{"gene":"GPR156","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2003,"finding":"GPR156 (GABABL) was identified as a novel class C GPCR with homology to GABA(B1) and GABA(B2), containing a putative coiled-coil domain, di-leucine and RXR(R) ER retention motifs in its C-terminus. When expressed alone or with GABA(B1) or GABA(B2), no receptor function in response to GABA(B) ligands was detected, indicating it does not form a functional GABA(B) receptor with known subunits.","method":"Homology-based cloning, heterologous expression, pharmacological assays","journal":"Brain research. Molecular brain research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assay with defined negative result, domain characterization, single lab","pmids":["12591167"],"is_preprint":false},{"year":2003,"finding":"GPR156 (GABABL) protein is broadly distributed in the rat CNS, with dense expression in cortex, hippocampus, dentate gyrus, cerebellum (granule cell layer and Purkinje cells), spinal cord substantia gelatinosa and ventral horn, and a subset of parvalbumin-positive hippocampal interneurons.","method":"Immunohistochemistry with rabbit polyclonal antisera specific to GABABL protein","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct immunohistochemical localization, single lab, replicated across multiple CNS regions","pmids":["14556935"],"is_preprint":false},{"year":2021,"finding":"GPR156 exhibits constitutive Gi/o protein coupling activity in the absence of any known ligand, as measured using luciferase reporter assays with G protein chimeras designed to detect intrinsically small constitutive Gi/o signaling.","method":"Luciferase reporter assays with Gα chimera proteins for Gi/o detection in unliganded receptors","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assay with engineered chimeras, single lab, single method","pmids":["33784795"],"is_preprint":false},{"year":2021,"finding":"The transcription factor EMX2 polarizes GPR156 distribution at hair cell boundaries; GPR156 then signals through Gαi to trigger a 180° reversal in hair cell orientation. This GPR156-Gαi signaling is essential for establishing mirror-image hair cell orientations in mouse vestibular otolith organs and in zebrafish lateral line neuromasts, and also instructs hair cell reversal in the auditory epithelium.","method":"Genetic loss-of-function (knockout mice and zebrafish mutants), immunofluorescence localization of GPR156, epistasis analysis with EMX2 and Gαi","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal genetic epistasis, direct protein localization, loss-of-function in two species with defined cellular phenotype","pmids":["34001891"],"is_preprint":false},{"year":2023,"finding":"STK32A is a downstream effector negatively regulated by EMX2 in hair cells, and STK32A regulates the apical localization of GPR156. In EMX2-positive hair cells, EMX2 represses Stk32a, permitting apical GPR156 enrichment and consequent orientation reversal; ectopic STK32A expression in EMX2-positive regions reorients bundles, and Stk32a loss disrupts GPR156 apical localization.","method":"Genetic epistasis in double mutant mice (Gpr156 × Stk32a), immunofluorescence, conditional ectopic expression","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with double mutants, direct localization experiments, ectopic expression rescue, single lab with multiple orthogonal approaches","pmids":["37144879"],"is_preprint":false},{"year":2023,"finding":"GPR156 is identified as a key inducer of lipid droplet accumulation in macrophages lacking MMGT1 during Mycobacterium tuberculosis infection, placing GPR156 in the MMGT1-GPR156-lipid droplet axis that promotes bacterial persistence.","method":"Genome-wide CRISPR screen, genetic knockdown/knockout validation, lipid droplet quantification in infected macrophages","journal":"Cell host & microbe","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR screen validated with targeted knockdown, specific cellular phenotype readout, single lab","pmids":["37269834"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of human GPR156 in Go-free and Go-coupled states reveal that an endogenous phospholipid molecule is located within each transmembrane domain (TMD) of the GPR156 dimer. Asymmetric binding of Gα to the phospholipid-bound GPR156 dimer restructures intracellular loops 1 and 2 and the C-terminal part of TM7 without altering dimer conformation, establishing GPR156 as a transducer for phospholipid signaling and explaining constitutive activity through constant phospholipid binding.","method":"Cryo-electron microscopy structure determination of apo and G-protein-coupled states, structural analysis of dimer interface and lipid binding site","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures in two functional states with defined lipid-binding pocket and G-protein interface, rigorous structural validation","pmids":["38332368"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of human apo GPR156 and the GPR156-Gi3 complex reveal a small extracellular region formed by ECL2 and the N-terminus. GPR156 forms a dimer via a TM5/6–TM5/6 interface in both apo and Gi3-coupled states, indicating high constitutive activity in the apo state. The C-terminus of the G-protein-bound subunit plays a dual role: promoting G protein binding in that subunit while preventing the G-free subunit from binding additional G protein.","method":"Cryo-EM structure determination of apo GPR156 and GPR156-Gi3 complex, structural analysis of dimer interface and C-terminus function","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures in two functional states, mechanistic inference from structural analysis of dimer and C-terminus, single lab with rigorous structural methods","pmids":["39638804"],"is_preprint":false},{"year":2024,"finding":"Loss of GPR156 in zebrafish neuromast hair cells eliminates the smaller mechanically evoked signals characteristic of EMX2-positive hair cells, while mechano-electrical transduction in mouse otolith organ hair cells is normal in Gpr156 mutants, indicating GPR156 relays both orientation and transduction properties downstream of EMX2 in zebrafish but is dispensable for transduction per se in mice.","method":"Electrophysiology (mechanically evoked signals), loss-of-function in Gpr156 zebrafish and mouse mutants","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct electrophysiological measurements in two animal models, loss-of-function with defined physiological readout, replicated across species","pmids":["39531034"],"is_preprint":false},{"year":2025,"finding":"GPR156 binds VANGL proteins (core planar cell polarity components) and depends on them for its function in hair cell orientation. However, inhibiting GPR156 signaling facilitates rather than prevents postnatal correction of orientation defects in Vangl and Fzd mutants, indicating GPR156 collaborates with core PCP components to establish orientation but is dispensable—and even inhibitory—for postnatal realignment.","method":"Genetic epistasis in double mutant mice (Gpr156 × Vangl1/2 and Gpr156 × Fzd3/6 conditional mutants), protein binding/localization assays","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis across multiple double-mutant combinations, direct protein interaction data, single lab with multiple orthogonal genetic approaches","pmids":["42063348"],"is_preprint":false},{"year":2025,"finding":"Genetic epistasis experiments in mice combining Gpr156 and Stk32a mutations demonstrate: (1) GPR156 functions to reverse stereociliary bundle orientation relative to the PCP axis but can be blocked by STK32A; (2) EMX2 establishes the boundary between the two hair cell groups by repressing Stk32a transcription. These functional relationships hold in both the utricle and cochlea.","method":"Genetic epistasis in double mutant mice (Gpr156 × Stk32a and Emx2 × Stk32a), hair cell orientation phenotyping","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple double-mutant combinations and defined cellular phenotype, functional relationships validated in two tissue contexts","pmids":["41208475"],"is_preprint":false},{"year":2025,"finding":"A rare missense variant in GPR156 (p.Glu533Asp) found in a Mennonite pedigree with major depressive disorder, when knocked into the murine Gpr156 locus, induces medial habenula hyperactivity and abnormal stress-related behaviors, placing GPR156 in the medial habenula circuitry for mood regulation.","method":"Humanized knock-in mouse model, in vivo electrophysiology (habenula activity), behavioral assays for stress sensitivity","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knock-in mouse model with defined electrophysiological and behavioral phenotypes, single lab","pmids":["40228124"],"is_preprint":false},{"year":2026,"finding":"Conditional inactivation of Gpr156 specifically in postmitotic hair cells recapitulates the misorientation phenotype of null mutants and results in similar auditory and vestibular dysfunction, demonstrating that GPR156 function is required in hair cells themselves (not surrounding cells) for proper orientation. Mechano-electrical transduction remains intact in the absence of GPR156, establishing that sensory deficits arise from misorientation rather than transduction failure.","method":"Conditional Cre-mediated knockout in postmitotic hair cells, auditory brainstem response, vestibulo-ocular reflex measurements, hair cell orientation phenotyping","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional cell-type-specific knockout with multiple orthogonal functional readouts, distinguishes cell-autonomous vs non-autonomous mechanism","pmids":["41547998"],"is_preprint":false}],"current_model":"GPR156 is a class C orphan GPCR that constitutively signals through Gαi/o proteins (driven by endogenous phospholipid binding within its transmembrane dimer interface) and is apically polarized in EMX2-expressing inner ear hair cells, where it acts downstream of EMX2—which represses the competing kinase STK32A—to reverse hair cell orientation 180° relative to core planar cell polarity cues, thereby establishing the mirror-image hair cell organization required for auditory and vestibular function; structurally, GPR156 dimerizes via a TM5/6 interface, and asymmetric G-protein coupling is modulated by the C-terminus of the G-protein-bound protomer, while in the medial habenula GPR156 also participates in mood regulation."},"narrative":{"mechanistic_narrative":"GPR156 is a constitutively active class C orphan GPCR that signals through Gi/o proteins and functions as the cell-autonomous switch that establishes mirror-image hair cell orientation in the inner ear [PMID:33784795, PMID:34001891, PMID:41547998]. Originally cloned as a GABA(B)-related receptor (GABABL) with C-terminal ER-retention motifs but no detectable GABA(B) function [PMID:12591167], it is now understood to couple to Gi/o without an exogenous agonist [PMID:33784795]. Cryo-EM structures show that GPR156 dimerizes through a TM5/6–TM5/6 interface and harbors an endogenous phospholipid within each transmembrane domain, accounting for its high constitutive activity; G protein engagement is asymmetric, with the C-terminus of the G-protein-bound protomer both promoting coupling in its own subunit and blocking coupling in the partner subunit [PMID:38332368, PMID:39638804]. In sensory epithelia, the transcription factor EMX2 polarizes GPR156 to the apical hair cell surface, where GPR156-Gαi signaling drives a 180° reversal of stereociliary bundle orientation relative to the core planar cell polarity axis, producing the mirror-image organization of vestibular otolith organs, the auditory epithelium, and zebrafish neuromasts [PMID:34001891]. EMX2 acts by repressing the kinase STK32A, which otherwise restricts apical GPR156 enrichment and blocks the reversal, so that the EMX2–STK32A–GPR156 hierarchy defines the boundary between oppositely oriented hair cell groups [PMID:37144879, PMID:41208475]. GPR156 binds and depends on the core PCP proteins VANGL to execute orientation, yet it is dispensable for mechano-electrical transduction in mammalian hair cells, indicating that its deficits arise from misorientation rather than transduction failure [PMID:42063348, PMID:41547998]. Beyond the inner ear, GPR156 has been implicated in macrophage lipid droplet accumulation during Mycobacterium tuberculosis infection [PMID:37269834] and in medial habenula circuitry for mood regulation through a depression-associated missense variant [PMID:40228124].","teleology":[{"year":2003,"claim":"Established that GPR156 is a class C GPCR related to GABA(B) subunits but does not reconstitute a functional GABA(B) receptor, framing it as an orphan receptor of unknown ligand and function.","evidence":"Homology-based cloning, heterologous expression, and pharmacological assays with GABA(B) ligands and subunits; broad CNS immunohistochemical mapping","pmids":["12591167","14556935"],"confidence":"Medium","gaps":["No endogenous ligand identified","No signaling output demonstrated","Function in any tissue undefined"]},{"year":2021,"claim":"Resolved that GPR156 is not silent but signals constitutively through Gi/o even without an added ligand, establishing an intrinsic signaling activity.","evidence":"Luciferase reporter assays with Gα chimeras engineered to detect small constitutive Gi/o signaling","pmids":["33784795"],"confidence":"Medium","gaps":["Mechanism driving constitutive activity not defined","Single method and lab","No physiological context for the signaling"]},{"year":2021,"claim":"Placed GPR156 in a developmental pathway by showing EMX2 polarizes GPR156 and that GPR156-Gαi signaling drives 180° hair cell orientation reversal, explaining mirror-image hair cell organization.","evidence":"Knockout mice and zebrafish mutants, GPR156 immunofluorescence localization, and epistasis with EMX2 and Gαi","pmids":["34001891"],"confidence":"High","gaps":["Molecular mechanism of EMX2-driven polarization unresolved","How Gαi signaling reorients the cytoskeleton unknown"]},{"year":2023,"claim":"Identified STK32A as the intermediary between EMX2 and GPR156, showing EMX2 represses Stk32a to permit apical GPR156 enrichment and orientation reversal.","evidence":"Gpr156 × Stk32a double-mutant epistasis, immunofluorescence, and conditional ectopic STK32A expression in mice","pmids":["37144879"],"confidence":"High","gaps":["How STK32A controls GPR156 apical localization mechanistically unknown","Direct STK32A substrates unidentified"]},{"year":2023,"claim":"Extended GPR156 beyond hair cells by implicating it in macrophage lipid droplet accumulation that promotes Mycobacterium tuberculosis persistence.","evidence":"Genome-wide CRISPR screen with knockdown/knockout validation and lipid droplet quantification in infected macrophages","pmids":["37269834"],"confidence":"Medium","gaps":["Signaling link between GPR156 and lipid droplet biogenesis undefined","Whether constitutive Gi/o activity drives this phenotype unknown"]},{"year":2024,"claim":"Provided the structural basis for constitutive activity, showing GPR156 is a phospholipid-bound TM5/6 dimer whose asymmetric G protein coupling is gated by the bound protomer's C-terminus.","evidence":"Cryo-EM structures of apo, Go-free, and Gi3/Go-coupled human GPR156 with analysis of the dimer interface, lipid pocket, and C-terminus","pmids":["38332368","39638804"],"confidence":"High","gaps":["Whether phospholipid occupancy is regulated in vivo unknown","How asymmetric coupling shapes downstream signaling output unresolved"]},{"year":2024,"claim":"Distinguished GPR156's role in orientation from transduction, showing it relays both orientation and transduction signatures in zebrafish but is dispensable for mammalian mechano-electrical transduction.","evidence":"Electrophysiology of mechanically evoked signals in Gpr156 zebrafish and mouse mutants","pmids":["39531034"],"confidence":"High","gaps":["Basis for species difference in transduction dependence unknown","Link between GPR156 signaling and transduction machinery in zebrafish undefined"]},{"year":2025,"claim":"Connected GPR156 to core PCP machinery by showing it binds and depends on VANGL, yet is dispensable and even inhibitory for postnatal orientation correction.","evidence":"Gpr156 × Vangl1/2 and Gpr156 × Fzd3/6 double-mutant epistasis with protein binding/localization assays in mice","pmids":["42063348"],"confidence":"High","gaps":["Molecular nature of the GPR156–VANGL interaction undefined","Why GPR156 inhibition aids postnatal realignment unexplained"]},{"year":2025,"claim":"Implicated GPR156 in mood regulation by showing a depression-associated missense variant induces medial habenula hyperactivity and abnormal stress behavior.","evidence":"Humanized p.Glu533Asp knock-in mouse with in vivo habenula electrophysiology and stress behavioral assays","pmids":["40228124"],"confidence":"Medium","gaps":["Whether the variant alters Gi/o signaling unknown","Causal link to human depression not established beyond single pedigree"]},{"year":2025,"claim":"Consolidated the regulatory hierarchy, confirming GPR156 reverses bundle orientation relative to the PCP axis but is blocked by STK32A, with EMX2 setting the boundary via Stk32a repression across utricle and cochlea.","evidence":"Gpr156 × Stk32a and Emx2 × Stk32a double-mutant epistasis with hair cell orientation phenotyping","pmids":["41208475"],"confidence":"High","gaps":["Biochemical mechanism by which STK32A blocks GPR156 unresolved"]},{"year":2026,"claim":"Demonstrated GPR156 acts cell-autonomously, with hair-cell-specific deletion recapitulating misorientation and sensory dysfunction while transduction remains intact.","evidence":"Conditional Cre knockout in postmitotic hair cells with auditory brainstem response, vestibulo-ocular reflex, and orientation phenotyping","pmids":["41547998"],"confidence":"High","gaps":["Downstream cytoskeletal effectors of GPR156-Gαi signaling unidentified"]},{"year":null,"claim":"How GPR156 constitutive Gi/o signaling is converted into directional cytoskeletal reorientation, and whether its non-cochlear roles in macrophages and habenula share this signaling logic, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No identified downstream effectors linking Gαi to bundle orientation","Endogenous regulation of phospholipid binding in vivo unknown","Mechanistic basis of lipid droplet and mood phenotypes undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[2,3,6,7]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,4,6,7]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3,6]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,4,9,12]}],"complexes":[],"partners":["VANGL1","VANGL2","STK32A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8NFN8","full_name":"Probable G-protein coupled receptor 156","aliases":["G-protein coupled receptor PGR28","GABAB-related G-protein coupled receptor"],"length_aa":814,"mass_kda":89.1,"function":"Orphan G-protein coupled receptor involved in the regulation of hair cell orientation in mechanosensory organs of the inner ear. It is required to trigger a 180 degree reversal in hair cell orientation, creating a virtual line of polarity reversal (LPR) across which stereociliary bundles are arranged in opposite orientations","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q8NFN8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR156","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GPR156","total_profiled":1310},"omim":[{"mim_id":"621310","title":"SERINE-THREONINE KINASE 32A; STK32A","url":"https://www.omim.org/entry/621310"},{"mim_id":"620551","title":"DEAFNESS, AUTOSOMAL RECESSIVE 121; DFNB121","url":"https://www.omim.org/entry/620551"},{"mim_id":"610464","title":"G PROTEIN-COUPLED RECEPTOR 156; GPR156","url":"https://www.omim.org/entry/610464"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Plasma membrane","reliability":"Supported"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in single","driving_tissues":[{"tissue":"testis","ntpm":2.5}],"url":"https://www.proteinatlas.org/search/GPR156"},"hgnc":{"alias_symbol":["PGR28","GABABL"],"prev_symbol":[]},"alphafold":{"accession":"Q8NFN8","domains":[{"cath_id":"1.20.1070.10","chopping":"45-189_233-332","consensus_level":"high","plddt":83.3006,"start":45,"end":332}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8NFN8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8NFN8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8NFN8-F1-predicted_aligned_error_v6.png","plddt_mean":55.47},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR156","jax_strain_url":"https://www.jax.org/strain/search?query=GPR156"},"sequence":{"accession":"Q8NFN8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8NFN8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8NFN8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8NFN8"}},"corpus_meta":[{"pmid":"36928819","id":"PMC_36928819","title":"Genetic association analysis of 77,539 genomes reveals rare disease etiologies.","date":"2023","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/36928819","citation_count":63,"is_preprint":false},{"pmid":"33784795","id":"PMC_33784795","title":"In vitro profiling of orphan G protein coupled receptor (GPCR) constitutive activity.","date":"2021","source":"British journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/33784795","citation_count":50,"is_preprint":false},{"pmid":"34001891","id":"PMC_34001891","title":"EMX2-GPR156-Gαi reverses hair cell orientation in mechanosensory epithelia.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/34001891","citation_count":49,"is_preprint":false},{"pmid":"12591167","id":"PMC_12591167","title":"Molecular cloning and characterisation of a novel GABAB-related G-protein coupled receptor.","date":"2003","source":"Brain research. 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When expressed alone or with GABA(B1) or GABA(B2), no receptor function in response to GABA(B) ligands was detected, indicating it does not form a functional GABA(B) receptor with known subunits.\",\n      \"method\": \"Homology-based cloning, heterologous expression, pharmacological assays\",\n      \"journal\": \"Brain research. Molecular brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assay with defined negative result, domain characterization, single lab\",\n      \"pmids\": [\"12591167\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"GPR156 (GABABL) protein is broadly distributed in the rat CNS, with dense expression in cortex, hippocampus, dentate gyrus, cerebellum (granule cell layer and Purkinje cells), spinal cord substantia gelatinosa and ventral horn, and a subset of parvalbumin-positive hippocampal interneurons.\",\n      \"method\": \"Immunohistochemistry with rabbit polyclonal antisera specific to GABABL protein\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct immunohistochemical localization, single lab, replicated across multiple CNS regions\",\n      \"pmids\": [\"14556935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"GPR156 exhibits constitutive Gi/o protein coupling activity in the absence of any known ligand, as measured using luciferase reporter assays with G protein chimeras designed to detect intrinsically small constitutive Gi/o signaling.\",\n      \"method\": \"Luciferase reporter assays with Gα chimera proteins for Gi/o detection in unliganded receptors\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assay with engineered chimeras, single lab, single method\",\n      \"pmids\": [\"33784795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The transcription factor EMX2 polarizes GPR156 distribution at hair cell boundaries; GPR156 then signals through Gαi to trigger a 180° reversal in hair cell orientation. This GPR156-Gαi signaling is essential for establishing mirror-image hair cell orientations in mouse vestibular otolith organs and in zebrafish lateral line neuromasts, and also instructs hair cell reversal in the auditory epithelium.\",\n      \"method\": \"Genetic loss-of-function (knockout mice and zebrafish mutants), immunofluorescence localization of GPR156, epistasis analysis with EMX2 and Gαi\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal genetic epistasis, direct protein localization, loss-of-function in two species with defined cellular phenotype\",\n      \"pmids\": [\"34001891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"STK32A is a downstream effector negatively regulated by EMX2 in hair cells, and STK32A regulates the apical localization of GPR156. In EMX2-positive hair cells, EMX2 represses Stk32a, permitting apical GPR156 enrichment and consequent orientation reversal; ectopic STK32A expression in EMX2-positive regions reorients bundles, and Stk32a loss disrupts GPR156 apical localization.\",\n      \"method\": \"Genetic epistasis in double mutant mice (Gpr156 × Stk32a), immunofluorescence, conditional ectopic expression\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with double mutants, direct localization experiments, ectopic expression rescue, single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"37144879\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GPR156 is identified as a key inducer of lipid droplet accumulation in macrophages lacking MMGT1 during Mycobacterium tuberculosis infection, placing GPR156 in the MMGT1-GPR156-lipid droplet axis that promotes bacterial persistence.\",\n      \"method\": \"Genome-wide CRISPR screen, genetic knockdown/knockout validation, lipid droplet quantification in infected macrophages\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR screen validated with targeted knockdown, specific cellular phenotype readout, single lab\",\n      \"pmids\": [\"37269834\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of human GPR156 in Go-free and Go-coupled states reveal that an endogenous phospholipid molecule is located within each transmembrane domain (TMD) of the GPR156 dimer. Asymmetric binding of Gα to the phospholipid-bound GPR156 dimer restructures intracellular loops 1 and 2 and the C-terminal part of TM7 without altering dimer conformation, establishing GPR156 as a transducer for phospholipid signaling and explaining constitutive activity through constant phospholipid binding.\",\n      \"method\": \"Cryo-electron microscopy structure determination of apo and G-protein-coupled states, structural analysis of dimer interface and lipid binding site\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures in two functional states with defined lipid-binding pocket and G-protein interface, rigorous structural validation\",\n      \"pmids\": [\"38332368\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of human apo GPR156 and the GPR156-Gi3 complex reveal a small extracellular region formed by ECL2 and the N-terminus. GPR156 forms a dimer via a TM5/6–TM5/6 interface in both apo and Gi3-coupled states, indicating high constitutive activity in the apo state. The C-terminus of the G-protein-bound subunit plays a dual role: promoting G protein binding in that subunit while preventing the G-free subunit from binding additional G protein.\",\n      \"method\": \"Cryo-EM structure determination of apo GPR156 and GPR156-Gi3 complex, structural analysis of dimer interface and C-terminus function\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures in two functional states, mechanistic inference from structural analysis of dimer and C-terminus, single lab with rigorous structural methods\",\n      \"pmids\": [\"39638804\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Loss of GPR156 in zebrafish neuromast hair cells eliminates the smaller mechanically evoked signals characteristic of EMX2-positive hair cells, while mechano-electrical transduction in mouse otolith organ hair cells is normal in Gpr156 mutants, indicating GPR156 relays both orientation and transduction properties downstream of EMX2 in zebrafish but is dispensable for transduction per se in mice.\",\n      \"method\": \"Electrophysiology (mechanically evoked signals), loss-of-function in Gpr156 zebrafish and mouse mutants\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct electrophysiological measurements in two animal models, loss-of-function with defined physiological readout, replicated across species\",\n      \"pmids\": [\"39531034\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"GPR156 binds VANGL proteins (core planar cell polarity components) and depends on them for its function in hair cell orientation. However, inhibiting GPR156 signaling facilitates rather than prevents postnatal correction of orientation defects in Vangl and Fzd mutants, indicating GPR156 collaborates with core PCP components to establish orientation but is dispensable—and even inhibitory—for postnatal realignment.\",\n      \"method\": \"Genetic epistasis in double mutant mice (Gpr156 × Vangl1/2 and Gpr156 × Fzd3/6 conditional mutants), protein binding/localization assays\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis across multiple double-mutant combinations, direct protein interaction data, single lab with multiple orthogonal genetic approaches\",\n      \"pmids\": [\"42063348\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Genetic epistasis experiments in mice combining Gpr156 and Stk32a mutations demonstrate: (1) GPR156 functions to reverse stereociliary bundle orientation relative to the PCP axis but can be blocked by STK32A; (2) EMX2 establishes the boundary between the two hair cell groups by repressing Stk32a transcription. These functional relationships hold in both the utricle and cochlea.\",\n      \"method\": \"Genetic epistasis in double mutant mice (Gpr156 × Stk32a and Emx2 × Stk32a), hair cell orientation phenotyping\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple double-mutant combinations and defined cellular phenotype, functional relationships validated in two tissue contexts\",\n      \"pmids\": [\"41208475\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A rare missense variant in GPR156 (p.Glu533Asp) found in a Mennonite pedigree with major depressive disorder, when knocked into the murine Gpr156 locus, induces medial habenula hyperactivity and abnormal stress-related behaviors, placing GPR156 in the medial habenula circuitry for mood regulation.\",\n      \"method\": \"Humanized knock-in mouse model, in vivo electrophysiology (habenula activity), behavioral assays for stress sensitivity\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knock-in mouse model with defined electrophysiological and behavioral phenotypes, single lab\",\n      \"pmids\": [\"40228124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Conditional inactivation of Gpr156 specifically in postmitotic hair cells recapitulates the misorientation phenotype of null mutants and results in similar auditory and vestibular dysfunction, demonstrating that GPR156 function is required in hair cells themselves (not surrounding cells) for proper orientation. Mechano-electrical transduction remains intact in the absence of GPR156, establishing that sensory deficits arise from misorientation rather than transduction failure.\",\n      \"method\": \"Conditional Cre-mediated knockout in postmitotic hair cells, auditory brainstem response, vestibulo-ocular reflex measurements, hair cell orientation phenotyping\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional cell-type-specific knockout with multiple orthogonal functional readouts, distinguishes cell-autonomous vs non-autonomous mechanism\",\n      \"pmids\": [\"41547998\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPR156 is a class C orphan GPCR that constitutively signals through Gαi/o proteins (driven by endogenous phospholipid binding within its transmembrane dimer interface) and is apically polarized in EMX2-expressing inner ear hair cells, where it acts downstream of EMX2—which represses the competing kinase STK32A—to reverse hair cell orientation 180° relative to core planar cell polarity cues, thereby establishing the mirror-image hair cell organization required for auditory and vestibular function; structurally, GPR156 dimerizes via a TM5/6 interface, and asymmetric G-protein coupling is modulated by the C-terminus of the G-protein-bound protomer, while in the medial habenula GPR156 also participates in mood regulation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GPR156 is a constitutively active class C orphan GPCR that signals through Gi/o proteins and functions as the cell-autonomous switch that establishes mirror-image hair cell orientation in the inner ear [#2, #3, #12]. Originally cloned as a GABA(B)-related receptor (GABABL) with C-terminal ER-retention motifs but no detectable GABA(B) function [#0], it is now understood to couple to Gi/o without an exogenous agonist [#2]. Cryo-EM structures show that GPR156 dimerizes through a TM5/6–TM5/6 interface and harbors an endogenous phospholipid within each transmembrane domain, accounting for its high constitutive activity; G protein engagement is asymmetric, with the C-terminus of the G-protein-bound protomer both promoting coupling in its own subunit and blocking coupling in the partner subunit [#6, #7]. In sensory epithelia, the transcription factor EMX2 polarizes GPR156 to the apical hair cell surface, where GPR156-Gαi signaling drives a 180° reversal of stereociliary bundle orientation relative to the core planar cell polarity axis, producing the mirror-image organization of vestibular otolith organs, the auditory epithelium, and zebrafish neuromasts [#3]. EMX2 acts by repressing the kinase STK32A, which otherwise restricts apical GPR156 enrichment and blocks the reversal, so that the EMX2–STK32A–GPR156 hierarchy defines the boundary between oppositely oriented hair cell groups [#4, #10]. GPR156 binds and depends on the core PCP proteins VANGL to execute orientation, yet it is dispensable for mechano-electrical transduction in mammalian hair cells, indicating that its deficits arise from misorientation rather than transduction failure [#9, #12]. Beyond the inner ear, GPR156 has been implicated in macrophage lipid droplet accumulation during Mycobacterium tuberculosis infection [#5] and in medial habenula circuitry for mood regulation through a depression-associated missense variant [#11].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Established that GPR156 is a class C GPCR related to GABA(B) subunits but does not reconstitute a functional GABA(B) receptor, framing it as an orphan receptor of unknown ligand and function.\",\n      \"evidence\": \"Homology-based cloning, heterologous expression, and pharmacological assays with GABA(B) ligands and subunits; broad CNS immunohistochemical mapping\",\n      \"pmids\": [\"12591167\", \"14556935\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No endogenous ligand identified\", \"No signaling output demonstrated\", \"Function in any tissue undefined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Resolved that GPR156 is not silent but signals constitutively through Gi/o even without an added ligand, establishing an intrinsic signaling activity.\",\n      \"evidence\": \"Luciferase reporter assays with Gα chimeras engineered to detect small constitutive Gi/o signaling\",\n      \"pmids\": [\"33784795\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Mechanism driving constitutive activity not defined\", \"Single method and lab\", \"No physiological context for the signaling\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Placed GPR156 in a developmental pathway by showing EMX2 polarizes GPR156 and that GPR156-Gαi signaling drives 180° hair cell orientation reversal, explaining mirror-image hair cell organization.\",\n      \"evidence\": \"Knockout mice and zebrafish mutants, GPR156 immunofluorescence localization, and epistasis with EMX2 and Gαi\",\n      \"pmids\": [\"34001891\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular mechanism of EMX2-driven polarization unresolved\", \"How Gαi signaling reorients the cytoskeleton unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified STK32A as the intermediary between EMX2 and GPR156, showing EMX2 represses Stk32a to permit apical GPR156 enrichment and orientation reversal.\",\n      \"evidence\": \"Gpr156 × Stk32a double-mutant epistasis, immunofluorescence, and conditional ectopic STK32A expression in mice\",\n      \"pmids\": [\"37144879\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"How STK32A controls GPR156 apical localization mechanistically unknown\", \"Direct STK32A substrates unidentified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended GPR156 beyond hair cells by implicating it in macrophage lipid droplet accumulation that promotes Mycobacterium tuberculosis persistence.\",\n      \"evidence\": \"Genome-wide CRISPR screen with knockdown/knockout validation and lipid droplet quantification in infected macrophages\",\n      \"pmids\": [\"37269834\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Signaling link between GPR156 and lipid droplet biogenesis undefined\", \"Whether constitutive Gi/o activity drives this phenotype unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided the structural basis for constitutive activity, showing GPR156 is a phospholipid-bound TM5/6 dimer whose asymmetric G protein coupling is gated by the bound protomer's C-terminus.\",\n      \"evidence\": \"Cryo-EM structures of apo, Go-free, and Gi3/Go-coupled human GPR156 with analysis of the dimer interface, lipid pocket, and C-terminus\",\n      \"pmids\": [\"38332368\", \"39638804\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Whether phospholipid occupancy is regulated in vivo unknown\", \"How asymmetric coupling shapes downstream signaling output unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Distinguished GPR156's role in orientation from transduction, showing it relays both orientation and transduction signatures in zebrafish but is dispensable for mammalian mechano-electrical transduction.\",\n      \"evidence\": \"Electrophysiology of mechanically evoked signals in Gpr156 zebrafish and mouse mutants\",\n      \"pmids\": [\"39531034\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Basis for species difference in transduction dependence unknown\", \"Link between GPR156 signaling and transduction machinery in zebrafish undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Connected GPR156 to core PCP machinery by showing it binds and depends on VANGL, yet is dispensable and even inhibitory for postnatal orientation correction.\",\n      \"evidence\": \"Gpr156 × Vangl1/2 and Gpr156 × Fzd3/6 double-mutant epistasis with protein binding/localization assays in mice\",\n      \"pmids\": [\"42063348\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular nature of the GPR156–VANGL interaction undefined\", \"Why GPR156 inhibition aids postnatal realignment unexplained\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Implicated GPR156 in mood regulation by showing a depression-associated missense variant induces medial habenula hyperactivity and abnormal stress behavior.\",\n      \"evidence\": \"Humanized p.Glu533Asp knock-in mouse with in vivo habenula electrophysiology and stress behavioral assays\",\n      \"pmids\": [\"40228124\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Whether the variant alters Gi/o signaling unknown\", \"Causal link to human depression not established beyond single pedigree\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Consolidated the regulatory hierarchy, confirming GPR156 reverses bundle orientation relative to the PCP axis but is blocked by STK32A, with EMX2 setting the boundary via Stk32a repression across utricle and cochlea.\",\n      \"evidence\": \"Gpr156 × Stk32a and Emx2 × Stk32a double-mutant epistasis with hair cell orientation phenotyping\",\n      \"pmids\": [\"41208475\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Biochemical mechanism by which STK32A blocks GPR156 unresolved\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Demonstrated GPR156 acts cell-autonomously, with hair-cell-specific deletion recapitulating misorientation and sensory dysfunction while transduction remains intact.\",\n      \"evidence\": \"Conditional Cre knockout in postmitotic hair cells with auditory brainstem response, vestibulo-ocular reflex, and orientation phenotyping\",\n      \"pmids\": [\"41547998\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Downstream cytoskeletal effectors of GPR156-Gαi signaling unidentified\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How GPR156 constitutive Gi/o signaling is converted into directional cytoskeletal reorientation, and whether its non-cochlear roles in macrophages and habenula share this signaling logic, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No identified downstream effectors linking Gαi to bundle orientation\", \"Endogenous regulation of phospholipid binding in vivo unknown\", \"Mechanistic basis of lipid droplet and mood phenotypes undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [2, 3, 6, 7]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 4, 6, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3, 6]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 4, 9, 12]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"VANGL1\",\n      \"VANGL2\",\n      \"STK32A\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}