{"gene":"GPR161","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2013,"finding":"GPR161 localizes to primary cilia in a Tulp3/IFT-A-dependent manner and constitutively increases cAMP levels, promoting PKA-dependent processing of Gli3 to its repressor form, thereby repressing Sonic hedgehog (Shh) signaling. Shh signaling directs GPR161 internalization from cilia, preventing its activity.","method":"Mouse knockout, cAMP reporter assays, Gli3 processing immunoblots, ciliary localization by immunofluorescence, epistasis with IFT-A/Tulp3 mutants","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (KO phenotype, cAMP assay, Gli3 processing, localization), highly cited foundational paper","pmids":["23332756"],"is_preprint":false},{"year":2016,"finding":"Shh pathway activation drives GPR161 removal from primary cilia in a two-step process: first, GRK2-mediated phosphorylation recruits β-arrestin to GPR161 (facilitated by ciliary Smoothened activation increasing GPR161-β-arrestin binding); second, clathrin-mediated endocytosis outside the cilium completes removal.","method":"β-arrestin recruitment assays, GRK2 inhibition/knockdown, clathrin inhibition, live-cell imaging, co-immunoprecipitation","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP, functional endocytosis assays, multiple orthogonal methods in a single well-cited study","pmids":["27002170"],"is_preprint":false},{"year":2016,"finding":"GPR161 functions as an A-kinase anchoring protein (AKAP) that directly binds type I PKA regulatory subunits (RI) via a hydrophobic interface in its cytoplasmic C-terminal tail. This binary complex compartmentalizes PKA to the plasma membrane and to primary cilia. GPR161 is itself a PKA phosphorylation target, and mutation of the PKA phosphorylation site affects its ciliary localization.","method":"Phosphoproteomics, cell-based protein-protein interaction reporters, direct binding assays, zebrafish in vivo rescue experiments, site-directed mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding mapped to domain, mutagenesis, phosphoproteomics, in vivo validation in zebrafish","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 mTORC1 signaling, decreases IQGAP1 phosphorylation, and promotes cell proliferation, migration, and intracellular accumulation of E-cadherin in mammary epithelial cells.","method":"Co-immunoprecipitation, knockdown/overexpression functional assays, 3D culture, signaling pathway analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2-3 — co-IP with functional follow-up, single lab study","pmids":["24599592"],"is_preprint":false},{"year":2008,"finding":"A C-terminal truncation mutation in Gpr161 (vacuolated lens allele) reduces receptor-mediated endocytosis and causes neural tube defects and cataracts in mice, establishing the C-terminal tail as required for normal endocytic trafficking.","method":"Positional cloning, endocytosis assays in mutant cells, mouse genetic analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — genetic + functional endocytosis assay in a single paper","pmids":["18250320"],"is_preprint":false},{"year":2019,"finding":"The IFT-B subunit IFT38 directly interacts with BBSome subunits BBS1, BBS2, and BBS9. This IFT-B–BBSome interaction is required for export of GPR161 from cilia upon Hedgehog signaling activation; cells expressing an IFT38 mutant that cannot bind the BBSome accumulate GPR161 within cilia.","method":"Visible immunoprecipitation assay, IFT38 knockout cell lines, rescue with wild-type vs. interaction-deficient IFT38 mutants, immunofluorescence","journal":"Biology open","confidence":"High","confidence_rationale":"Tier 1-2 — reconstituted interaction, KO + rescue with separation-of-function mutant","pmids":["31471295"],"is_preprint":false},{"year":2018,"finding":"Gpr161 deletion in mouse neural stem cells or granule cell (GC) progenitors increases Shh pathway activity by restricting Gli3-mediated repression, causing excessive GC progenitor proliferation and medulloblastoma; the overproduction phenotype is cilium-dependent.","method":"Conditional knockout mouse models (neural stem cell- and GC progenitor-specific), cilium-dependency tested by genetic cilia ablation, pathway analysis","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with defined cellular phenotype, cilium dependency established by epistasis","pmids":["29386106"],"is_preprint":false},{"year":2018,"finding":"Limb-specific deletion of Gpr161 causes premature expansion of Shh signaling and ectopic Shh-dependent patterning defects; loss of Gpr161 in chondrocytes prevents columnar differentiation and Ihh signaling. All limb and skeletal morphogenesis defects are suppressed in the absence of cilia, placing Gpr161 upstream of cilia-dependent hedgehog signaling in these tissues.","method":"Conditional knockout mice, genetic epistasis with cilia mutants, histological and molecular analysis of signaling","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — conditional KO + cilium epistasis (cilia-ablation rescue) with multiple tissue readouts","pmids":["29222391"],"is_preprint":false},{"year":2021,"finding":"A ciliary localization-defective but cAMP-signaling-competent knock-in variant of Gpr161 (Gpr161mut1) shows that ciliary and extraciliary pools of Gpr161 establish tissue-specific Gli repressor thresholds: loss of ciliary localization causes intermediate neural tube progenitor expansion (Gli3 repressor-dependent) but not the ventral-most Gli2 activator-dependent expansion seen in full knockouts. Polydactyly and midfacial widening result from loss of ciliary localization.","method":"Knock-in mouse engineering, conditional allele comparisons (KO vs. cilia-defective mutant), immunofluorescence, pathway target analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1-2 — separation-of-function knock-in with precise mechanistic dissection across tissues","pmids":["34346313"],"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 orthosteric ligand pocket; (2) a sterol binds adjacent to transmembrane helices 6 and 7 and stabilizes the Gs-coupling conformation—mutations preventing sterol binding suppress cAMP signaling but retain the ability to suppress GLI2 accumulation in cilia; (3) the 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 site and PKA-binding site, cAMP assays, GLI2 ciliary accumulation assays","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with mutagenesis and functional validation across multiple orthogonal assays","pmids":["38326651"],"is_preprint":false},{"year":2021,"finding":"PKA feedback phosphorylation of Gpr161 fine-tunes its ciliary localization and PKA activity. PKA phosphorylation-deficient Gpr161 shows increased sensitivity to Shh, resulting in excess high-level Hh target gene expression in zebrafish, demonstrating a feedback loop controlling Gpr161 activity.","method":"Zebrafish loss-of-function, transgenic rescue with phosphorylation-deficient Gpr161 mutants, BRET-based Gαs coupling assay, Hh target gene expression analysis","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 — separation-of-function mutants in vivo with direct Gαs coupling assay, single lab","pmids":["33531430"],"is_preprint":false},{"year":2019,"finding":"Novel rare GPR161 variants identified in spina bifida patients mislocalize to primary cilia, dysregulate Shh and Wnt signaling, and inhibit cell proliferation in vitro, acting as dominant negatives.","method":"Sanger sequencing, in vitro ciliary localization by immunofluorescence, Shh/Wnt pathway reporter assays, cell proliferation assays","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2-3 — functional variant characterization with ciliary localization and pathway assays, single lab","pmids":["30256984"],"is_preprint":false},{"year":2021,"finding":"Gpr161 is a mechanoresponsive receptor in mesenchymal stem cells (MSCs) that localizes to the primary cilium and mediates fluid shear stress-induced cAMP signaling and osteogenesis through adenylyl cyclase 6 (AC6). Hh signaling downstream of this Gpr161-AC6-cAMP axis is required for loading-induced osteogenic differentiation.","method":"Fluid shear stress assay, siRNA knockdown of Gpr161, AC6, and IFT88, cAMP measurement, osteogenesis assays","journal":"Bone","confidence":"Medium","confidence_rationale":"Tier 2-3 — functional knockdown with defined mechanotransduction pathway, single lab","pmids":["33450431"],"is_preprint":false},{"year":2025,"finding":"GPR161 acts as a mechanical sensor at the primary cilium with its helix 8 being essential for mechanosensitivity. Fluid shear stress activates GPR161, triggering a cAMP/PKA signaling cascade that phosphorylates NDE1 (a dynein complex regulator) and reorganizes microtubules to drive the saltatory migration of neurons.","method":"Ex vivo neuronal migration model, microfluidic assays, helix 8 mutagenesis, NDE1 phosphorylation analysis, live imaging","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 — domain mutagenesis + functional migration assay + downstream phosphorylation target identified, single paper","pmids":["40737401"],"is_preprint":false},{"year":2025,"finding":"β-arrestins (ARRB1 and ARRB2) are required for GPR161 export from cilia. Activation-mimetic β-arrestin mutants interact with both the BBSome and ciliary GPR161 and cause constitutive GPR161 export. GRK2 phosphorylates GPR161 to recruit β-arrestins, which in their activated conformation interact with the BBSome to connect GPR161 to the IFT machinery for export.","method":"ARRB1/ARRB2 double-knockout cells, expression of activation-mimetic β-arrestin mutants, co-immunoprecipitation, IFT27/BBSome-KO analysis, immunofluorescence","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — KO + activation-mimetic mutant rescue + co-IP demonstrating BBSome-β-arrestin-GPR161 complex, multiple orthogonal methods","pmids":["40384633"],"is_preprint":false},{"year":2024,"finding":"Fuz is genetically epistatic to Gpr161 in Shh signaling during mouse neural tube development. The Fuz protein biochemically interacts with GPR161, and Fuz regulates GPR161-mediated ciliary localization through a mechanism involving β-arrestin 2.","method":"Double-mutant epistasis in mice, co-immunoprecipitation, ciliary localization assays, β-arrestin 2 functional analysis","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2-3 — genetic epistasis + biochemical interaction + localization assay, single lab","pmids":["39369306"],"is_preprint":false},{"year":2025,"finding":"Cranial neural tube closure requires GPR161 ciliary localization for GLI3 repressor (GLI3R) formation. Epistasis experiments show that Gli3R expression, but not Gli2 loss, rescues exencephaly in Gpr161 knockout mice. GLI3R restricts forebrain ventral floor plate expansion and mediates apical constriction in lateral midbrain neural folds prior to closure.","method":"Gpr161 mutant allelic series, genetic epistasis (Gpr161 KO × GLI effector mutants), non-ciliary Gpr161 knock-in, in toto live imaging","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — allelic series + epistasis + separation-of-function knock-in + live imaging establishing mechanistic pathway","pmids":["41417007"],"is_preprint":false},{"year":2015,"finding":"In the context of a hypomorphic Gpr161 allele, Gpr161 regulates retinoic acid (RA) and canonical Wnt pathway gene expression during neurulation independently of severe Shh pathway effects. RA injection rescues Wnt markers and neural tube defects in Gpr161 hypomorphs.","method":"QRT-PCR, ISH, IHC, RA injection rescue experiments, modifier QTL analysis in mice","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2-3 — pharmacological rescue + expression analysis, but pathway connection is indirect","pmids":["25753732"],"is_preprint":false}],"current_model":"GPR161 is a constitutively active orphan GPCR that localizes to primary cilia via Tulp3/IFT-A-dependent trafficking, where it couples to Gs to elevate cAMP and activate PKA—which it anchors directly via an AKAP motif in its C-terminus—thereby promoting Gli3 repressor formation and suppressing Hedgehog signaling; upon Hedgehog pathway activation, Smoothened facilitates GRK2-mediated phosphorylation of GPR161, recruiting β-arrestins that interact with the BBSome–IFT machinery to export GPR161 from cilia via clathrin-mediated endocytosis, and a sterol binding at transmembrane helices 6/7 stabilizes the Gs-coupling conformation as revealed by cryo-EM structure, while the PKA-binding C-terminal site (rather than sterol-dependent cAMP) is critical for suppressing GLI2 accumulation in cilia."},"narrative":{"teleology":[{"year":2008,"claim":"Identification of a C-terminal truncation mutation in Gpr161 (vacuolated lens allele) established that the receptor's C-terminal tail is essential for endocytic trafficking and that its loss causes neural tube defects and cataracts, providing the first link between GPR161 and developmental signaling.","evidence":"Positional cloning and endocytosis assays in mutant mouse cells","pmids":["18250320"],"confidence":"Medium","gaps":["No pathway mechanism identified","No ciliary localization reported","Single mouse allele"]},{"year":2013,"claim":"The foundational mechanistic framework was established: GPR161 localizes to primary cilia via Tulp3/IFT-A, constitutively activates cAMP/PKA to promote Gli3 repressor formation, and is removed from cilia upon Shh stimulation—resolving how the cilium suppresses Hh signaling in the absence of ligand.","evidence":"Mouse Gpr161 knockout, cAMP reporter assays, Gli3 processing immunoblots, ciliary immunofluorescence, epistasis with IFT-A/Tulp3 mutants","pmids":["23332756"],"confidence":"High","gaps":["Mechanism of ciliary exit unknown","Coupling to PKA not mapped to a direct binding interface","No structural information on receptor"]},{"year":2015,"claim":"A hypomorphic Gpr161 allele revealed Shh-independent regulation of retinoic acid and Wnt pathway gene expression during neurulation, suggesting broader signaling roles for GPR161 beyond Hedgehog repression.","evidence":"QRT-PCR, in situ hybridization, retinoic acid rescue of neural tube defects in Gpr161 hypomorphic mice","pmids":["25753732"],"confidence":"Medium","gaps":["Pathway connection to RA and Wnt is indirect","Not confirmed in independent genetic backgrounds","Mechanism linking cAMP to Wnt/RA regulation unclear"]},{"year":2016,"claim":"Two key mechanisms were resolved in parallel: (1) Shh-induced ciliary removal proceeds through GRK2 phosphorylation of GPR161, β-arrestin recruitment, and clathrin-mediated endocytosis; (2) GPR161 directly binds type I PKA regulatory subunits via a C-terminal AKAP motif, compartmentalizing PKA to cilia and the plasma membrane.","evidence":"β-arrestin recruitment and clathrin inhibition assays with live-cell imaging; phosphoproteomics, direct binding assays, AKAP domain mutagenesis, zebrafish in vivo rescue","pmids":["27002170","27357676"],"confidence":"High","gaps":["Role of BBSome in β-arrestin-mediated export not yet defined","Relative contributions of cAMP production vs. PKA anchoring to Gli repression unresolved"]},{"year":2018,"claim":"Conditional knockouts demonstrated that Gpr161 loss causes cilium-dependent tissue-specific Hh pathway hyperactivation, driving medulloblastoma from cerebellar granule cell progenitors and limb/skeletal patterning defects—establishing GPR161 as a bona fide Hh pathway tumor suppressor.","evidence":"Cell-type-specific conditional knockout mice with genetic cilia ablation epistasis, histological and molecular pathway analysis","pmids":["29386106","29222391"],"confidence":"High","gaps":["Human tumor suppressor role not validated by patient mutation data at this point","Hh-independent functions in these tissues not tested"]},{"year":2019,"claim":"The IFT-B–BBSome connection required for GPR161 ciliary export was mapped: IFT38 directly binds BBSome subunits, and disrupting this interaction traps GPR161 in cilia. Separately, rare GPR161 variants in human spina bifida patients were shown to mislocalize and dominantly disrupt Shh signaling.","evidence":"IFT38 knockout + rescue with BBSome-interaction-deficient mutant; Sanger sequencing of spina bifida patients with in vitro functional characterization","pmids":["31471295","30256984"],"confidence":"High","gaps":["Full reconstitution of the GPR161–β-arrestin–BBSome–IFT export complex not achieved","Spina bifida variants characterized in vitro only, lacking in vivo rescue"]},{"year":2021,"claim":"Separation-of-function studies dissected ciliary vs. extraciliary GPR161 roles: a ciliary-localization-defective but cAMP-competent knock-in showed that the ciliary pool specifically controls Gli3 repressor thresholds in the neural tube, while PKA feedback phosphorylation of GPR161 fine-tunes its own ciliary retention and Hh sensitivity.","evidence":"Knock-in mouse allelic series with tissue-level pathway readouts; zebrafish transgenic rescue with phospho-deficient GPR161 mutants and BRET-based Gαs coupling","pmids":["34346313","33450431","33531430"],"confidence":"High","gaps":["Extraciliary GPR161 function mechanism remains molecularly undefined","Mechanoresponsive role via AC6 seen in MSCs not validated in other cell types"]},{"year":2024,"claim":"Cryo-EM structure of GPR161–Gs resolved the molecular basis of constitutive activity: ECL2 occupies the orthosteric pocket, a sterol at TM6/7 stabilizes the active conformation for Gs coupling, and critically, the C-terminal PKA-binding site—not sterol-dependent cAMP—is the essential feature for suppressing GLI2 ciliary accumulation, separating cAMP production from PKA anchoring as distinct functional outputs.","evidence":"Cryo-EM structure, sterol-binding and PKA-binding site mutagenesis, cAMP and GLI2 ciliary accumulation assays","pmids":["38326651"],"confidence":"High","gaps":["Endogenous sterol ligand identity not confirmed","No structure of inactive or β-arrestin-bound state","How PKA anchoring mechanistically suppresses GLI2 at cilia is unresolved"]},{"year":2025,"claim":"The complete β-arrestin–BBSome export pathway was reconstituted: β-arrestins are required for GPR161 ciliary exit, activation-mimetic β-arrestin mutants constitutively interact with the BBSome and GPR161 to drive export, and GRK2 phosphorylation initiates this cascade. Separately, GPR161 was shown to function as a ciliary mechanosensor whose helix 8 mediates shear-stress responsiveness to drive NDE1 phosphorylation and saltatory neuronal migration.","evidence":"ARRB1/ARRB2 double-KO cells with activation-mimetic rescue and co-IP; microfluidic shear-stress assays with helix 8 mutagenesis and NDE1 phosphorylation analysis","pmids":["40384633","40737401"],"confidence":"High","gaps":["Structural basis of β-arrestin–BBSome interaction unknown","Helix 8 mechanosensing mechanism not structurally defined","In vivo relevance of mechanosensing for brain development needs confirmation"]},{"year":2025,"claim":"GLI3 repressor was identified as the essential effector of GPR161-dependent cranial neural tube closure: GLI3R expression, but not GLI2 loss, rescues exencephaly in Gpr161 knockouts, with GLI3R restricting forebrain floor plate expansion and mediating apical constriction for neural fold closure.","evidence":"Gpr161 allelic series × GLI effector mutant epistasis, non-ciliary knock-in, in toto live imaging","pmids":["41417007"],"confidence":"High","gaps":["Molecular mechanism linking GLI3R to apical constriction machinery unknown","Whether this mechanism operates in human cranial closure untested"]},{"year":null,"claim":"Key open questions include the identity of the endogenous sterol or lipid ligand that activates GPR161, the structural basis of the inactive and β-arrestin-bound receptor states, how PKA anchoring at cilia mechanistically controls GLI2 accumulation independently of cAMP, and the full extent of Hedgehog-independent GPR161 signaling roles (Wnt, RA, mTORC1).","evidence":"","pmids":[],"confidence":"Medium","gaps":["Endogenous activating ligand unidentified","No inactive-state or arrestin-bound structure","PKA-anchoring-to-GLI2 mechanism unresolved","Hh-independent roles lack in vivo mechanistic validation"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,9,13]},{"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":[13,12]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[0,2,5,8,9,14]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,6,7,8,9]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6,7,8,16]}],"complexes":["GPR161–PKA-RI (AKAP complex)","GPR161–β-arrestin–BBSome export complex"],"partners":["GNAS","PRKAR1A","ARRB2","GRK2","TULP3","IQGAP1","FUZ","IFT38"],"other_free_text":[]},"mechanistic_narrative":"GPR161 is a constitutively active orphan G-protein-coupled receptor that localizes to primary cilia and functions as a central negative regulator of Hedgehog (Hh) signaling by coupling to Gαs to elevate cAMP and by directly scaffolding type I PKA via an A-kinase anchoring protein (AKAP) motif in its C-terminal tail, thereby promoting GLI3 processing to its repressor form [PMID:23332756, PMID:27357676, PMID:38326651]. A cryo-EM structure reveals that a sterol bound at transmembrane helices 6/7 stabilizes the Gs-coupling conformation, while the PKA-binding C-terminal domain—rather than sterol-dependent cAMP production—is the critical determinant for suppressing GLI2 accumulation in cilia [PMID:38326651]. Ciliary trafficking of GPR161 depends on Tulp3/IFT-A for import [PMID:23332756], and Hh-triggered export requires GRK2 phosphorylation, β-arrestin recruitment, and coupling to the BBSome–IFT machinery for clathrin-mediated removal [PMID:27002170, PMID:40384633]. Loss of Gpr161 causes cilium-dependent ectopic Hh pathway activation leading to neural tube defects, medulloblastoma, polydactyly, and skeletal patterning abnormalities in mice, and rare GPR161 variants have been identified in human spina bifida patients [PMID:29386106, PMID:29222391, PMID:30256984]."},"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 muscle","ntpm":25.7}],"url":"https://www.proteinatlas.org/search/GPR161"},"hgnc":{"alias_symbol":["RE2"],"prev_symbol":[]},"alphafold":{"accession":"Q8N6U8","domains":[{"cath_id":"1.20.1070.10","chopping":"8-227_258-349","consensus_level":"medium","plddt":84.9289,"start":8,"end":349}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N6U8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N6U8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N6U8-F1-predicted_aligned_error_v6.png","plddt_mean":68.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR161","jax_strain_url":"https://www.jax.org/strain/search?query=GPR161"},"sequence":{"accession":"Q8N6U8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8N6U8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8N6U8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N6U8"}},"corpus_meta":[{"pmid":"23332756","id":"PMC_23332756","title":"The ciliary G-protein-coupled receptor Gpr161 negatively regulates the Sonic hedgehog pathway via cAMP signaling.","date":"2013","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/23332756","citation_count":402,"is_preprint":false},{"pmid":"27002170","id":"PMC_27002170","title":"Smoothened determines β-arrestin-mediated removal of the G protein-coupled receptor Gpr161 from the primary cilium.","date":"2016","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/27002170","citation_count":118,"is_preprint":false},{"pmid":"27357676","id":"PMC_27357676","title":"Gpr161 anchoring of PKA consolidates GPCR and cAMP signaling.","date":"2016","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/27357676","citation_count":94,"is_preprint":false},{"pmid":"24599592","id":"PMC_24599592","title":"G-protein-coupled receptor GPR161 is overexpressed in breast cancer and is a promoter of cell proliferation and invasion.","date":"2014","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/24599592","citation_count":65,"is_preprint":false},{"pmid":"18250320","id":"PMC_18250320","title":"The orphan G protein-coupled receptor, Gpr161, encodes the vacuolated lens locus and controls neurulation and lens development.","date":"2008","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/18250320","citation_count":60,"is_preprint":false},{"pmid":"25322266","id":"PMC_25322266","title":"Whole-exome sequencing identifies homozygous GPR161 mutation in a family with pituitary stalk interruption syndrome.","date":"2015","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/25322266","citation_count":55,"is_preprint":false},{"pmid":"31609649","id":"PMC_31609649","title":"Germline GPR161 Mutations Predispose to Pediatric Medulloblastoma.","date":"2019","source":"Journal of clinical oncology : official journal of the American Society of Clinical Oncology","url":"https://pubmed.ncbi.nlm.nih.gov/31609649","citation_count":55,"is_preprint":false},{"pmid":"29222391","id":"PMC_29222391","title":"The G protein-coupled receptor Gpr161 regulates forelimb formation, limb patterning and skeletal morphogenesis in a primary cilium-dependent manner.","date":"2018","source":"Development (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/29222391","citation_count":54,"is_preprint":false},{"pmid":"29386106","id":"PMC_29386106","title":"Basal Suppression of the Sonic Hedgehog Pathway by the G-Protein-Coupled Receptor Gpr161 Restricts Medulloblastoma Pathogenesis.","date":"2018","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/29386106","citation_count":47,"is_preprint":false},{"pmid":"31471295","id":"PMC_31471295","title":"Requirement of IFT-B-BBSome complex interaction in export of GPR161 from cilia.","date":"2019","source":"Biology open","url":"https://pubmed.ncbi.nlm.nih.gov/31471295","citation_count":41,"is_preprint":false},{"pmid":"31947770","id":"PMC_31947770","title":"Hedgehog and Gpr161: Regulating cAMP Signaling in the Primary Cilium.","date":"2020","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/31947770","citation_count":34,"is_preprint":false},{"pmid":"32914682","id":"PMC_32914682","title":"Enterotoxigenic Escherichia coli infection promotes enteric defensin expression via FOXO6-METTL3-m6A-GPR161 signalling axis.","date":"2020","source":"RNA biology","url":"https://pubmed.ncbi.nlm.nih.gov/32914682","citation_count":31,"is_preprint":false},{"pmid":"30256984","id":"PMC_30256984","title":"Dominant negative GPR161 rare variants are risk factors of human spina bifida.","date":"2019","source":"Human molecular 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increases cAMP levels, promoting PKA-dependent processing of Gli3 to its repressor form, thereby repressing Sonic hedgehog (Shh) signaling. Shh signaling directs GPR161 internalization from cilia, preventing its activity.\",\n      \"method\": \"Mouse knockout, cAMP reporter assays, Gli3 processing immunoblots, ciliary localization by immunofluorescence, epistasis with IFT-A/Tulp3 mutants\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (KO phenotype, cAMP assay, Gli3 processing, localization), highly cited foundational paper\",\n      \"pmids\": [\"23332756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Shh pathway activation drives GPR161 removal from primary cilia in a two-step process: first, GRK2-mediated phosphorylation recruits β-arrestin to GPR161 (facilitated by ciliary Smoothened activation increasing GPR161-β-arrestin binding); second, clathrin-mediated endocytosis outside the cilium completes removal.\",\n      \"method\": \"β-arrestin recruitment assays, GRK2 inhibition/knockdown, clathrin inhibition, live-cell imaging, co-immunoprecipitation\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP, functional endocytosis assays, multiple orthogonal methods in a single well-cited study\",\n      \"pmids\": [\"27002170\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GPR161 functions as an A-kinase anchoring protein (AKAP) that directly binds type I PKA regulatory subunits (RI) via a hydrophobic interface in its cytoplasmic C-terminal tail. This binary complex compartmentalizes PKA to the plasma membrane and to primary cilia. GPR161 is itself a PKA phosphorylation target, and mutation of the PKA phosphorylation site affects its ciliary localization.\",\n      \"method\": \"Phosphoproteomics, cell-based protein-protein interaction reporters, direct binding assays, zebrafish in vivo rescue experiments, site-directed mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding mapped to domain, mutagenesis, phosphoproteomics, in vivo validation in zebrafish\",\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 mTORC1 signaling, decreases IQGAP1 phosphorylation, and promotes cell proliferation, migration, and intracellular accumulation of E-cadherin in mammary epithelial cells.\",\n      \"method\": \"Co-immunoprecipitation, knockdown/overexpression functional assays, 3D culture, signaling pathway analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — co-IP with functional follow-up, single lab study\",\n      \"pmids\": [\"24599592\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"A C-terminal truncation mutation in Gpr161 (vacuolated lens allele) reduces receptor-mediated endocytosis and causes neural tube defects and cataracts in mice, establishing the C-terminal tail as required for normal endocytic trafficking.\",\n      \"method\": \"Positional cloning, endocytosis assays in mutant cells, mouse genetic analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic + functional endocytosis assay in a single paper\",\n      \"pmids\": [\"18250320\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The IFT-B subunit IFT38 directly interacts with BBSome subunits BBS1, BBS2, and BBS9. This IFT-B–BBSome interaction is required for export of GPR161 from cilia upon Hedgehog signaling activation; cells expressing an IFT38 mutant that cannot bind the BBSome accumulate GPR161 within cilia.\",\n      \"method\": \"Visible immunoprecipitation assay, IFT38 knockout cell lines, rescue with wild-type vs. interaction-deficient IFT38 mutants, immunofluorescence\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted interaction, KO + rescue with separation-of-function mutant\",\n      \"pmids\": [\"31471295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Gpr161 deletion in mouse neural stem cells or granule cell (GC) progenitors increases Shh pathway activity by restricting Gli3-mediated repression, causing excessive GC progenitor proliferation and medulloblastoma; the overproduction phenotype is cilium-dependent.\",\n      \"method\": \"Conditional knockout mouse models (neural stem cell- and GC progenitor-specific), cilium-dependency tested by genetic cilia ablation, pathway analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with defined cellular phenotype, cilium dependency established by epistasis\",\n      \"pmids\": [\"29386106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Limb-specific deletion of Gpr161 causes premature expansion of Shh signaling and ectopic Shh-dependent patterning defects; loss of Gpr161 in chondrocytes prevents columnar differentiation and Ihh signaling. All limb and skeletal morphogenesis defects are suppressed in the absence of cilia, placing Gpr161 upstream of cilia-dependent hedgehog signaling in these tissues.\",\n      \"method\": \"Conditional knockout mice, genetic epistasis with cilia mutants, histological and molecular analysis of signaling\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO + cilium epistasis (cilia-ablation rescue) with multiple tissue readouts\",\n      \"pmids\": [\"29222391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A ciliary localization-defective but cAMP-signaling-competent knock-in variant of Gpr161 (Gpr161mut1) shows that ciliary and extraciliary pools of Gpr161 establish tissue-specific Gli repressor thresholds: loss of ciliary localization causes intermediate neural tube progenitor expansion (Gli3 repressor-dependent) but not the ventral-most Gli2 activator-dependent expansion seen in full knockouts. Polydactyly and midfacial widening result from loss of ciliary localization.\",\n      \"method\": \"Knock-in mouse engineering, conditional allele comparisons (KO vs. cilia-defective mutant), immunofluorescence, pathway target analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — separation-of-function knock-in with precise mechanistic dissection across tissues\",\n      \"pmids\": [\"34346313\"],\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 orthosteric ligand pocket; (2) a sterol binds adjacent to transmembrane helices 6 and 7 and stabilizes the Gs-coupling conformation—mutations preventing sterol binding suppress cAMP signaling but retain the ability to suppress GLI2 accumulation in cilia; (3) the 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 site and PKA-binding site, cAMP assays, GLI2 ciliary accumulation assays\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with mutagenesis and functional validation across multiple orthogonal assays\",\n      \"pmids\": [\"38326651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PKA feedback phosphorylation of Gpr161 fine-tunes its ciliary localization and PKA activity. PKA phosphorylation-deficient Gpr161 shows increased sensitivity to Shh, resulting in excess high-level Hh target gene expression in zebrafish, demonstrating a feedback loop controlling Gpr161 activity.\",\n      \"method\": \"Zebrafish loss-of-function, transgenic rescue with phosphorylation-deficient Gpr161 mutants, BRET-based Gαs coupling assay, Hh target gene expression analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — separation-of-function mutants in vivo with direct Gαs coupling assay, single lab\",\n      \"pmids\": [\"33531430\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Novel rare GPR161 variants identified in spina bifida patients mislocalize to primary cilia, dysregulate Shh and Wnt signaling, and inhibit cell proliferation in vitro, acting as dominant negatives.\",\n      \"method\": \"Sanger sequencing, in vitro ciliary localization by immunofluorescence, Shh/Wnt pathway reporter assays, cell proliferation assays\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — functional variant characterization with ciliary localization and pathway assays, single lab\",\n      \"pmids\": [\"30256984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Gpr161 is a mechanoresponsive receptor in mesenchymal stem cells (MSCs) that localizes to the primary cilium and mediates fluid shear stress-induced cAMP signaling and osteogenesis through adenylyl cyclase 6 (AC6). Hh signaling downstream of this Gpr161-AC6-cAMP axis is required for loading-induced osteogenic differentiation.\",\n      \"method\": \"Fluid shear stress assay, siRNA knockdown of Gpr161, AC6, and IFT88, cAMP measurement, osteogenesis assays\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — functional knockdown with defined mechanotransduction pathway, single lab\",\n      \"pmids\": [\"33450431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR161 acts as a mechanical sensor at the primary cilium with its helix 8 being essential for mechanosensitivity. Fluid shear stress activates GPR161, triggering a cAMP/PKA signaling cascade that phosphorylates NDE1 (a dynein complex regulator) and reorganizes microtubules to drive the saltatory migration of neurons.\",\n      \"method\": \"Ex vivo neuronal migration model, microfluidic assays, helix 8 mutagenesis, NDE1 phosphorylation analysis, live imaging\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain mutagenesis + functional migration assay + downstream phosphorylation target identified, single paper\",\n      \"pmids\": [\"40737401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"β-arrestins (ARRB1 and ARRB2) are required for GPR161 export from cilia. Activation-mimetic β-arrestin mutants interact with both the BBSome and ciliary GPR161 and cause constitutive GPR161 export. GRK2 phosphorylates GPR161 to recruit β-arrestins, which in their activated conformation interact with the BBSome to connect GPR161 to the IFT machinery for export.\",\n      \"method\": \"ARRB1/ARRB2 double-knockout cells, expression of activation-mimetic β-arrestin mutants, co-immunoprecipitation, IFT27/BBSome-KO analysis, immunofluorescence\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO + activation-mimetic mutant rescue + co-IP demonstrating BBSome-β-arrestin-GPR161 complex, multiple orthogonal methods\",\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. The Fuz protein biochemically interacts with GPR161, and Fuz regulates GPR161-mediated ciliary localization through a mechanism involving β-arrestin 2.\",\n      \"method\": \"Double-mutant epistasis in mice, co-immunoprecipitation, ciliary localization assays, β-arrestin 2 functional analysis\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — genetic epistasis + biochemical interaction + localization assay, single lab\",\n      \"pmids\": [\"39369306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cranial neural tube closure requires GPR161 ciliary localization for GLI3 repressor (GLI3R) formation. Epistasis experiments show that Gli3R expression, but not Gli2 loss, rescues exencephaly in Gpr161 knockout mice. GLI3R restricts forebrain ventral floor plate expansion and mediates apical constriction in lateral midbrain neural folds prior to closure.\",\n      \"method\": \"Gpr161 mutant allelic series, genetic epistasis (Gpr161 KO × GLI effector mutants), non-ciliary Gpr161 knock-in, in toto live imaging\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — allelic series + epistasis + separation-of-function knock-in + live imaging establishing mechanistic pathway\",\n      \"pmids\": [\"41417007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In the context of a hypomorphic Gpr161 allele, Gpr161 regulates retinoic acid (RA) and canonical Wnt pathway gene expression during neurulation independently of severe Shh pathway effects. RA injection rescues Wnt markers and neural tube defects in Gpr161 hypomorphs.\",\n      \"method\": \"QRT-PCR, ISH, IHC, RA injection rescue experiments, modifier QTL analysis in mice\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — pharmacological rescue + expression analysis, but pathway connection is indirect\",\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 Tulp3/IFT-A-dependent trafficking, where it couples to Gs to elevate cAMP and activate PKA—which it anchors directly via an AKAP motif in its C-terminus—thereby promoting Gli3 repressor formation and suppressing Hedgehog signaling; upon Hedgehog pathway activation, Smoothened facilitates GRK2-mediated phosphorylation of GPR161, recruiting β-arrestins that interact with the BBSome–IFT machinery to export GPR161 from cilia via clathrin-mediated endocytosis, and a sterol binding at transmembrane helices 6/7 stabilizes the Gs-coupling conformation as revealed by cryo-EM structure, while the PKA-binding C-terminal site (rather than sterol-dependent cAMP) is critical for suppressing GLI2 accumulation in cilia.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GPR161 is a constitutively active orphan G-protein-coupled receptor that localizes to primary cilia and functions as a central negative regulator of Hedgehog (Hh) signaling by coupling to Gαs to elevate cAMP and by directly scaffolding type I PKA via an A-kinase anchoring protein (AKAP) motif in its C-terminal tail, thereby promoting GLI3 processing to its repressor form [PMID:23332756, PMID:27357676, PMID:38326651]. A cryo-EM structure reveals that a sterol bound at transmembrane helices 6/7 stabilizes the Gs-coupling conformation, while the PKA-binding C-terminal domain—rather than sterol-dependent cAMP production—is the critical determinant for suppressing GLI2 accumulation in cilia [PMID:38326651]. Ciliary trafficking of GPR161 depends on Tulp3/IFT-A for import [PMID:23332756], and Hh-triggered export requires GRK2 phosphorylation, β-arrestin recruitment, and coupling to the BBSome–IFT machinery for clathrin-mediated removal [PMID:27002170, PMID:40384633]. Loss of Gpr161 causes cilium-dependent ectopic Hh pathway activation leading to neural tube defects, medulloblastoma, polydactyly, and skeletal patterning abnormalities in mice, and rare GPR161 variants have been identified in human spina bifida patients [PMID:29386106, PMID:29222391, PMID:30256984].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of a C-terminal truncation mutation in Gpr161 (vacuolated lens allele) established that the receptor's C-terminal tail is essential for endocytic trafficking and that its loss causes neural tube defects and cataracts, providing the first link between GPR161 and developmental signaling.\",\n      \"evidence\": \"Positional cloning and endocytosis assays in mutant mouse cells\",\n      \"pmids\": [\"18250320\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No pathway mechanism identified\", \"No ciliary localization reported\", \"Single mouse allele\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The foundational mechanistic framework was established: GPR161 localizes to primary cilia via Tulp3/IFT-A, constitutively activates cAMP/PKA to promote Gli3 repressor formation, and is removed from cilia upon Shh stimulation—resolving how the cilium suppresses Hh signaling in the absence of ligand.\",\n      \"evidence\": \"Mouse Gpr161 knockout, cAMP reporter assays, Gli3 processing immunoblots, ciliary immunofluorescence, epistasis with IFT-A/Tulp3 mutants\",\n      \"pmids\": [\"23332756\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of ciliary exit unknown\", \"Coupling to PKA not mapped to a direct binding interface\", \"No structural information on receptor\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"A hypomorphic Gpr161 allele revealed Shh-independent regulation of retinoic acid and Wnt pathway gene expression during neurulation, suggesting broader signaling roles for GPR161 beyond Hedgehog repression.\",\n      \"evidence\": \"QRT-PCR, in situ hybridization, retinoic acid rescue of neural tube defects in Gpr161 hypomorphic mice\",\n      \"pmids\": [\"25753732\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Pathway connection to RA and Wnt is indirect\", \"Not confirmed in independent genetic backgrounds\", \"Mechanism linking cAMP to Wnt/RA regulation unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Two key mechanisms were resolved in parallel: (1) Shh-induced ciliary removal proceeds through GRK2 phosphorylation of GPR161, β-arrestin recruitment, and clathrin-mediated endocytosis; (2) GPR161 directly binds type I PKA regulatory subunits via a C-terminal AKAP motif, compartmentalizing PKA to cilia and the plasma membrane.\",\n      \"evidence\": \"β-arrestin recruitment and clathrin inhibition assays with live-cell imaging; phosphoproteomics, direct binding assays, AKAP domain mutagenesis, zebrafish in vivo rescue\",\n      \"pmids\": [\"27002170\", \"27357676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Role of BBSome in β-arrestin-mediated export not yet defined\", \"Relative contributions of cAMP production vs. PKA anchoring to Gli repression unresolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Conditional knockouts demonstrated that Gpr161 loss causes cilium-dependent tissue-specific Hh pathway hyperactivation, driving medulloblastoma from cerebellar granule cell progenitors and limb/skeletal patterning defects—establishing GPR161 as a bona fide Hh pathway tumor suppressor.\",\n      \"evidence\": \"Cell-type-specific conditional knockout mice with genetic cilia ablation epistasis, histological and molecular pathway analysis\",\n      \"pmids\": [\"29386106\", \"29222391\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human tumor suppressor role not validated by patient mutation data at this point\", \"Hh-independent functions in these tissues not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The IFT-B–BBSome connection required for GPR161 ciliary export was mapped: IFT38 directly binds BBSome subunits, and disrupting this interaction traps GPR161 in cilia. Separately, rare GPR161 variants in human spina bifida patients were shown to mislocalize and dominantly disrupt Shh signaling.\",\n      \"evidence\": \"IFT38 knockout + rescue with BBSome-interaction-deficient mutant; Sanger sequencing of spina bifida patients with in vitro functional characterization\",\n      \"pmids\": [\"31471295\", \"30256984\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full reconstitution of the GPR161–β-arrestin–BBSome–IFT export complex not achieved\", \"Spina bifida variants characterized in vitro only, lacking in vivo rescue\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Separation-of-function studies dissected ciliary vs. extraciliary GPR161 roles: a ciliary-localization-defective but cAMP-competent knock-in showed that the ciliary pool specifically controls Gli3 repressor thresholds in the neural tube, while PKA feedback phosphorylation of GPR161 fine-tunes its own ciliary retention and Hh sensitivity.\",\n      \"evidence\": \"Knock-in mouse allelic series with tissue-level pathway readouts; zebrafish transgenic rescue with phospho-deficient GPR161 mutants and BRET-based Gαs coupling\",\n      \"pmids\": [\"34346313\", \"33450431\", \"33531430\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Extraciliary GPR161 function mechanism remains molecularly undefined\", \"Mechanoresponsive role via AC6 seen in MSCs not validated in other cell types\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Cryo-EM structure of GPR161–Gs resolved the molecular basis of constitutive activity: ECL2 occupies the orthosteric pocket, a sterol at TM6/7 stabilizes the active conformation for Gs coupling, and critically, the C-terminal PKA-binding site—not sterol-dependent cAMP—is the essential feature for suppressing GLI2 ciliary accumulation, separating cAMP production from PKA anchoring as distinct functional outputs.\",\n      \"evidence\": \"Cryo-EM structure, sterol-binding and PKA-binding site mutagenesis, cAMP and GLI2 ciliary accumulation assays\",\n      \"pmids\": [\"38326651\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous sterol ligand identity not confirmed\", \"No structure of inactive or β-arrestin-bound state\", \"How PKA anchoring mechanistically suppresses GLI2 at cilia is unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The complete β-arrestin–BBSome export pathway was reconstituted: β-arrestins are required for GPR161 ciliary exit, activation-mimetic β-arrestin mutants constitutively interact with the BBSome and GPR161 to drive export, and GRK2 phosphorylation initiates this cascade. Separately, GPR161 was shown to function as a ciliary mechanosensor whose helix 8 mediates shear-stress responsiveness to drive NDE1 phosphorylation and saltatory neuronal migration.\",\n      \"evidence\": \"ARRB1/ARRB2 double-KO cells with activation-mimetic rescue and co-IP; microfluidic shear-stress assays with helix 8 mutagenesis and NDE1 phosphorylation analysis\",\n      \"pmids\": [\"40384633\", \"40737401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of β-arrestin–BBSome interaction unknown\", \"Helix 8 mechanosensing mechanism not structurally defined\", \"In vivo relevance of mechanosensing for brain development needs confirmation\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"GLI3 repressor was identified as the essential effector of GPR161-dependent cranial neural tube closure: GLI3R expression, but not GLI2 loss, rescues exencephaly in Gpr161 knockouts, with GLI3R restricting forebrain floor plate expansion and mediating apical constriction for neural fold closure.\",\n      \"evidence\": \"Gpr161 allelic series × GLI effector mutant epistasis, non-ciliary knock-in, in toto live imaging\",\n      \"pmids\": [\"41417007\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking GLI3R to apical constriction machinery unknown\", \"Whether this mechanism operates in human cranial closure untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the identity of the endogenous sterol or lipid ligand that activates GPR161, the structural basis of the inactive and β-arrestin-bound receptor states, how PKA anchoring at cilia mechanistically controls GLI2 accumulation independently of cAMP, and the full extent of Hedgehog-independent GPR161 signaling roles (Wnt, RA, mTORC1).\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous activating ligand unidentified\", \"No inactive-state or arrestin-bound structure\", \"PKA-anchoring-to-GLI2 mechanism unresolved\", \"Hh-independent roles lack in vivo mechanistic validation\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 9, 13]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [13, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [0, 2, 5, 8, 9, 14]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 6, 7, 8, 9]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 7, 8, 16]}\n    ],\n    \"complexes\": [\n      \"GPR161–PKA-RI (AKAP complex)\",\n      \"GPR161–β-arrestin–BBSome export complex\"\n    ],\n    \"partners\": [\n      \"GNAS\",\n      \"PRKAR1A\",\n      \"ARRB2\",\n      \"GRK2\",\n      \"TULP3\",\n      \"IQGAP1\",\n      \"FUZ\",\n      \"IFT38\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}