; intracellularly, LGR4 recruits IQGAP1 to the Wnt signalosome to drive MEK1/2-mediated LRP5/6 phosphorylation.\",\n      \"evidence\": \"Mutagenesis of R-spondin dual-binding motifs, co-immunoprecipitation of ternary complex, ZNRF3 clearance assays, IQGAP1 pulldown and signaling assays, in vitro reconstitution of ternary complexes\",\n      \"pmids\": [\"24165923\", \"24050775\", \"24639526\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of ternary complex in native membranes unclear\", \"Role of clathrin-mediated endocytosis in ternary complex turnover not fully resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"LGR4-dependent Wnt/β-catenin signaling was shown to be essential for intestinal stem cell maintenance, Paneth cell differentiation, spermatogenesis, mammary gland development, and myogenic differentiation, establishing LGR4 as a master amplifier of Wnt signaling across multiple stem/progenitor cell compartments.\",\n      \"evidence\": \"Lgr4 knockout and conditional knockout mice, ex vivo crypt cultures, genetic epistasis with Apc(min/+), Sox2 and follistatin as downstream targets identified by ChIP and promoter analysis\",\n      \"pmids\": [\"21508962\", \"23533175\", \"23712846\", \"24344199\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative contributions of LGR4 vs LGR5 in each tissue not fully delineated\", \"Whether RSPO isoform selectivity differs across tissues not resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"LGR4 was identified as a second receptor for RANKL that signals through Gαq-GSK3β to suppress NFATc1 and osteoclastogenesis, revealing a Wnt-independent G protein-coupled signaling axis and establishing LGR4 as a multi-ligand receptor.\",\n      \"evidence\": \"Co-immunoprecipitation, competition binding with RANK, Gαq signaling assays, conditional knockout mice, soluble LGR4-ECD treatment in vivo\",\n      \"pmids\": [\"27064449\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for RANKL–LGR4 interaction unknown\", \"Whether RANKL and RSPO signaling through LGR4 are mutually exclusive not tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The RSPO-LGR4/5-ZNRF3/RNF43 module was demonstrated to control hepatic metabolic zonation by maintaining the Wnt/β-catenin gradient across the liver lobule, showing that the ternary complex mechanism operates in terminally differentiated tissue homeostasis, not only in stem cells.\",\n      \"evidence\": \"Liver-specific LGR4/5 conditional knockout, recombinant RSPO1 treatment reversibly expanding the Wnt zone\",\n      \"pmids\": [\"27088858\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether RSPO dosage titrates zone boundaries quantitatively not fully modeled\", \"Relative contributions of LGR4 vs LGR5 in hepatocytes not separated\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"LGR4 was shown to regulate macrophage polarization: RSPO-LGR4-ERK-STAT3 signaling promotes M2 polarization of tumor-associated macrophages, while LGR4-CREB-driven AP-1 transactivation governs pro-inflammatory macrophage programs, demonstrating context-dependent immune signaling outputs.\",\n      \"evidence\": \"Macrophage-specific Lgr4 conditional knockout mice, tumor models, cardiac injury models, transcriptomics, AP-1 and CREB activity assays\",\n      \"pmids\": [\"29967265\", \"32600176\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How the same receptor achieves opposing macrophage polarization outcomes (M2 vs pro-inflammatory) in different contexts not mechanistically reconciled\", \"Whether RSPO is the relevant ligand in both contexts not confirmed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Domain-swap experiments revealed that the LGR4 seven-transmembrane domain constitutively interacts with ZNRF3/RNF43 even without R-spondin, distinguishing LGR4 mechanistically from LGR5, which instead enhances Wnt signaling by interacting with FZD-LRP6; both receptors exist as dimers.\",\n      \"evidence\": \"Co-immunoprecipitation, proximity ligation, time-resolved FRET, domain-swap chimeras between LGR4 and LGR5\",\n      \"pmids\": [\"33262293\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of TM domain–ZNRF3 interface not determined\", \"Functional significance of LGR4 dimerization not established\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"A Wnt-independent LGR4-EGFR axis was uncovered: LGR4 physically interacts with EGFR and blocks its ubiquitination, sustaining EGFR activation to drive cancer metastasis; LGR4 mutants defective in Wnt potentiation retain full pro-metastatic capacity, functionally separating Wnt and EGFR outputs of LGR4.\",\n      \"evidence\": \"Co-immunoprecipitation, EGFR ubiquitination assay, Wnt-signaling-dead LGR4 mutants, in vivo xenograft metastasis\",\n      \"pmids\": [\"34099494\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which LGR4 domain mediates EGFR interaction unknown\", \"Whether EGFR-protective function occurs in normal physiology or is cancer-specific not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Nidogen-2 was identified as a novel endogenous ligand of LGR4, engaging Gαq-PKCα-AMPKα1 biased signaling to counteract vascular smooth muscle cell osteogenic transition and calcification, expanding the ligand repertoire beyond R-spondins and RANKL.\",\n      \"evidence\": \"Membrane protein interactome screen, SPR binding (KD ~27 nM), G protein signaling assays, conditional knockout\",\n      \"pmids\": [\"36354004\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether nidogen-2 and RSPO compete for LGR4 binding unknown\", \"Structural basis for nidogen-2–LGR4 recognition not determined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"LGR4-Wnt signaling was shown to transcriptionally upregulate SLC7A11 to inhibit ferroptosis, conferring acquired drug resistance in colorectal cancer; a therapeutic LGR4 monoclonal antibody reversed this resistance by sensitizing tumors to ferroptosis.\",\n      \"evidence\": \"Organoid drug resistance model, transcriptomics, LGR4-mAb treatment, ferroptosis assays\",\n      \"pmids\": [\"38291304\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether SLC7A11 upregulation is a direct β-catenin/TCF target or indirect not fully dissected\", \"In vivo efficacy of LGR4-mAb across tumor types not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full-length structure of LGR4 including its transmembrane domain in complex with ZNRF3/RNF43 or EGFR, how a single receptor achieves biased signaling through Gαq, cAMP, NF-κB, and β-catenin depending on ligand and cellular context, and whether the diverse ligands (R-spondins, RANKL, norrin, nidogen-2) compete for or cooperate through overlapping or distinct binding sites.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No full-length LGR4 structure available\", \"Biased signaling mechanism across multiple ligands not reconciled\", \"Functional interplay among four known ligand classes not systematically tested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 7, 9, 13]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5, 10, 20]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 5, 10, 29]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 5, 6, 9, 10, 13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [21, 22, 27, 30]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [14, 15, 24, 33]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [20, 37, 38]}\n    ],\n    \"complexes\": [\n      \"RSPO–LGR4–ZNRF3/RNF43 ternary complex\",\n      \"LGR4–IQGAP1–DVL signalosome\"\n    ],\n    \"partners\": [\n      \"ZNRF3\",\n      \"RNF43\",\n      \"IQGAP1\",\n      \"EGFR\",\n      \"RSPO1\",\n      \"RSPO2\",\n      \"RANKL\",\n      \"NID2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}