{"gene":"GPR161","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2013,"finding":"GPR161 localizes to primary cilia in a Tulp3/IFT-A-dependent manner. Constitutive GPR161 activity increases cAMP levels and promotes processing of Gli3 to its repressor form, thereby repressing Shh signaling. Shh pathway activation directs GPR161 internalization from cilia, preventing its activity. Thus GPR161 couples PKA activation to Shh pathway repression during neural tube development.","method":"Mouse knockout/loss-of-function, cAMP reporter assays, immunofluorescence localization in primary cilia, Gli3 processing western blot, genetic epistasis with Tulp3/IFT-A mutants","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (KO phenotype, cAMP assays, Gli3 processing, localization), replicated across multiple contexts, foundational study","pmids":["23332756"],"is_preprint":false},{"year":2016,"finding":"Removal of GPR161 from primary cilia upon Shh pathway activation is a two-step process: (1) β-arrestin recruitment to the signaling-competent receptor facilitated by GPCR kinase Grk2, promoted by ciliary Smoothened activation; (2) clathrin-mediated endocytosis outside the ciliary compartment. Smoothened activity in cilia increases GPR161–β-arrestin binding to promote GPR161 removal both at rest and upon Shh stimulation.","method":"Co-immunoprecipitation, dominant-negative and knockout cell lines, live-cell imaging, pharmacological inhibitors of clathrin-mediated endocytosis, overexpression of Smoothened mutants","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (Co-IP, genetic KO, pharmacological inhibition, live imaging) in a single focused study","pmids":["27002170"],"is_preprint":false},{"year":2016,"finding":"GPR161 functions as an A-kinase anchoring protein (AKAP) for type I PKA regulatory subunits (RI), binding directly via a hydrophobic interaction interface in its cytoplasmic C-terminal tail. The GPR161–RI binary complex promotes GPR161 compartmentalization at the plasma membrane and recruits PKA RI to primary cilia in zebrafish. GPR161 is itself a substrate of PKA phosphorylation, and mutation of the PKA phosphorylation site affects its ciliary localization.","method":"Phosphoproteomics, cell-based protein-protein interaction reporters, direct binding assays (RI pull-down), site-directed mutagenesis, zebrafish in vivo imaging","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — phosphoproteomics, direct binding assays, mutagenesis, and in vivo zebrafish validation; multiple orthogonal methods","pmids":["27357676"],"is_preprint":false},{"year":2014,"finding":"GPR161 forms a signaling complex with β-arrestin 2 and IQGAP1 (a regulator of mTORC1 and E-cadherin). GPR161 overexpression activates mTOR signaling and decreases IQGAP1 phosphorylation. Knockdown of GPR161 impairs proliferation of basal breast cancer cell lines.","method":"Co-immunoprecipitation (GPR161–β-arrestin 2–IQGAP1 complex), shRNA knockdown, overexpression in mammary epithelial cells, 3D culture, western blot for mTOR pathway","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — Co-IP complex identification plus functional KD phenotype; single lab, two orthogonal methods","pmids":["24599592"],"is_preprint":false},{"year":2008,"finding":"A deletion/frameshift mutation in Gpr161 truncates its C-terminal tail, causing reduced receptor-mediated endocytosis and leading to neural tube defects and cataracts in the vacuolated lens mouse mutant.","method":"Positional cloning, characterization of endocytosis by functional assay of C-terminally truncated receptor","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — positional cloning plus functional endocytosis assay; single lab","pmids":["18250320"],"is_preprint":false},{"year":2018,"finding":"Gpr161 promotes Gli transcriptional repressor formation over activator formation in limb/skeletal morphogenesis in a primary cilium-dependent manner. Limb-specific deletion causes prematurely expanded Shh signaling and polysyndactyly; endochondral bone formation defects result from accumulation of proliferating periarticular-like chondrocytes and lack of Ihh signaling. All defects are suppressed in the absence of cilia.","method":"Conditional knockout mice (limb-specific, craniofacial mesenchyme-specific), genetic epistasis with cilia mutants, histology, in situ hybridization for Shh/Ihh targets","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — tissue-specific KO with epistasis to cilia mutants; multiple tissue contexts and pathway readouts","pmids":["29222391"],"is_preprint":false},{"year":2018,"finding":"Gpr161 acts as a tumor suppressor in Shh-subtype medulloblastoma by restricting Gli3-mediated repression. Deletion in neural stem cells or granule cell (GC) progenitors increases downstream Shh pathway activity and GC progenitor proliferation in a cilium-dependent manner.","method":"Conditional knockout mice (neural stem cell- and GC progenitor-specific), immunofluorescence, pathway target gene expression, Gli3 repressor analysis","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple conditional KO models with defined cellular phenotypes, cilium-dependence confirmed genetically","pmids":["29386106"],"is_preprint":false},{"year":2019,"finding":"The IFT-B complex subunit IFT38 interacts with BBSome subunits BBS1, BBS2, and BBS9. This IFT-B–BBSome interaction is required for GPR161 export from cilia upon Hedgehog signaling stimulation; cells expressing an IFT38 mutant lacking this interaction show significant accumulation of GPR161 within cilia.","method":"Visible immunoprecipitation assay, IFT38-knockout cell lines expressing wild-type or interaction-deficient IFT38 mutant, immunofluorescence for ciliary GPR161 accumulation","journal":"Biology open","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal interaction assay plus rescue experiment with separation-of-function mutant, clean mechanistic readout","pmids":["31471295"],"is_preprint":false},{"year":2019,"finding":"Novel rare GPR161 variants found in spina bifida patients mislocalize to primary cilia, dysregulate both Shh and Wnt signaling, and inhibit cell proliferation in vitro, acting in a dominant-negative manner.","method":"Sanger sequencing of patient cohort, immunofluorescence for ciliary localization of variant proteins, Shh/Wnt pathway reporter assays, cell proliferation assay","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — functional characterization of patient variants by localization and pathway assays; single lab","pmids":["30256984"],"is_preprint":false},{"year":2019,"finding":"Gpr161 deletion in mouse neuroepithelial cells/radial glia leads to Shh pathway derepression, radial glial overproliferation, ventriculomegaly, polymicrogyria, and disrupted neuronal migration, with periventricular nodular heterotopia.","method":"Conditional knockout mice (neuroepithelial/radial glia-specific), in toto imaging, BrdU/EdU proliferation assays, immunofluorescence for progenitor markers","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with defined cellular phenotypes; single lab","pmids":["30914320"],"is_preprint":false},{"year":2021,"finding":"A ciliary-localization-defective but cAMP-signaling-competent knock-in variant (Gpr161mut1) revealed that ciliary and extraciliary GPR161 pools establish tissue-specific Gli repressor thresholds. Ciliary GPR161 is required for Gli2 activator-dependent suppression in the ventral-most neural tube progenitors, while extraciliary GPR161 prevents ventralization; limb and midface morphogenesis depend on Gli repressor thresholds set by ciliary GPR161.","method":"CRISPR/Cas9 knock-in mice, immunofluorescence, pathway target gene expression, genetic epistasis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — separation-of-function knock-in with multiple tissue readouts and epistasis; rigorous mechanistic dissection","pmids":["34346313"],"is_preprint":false},{"year":2021,"finding":"PKA feedback-mediated phosphorylation of Gpr161 fine-tunes ciliary localization and PKA activity. PKA phosphorylation-deficient Gpr161 forms directly couple to Gαs and display increased sensitivity to Shh, causing excess high-level Hh target gene expression in zebrafish. Loss of Gpr161 in zebrafish leads to constitutive activation of medium and low, but not maximal, levels of Hh target gene expression.","method":"Zebrafish gpr161 morphants/mutants, BRET Gαs coupling assay, phosphorylation-deficient mutant analysis, in situ hybridization for Hh targets","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Gαs coupling assay plus in vivo zebrafish mutant analysis; single lab","pmids":["33531430"],"is_preprint":false},{"year":2021,"finding":"Gpr161 is a mechanoresponsive GPCR at the primary cilium of mesenchymal stem cells (MSCs). Fluid shear stress activates Gpr161-mediated cAMP signaling, which requires IFT88/cilia and adenylyl cyclase 6 (AC6), to drive osteogenic differentiation. Hh signaling downstream of this axis is required for loading-induced osteogenesis.","method":"Gpr161 siRNA knockdown, fluid shear stress application, cAMP measurement, IFT88 knockdown, AC6 inhibition, osteogenic differentiation assays","journal":"Bone","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — multiple knockdowns with functional readout; mechanistic pathway placed; single lab","pmids":["33450431"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure of active human GPR161 bound to heterotrimeric Gs revealed: (1) extracellular loop 2 occupies the canonical GPCR orthosteric ligand pocket; (2) a sterol binds adjacent to transmembrane helices 6 and 7, stabilizing the Gs-coupling conformation — mutations preventing sterol binding suppress cAMP signaling but retain GPR161's ability to suppress GLI2 accumulation in cilia; (3) a PKA-binding site in the GPR161 C-terminus is critical for suppressing GLI2 ciliary accumulation.","method":"Cryo-EM structure determination, site-directed mutagenesis of sterol-binding and PKA-binding sites, cAMP reporter assays, immunofluorescence for GLI2 ciliary localization","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure plus mutagenesis plus functional assays; multiple orthogonal methods in a single rigorous study","pmids":["38326651"],"is_preprint":false},{"year":2025,"finding":"β-arrestin (ARRB1/ARRB2) is required for GPR161 export from cilia. GRK2 phosphorylates GPR161, enabling β-arrestin recruitment in its activated conformation, which then interacts with the BBSome to connect GPR161 to the IFT machinery for export. Activation-mimetic β-arrestin mutants cause constitutive GPR161 export. ARRB1/ARRB2 double knockout impairs GPR161 export.","method":"β-arrestin double KO cell lines, IFT27 and BBSome-subunit KO cells, overexpression of activation-mimetic β-arrestin mutants, Co-IP of β-arrestin with BBSome, immunofluorescence for ciliary GPR161","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple KO lines, interaction assays, separation-of-function mutants; multiple orthogonal approaches in one study","pmids":["40384633"],"is_preprint":false},{"year":2024,"finding":"Fuz is genetically epistatic to Gpr161 in Shh signaling during mouse neural tube development. FUZ protein biochemically interacts with GPR161, and Fuz regulates GPR161-mediated ciliary localization via β-arrestin 2.","method":"Genetic epistasis in double-mutant mice, co-immunoprecipitation of FUZ and GPR161, β-arrestin 2 interaction assay, ciliary localization immunofluorescence","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis plus Co-IP; single lab, two complementary methods","pmids":["39369306"],"is_preprint":false},{"year":2025,"finding":"GPR161 acts as a mechanoreceptor at the primary cilium driving saltatory neuronal migration. Fluid shear stress induces GPR161-dependent cAMP/PKA signaling, leading to phosphorylation of NDE1 (a dynein complex regulator) and microtubule reorganization to regulate the rhythmicity of neuronal migration. The mechanosensitive helix 8 of GPR161 is essential for this function.","method":"Ex vivo neuronal migration model, microfluidic shear stress assays, GPR161 knockout/knockdown, helix-8 mutagenesis, phospho-NDE1 western blot, immunofluorescence for microtubule organization","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional assays with KO/mutagenesis and defined downstream substrate (NDE1); single lab, multiple methods","pmids":["40737401"],"is_preprint":false},{"year":2025,"finding":"GPR161 promotes melanoma proliferation and metabolic activity through a STAT3–GPR161–TXNIP regulatory axis: STAT3 binds the GPR161 promoter and transcriptionally activates GPR161; GPR161 in turn negatively regulates TXNIP expression, reducing glycolytic capacity and proliferation when GPR161 is depleted.","method":"Promoter analysis, chromatin immunoprecipitation-qPCR for STAT3 binding, siRNA/shRNA knockdown, TXNIP overexpression/knockdown, glycolytic capacity assays","journal":"Journal of microbiology and biotechnology","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — ChIP-qPCR plus functional knockdown with defined downstream readout; single lab","pmids":["41834581"],"is_preprint":false},{"year":2026,"finding":"GPR161 promotes glycolytic reprogramming in macrophages during acute lung injury through suppression of complement component 5a receptor 1 (C5aR1). Co-immunoprecipitation and surface plasmon resonance identified C5aR1 as a downstream target/interactor of GPR161; macrophage-specific GPR161 knockout attenuates pulmonary inflammatory damage.","method":"Global and macrophage-specific conditional KO mice, RNA sequencing, co-immunoprecipitation, surface plasmon resonance, glycolytic capacity assays","journal":"Cellular & molecular biology letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding assay (SPR + Co-IP) plus conditional KO with defined cellular phenotype; single lab","pmids":["42185764"],"is_preprint":false},{"year":2025,"finding":"FKBP8 was identified as a novel interacting protein of GPR161 (and FUZ), validated by affinity purification-mass spectrometry; GPR161 interactome is enriched for proteins associated with proteasomal catabolic processes, trafficking, receptor complex, and ER-Golgi transport.","method":"Affinity-based LC-MS/MS immunoprecipitation, STRING network analysis, co-IP validation of FKBP8","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/MS study; no functional follow-up on GPR161-FKBP8 interaction; preprint only","pmids":["41000683"],"is_preprint":true},{"year":2025,"finding":"Tumors arising from Gαs pathway inactivation are independent of GPR161 (and of canonical Smoothened), establishing an SMO-independent oncogenic Hedgehog signaling model that bypasses GPR161's negative regulatory role.","method":"Genetic epistasis in mouse BCC-like tumor models (Gαs pathway inactivation combined with GPR161 deletion), histology, bulk and single-cell RNA sequencing","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in vivo with transcriptomic readout; preprint, single lab","pmids":["bio_10.1101_2025.02.21.639530"],"is_preprint":true},{"year":2024,"finding":"ARL13B maintains GPR161 ciliary localization; ARL13B KO reduces ciliary GPR161 and increases Shh signaling. GPR161 deletion in human iPSC-derived brain organoids decreases Gli3 repressor formation and increases Shh signaling, causing ventralization of dorsal cortical progenitors. Pharmacological or optogenetic elevation of ciliary cAMP rescues Gli3 repressor formation and dorsal neural stem cell identity in GPR161 KO organoids.","method":"CRISPR KO in human iPSC-derived brain organoids, immunofluorescence for GPR161 and Gli3 repressor, cAMP pharmacology, optogenetic cAMP elevation","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO in human organoids with pathway rescue by cAMP; multiple orthogonal interventions; preprint, single lab","pmids":["bio_10.1101_2024.07.18.604098"],"is_preprint":true},{"year":2025,"finding":"GPR161-driven GLI3 repressor (GLI3R) signaling at the primary cilium controls apical constriction and cell fate during cranial neural tube closure. A functional non-ciliary Gpr161 knock-in implicated ciliary GPR161 localization in initiation and maintenance of cranial closure. GLI3R expression (but not GLI2 loss) rescued exencephaly in Gpr161 KO mice, and GLI3R restricted forebrain ventral floor plate expansion and mediated apical constriction in lateral midbrain neural folds.","method":"Gpr161 mutant allelic series including non-ciliary knock-in, genetic epistasis with Gli2 KO and Gli3R expression, in toto imaging of cell behavior, immunofluorescence","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — allelic series plus genetic epistasis plus live imaging; multiple orthogonal approaches; single lab but very rigorous","pmids":["41417007"],"is_preprint":false},{"year":2015,"finding":"Gpr161 regulates the retinoic acid (RA) and canonical Wnt pathways during neurulation, downstream of Gpr161 activity. RA injection restores canonical Wnt marker expression and rescues Gpr161 hypomorphic neural tube defects, placing RA upstream of Wnt as part of the Gpr161 downstream cascade.","method":"Hypomorphic Gpr161vl allele, modifier QTL mapping, QRT-PCR, in situ hybridization, IHC, intraperitoneal RA injection rescue experiment","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological rescue plus gene expression analysis; single lab, two orthogonal approaches","pmids":["25753732"],"is_preprint":false}],"current_model":"GPR161 is a constitutively active orphan GPCR that localizes to primary cilia via IFT-A/Tulp3, where it couples to Gαs to elevate cAMP and anchor type I PKA (acting as an AKAP) through its C-terminal tail, thereby promoting Gli3 transcriptional repressor formation and suppressing basal Hedgehog signaling; upon Shh pathway activation, Smoothened accumulates in cilia and facilitates GRK2-mediated phosphorylation of GPR161 followed by β-arrestin recruitment, BBSome engagement, and clathrin-mediated removal from cilia, relieving pathway repression. A cryo-EM structure revealed that an extrahelical sterol stabilizes GPR161's Gs-coupling conformation while the PKA-binding C-terminal site is specifically required for suppressing GLI2 accumulation in cilia. Beyond Hedgehog, GPR161 also serves as a mechanosensor driving neuronal saltatory migration via NDE1 phosphorylation, regulates osteogenesis in mesenchymal stem cells via adenylyl cyclase 6, forms signaling complexes with β-arrestin 2/IQGAP1 to activate mTOR in cancer cells, and in macrophages suppresses C5aR1 to control glycolytic reprogramming."},"narrative":{"mechanistic_narrative":"GPR161 is a constitutively active, ciliary GPCR that functions as the principal negative regulator of Hedgehog (Hh) signaling by coupling cAMP/PKA activity to formation of the Gli3 transcriptional repressor [PMID:23332756]. Localized to primary cilia in a Tulp3/IFT-A-dependent manner, its constitutive Gαs coupling raises cAMP and drives Gli3 processing to its repressor form, suppressing basal Shh output during neural tube, limb, and skeletal development [PMID:23332756, PMID:29222391]. A cryo-EM structure of active GPR161–Gs showed extracellular loop 2 occupying the orthosteric pocket and an extrahelical sterol adjacent to TM6/TM7 stabilizing the Gs-coupling conformation; sterol-binding mutations abolish cAMP signaling yet retain suppression of GLI2 ciliary accumulation, which instead depends on a C-terminal PKA-binding site [PMID:38326651]. GPR161 acts as an A-kinase anchoring protein for type I PKA regulatory subunits through this cytoplasmic C-tail and is itself a PKA substrate, establishing a feedback loop tuning its ciliary residence [PMID:27357676, PMID:33531430]. Upon Shh activation, ciliary Smoothened promotes GRK2-mediated phosphorylation of GPR161 and β-arrestin recruitment, which engages the BBSome–IFT machinery to drive clathrin-mediated removal of the receptor from cilia, relieving repression [PMID:27002170, PMID:40384633, PMID:31471295]. Separation-of-function alleles establish that distinct ciliary and extraciliary GPR161 pools set tissue-specific Gli repressor thresholds governing neural tube patterning, neural tube closure, and limb/craniofacial morphogenesis [PMID:34346313, PMID:41417007]. Loss of GPR161 derepresses Shh signaling and causes overproliferation phenotypes, defining it as a tumor suppressor in Shh-subtype medulloblastoma [PMID:29386106, PMID:30914320]. Beyond Hedgehog, GPR161 functions as a mechanoresponsive ciliary receptor: fluid shear stress triggers GPR161-dependent cAMP/PKA signaling that drives osteogenic differentiation of mesenchymal stem cells via adenylyl cyclase 6 [PMID:33450431] and regulates saltatory neuronal migration via phosphorylation of the dynein regulator NDE1 through its mechanosensitive helix 8 [PMID:40737401]. Truncating and rare patient variants link GPR161 to neural tube defects, cataracts, and spina bifida [PMID:18250320, PMID:30256984].","teleology":[{"year":2008,"claim":"Before its signaling role was known, positional cloning of a spontaneous mouse mutant tied Gpr161 to a developmental disease phenotype and implicated its C-terminal tail in receptor trafficking.","evidence":"Positional cloning of the vacuolated lens mutant and endocytosis assay of a C-terminally truncated receptor","pmids":["18250320"],"confidence":"Medium","gaps":["No signaling mechanism or pathway placement","Did not connect endocytosis defect to a downstream transduction cascade"]},{"year":2013,"claim":"Established the core mechanism: GPR161 is a ciliary, constitutively active GPCR that raises cAMP to drive Gli3 repressor formation and suppress Shh signaling, with Shh-triggered ciliary removal relieving repression.","evidence":"Mouse knockout, cAMP reporters, Gli3 processing westerns, and genetic epistasis with Tulp3/IFT-A in neural tube development","pmids":["23332756"],"confidence":"High","gaps":["G protein coupling not directly demonstrated biochemically","Endogenous ligand not identified","Molecular machinery of ciliary export undefined"]},{"year":2014,"claim":"Identified a Hedgehog-independent role by placing GPR161 in a β-arrestin 2/IQGAP1 complex that activates mTOR and supports cancer cell proliferation.","evidence":"Co-IP of the GPR161–β-arrestin 2–IQGAP1 complex and shRNA knockdown in basal breast cancer cells","pmids":["24599592"],"confidence":"Medium","gaps":["Single lab, Co-IP-based complex","Directness of IQGAP1/mTOR coupling not resolved","Relationship to ciliary cAMP role unclear"]},{"year":2015,"claim":"Extended the downstream cascade by linking GPR161 activity to retinoic acid and canonical Wnt regulation during neurulation.","evidence":"Hypomorphic Gpr161vl allele with RA injection rescue and gene-expression analysis in mouse neural tube","pmids":["25753732"],"confidence":"Medium","gaps":["Mechanism connecting GPR161 to RA/Wnt not molecular","Single lab pharmacological rescue"]},{"year":2016,"claim":"Resolved how GPR161 is removed from cilia: a two-step GRK2/β-arrestin recruitment followed by extraciliary clathrin-mediated endocytosis, promoted by ciliary Smoothened activity.","evidence":"Co-IP, dominant-negative and KO cells, live imaging, and clathrin inhibitors","pmids":["27002170"],"confidence":"High","gaps":["BBSome/IFT linkage to export not yet defined","GRK2 phosphosites on GPR161 not mapped"]},{"year":2016,"claim":"Defined GPR161 as an AKAP that directly anchors type I PKA via its C-tail and is reciprocally phosphorylated by PKA, explaining how it compartmentalizes PKA activity at cilia.","evidence":"Phosphoproteomics, RI pull-down binding assays, mutagenesis, and zebrafish imaging","pmids":["27357676"],"confidence":"High","gaps":["Structural basis of the RI interface not resolved","Functional separation of AKAP versus cAMP-generating roles not yet tested"]},{"year":2018,"claim":"Demonstrated tissue-specific, cilium-dependent Gli-repressor regulation by GPR161 in limb/skeletal morphogenesis and its tumor-suppressor function in Shh-medulloblastoma.","evidence":"Conditional KO mice in limb, craniofacial, neural stem cell, and granule progenitor lineages with epistasis to cilia mutants","pmids":["29222391","29386106"],"confidence":"High","gaps":["Whether different tissues use distinct GPR161 effector pools unresolved","Quantitative link between cAMP levels and Gli thresholds not established"]},{"year":2019,"claim":"Defined the molecular export route by showing IFT-B subunit IFT38 bridges to the BBSome to drive Hh-induced GPR161 export, and extended disease relevance with dominant-negative spina bifida variants and cortical malformation phenotypes.","evidence":"Reciprocal IP and rescue with interaction-deficient IFT38; patient variant localization/reporter assays; conditional KO mice with proliferation and migration readouts","pmids":["31471295","30256984","30914320"],"confidence":"Medium","gaps":["Order of β-arrestin versus BBSome engagement not fully resolved","Patient variant studies single-lab","Wnt dysregulation mechanism in variants unclear"]},{"year":2021,"claim":"Separated ciliary from extraciliary GPR161 functions and established a PKA-phosphorylation feedback loop tuning ciliary localization and Gαs coupling sensitivity, while extending function to mechanosensation in osteogenesis.","evidence":"CRISPR knock-in separation-of-function mice, BRET Gαs coupling and phospho-deficient mutants in zebrafish, and shear-stress/AC6 knockdown in MSCs","pmids":["34346313","33531430","33450431"],"confidence":"High","gaps":["How extraciliary pool localizes and signals mechanistically undefined","Mechanotransduction sensor element not yet identified"]},{"year":2024,"claim":"Provided the structural mechanism: a cryo-EM active GPR161–Gs structure showing ELL2 in the orthosteric pocket and a sterol stabilizing Gs coupling, and dissociating cAMP signaling from a C-terminal PKA-binding site required for GLI2 suppression.","evidence":"Cryo-EM with sterol/PKA-site mutagenesis, cAMP assays, and GLI2 ciliary localization; FUZ identified as an interactor regulating ciliary localization via β-arrestin 2","pmids":["38326651","39369306"],"confidence":"High","gaps":["Identity of any endogenous diffusible ligand unresolved","How sterol availability is regulated in vivo unknown","FUZ regulatory mechanism only partly defined"]},{"year":2025,"claim":"Mechanistically completed the export pathway (GRK2→β-arrestin→BBSome→IFT) and broadened GPR161's roles into neuronal migration mechanosensing, melanoma metabolism, and macrophage glycolytic reprogramming.","evidence":"β-arrestin/IFT27/BBSome KO cells and activation-mimetic mutants; helix-8 mutagenesis with phospho-NDE1; STAT3–GPR161–TXNIP ChIP/knockdown; macrophage-specific KO with Co-IP/SPR for C5aR1","pmids":["40384633","40737401","41834581","42185764"],"confidence":"Medium","gaps":["Non-Hedgehog effector mechanisms are single-lab","Whether these roles share the ciliary cAMP module is unclear","Directness of some interactions (C5aR1, NDE1) needs orthogonal confirmation"]},{"year":null,"claim":"The endogenous activating ligand of GPR161 and the unifying molecular basis linking its ciliary cAMP/AKAP function to its diverse non-Hedgehog roles remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No deorphanized endogenous agonist established","Mechanism switching between mechanosensation and constitutive signaling unknown","Therapeutic targetability untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,11,13]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[12,16]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[0,1,10,13]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,10]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0,5,10,22]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[1,7,14]}],"complexes":["BBSome (functional engagement for ciliary export)"],"partners":["ARRB2","ARRB1","GRK2","IQGAP1","IFT38","FUZ","FKBP8","C5AR1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8N6U8","full_name":"G-protein coupled receptor 161","aliases":["G-protein coupled receptor RE2"],"length_aa":529,"mass_kda":58.6,"function":"Key negative regulator of Shh signaling, which promotes the processing of GLI3 into GLI3R during neural tube development. Recruited by TULP3 and the IFT-A complex to primary cilia and acts as a regulator of the PKA-dependent basal repression machinery in Shh signaling by increasing cAMP levels, leading to promote the PKA-dependent processing of GLI3 into GLI3R and repress the Shh signaling. In presence of SHH, it is removed from primary cilia and is internalized into recycling endosomes, preventing its activity and allowing activation of the Shh signaling. Its ligand is unknown (By similarity)","subcellular_location":"Cell projection, cilium membrane; Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q8N6U8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR161","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GPR161","total_profiled":1310},"omim":[{"mim_id":"612250","title":"G PROTEIN-COUPLED RECEPTOR 161: GPR161","url":"https://www.omim.org/entry/612250"},{"mim_id":"608040","title":"INTRAFLAGELLAR TRANSPORT 74; IFT74","url":"https://www.omim.org/entry/608040"},{"mim_id":"604730","title":"TUB-LIKE PROTEIN 3; TULP3","url":"https://www.omim.org/entry/604730"},{"mim_id":"601094","title":"FORKHEAD BOX E3; FOXE3","url":"https://www.omim.org/entry/601094"},{"mim_id":"155255","title":"MEDULLOBLASTOMA; MDB","url":"https://www.omim.org/entry/155255"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Primary cilium","reliability":"Approved"},{"location":"Primary cilium transition zone","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"endometrium 1","ntpm":24.0},{"tissue":"smooth 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Constitutive GPR161 activity increases cAMP levels and promotes processing of Gli3 to its repressor form, thereby repressing Shh signaling. Shh pathway activation directs GPR161 internalization from cilia, preventing its activity. Thus GPR161 couples PKA activation to Shh pathway repression during neural tube development.\",\n      \"method\": \"Mouse knockout/loss-of-function, cAMP reporter assays, immunofluorescence localization in primary cilia, Gli3 processing western blot, genetic epistasis with Tulp3/IFT-A mutants\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (KO phenotype, cAMP assays, Gli3 processing, localization), replicated across multiple contexts, foundational study\",\n      \"pmids\": [\"23332756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Removal of GPR161 from primary cilia upon Shh pathway activation is a two-step process: (1) β-arrestin recruitment to the signaling-competent receptor facilitated by GPCR kinase Grk2, promoted by ciliary Smoothened activation; (2) clathrin-mediated endocytosis outside the ciliary compartment. Smoothened activity in cilia increases GPR161–β-arrestin binding to promote GPR161 removal both at rest and upon Shh stimulation.\",\n      \"method\": \"Co-immunoprecipitation, dominant-negative and knockout cell lines, live-cell imaging, pharmacological inhibitors of clathrin-mediated endocytosis, overexpression of Smoothened mutants\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (Co-IP, genetic KO, pharmacological inhibition, live imaging) in a single focused study\",\n      \"pmids\": [\"27002170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GPR161 functions as an A-kinase anchoring protein (AKAP) for type I PKA regulatory subunits (RI), binding directly via a hydrophobic interaction interface in its cytoplasmic C-terminal tail. The GPR161–RI binary complex promotes GPR161 compartmentalization at the plasma membrane and recruits PKA RI to primary cilia in zebrafish. GPR161 is itself a substrate of PKA phosphorylation, and mutation of the PKA phosphorylation site affects its ciliary localization.\",\n      \"method\": \"Phosphoproteomics, cell-based protein-protein interaction reporters, direct binding assays (RI pull-down), site-directed mutagenesis, zebrafish in vivo imaging\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — phosphoproteomics, direct binding assays, mutagenesis, and in vivo zebrafish validation; multiple orthogonal methods\",\n      \"pmids\": [\"27357676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GPR161 forms a signaling complex with β-arrestin 2 and IQGAP1 (a regulator of mTORC1 and E-cadherin). GPR161 overexpression activates mTOR signaling and decreases IQGAP1 phosphorylation. Knockdown of GPR161 impairs proliferation of basal breast cancer cell lines.\",\n      \"method\": \"Co-immunoprecipitation (GPR161–β-arrestin 2–IQGAP1 complex), shRNA knockdown, overexpression in mammary epithelial cells, 3D culture, western blot for mTOR pathway\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — Co-IP complex identification plus functional KD phenotype; single lab, two orthogonal methods\",\n      \"pmids\": [\"24599592\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"A deletion/frameshift mutation in Gpr161 truncates its C-terminal tail, causing reduced receptor-mediated endocytosis and leading to neural tube defects and cataracts in the vacuolated lens mouse mutant.\",\n      \"method\": \"Positional cloning, characterization of endocytosis by functional assay of C-terminally truncated receptor\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — positional cloning plus functional endocytosis assay; single lab\",\n      \"pmids\": [\"18250320\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Gpr161 promotes Gli transcriptional repressor formation over activator formation in limb/skeletal morphogenesis in a primary cilium-dependent manner. Limb-specific deletion causes prematurely expanded Shh signaling and polysyndactyly; endochondral bone formation defects result from accumulation of proliferating periarticular-like chondrocytes and lack of Ihh signaling. All defects are suppressed in the absence of cilia.\",\n      \"method\": \"Conditional knockout mice (limb-specific, craniofacial mesenchyme-specific), genetic epistasis with cilia mutants, histology, in situ hybridization for Shh/Ihh targets\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — tissue-specific KO with epistasis to cilia mutants; multiple tissue contexts and pathway readouts\",\n      \"pmids\": [\"29222391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Gpr161 acts as a tumor suppressor in Shh-subtype medulloblastoma by restricting Gli3-mediated repression. Deletion in neural stem cells or granule cell (GC) progenitors increases downstream Shh pathway activity and GC progenitor proliferation in a cilium-dependent manner.\",\n      \"method\": \"Conditional knockout mice (neural stem cell- and GC progenitor-specific), immunofluorescence, pathway target gene expression, Gli3 repressor analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple conditional KO models with defined cellular phenotypes, cilium-dependence confirmed genetically\",\n      \"pmids\": [\"29386106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The IFT-B complex subunit IFT38 interacts with BBSome subunits BBS1, BBS2, and BBS9. This IFT-B–BBSome interaction is required for GPR161 export from cilia upon Hedgehog signaling stimulation; cells expressing an IFT38 mutant lacking this interaction show significant accumulation of GPR161 within cilia.\",\n      \"method\": \"Visible immunoprecipitation assay, IFT38-knockout cell lines expressing wild-type or interaction-deficient IFT38 mutant, immunofluorescence for ciliary GPR161 accumulation\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal interaction assay plus rescue experiment with separation-of-function mutant, clean mechanistic readout\",\n      \"pmids\": [\"31471295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Novel rare GPR161 variants found in spina bifida patients mislocalize to primary cilia, dysregulate both Shh and Wnt signaling, and inhibit cell proliferation in vitro, acting in a dominant-negative manner.\",\n      \"method\": \"Sanger sequencing of patient cohort, immunofluorescence for ciliary localization of variant proteins, Shh/Wnt pathway reporter assays, cell proliferation assay\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — functional characterization of patient variants by localization and pathway assays; single lab\",\n      \"pmids\": [\"30256984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Gpr161 deletion in mouse neuroepithelial cells/radial glia leads to Shh pathway derepression, radial glial overproliferation, ventriculomegaly, polymicrogyria, and disrupted neuronal migration, with periventricular nodular heterotopia.\",\n      \"method\": \"Conditional knockout mice (neuroepithelial/radial glia-specific), in toto imaging, BrdU/EdU proliferation assays, immunofluorescence for progenitor markers\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with defined cellular phenotypes; single lab\",\n      \"pmids\": [\"30914320\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A ciliary-localization-defective but cAMP-signaling-competent knock-in variant (Gpr161mut1) revealed that ciliary and extraciliary GPR161 pools establish tissue-specific Gli repressor thresholds. Ciliary GPR161 is required for Gli2 activator-dependent suppression in the ventral-most neural tube progenitors, while extraciliary GPR161 prevents ventralization; limb and midface morphogenesis depend on Gli repressor thresholds set by ciliary GPR161.\",\n      \"method\": \"CRISPR/Cas9 knock-in mice, immunofluorescence, pathway target gene expression, genetic epistasis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — separation-of-function knock-in with multiple tissue readouts and epistasis; rigorous mechanistic dissection\",\n      \"pmids\": [\"34346313\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PKA feedback-mediated phosphorylation of Gpr161 fine-tunes ciliary localization and PKA activity. PKA phosphorylation-deficient Gpr161 forms directly couple to Gαs and display increased sensitivity to Shh, causing excess high-level Hh target gene expression in zebrafish. Loss of Gpr161 in zebrafish leads to constitutive activation of medium and low, but not maximal, levels of Hh target gene expression.\",\n      \"method\": \"Zebrafish gpr161 morphants/mutants, BRET Gαs coupling assay, phosphorylation-deficient mutant analysis, in situ hybridization for Hh targets\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Gαs coupling assay plus in vivo zebrafish mutant analysis; single lab\",\n      \"pmids\": [\"33531430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Gpr161 is a mechanoresponsive GPCR at the primary cilium of mesenchymal stem cells (MSCs). Fluid shear stress activates Gpr161-mediated cAMP signaling, which requires IFT88/cilia and adenylyl cyclase 6 (AC6), to drive osteogenic differentiation. Hh signaling downstream of this axis is required for loading-induced osteogenesis.\",\n      \"method\": \"Gpr161 siRNA knockdown, fluid shear stress application, cAMP measurement, IFT88 knockdown, AC6 inhibition, osteogenic differentiation assays\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — multiple knockdowns with functional readout; mechanistic pathway placed; single lab\",\n      \"pmids\": [\"33450431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure of active human GPR161 bound to heterotrimeric Gs revealed: (1) extracellular loop 2 occupies the canonical GPCR orthosteric ligand pocket; (2) a sterol binds adjacent to transmembrane helices 6 and 7, stabilizing the Gs-coupling conformation — mutations preventing sterol binding suppress cAMP signaling but retain GPR161's ability to suppress GLI2 accumulation in cilia; (3) a PKA-binding site in the GPR161 C-terminus is critical for suppressing GLI2 ciliary accumulation.\",\n      \"method\": \"Cryo-EM structure determination, site-directed mutagenesis of sterol-binding and PKA-binding sites, cAMP reporter assays, immunofluorescence for GLI2 ciliary localization\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure plus mutagenesis plus functional assays; multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"38326651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"β-arrestin (ARRB1/ARRB2) is required for GPR161 export from cilia. GRK2 phosphorylates GPR161, enabling β-arrestin recruitment in its activated conformation, which then interacts with the BBSome to connect GPR161 to the IFT machinery for export. Activation-mimetic β-arrestin mutants cause constitutive GPR161 export. ARRB1/ARRB2 double knockout impairs GPR161 export.\",\n      \"method\": \"β-arrestin double KO cell lines, IFT27 and BBSome-subunit KO cells, overexpression of activation-mimetic β-arrestin mutants, Co-IP of β-arrestin with BBSome, immunofluorescence for ciliary GPR161\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple KO lines, interaction assays, separation-of-function mutants; multiple orthogonal approaches in one study\",\n      \"pmids\": [\"40384633\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Fuz is genetically epistatic to Gpr161 in Shh signaling during mouse neural tube development. FUZ protein biochemically interacts with GPR161, and Fuz regulates GPR161-mediated ciliary localization via β-arrestin 2.\",\n      \"method\": \"Genetic epistasis in double-mutant mice, co-immunoprecipitation of FUZ and GPR161, β-arrestin 2 interaction assay, ciliary localization immunofluorescence\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis plus Co-IP; single lab, two complementary methods\",\n      \"pmids\": [\"39369306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR161 acts as a mechanoreceptor at the primary cilium driving saltatory neuronal migration. Fluid shear stress induces GPR161-dependent cAMP/PKA signaling, leading to phosphorylation of NDE1 (a dynein complex regulator) and microtubule reorganization to regulate the rhythmicity of neuronal migration. The mechanosensitive helix 8 of GPR161 is essential for this function.\",\n      \"method\": \"Ex vivo neuronal migration model, microfluidic shear stress assays, GPR161 knockout/knockdown, helix-8 mutagenesis, phospho-NDE1 western blot, immunofluorescence for microtubule organization\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional assays with KO/mutagenesis and defined downstream substrate (NDE1); single lab, multiple methods\",\n      \"pmids\": [\"40737401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR161 promotes melanoma proliferation and metabolic activity through a STAT3–GPR161–TXNIP regulatory axis: STAT3 binds the GPR161 promoter and transcriptionally activates GPR161; GPR161 in turn negatively regulates TXNIP expression, reducing glycolytic capacity and proliferation when GPR161 is depleted.\",\n      \"method\": \"Promoter analysis, chromatin immunoprecipitation-qPCR for STAT3 binding, siRNA/shRNA knockdown, TXNIP overexpression/knockdown, glycolytic capacity assays\",\n      \"journal\": \"Journal of microbiology and biotechnology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — ChIP-qPCR plus functional knockdown with defined downstream readout; single lab\",\n      \"pmids\": [\"41834581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"GPR161 promotes glycolytic reprogramming in macrophages during acute lung injury through suppression of complement component 5a receptor 1 (C5aR1). Co-immunoprecipitation and surface plasmon resonance identified C5aR1 as a downstream target/interactor of GPR161; macrophage-specific GPR161 knockout attenuates pulmonary inflammatory damage.\",\n      \"method\": \"Global and macrophage-specific conditional KO mice, RNA sequencing, co-immunoprecipitation, surface plasmon resonance, glycolytic capacity assays\",\n      \"journal\": \"Cellular & molecular biology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding assay (SPR + Co-IP) plus conditional KO with defined cellular phenotype; single lab\",\n      \"pmids\": [\"42185764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FKBP8 was identified as a novel interacting protein of GPR161 (and FUZ), validated by affinity purification-mass spectrometry; GPR161 interactome is enriched for proteins associated with proteasomal catabolic processes, trafficking, receptor complex, and ER-Golgi transport.\",\n      \"method\": \"Affinity-based LC-MS/MS immunoprecipitation, STRING network analysis, co-IP validation of FKBP8\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/MS study; no functional follow-up on GPR161-FKBP8 interaction; preprint only\",\n      \"pmids\": [\"41000683\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Tumors arising from Gαs pathway inactivation are independent of GPR161 (and of canonical Smoothened), establishing an SMO-independent oncogenic Hedgehog signaling model that bypasses GPR161's negative regulatory role.\",\n      \"method\": \"Genetic epistasis in mouse BCC-like tumor models (Gαs pathway inactivation combined with GPR161 deletion), histology, bulk and single-cell RNA sequencing\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in vivo with transcriptomic readout; preprint, single lab\",\n      \"pmids\": [\"bio_10.1101_2025.02.21.639530\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ARL13B maintains GPR161 ciliary localization; ARL13B KO reduces ciliary GPR161 and increases Shh signaling. GPR161 deletion in human iPSC-derived brain organoids decreases Gli3 repressor formation and increases Shh signaling, causing ventralization of dorsal cortical progenitors. Pharmacological or optogenetic elevation of ciliary cAMP rescues Gli3 repressor formation and dorsal neural stem cell identity in GPR161 KO organoids.\",\n      \"method\": \"CRISPR KO in human iPSC-derived brain organoids, immunofluorescence for GPR161 and Gli3 repressor, cAMP pharmacology, optogenetic cAMP elevation\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO in human organoids with pathway rescue by cAMP; multiple orthogonal interventions; preprint, single lab\",\n      \"pmids\": [\"bio_10.1101_2024.07.18.604098\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR161-driven GLI3 repressor (GLI3R) signaling at the primary cilium controls apical constriction and cell fate during cranial neural tube closure. A functional non-ciliary Gpr161 knock-in implicated ciliary GPR161 localization in initiation and maintenance of cranial closure. GLI3R expression (but not GLI2 loss) rescued exencephaly in Gpr161 KO mice, and GLI3R restricted forebrain ventral floor plate expansion and mediated apical constriction in lateral midbrain neural folds.\",\n      \"method\": \"Gpr161 mutant allelic series including non-ciliary knock-in, genetic epistasis with Gli2 KO and Gli3R expression, in toto imaging of cell behavior, immunofluorescence\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — allelic series plus genetic epistasis plus live imaging; multiple orthogonal approaches; single lab but very rigorous\",\n      \"pmids\": [\"41417007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Gpr161 regulates the retinoic acid (RA) and canonical Wnt pathways during neurulation, downstream of Gpr161 activity. RA injection restores canonical Wnt marker expression and rescues Gpr161 hypomorphic neural tube defects, placing RA upstream of Wnt as part of the Gpr161 downstream cascade.\",\n      \"method\": \"Hypomorphic Gpr161vl allele, modifier QTL mapping, QRT-PCR, in situ hybridization, IHC, intraperitoneal RA injection rescue experiment\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological rescue plus gene expression analysis; single lab, two orthogonal approaches\",\n      \"pmids\": [\"25753732\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPR161 is a constitutively active orphan GPCR that localizes to primary cilia via IFT-A/Tulp3, where it couples to Gαs to elevate cAMP and anchor type I PKA (acting as an AKAP) through its C-terminal tail, thereby promoting Gli3 transcriptional repressor formation and suppressing basal Hedgehog signaling; upon Shh pathway activation, Smoothened accumulates in cilia and facilitates GRK2-mediated phosphorylation of GPR161 followed by β-arrestin recruitment, BBSome engagement, and clathrin-mediated removal from cilia, relieving pathway repression. A cryo-EM structure revealed that an extrahelical sterol stabilizes GPR161's Gs-coupling conformation while the PKA-binding C-terminal site is specifically required for suppressing GLI2 accumulation in cilia. Beyond Hedgehog, GPR161 also serves as a mechanosensor driving neuronal saltatory migration via NDE1 phosphorylation, regulates osteogenesis in mesenchymal stem cells via adenylyl cyclase 6, forms signaling complexes with β-arrestin 2/IQGAP1 to activate mTOR in cancer cells, and in macrophages suppresses C5aR1 to control glycolytic reprogramming.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GPR161 is a constitutively active, ciliary GPCR that functions as the principal negative regulator of Hedgehog (Hh) signaling by coupling cAMP/PKA activity to formation of the Gli3 transcriptional repressor [#0]. Localized to primary cilia in a Tulp3/IFT-A-dependent manner, its constitutive Gαs coupling raises cAMP and drives Gli3 processing to its repressor form, suppressing basal Shh output during neural tube, limb, and skeletal development [#0, #5]. A cryo-EM structure of active GPR161–Gs showed extracellular loop 2 occupying the orthosteric pocket and an extrahelical sterol adjacent to TM6/TM7 stabilizing the Gs-coupling conformation; sterol-binding mutations abolish cAMP signaling yet retain suppression of GLI2 ciliary accumulation, which instead depends on a C-terminal PKA-binding site [#13]. GPR161 acts as an A-kinase anchoring protein for type I PKA regulatory subunits through this cytoplasmic C-tail and is itself a PKA substrate, establishing a feedback loop tuning its ciliary residence [#2, #11]. Upon Shh activation, ciliary Smoothened promotes GRK2-mediated phosphorylation of GPR161 and β-arrestin recruitment, which engages the BBSome–IFT machinery to drive clathrin-mediated removal of the receptor from cilia, relieving repression [#1, #14, #7]. Separation-of-function alleles establish that distinct ciliary and extraciliary GPR161 pools set tissue-specific Gli repressor thresholds governing neural tube patterning, neural tube closure, and limb/craniofacial morphogenesis [#10, #22]. Loss of GPR161 derepresses Shh signaling and causes overproliferation phenotypes, defining it as a tumor suppressor in Shh-subtype medulloblastoma [#6, #9]. Beyond Hedgehog, GPR161 functions as a mechanoresponsive ciliary receptor: fluid shear stress triggers GPR161-dependent cAMP/PKA signaling that drives osteogenic differentiation of mesenchymal stem cells via adenylyl cyclase 6 [#12] and regulates saltatory neuronal migration via phosphorylation of the dynein regulator NDE1 through its mechanosensitive helix 8 [#16]. Truncating and rare patient variants link GPR161 to neural tube defects, cataracts, and spina bifida [#4, #8].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Before its signaling role was known, positional cloning of a spontaneous mouse mutant tied Gpr161 to a developmental disease phenotype and implicated its C-terminal tail in receptor trafficking.\",\n      \"evidence\": \"Positional cloning of the vacuolated lens mutant and endocytosis assay of a C-terminally truncated receptor\",\n      \"pmids\": [\"18250320\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No signaling mechanism or pathway placement\", \"Did not connect endocytosis defect to a downstream transduction cascade\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Established the core mechanism: GPR161 is a ciliary, constitutively active GPCR that raises cAMP to drive Gli3 repressor formation and suppress Shh signaling, with Shh-triggered ciliary removal relieving repression.\",\n      \"evidence\": \"Mouse knockout, cAMP reporters, Gli3 processing westerns, and genetic epistasis with Tulp3/IFT-A in neural tube development\",\n      \"pmids\": [\"23332756\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G protein coupling not directly demonstrated biochemically\", \"Endogenous ligand not identified\", \"Molecular machinery of ciliary export undefined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified a Hedgehog-independent role by placing GPR161 in a β-arrestin 2/IQGAP1 complex that activates mTOR and supports cancer cell proliferation.\",\n      \"evidence\": \"Co-IP of the GPR161–β-arrestin 2–IQGAP1 complex and shRNA knockdown in basal breast cancer cells\",\n      \"pmids\": [\"24599592\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, Co-IP-based complex\", \"Directness of IQGAP1/mTOR coupling not resolved\", \"Relationship to ciliary cAMP role unclear\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extended the downstream cascade by linking GPR161 activity to retinoic acid and canonical Wnt regulation during neurulation.\",\n      \"evidence\": \"Hypomorphic Gpr161vl allele with RA injection rescue and gene-expression analysis in mouse neural tube\",\n      \"pmids\": [\"25753732\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting GPR161 to RA/Wnt not molecular\", \"Single lab pharmacological rescue\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Resolved how GPR161 is removed from cilia: a two-step GRK2/β-arrestin recruitment followed by extraciliary clathrin-mediated endocytosis, promoted by ciliary Smoothened activity.\",\n      \"evidence\": \"Co-IP, dominant-negative and KO cells, live imaging, and clathrin inhibitors\",\n      \"pmids\": [\"27002170\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"BBSome/IFT linkage to export not yet defined\", \"GRK2 phosphosites on GPR161 not mapped\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined GPR161 as an AKAP that directly anchors type I PKA via its C-tail and is reciprocally phosphorylated by PKA, explaining how it compartmentalizes PKA activity at cilia.\",\n      \"evidence\": \"Phosphoproteomics, RI pull-down binding assays, mutagenesis, and zebrafish imaging\",\n      \"pmids\": [\"27357676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the RI interface not resolved\", \"Functional separation of AKAP versus cAMP-generating roles not yet tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrated tissue-specific, cilium-dependent Gli-repressor regulation by GPR161 in limb/skeletal morphogenesis and its tumor-suppressor function in Shh-medulloblastoma.\",\n      \"evidence\": \"Conditional KO mice in limb, craniofacial, neural stem cell, and granule progenitor lineages with epistasis to cilia mutants\",\n      \"pmids\": [\"29222391\", \"29386106\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether different tissues use distinct GPR161 effector pools unresolved\", \"Quantitative link between cAMP levels and Gli thresholds not established\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Defined the molecular export route by showing IFT-B subunit IFT38 bridges to the BBSome to drive Hh-induced GPR161 export, and extended disease relevance with dominant-negative spina bifida variants and cortical malformation phenotypes.\",\n      \"evidence\": \"Reciprocal IP and rescue with interaction-deficient IFT38; patient variant localization/reporter assays; conditional KO mice with proliferation and migration readouts\",\n      \"pmids\": [\"31471295\", \"30256984\", \"30914320\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Order of β-arrestin versus BBSome engagement not fully resolved\", \"Patient variant studies single-lab\", \"Wnt dysregulation mechanism in variants unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Separated ciliary from extraciliary GPR161 functions and established a PKA-phosphorylation feedback loop tuning ciliary localization and Gαs coupling sensitivity, while extending function to mechanosensation in osteogenesis.\",\n      \"evidence\": \"CRISPR knock-in separation-of-function mice, BRET Gαs coupling and phospho-deficient mutants in zebrafish, and shear-stress/AC6 knockdown in MSCs\",\n      \"pmids\": [\"34346313\", \"33531430\", \"33450431\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How extraciliary pool localizes and signals mechanistically undefined\", \"Mechanotransduction sensor element not yet identified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided the structural mechanism: a cryo-EM active GPR161–Gs structure showing ELL2 in the orthosteric pocket and a sterol stabilizing Gs coupling, and dissociating cAMP signaling from a C-terminal PKA-binding site required for GLI2 suppression.\",\n      \"evidence\": \"Cryo-EM with sterol/PKA-site mutagenesis, cAMP assays, and GLI2 ciliary localization; FUZ identified as an interactor regulating ciliary localization via β-arrestin 2\",\n      \"pmids\": [\"38326651\", \"39369306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of any endogenous diffusible ligand unresolved\", \"How sterol availability is regulated in vivo unknown\", \"FUZ regulatory mechanism only partly defined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Mechanistically completed the export pathway (GRK2→β-arrestin→BBSome→IFT) and broadened GPR161's roles into neuronal migration mechanosensing, melanoma metabolism, and macrophage glycolytic reprogramming.\",\n      \"evidence\": \"β-arrestin/IFT27/BBSome KO cells and activation-mimetic mutants; helix-8 mutagenesis with phospho-NDE1; STAT3–GPR161–TXNIP ChIP/knockdown; macrophage-specific KO with Co-IP/SPR for C5aR1\",\n      \"pmids\": [\"40384633\", \"40737401\", \"41834581\", \"42185764\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Non-Hedgehog effector mechanisms are single-lab\", \"Whether these roles share the ciliary cAMP module is unclear\", \"Directness of some interactions (C5aR1, NDE1) needs orthogonal confirmation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The endogenous activating ligand of GPR161 and the unifying molecular basis linking its ciliary cAMP/AKAP function to its diverse non-Hedgehog roles remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No deorphanized endogenous agonist established\", \"Mechanism switching between mechanosensation and constitutive signaling unknown\", \"Therapeutic targetability untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 11, 13]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [12, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [0, 1, 10, 13]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 5, 10, 22]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [1, 7, 14]}\n    ],\n    \"complexes\": [\"BBSome (functional engagement for ciliary export)\"],\n    \"partners\": [\"ARRB2\", \"ARRB1\", \"GRK2\", \"IQGAP1\", \"IFT38\", \"FUZ\", \"FKBP8\", \"C5aR1\"],\n    \"other_free_text\": []\n  }\n}\n```","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}