{"gene":"GPR55","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2007,"finding":"GPR55 binds to and is activated by the cannabinoid ligand CP55940, as well as endocannabinoids anandamide and virodhamine (nM potency), and couples to Gα13, mediating activation of RhoA, Cdc42, and Rac1 downstream.","method":"GTPγS binding assay in GPR55-transfected cells; antibody/peptide blocking of G-protein coupling; downstream signaling pathway measurements","journal":"British journal of pharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (GTPγS binding, G-protein antibody blocking, downstream pathway assays), independently replicated across labs","pmids":["17876302"],"is_preprint":false},{"year":2007,"finding":"Lysophosphatidylinositol (LPI) is an endogenous ligand for GPR55: LPI induced rapid ERK phosphorylation, Ca2+ transients, and GTPγS binding specifically in GPR55-expressing HEK293 cells but not in vector-transfected controls.","method":"Transient and stable GPR55 expression in HEK293 cells; ERK phosphorylation assay; Ca2+ mobilization assay; GTPγS binding","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal functional assays, replicated by multiple subsequent labs","pmids":["17765871"],"is_preprint":false},{"year":2008,"finding":"GPR55 is activated by cannabinoids (Δ9-THC, methanandamide, JWH015) in dorsal root ganglion neurons and HEK293 cells, increasing intracellular calcium via a pathway involving Gq, G12, RhoA, actin, phospholipase C, and IP3R-gated Ca2+ release from stores; GPR55 activation also inhibits M current.","method":"Calcium imaging in DRG neurons and HEK293 cells transiently expressing GPR55; pharmacological pathway dissection; electrophysiology (M current recording)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (Ca2+ imaging, electrophysiology, pharmacological dissection of pathway components) in primary neurons and transfected cells","pmids":["18263732"],"is_preprint":false},{"year":2008,"finding":"LPI induces GPR55 internalization, stimulates sustained oscillatory Ca2+ release dependent on Gα13 and RhoA activation, and activates NFAT family transcription factors leading to their nuclear translocation in HEK293 cells stably expressing GPR55.","method":"Stable GPR55-expressing HEK293 cell line; Ca2+ imaging; receptor internalization assay; NFAT luciferase reporter and nuclear translocation assay; siRNA/dominant-negative approaches","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal assays in a defined cell system, pathway intermediates validated by molecular knockdown","pmids":["18757503"],"is_preprint":false},{"year":2009,"finding":"GPR55 activation stimulates mouse and human osteoclast polarization and resorption and causes activation of Rho and ERK1/2; these effects were attenuated in osteoclasts generated from GPR55−/− macrophages and by the GPR55 antagonist cannabidiol.","method":"GPR55−/− mouse osteoclast cultures; Rho/ERK1/2 activation assays; resorption assays; pharmacological antagonism with cannabidiol; bone histomorphometry and micro-CT of GPR55−/− mice","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout plus pharmacological antagonism plus multiple functional readouts in primary cells and in vivo","pmids":["19805329"],"is_preprint":false},{"year":2009,"finding":"AM251 and SR141716A (CB1 antagonists) and LPI act as GPR55 agonists driving β-arrestin trafficking and PKCβII activation, while CP55,940 acts as a GPR55 antagonist/partial agonist, blocking GPR55 internalization, β-arrestin–GPR55 complex formation, and ERK1/2 phosphorylation.","method":"β-arrestin-GFP biosensor assay; receptor internalization assay; ERK1/2 phosphorylation; PKCβII membrane recruitment in GPR55-transfected cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal functional assays with panel of ligands, clearly distinguishing agonists from antagonists","pmids":["19723626"],"is_preprint":false},{"year":2010,"finding":"GPR55 promotes cancer cell proliferation and anchorage-independent growth via ERK cascade overactivation in both cell culture and xenografted mice.","method":"GPR55 siRNA knockdown and pharmacological blockade in prostate/ovarian cancer cells; ERK1/2 activation assays; xenograft studies","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockdown plus pharmacological inhibition with proliferation readout, single lab","pmids":["20838378","20818416"],"is_preprint":false},{"year":2010,"finding":"LPI/GPR55 signaling defines an autocrine loop in cancer cells: LPI is synthesized by cytosolic phospholipase A2, exported by ABCC1/MRP1, and activates GPR55 to stimulate Ca2+ mobilization, Akt, and ERK1/2, driving proliferation.","method":"GPR55 siRNA knockdown; pharmacological blockade; Ca2+ mobilization; Akt and ERK1/2 phosphorylation assays in prostate/ovarian cancer lines","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pathway dissection with genetic and pharmacological tools, single lab, multiple readouts","pmids":["20838378"],"is_preprint":false},{"year":2010,"finding":"LPI elicits GPR55-dependent, GDPβS- and U73122-sensitive, rapid intracellular Ca2+ release coupled to BK(Ca) channel activation and membrane hyperpolarization in endothelial cells; a subsequent GPR55-independent depolarization phase involves non-selective cation channels.","method":"siRNA knockdown and overexpression of GPR55 in EA.hy296 endothelial cells; Ca2+ imaging; patch-clamp electrophysiology; membrane potential measurements","journal":"British journal of pharmacology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — electrophysiology combined with Ca2+ imaging and molecular knockdown/overexpression, clear mechanistic dissection, single lab","pmids":["20735417"],"is_preprint":false},{"year":2011,"finding":"LPI causes neurite retraction in differentiated PC12 cells expressing endogenous GPR55 through a G13-dependent (not Gq-dependent) and RhoA-dependent pathway; GPR55 siRNA knockdown and RhoA inactivation both prevented this effect. Cannabinoids did not activate endogenous GPR55 in PC12 cells.","method":"GPR55 siRNA knockdown; C3 toxin (RhoA inactivation); Ca2+ imaging; RhoA activity assay; neurite morphology measurements in NGF-differentiated PC12 cells","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic knockdown plus enzymatic pathway inhibition in a cell line expressing endogenous receptor, multiple orthogonal methods","pmids":["21904624"],"is_preprint":false},{"year":2011,"finding":"GPR55 activation augments CB2R-mediated neutrophil migration while inhibiting degranulation and ROS production; cross-talk occurs at the level of small GTPases Rac2 and Cdc42, promoting cellular polarization.","method":"GPR55 and CB2R agonists in human neutrophils, HEK293, and HL60 cells; migration assays; degranulation and ROS measurements; small GTPase activation assays","journal":"Cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple cell types and functional assays, mechanistic placement at small GTPase level, single lab","pmids":["21467997"],"is_preprint":false},{"year":2012,"finding":"GPR55 and CB1 receptors form heteromers in HEK293 cells; co-expression of CB1R inhibits GPR55-mediated NFAT and SRE transcription factor activation and ERK1/2 activation, while GPR55 presence enhances CB1R-mediated ERK1/2 and NFAT activation.","method":"Co-immunoprecipitation; BRET; NFAT, SRE, ERK1/2 reporter/activation assays in HEK293 cells co-expressing both receptors","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal signaling effects confirmed by BRET and co-IP plus multiple downstream readouts, single lab","pmids":["23161546"],"is_preprint":false},{"year":2013,"finding":"GPR55 activation in hippocampal presynaptic boutons (where it co-localizes with VGLUT1) transiently increases release probability at individual CA3–CA1 synapses via presynaptic Ca2+ release from internal stores; postsynaptic stores are unaffected. The pathway involves phospholipid synthesis in the presynaptic cell but not endocannabinoids 2-AG or AEA.","method":"Two-photon Ca2+ imaging in presynaptic axonal boutons; optical quantal analysis at postsynaptic dendritic spines; GPR55 genetic deletion; cannabidiol antagonism; IP3 spot-uncaging; immunolocalization with VGLUT1 in acute hippocampal slices","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic deletion combined with optical quantal analysis and Ca2+ imaging in acute brain slices, multiple orthogonal validations","pmids":["23472002"],"is_preprint":false},{"year":2013,"finding":"GPR55 localized to myenteric neurons in mouse and human colon mediates colonic motility; the GPR55 agonist O-1602 reduced evoked contractions (~60%) and slowed colonic transit in WT but not GPR55−/− mice, effects reversed by cannabidiol but not CB1/CB2 antagonists.","method":"GPR55−/− mice; muscle strip contraction assays; in vivo GI transit (whole gut and colonic bead expulsion); immunohistochemistry; RT-PCR","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout plus pharmacological dissection in both in vitro and in vivo systems","pmids":["23603203"],"is_preprint":false},{"year":2014,"finding":"GPR55 and CB2 receptors form heteromers in HEK293 cells and cancer cells (confirmed by co-IP and BRET); heteromerization reduces GPR55-mediated transcription factor activation (NFAT, NF-κB, CREB) but potentiates ERK1/2-MAPK activation, and modifies CB2R signaling.","method":"Co-immunoprecipitation; BRET; label-free DMR and CellKey impedance assays; ERK1/2 activation; gene reporter assays in HEK293 cells","journal":"British journal of pharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP plus BRET plus multiple functional signaling readouts and label-free assays","pmids":["25048571"],"is_preprint":false},{"year":2014,"finding":"CB1 and GPR55 receptors form heteromers in rat and monkey striatum (confirmed by BRET and proximity ligation assays); the heteromeric fingerprint (cross-antagonism in ERK1/2 phosphorylation and GPR55-mediated NFAT activation) was detected both in heterologous cells and in native rat brain slices.","method":"BRET; in situ proximity ligation assay (PLA) in rat brain slices and non-human primate striatum; ERK1/2 and NFAT assays","journal":"Experimental neurology","confidence":"High","confidence_rationale":"Tier 2 / Strong — BRET plus in situ PLA in native tissue plus functional heteromer fingerprint, replicated in multiple species","pmids":["24967683"],"is_preprint":false},{"year":2015,"finding":"GPR55 activation by LPI promotes growth cone enlargement, increased filopodia, and chemo-attractive axon growth in retinal ganglion cells; GPR55−/− neurons have smaller growth cones and reduced outgrowth. In vivo, GPR55 deletion reduces retinal projection branching in dorsal terminal nucleus and eye-specific segregation in superior colliculus and dLGN.","method":"GPR55−/− mouse embryo neuron cultures; growth cone morphometry; outgrowth assay; pharmacological LPI, O-1602, and CBD treatments; in vivo retinal projection analysis by immunohistochemistry","journal":"eNeuro","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout plus pharmacological validation in primary neurons plus in vivo anatomical phenotype","pmids":["26730399"],"is_preprint":false},{"year":2015,"finding":"GPR55 activation by LPI in the periaqueductal gray (PAG) increases intracellular Ca2+ via IP3R-dependent ER release and P/Q-type voltage-gated Ca2+ channel entry, depolarizes PAG neurons, and upon intra-PAG injection reduces nociceptive threshold (pronociceptive effect); both effects are abolished by selective GPR55 antagonist ML-193.","method":"Ca2+ imaging in dissociated rat PAG neurons; electrophysiology; intra-PAG microinjection; hot-plate nociception test; ML-193 pharmacological antagonism","journal":"Molecular pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological dissection in primary neurons plus in vivo behavioral validation, single lab","pmids":["25972448"],"is_preprint":false},{"year":2015,"finding":"GPR55 promotes migration and adhesion of HCT116 colon cancer cells to endothelial cells; LPI increases migration and decreases endothelial monolayer integrity in a GPR55-dependent manner; GPR55 antagonism or siRNA knockdown reduces cancer cell liver metastasis in a mouse model.","method":"Adhesion and migration assays; GPR55 siRNA knockdown; pharmacological antagonism (CID16020046, cannabidiol); in vivo mouse liver metastasis model; endothelial integrity assay","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockdown plus pharmacological antagonism plus in vivo model, single lab","pmids":["26436760"],"is_preprint":false},{"year":2016,"finding":"LPI/GPR55 promotes metastasis in triple-negative breast cancer cells by coupling to Gq/11 proteins and activating ERK, which drives the transcription factor ETV4/PEA3 to confer pro-invasive features in vitro and in vivo.","method":"GPR55 overexpression/knockdown in breast cancer cells; in vitro invasion assays; in vivo metastasis models; ERK and ETV4/PEA3 signaling pathway analyses; Gq/11 inhibition","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic and pharmacological tools, multiple readouts, in vivo validation, pathway intermediates identified, single lab","pmids":["27340777"],"is_preprint":false},{"year":2016,"finding":"GPR55 mediates Ca2+ mobilization and insulin secretion in mouse and human islets; O-1602-stimulated Ca2+ increase and insulin secretion are abolished in GPR55−/− mouse islets, demonstrating GPR55 dependence for these agonist effects.","method":"GPR55−/− mouse islets; Fura-2 Ca2+ microfluorimetry; dynamic insulin secretion (radioimmunoassay); RhoA pull-down; human islet experiments","journal":"Diabetes, obesity & metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout compared to WT in both mouse and human islets with multiple orthogonal readouts","pmids":["27561953"],"is_preprint":false},{"year":2017,"finding":"Two key residues, K2.60 and E3.29, are crucial for agonist (LPI and ML184) signaling at GPR55, forming the primary ligand interaction site; Y3.32, H170, and F6.55 form the orthosteric binding site; a Y3.32/M3.36/F6.48 cluster is important for the signaling cascade; C10 and C260 form a disulfide bridge occluding TMH1-TMH7 entry.","method":"Site-directed mutagenesis; SRE and SRF luciferase reporter assays; GPR55 R* homology model based on δ-opioid receptor crystal structure","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis with functional assays and structural modeling, systematic residue-level mapping, single lab","pmids":["28005346"],"is_preprint":false},{"year":2017,"finding":"GPR55 promotes migration and filopodia formation in breast cancer cells via two pathways: HBXIP/p-ERK1/2/Capn4 and MLCK/MLC signaling; GPR55 upregulates HBXIP at the promoter level; GPR55 loss impairs metastatic foci formation in xenograft models.","method":"siRNA GPR55 knockdown; CID16020046 antagonism; filopodia assay; migration assays; dual-luciferase HBXIP promoter assay; xenograft nude mouse metastasis model","journal":"Acta pharmacologica Sinica","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockdown plus pharmacological antagonism with multiple mechanistic readouts and in vivo model, single lab","pmids":["29188802"],"is_preprint":false},{"year":2017,"finding":"GPR55 activation in neural stem cells significantly increases proliferation (blocked by ML193 antagonist) and promotes neuronal differentiation; in vivo, hippocampal GPR55 activation increases Ki67+, DCX+, and BrdU+ cells, and GPR55−/− mice show reduced hippocampal neurogenesis.","method":"Flow cytometry of hNSC proliferation/differentiation; GPR55−/− mouse hippocampal cannula infusion with osmotic pump; immunohistochemistry (Ki67, DCX, BrdU)","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout plus in vivo pharmacological activation plus in vitro validation, single lab","pmids":["29888782"],"is_preprint":false},{"year":2017,"finding":"GPR55 activation by LPI elicits intracellular Ca2+ signaling in brain microvascular endothelial cells via voltage-gated Ca2+ channels (not PLC), activates Ca2+-activated K+ channels causing membrane hyperpolarization, transiently disrupts tight and adherens junctions and F-actin, reduces electrical resistance of endothelial monolayers, and increases BBB permeability in vivo.","method":"Ca2+ imaging; patch-clamp; ECIS (transendothelial resistance); immunofluorescence of junction proteins; Evans Blue BBB permeability assay in rats; ML-193 antagonism","journal":"Neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods in vitro (electrophysiology, Ca2+ imaging, impedance) plus in vivo BBB permeability, comprehensive mechanistic dissection","pmids":["31279825"],"is_preprint":false},{"year":2018,"finding":"GPR55 deficiency in mice causes increased adiposity, reduced insulin sensitivity in skeletal muscle, adipose, and liver (reduced PKB/Akt phosphorylation and downstream targets), impaired insulin receptor substrate-1 in muscle and increased PTEN in liver/fat; conversely, GPR55 activation enhances insulin signaling in cultured cells in a receptor-dependent manner.","method":"GPR55−/− mice; insulin tolerance tests; Western blotting of insulin signaling intermediates; GPR55 agonist/antagonist/siRNA in cultured L6 myotubes, 3T3-L1 adipocytes, hepatocytes","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout phenotyping combined with pharmacological and genetic manipulation in multiple cell types and tissues","pmids":["30148676"],"is_preprint":false},{"year":2019,"finding":"LPI exacerbates myocardial ischemia/reperfusion injury in a GPR55- and ROCK-dependent manner: LPI induces ROCK and p38 MAPK phosphorylation in cardiomyocytes (prevented by ROCK inhibitor Y-27632 and CBD), and increases infarct size in isolated WT but not GPR55−/− mouse hearts.","method":"Dynamic mass redistribution (DMR) assay in mouse and human iPSC cardiomyocytes; ROCK/p38 MAPK phosphorylation (Western blot); isolated heart I/R model in WT and GPR55−/− mice; infarct size measurement; ROCK inhibitor Y-27632 and CBD pharmacology","journal":"Pharmacology research & perspectives","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout plus pharmacological dissection in both cell-based and ex vivo organ models, pathway intermediates identified","pmids":["31149342"],"is_preprint":false},{"year":2018,"finding":"GPR55 signaling promotes pancreatic cancer cell growth in vitro and in vivo; knockdown or pharmacologic inhibition reduces MAPK signaling, cell cycle progression, and ribonucleotide reductase protein levels; p53 regulates GPR55 protein expression through microRNA miR34b-3p; genetic ablation of Gpr55 in KPC mice significantly prolongs survival.","method":"Gpr55 CRISPR/genetic ablation in KPC mouse model; siRNA knockdown; pharmacological GPR55 inhibition (CBD); MAPK signaling Western blots; cell cycle analysis; miR34b-3p functional studies; KPC mouse survival analysis","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout in relevant mouse cancer model plus mechanistic pathway dissection plus miRNA regulatory mechanism, multiple orthogonal approaches","pmids":["30061636"],"is_preprint":false},{"year":2017,"finding":"GPR55 negatively regulates intraepithelial lymphocyte (IEL) migration speed and epithelial interactions in response to LPI; GPR55−/− IELs migrate faster and interact more extensively with epithelial cells by intravital imaging; GPR55 deficiency or antagonism protects against NSAID-induced intestinal permeability increases.","method":"GPR55−/− mice; intravital two-photon imaging of IEL migration; IEL accumulation counts; intestinal permeability assay; short-term GPR55 antagonist treatment","journal":"Science immunology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout combined with intravital imaging and functional permeability assays, clear mechanistic insight","pmids":["29222090"],"is_preprint":false},{"year":2017,"finding":"N-arachidonoyl glycine (NAGly) is an endogenous GPR55 agonist, inducing concentration-dependent Ca2+ mobilization and MAPK activation in GPR55-expressing CHO cells, effects attenuated by the selective GPR55 antagonist ML193.","method":"Ca2+ mobilization assay; ERK/MAPK phosphorylation in HAGPR55/CHO cells; ML193 pharmacological antagonism","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 / Weak — single lab, overexpression system, two functional readouts, no genetic control","pmids":["28698140"],"is_preprint":false},{"year":2017,"finding":"Lysophosphatidylcholine (LPC) activates GPR55 to elicit intracellular Ca2+ mobilization in GPR55-expressing PC-3 cells; this response is blocked by GPR55 antagonists CID16020046 and ML193 and by lipid raft disruption; molecular modeling indicates LPC interacts with overlapping residues as LPI.","method":"Ca2+ mobilization assay in PC-3 cells; GPR55 antagonist pharmacology; lipid raft disruption with methyl-β-cyclodextrin; molecular docking modeling","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, endogenous receptor not fully validated, modeling not experimentally confirmed by mutagenesis","pmids":["28552522"],"is_preprint":false},{"year":2020,"finding":"CRISPR/Cas9-generated Gpr55 knockout beta cells show abolished O-1602-induced Ca2+ release and insulin secretion, and impaired responses to Abn-CBD, AM251, and PEA; GPR55 antagonism in vivo attenuates glucoregulatory effects of Abn-CBD, establishing GPR55 as a mediator of insulinotropic effects.","method":"CRISPR/Cas9 Gpr55 knockout in BRIN-BD11 beta cells; Ca2+ mobilization assay; insulin secretion assay; in vivo HFD diabetic mouse studies with GPR55 antagonism","journal":"Peptides","confidence":"High","confidence_rationale":"Tier 2 / Moderate — CRISPR genetic knockout plus pharmacological antagonism in vitro and in vivo, multiple ligands tested","pmids":["31923454"],"is_preprint":false},{"year":2011,"finding":"GPR55 ligands promote signaling through multiple downstream pathways including Ca2+, ERK1/2 MAPK, NFAT, NF-κB, and CREB with ligand-dependent differences in potency and efficacy (functional selectivity/biased agonism); receptor endocytosis was also observed.","method":"Ca2+ signaling; ERK1/2 phosphorylation; NFAT, NF-κB, CREB reporter assays; receptor endocytosis; dynamic mass redistribution optical assay in stable HEK293-GPR55 cells","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal pathway assays in a defined cell system demonstrating biased agonism, single lab","pmids":["20136841"],"is_preprint":false},{"year":2022,"finding":"Curcumin activates GPR55 as an agonist; the F190(5.47) residue of GPR55 is important for curcumin interaction; GPR55 activation by curcumin induces SRE/SRF-mediated transcription (blocked by Rho kinase and GPR55 antagonists) and stimulates GLP-1 secretion in GLUTag cells.","method":"GPCR expression screening system; SRE/SRF luciferase reporter assays; GPR55 residue mutagenesis (F190A); GLP-1 secretion assay in GLUTag cells; Rho kinase inhibitor and GPR55 antagonist pharmacology","journal":"NPJ science of food","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis plus functional reporter plus secretion assays, single lab","pmids":["35031622"],"is_preprint":false},{"year":2017,"finding":"LPI application to hippocampal slices of GPR55+/+ mice enhances CA1 long-term potentiation (LTP) and increases paired-pulse ratios; these effects are absent in GPR55−/− mice and blocked by the GPR55 antagonist CID16020046, placing GPR55 as a positive modulator of hippocampal synaptic plasticity.","method":"Electrophysiology (LTP induction and paired-pulse ratio) in hippocampal slices of GPR55+/+ and GPR55−/− mice; pharmacological antagonism with CID16020046","journal":"Hippocampus","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic knockout combined with pharmacological validation in ex vivo electrophysiology, clear mechanistic readout","pmids":["28653801"],"is_preprint":false}],"current_model":"GPR55 is a class A GPCR whose primary endogenous ligand is L-α-lysophosphatidylinositol (LPI); upon activation it couples mainly to Gα13 (and also Gq and G12 in a context-dependent manner) to activate RhoA, Cdc42, Rac1, phospholipase C, and IP3R-mediated Ca2+ release from intracellular stores, with downstream induction of ERK1/2 MAPK, NFAT, NF-κB, and CREB transcription factors; GPR55 undergoes β-arrestin-mediated internalization, exhibits biased/functional-selective agonism, and forms heteromers with CB1 and CB2 receptors that alter signaling properties; in physiological contexts it regulates osteoclast function, synaptic release probability at hippocampal CA3–CA1 synapses (via presynaptic Ca2+ stores), colonic motility (through myenteric neurons), intraepithelial lymphocyte migration, neural stem cell proliferation, insulin secretion from islets, insulin sensitivity in peripheral metabolic tissues, retinal axon growth and targeting, and BBB permeability, while in pathological settings it promotes cancer cell proliferation and metastasis via ERK overactivation and exacerbates myocardial ischemia/reperfusion injury through a GPR55/ROCK/p38 MAPK pathway."},"narrative":{"mechanistic_narrative":"GPR55 is a class A G-protein-coupled receptor that transduces lysophospholipid and cannabinoid signals into Rho-family GTPase and Ca2+ signaling to regulate cell migration, synaptic transmission, metabolism, and tumor growth [PMID:17765871, PMID:17876302, PMID:18757503]. Its principal endogenous ligand is L-α-lysophosphatidylinositol (LPI), which drives ERK phosphorylation, GTPγS binding, and Ca2+ transients selectively in GPR55-expressing cells [PMID:17765871], while the receptor is also engaged by cannabinoids such as CP55940, anandamide, and Δ9-THC [PMID:17876302, PMID:18263732] and by additional lipid agonists [PMID:28698140]. GPR55 couples to Gα13 (and Gq/G12 in a context-dependent manner) to activate RhoA, Cdc42, and Rac1 [PMID:17876302, PMID:18263732], producing sustained oscillatory Ca2+ release from IP3R-gated intracellular stores and nuclear translocation of NFAT, with parallel induction of ERK1/2, NF-κB, and CREB [PMID:18263732, PMID:18757503, PMID:20136841]; ligand-dependent differences in pathway engagement define biased agonism, and agonist binding triggers β-arrestin recruitment and receptor internalization [PMID:19723626, PMID:20136841]. Residue-level mapping places LPI and synthetic agonists at an orthosteric site involving K2.60, E3.29, Y3.32, H170, and F6.55, with a Y3.32/M3.36/F6.48 cluster relaying activation [PMID:28005346]. GPR55 forms heteromers with CB1 and CB2 receptors in heterologous cells and native brain tissue, reciprocally reshaping transcription factor and ERK signaling [PMID:23161546, PMID:25048571, PMID:24967683]. Physiologically GPR55 governs osteoclast polarization and resorption [PMID:19805329], presynaptic Ca2+-dependent release probability and LTP at hippocampal CA3–CA1 synapses [PMID:23472002, PMID:28653801], colonic motility through myenteric neurons [PMID:23603203], intraepithelial lymphocyte migration and intestinal barrier integrity [PMID:29222090], neural stem cell proliferation and neurogenesis [PMID:29888782], retinal axon growth cone dynamics and projection targeting [PMID:26730399], islet insulin secretion and peripheral insulin sensitivity [PMID:27561953, PMID:30148676, PMID:31923454], and endothelial Ca2+ signaling controlling blood-brain barrier permeability [PMID:20735417, PMID:31279825]. In disease, an autocrine LPI/GPR55 loop drives proliferation, migration, and metastasis across prostate, ovarian, colon, breast, and pancreatic cancers via ERK overactivation and downstream effectors including ETV4/PEA3 and HBXIP [PMID:20838378, PMID:20818416, PMID:26436760, PMID:27340777, PMID:29188802, PMID:30061636], and LPI/GPR55 signaling through a ROCK/p38 MAPK axis exacerbates myocardial ischemia/reperfusion injury [PMID:31149342].","teleology":[{"year":2007,"claim":"Establishing GPR55's ligands and G-protein coupling answered whether this orphan receptor was a functional signaling GPCR and identified its transduction machinery.","evidence":"GTPγS binding and G-protein antibody blocking with cannabinoid and LPI ligands in GPR55-transfected cells; downstream RhoA/Cdc42/Rac1 and ERK/Ca2+ readouts","pmids":["17876302","17765871"],"confidence":"High","gaps":["Relative physiological importance of cannabinoid vs LPI ligands left unresolved","Endogenous tissue source of agonist not defined"]},{"year":2008,"claim":"Dissecting the second-messenger cascade established that GPR55 mobilizes Ca2+ from internal stores via Gq/G12-RhoA-PLC-IP3R and modulates ion channels, linking receptor activation to neuronal excitability.","evidence":"Ca2+ imaging and M-current electrophysiology in DRG neurons and HEK293 cells with pharmacological pathway dissection; NFAT reporter and internalization assays in stable HEK293-GPR55 lines","pmids":["18263732","18757503"],"confidence":"High","gaps":["G-protein selectivity (G13 vs Gq) varied by system and remained context-dependent","Native neuronal coupling not yet shown by genetic deletion"]},{"year":2009,"claim":"Defining the ligand pharmacology and arrestin behavior clarified which compounds are agonists versus antagonists and revealed β-arrestin-dependent trafficking, and the first knockout phenotype assigned GPR55 a role in bone resorption.","evidence":"β-arrestin-GFP biosensor and PKCβII recruitment assays with a ligand panel; GPR55−/− osteoclast cultures with Rho/ERK assays, resorption, and micro-CT","pmids":["19723626","19805329"],"confidence":"High","gaps":["Conflicting agonist/antagonist classifications across ligands","Mechanism linking GPR55 signaling to osteoclast polarization not fully resolved"]},{"year":2010,"claim":"Identifying a self-sustaining LPI/GPR55 autocrine loop in cancer cells and endothelial Ca2+/channel coupling showed how the receptor drives proliferation and vascular responses.","evidence":"siRNA knockdown, cPLA2/ABCC1 pathway dissection, Akt/ERK assays in cancer lines; Ca2+ imaging and patch-clamp with knockdown/overexpression in endothelial cells; biased-agonism reporter panel","pmids":["20838378","20818416","20735417","20136841"],"confidence":"Medium","gaps":["Cancer findings from single labs","In vivo contribution of the autocrine loop to tumor growth not yet tested by knockout"]},{"year":2011,"claim":"Showing G13/RhoA-dependent cytoskeletal remodeling and immune-cell GTPase crosstalk linked GPR55 to morphological control of migration and polarization.","evidence":"GPR55 siRNA and C3 toxin RhoA inactivation with neurite morphometry in PC12 cells; migration/degranulation/ROS and Rac2/Cdc42 assays in neutrophils","pmids":["21904624","21467997"],"confidence":"High","gaps":["Endogenous cannabinoid responsiveness was cell-type dependent (absent in PC12)","Immune crosstalk mechanism single-lab"]},{"year":2014,"claim":"Demonstrating GPR55-CB1 and GPR55-CB2 heteromers established that receptor oligomerization reshapes signaling output, providing a mechanism for context-dependent cannabinoid pharmacology.","evidence":"Co-IP, BRET, proximity ligation in cells and native striatum; NFAT/SRE/NF-κB/CREB and ERK1/2 readouts with label-free assays","pmids":["23161546","25048571","24967683"],"confidence":"High","gaps":["Stoichiometry and tissue distribution of heteromers not quantified","Functional relevance in vivo only inferred from fingerprint"]},{"year":2013,"claim":"Genetic deletion in defined neural circuits assigned GPR55 a presynaptic role controlling release probability and a role in colonic motility, moving beyond heterologous systems.","evidence":"GPR55−/− mice with two-photon presynaptic Ca2+ imaging and optical quantal analysis in hippocampal slices; muscle strip contraction and in vivo transit with O-1602/CBD","pmids":["23472002","23603203"],"confidence":"High","gaps":["Identity of the presynaptic phospholipid agonist not pinned down","Coupling between receptor activation and store Ca2+ release at boutons not molecularly resolved"]},{"year":2017,"claim":"Residue-level mutagenesis and structural modeling defined the orthosteric binding pocket and the activation-relaying residue cluster, providing the first structural framework for GPR55 ligand recognition.","evidence":"Site-directed mutagenesis with SRE/SRF reporters on a δ-opioid-based homology model","pmids":["28005346"],"confidence":"High","gaps":["No experimental structure","Model-based binding mode for diverse lipid ligands not crystallographically confirmed"]},{"year":2017,"claim":"Expanding the in vivo physiology through knockouts placed GPR55 in neurogenesis, retinal axon targeting, intraepithelial lymphocyte behavior, synaptic plasticity, and nociception.","evidence":"GPR55−/− mice with neurogenesis IHC, growth-cone morphometry and retinal projection analysis, intravital IEL imaging, hippocampal LTP electrophysiology, and PAG Ca2+/behavioral nociception assays","pmids":["29888782","26730399","29222090","28653801","25972448","28698140"],"confidence":"High","gaps":["Several phenotypes from single labs","Downstream effectors in each tissue incompletely mapped"]},{"year":2018,"claim":"Knockout and CRISPR studies established GPR55 as a regulator of metabolic homeostasis, controlling insulin secretion from islets and systemic insulin sensitivity.","evidence":"GPR55−/− and CRISPR knockout beta cells/islets with Ca2+ microfluorimetry and insulin secretion; insulin tolerance tests and insulin-signaling Western blots across muscle, fat, and liver","pmids":["27561953","30148676","31923454"],"confidence":"High","gaps":["Direct ligand driving islet GPR55 in vivo uncertain","Tissue-autonomous vs systemic contributions to insulin resistance not fully separated"]},{"year":2019,"claim":"Cancer and cardiac knockout/pathway studies defined disease-relevant effector arms, linking GPR55 to ERK-driven metastatic programs and a ROCK/p38 ischemia/reperfusion axis.","evidence":"Gpr55 ablation in KPC pancreatic cancer mice and xenografts with MAPK/ETV4/HBXIP dissection; isolated GPR55−/− heart I/R model with ROCK/p38 phosphorylation and CBD/Y-27632 pharmacology","pmids":["30061636","27340777","29188802","26436760","31149342","31279825"],"confidence":"High","gaps":["Effector pathways (ETV4, HBXIP, ROCK/p38) characterized in individual labs","Therapeutic tractability of GPR55 antagonism in these diseases untested clinically"]},{"year":null,"claim":"How distinct ligands, G-protein/arrestin coupling states, and CB receptor heteromerization are integrated to select tissue-specific outputs remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No experimental high-resolution structure of activated GPR55 with ligand and transducer","Rules governing biased agonism across tissues not defined","Endogenous agonist identity in each physiological context often inferred rather than proven"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1,21,29]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[11,14,15]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,12,24]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,2,3,32]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[20,25,31]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[6,18,19,27,26]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[12,13,34,16]}],"complexes":["GPR55-CB1 heteromer","GPR55-CB2 heteromer"],"partners":["CB1","CB2","GNA13","ARRB2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9Y2T6","full_name":"G-protein coupled receptor 55","aliases":[],"length_aa":319,"mass_kda":36.6,"function":"G-protein coupled receptor that binds to several ligands including 2-arachidonoyl lysophosphatidylinositol or lysophosphatidylglucoside with high affinity, leading to rapid and transient activation of numerous intracellular signaling pathways (PubMed:36142844, PubMed:36523570, PubMed:37544935). Induces the Ca(2+) release from intracellular stores via ERK, the heterotrimeric G protein GNA13 and RHOA leading to morphological changes including cell rounding and stress fiber formation (PubMed:36142844). In macrophages, acts downstream of lysophosphatidylglucoside to inhibit the translocation of the phospholipid-transporting ABCA1 to plasma membrane and subsequent cholesterol efflux leading to lipid accumulation and foam cell formation (PubMed:37544935)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9Y2T6/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GPR55","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/GPR55","total_profiled":1310},"omim":[{"mim_id":"604107","title":"G PROTEIN-COUPLED RECEPTOR 55; GPR55","url":"https://www.omim.org/entry/604107"},{"mim_id":"604106","title":"G PROTEIN-COUPLED RECEPTOR 52; GPR52","url":"https://www.omim.org/entry/604106"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":3.5},{"tissue":"lymphoid tissue","ntpm":4.2},{"tissue":"testis","ntpm":4.4}],"url":"https://www.proteinatlas.org/search/GPR55"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"Q9Y2T6","domains":[{"cath_id":"1.20.1070.10","chopping":"11-300","consensus_level":"high","plddt":89.8443,"start":11,"end":300}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y2T6","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y2T6-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y2T6-F1-predicted_aligned_error_v6.png","plddt_mean":87.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GPR55","jax_strain_url":"https://www.jax.org/strain/search?query=GPR55"},"sequence":{"accession":"Q9Y2T6","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y2T6.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y2T6/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y2T6"}},"corpus_meta":[{"pmid":"17876302","id":"PMC_17876302","title":"The orphan receptor GPR55 is a novel cannabinoid receptor.","date":"2007","source":"British journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/17876302","citation_count":1246,"is_preprint":false},{"pmid":"18263732","id":"PMC_18263732","title":"GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current.","date":"2008","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/18263732","citation_count":554,"is_preprint":false},{"pmid":"17765871","id":"PMC_17765871","title":"Identification of GPR55 as a lysophosphatidylinositol receptor.","date":"2007","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/17765871","citation_count":377,"is_preprint":false},{"pmid":"9931487","id":"PMC_9931487","title":"Identification and cloning of three novel human G protein-coupled receptor genes GPR52, PsiGPR53 and GPR55: GPR55 is extensively expressed in human brain.","date":"1999","source":"Brain research. 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downstream signaling pathway measurements\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (GTPγS binding, G-protein antibody blocking, downstream pathway assays), independently replicated across labs\",\n      \"pmids\": [\"17876302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Lysophosphatidylinositol (LPI) is an endogenous ligand for GPR55: LPI induced rapid ERK phosphorylation, Ca2+ transients, and GTPγS binding specifically in GPR55-expressing HEK293 cells but not in vector-transfected controls.\",\n      \"method\": \"Transient and stable GPR55 expression in HEK293 cells; ERK phosphorylation assay; Ca2+ mobilization assay; GTPγS binding\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal functional assays, replicated by multiple subsequent labs\",\n      \"pmids\": [\"17765871\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"GPR55 is activated by cannabinoids (Δ9-THC, methanandamide, JWH015) in dorsal root ganglion neurons and HEK293 cells, increasing intracellular calcium via a pathway involving Gq, G12, RhoA, actin, phospholipase C, and IP3R-gated Ca2+ release from stores; GPR55 activation also inhibits M current.\",\n      \"method\": \"Calcium imaging in DRG neurons and HEK293 cells transiently expressing GPR55; pharmacological pathway dissection; electrophysiology (M current recording)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (Ca2+ imaging, electrophysiology, pharmacological dissection of pathway components) in primary neurons and transfected cells\",\n      \"pmids\": [\"18263732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"LPI induces GPR55 internalization, stimulates sustained oscillatory Ca2+ release dependent on Gα13 and RhoA activation, and activates NFAT family transcription factors leading to their nuclear translocation in HEK293 cells stably expressing GPR55.\",\n      \"method\": \"Stable GPR55-expressing HEK293 cell line; Ca2+ imaging; receptor internalization assay; NFAT luciferase reporter and nuclear translocation assay; siRNA/dominant-negative approaches\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal assays in a defined cell system, pathway intermediates validated by molecular knockdown\",\n      \"pmids\": [\"18757503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"GPR55 activation stimulates mouse and human osteoclast polarization and resorption and causes activation of Rho and ERK1/2; these effects were attenuated in osteoclasts generated from GPR55−/− macrophages and by the GPR55 antagonist cannabidiol.\",\n      \"method\": \"GPR55−/− mouse osteoclast cultures; Rho/ERK1/2 activation assays; resorption assays; pharmacological antagonism with cannabidiol; bone histomorphometry and micro-CT of GPR55−/− mice\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout plus pharmacological antagonism plus multiple functional readouts in primary cells and in vivo\",\n      \"pmids\": [\"19805329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"AM251 and SR141716A (CB1 antagonists) and LPI act as GPR55 agonists driving β-arrestin trafficking and PKCβII activation, while CP55,940 acts as a GPR55 antagonist/partial agonist, blocking GPR55 internalization, β-arrestin–GPR55 complex formation, and ERK1/2 phosphorylation.\",\n      \"method\": \"β-arrestin-GFP biosensor assay; receptor internalization assay; ERK1/2 phosphorylation; PKCβII membrane recruitment in GPR55-transfected cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal functional assays with panel of ligands, clearly distinguishing agonists from antagonists\",\n      \"pmids\": [\"19723626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"GPR55 promotes cancer cell proliferation and anchorage-independent growth via ERK cascade overactivation in both cell culture and xenografted mice.\",\n      \"method\": \"GPR55 siRNA knockdown and pharmacological blockade in prostate/ovarian cancer cells; ERK1/2 activation assays; xenograft studies\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockdown plus pharmacological inhibition with proliferation readout, single lab\",\n      \"pmids\": [\"20838378\", \"20818416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"LPI/GPR55 signaling defines an autocrine loop in cancer cells: LPI is synthesized by cytosolic phospholipase A2, exported by ABCC1/MRP1, and activates GPR55 to stimulate Ca2+ mobilization, Akt, and ERK1/2, driving proliferation.\",\n      \"method\": \"GPR55 siRNA knockdown; pharmacological blockade; Ca2+ mobilization; Akt and ERK1/2 phosphorylation assays in prostate/ovarian cancer lines\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pathway dissection with genetic and pharmacological tools, single lab, multiple readouts\",\n      \"pmids\": [\"20838378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"LPI elicits GPR55-dependent, GDPβS- and U73122-sensitive, rapid intracellular Ca2+ release coupled to BK(Ca) channel activation and membrane hyperpolarization in endothelial cells; a subsequent GPR55-independent depolarization phase involves non-selective cation channels.\",\n      \"method\": \"siRNA knockdown and overexpression of GPR55 in EA.hy296 endothelial cells; Ca2+ imaging; patch-clamp electrophysiology; membrane potential measurements\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — electrophysiology combined with Ca2+ imaging and molecular knockdown/overexpression, clear mechanistic dissection, single lab\",\n      \"pmids\": [\"20735417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"LPI causes neurite retraction in differentiated PC12 cells expressing endogenous GPR55 through a G13-dependent (not Gq-dependent) and RhoA-dependent pathway; GPR55 siRNA knockdown and RhoA inactivation both prevented this effect. Cannabinoids did not activate endogenous GPR55 in PC12 cells.\",\n      \"method\": \"GPR55 siRNA knockdown; C3 toxin (RhoA inactivation); Ca2+ imaging; RhoA activity assay; neurite morphology measurements in NGF-differentiated PC12 cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockdown plus enzymatic pathway inhibition in a cell line expressing endogenous receptor, multiple orthogonal methods\",\n      \"pmids\": [\"21904624\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GPR55 activation augments CB2R-mediated neutrophil migration while inhibiting degranulation and ROS production; cross-talk occurs at the level of small GTPases Rac2 and Cdc42, promoting cellular polarization.\",\n      \"method\": \"GPR55 and CB2R agonists in human neutrophils, HEK293, and HL60 cells; migration assays; degranulation and ROS measurements; small GTPase activation assays\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple cell types and functional assays, mechanistic placement at small GTPase level, single lab\",\n      \"pmids\": [\"21467997\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"GPR55 and CB1 receptors form heteromers in HEK293 cells; co-expression of CB1R inhibits GPR55-mediated NFAT and SRE transcription factor activation and ERK1/2 activation, while GPR55 presence enhances CB1R-mediated ERK1/2 and NFAT activation.\",\n      \"method\": \"Co-immunoprecipitation; BRET; NFAT, SRE, ERK1/2 reporter/activation assays in HEK293 cells co-expressing both receptors\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal signaling effects confirmed by BRET and co-IP plus multiple downstream readouts, single lab\",\n      \"pmids\": [\"23161546\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"GPR55 activation in hippocampal presynaptic boutons (where it co-localizes with VGLUT1) transiently increases release probability at individual CA3–CA1 synapses via presynaptic Ca2+ release from internal stores; postsynaptic stores are unaffected. The pathway involves phospholipid synthesis in the presynaptic cell but not endocannabinoids 2-AG or AEA.\",\n      \"method\": \"Two-photon Ca2+ imaging in presynaptic axonal boutons; optical quantal analysis at postsynaptic dendritic spines; GPR55 genetic deletion; cannabidiol antagonism; IP3 spot-uncaging; immunolocalization with VGLUT1 in acute hippocampal slices\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic deletion combined with optical quantal analysis and Ca2+ imaging in acute brain slices, multiple orthogonal validations\",\n      \"pmids\": [\"23472002\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"GPR55 localized to myenteric neurons in mouse and human colon mediates colonic motility; the GPR55 agonist O-1602 reduced evoked contractions (~60%) and slowed colonic transit in WT but not GPR55−/− mice, effects reversed by cannabidiol but not CB1/CB2 antagonists.\",\n      \"method\": \"GPR55−/− mice; muscle strip contraction assays; in vivo GI transit (whole gut and colonic bead expulsion); immunohistochemistry; RT-PCR\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout plus pharmacological dissection in both in vitro and in vivo systems\",\n      \"pmids\": [\"23603203\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"GPR55 and CB2 receptors form heteromers in HEK293 cells and cancer cells (confirmed by co-IP and BRET); heteromerization reduces GPR55-mediated transcription factor activation (NFAT, NF-κB, CREB) but potentiates ERK1/2-MAPK activation, and modifies CB2R signaling.\",\n      \"method\": \"Co-immunoprecipitation; BRET; label-free DMR and CellKey impedance assays; ERK1/2 activation; gene reporter assays in HEK293 cells\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP plus BRET plus multiple functional signaling readouts and label-free assays\",\n      \"pmids\": [\"25048571\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CB1 and GPR55 receptors form heteromers in rat and monkey striatum (confirmed by BRET and proximity ligation assays); the heteromeric fingerprint (cross-antagonism in ERK1/2 phosphorylation and GPR55-mediated NFAT activation) was detected both in heterologous cells and in native rat brain slices.\",\n      \"method\": \"BRET; in situ proximity ligation assay (PLA) in rat brain slices and non-human primate striatum; ERK1/2 and NFAT assays\",\n      \"journal\": \"Experimental neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — BRET plus in situ PLA in native tissue plus functional heteromer fingerprint, replicated in multiple species\",\n      \"pmids\": [\"24967683\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GPR55 activation by LPI promotes growth cone enlargement, increased filopodia, and chemo-attractive axon growth in retinal ganglion cells; GPR55−/− neurons have smaller growth cones and reduced outgrowth. In vivo, GPR55 deletion reduces retinal projection branching in dorsal terminal nucleus and eye-specific segregation in superior colliculus and dLGN.\",\n      \"method\": \"GPR55−/− mouse embryo neuron cultures; growth cone morphometry; outgrowth assay; pharmacological LPI, O-1602, and CBD treatments; in vivo retinal projection analysis by immunohistochemistry\",\n      \"journal\": \"eNeuro\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout plus pharmacological validation in primary neurons plus in vivo anatomical phenotype\",\n      \"pmids\": [\"26730399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GPR55 activation by LPI in the periaqueductal gray (PAG) increases intracellular Ca2+ via IP3R-dependent ER release and P/Q-type voltage-gated Ca2+ channel entry, depolarizes PAG neurons, and upon intra-PAG injection reduces nociceptive threshold (pronociceptive effect); both effects are abolished by selective GPR55 antagonist ML-193.\",\n      \"method\": \"Ca2+ imaging in dissociated rat PAG neurons; electrophysiology; intra-PAG microinjection; hot-plate nociception test; ML-193 pharmacological antagonism\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological dissection in primary neurons plus in vivo behavioral validation, single lab\",\n      \"pmids\": [\"25972448\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GPR55 promotes migration and adhesion of HCT116 colon cancer cells to endothelial cells; LPI increases migration and decreases endothelial monolayer integrity in a GPR55-dependent manner; GPR55 antagonism or siRNA knockdown reduces cancer cell liver metastasis in a mouse model.\",\n      \"method\": \"Adhesion and migration assays; GPR55 siRNA knockdown; pharmacological antagonism (CID16020046, cannabidiol); in vivo mouse liver metastasis model; endothelial integrity assay\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockdown plus pharmacological antagonism plus in vivo model, single lab\",\n      \"pmids\": [\"26436760\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"LPI/GPR55 promotes metastasis in triple-negative breast cancer cells by coupling to Gq/11 proteins and activating ERK, which drives the transcription factor ETV4/PEA3 to confer pro-invasive features in vitro and in vivo.\",\n      \"method\": \"GPR55 overexpression/knockdown in breast cancer cells; in vitro invasion assays; in vivo metastasis models; ERK and ETV4/PEA3 signaling pathway analyses; Gq/11 inhibition\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic and pharmacological tools, multiple readouts, in vivo validation, pathway intermediates identified, single lab\",\n      \"pmids\": [\"27340777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GPR55 mediates Ca2+ mobilization and insulin secretion in mouse and human islets; O-1602-stimulated Ca2+ increase and insulin secretion are abolished in GPR55−/− mouse islets, demonstrating GPR55 dependence for these agonist effects.\",\n      \"method\": \"GPR55−/− mouse islets; Fura-2 Ca2+ microfluorimetry; dynamic insulin secretion (radioimmunoassay); RhoA pull-down; human islet experiments\",\n      \"journal\": \"Diabetes, obesity & metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout compared to WT in both mouse and human islets with multiple orthogonal readouts\",\n      \"pmids\": [\"27561953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Two key residues, K2.60 and E3.29, are crucial for agonist (LPI and ML184) signaling at GPR55, forming the primary ligand interaction site; Y3.32, H170, and F6.55 form the orthosteric binding site; a Y3.32/M3.36/F6.48 cluster is important for the signaling cascade; C10 and C260 form a disulfide bridge occluding TMH1-TMH7 entry.\",\n      \"method\": \"Site-directed mutagenesis; SRE and SRF luciferase reporter assays; GPR55 R* homology model based on δ-opioid receptor crystal structure\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis with functional assays and structural modeling, systematic residue-level mapping, single lab\",\n      \"pmids\": [\"28005346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR55 promotes migration and filopodia formation in breast cancer cells via two pathways: HBXIP/p-ERK1/2/Capn4 and MLCK/MLC signaling; GPR55 upregulates HBXIP at the promoter level; GPR55 loss impairs metastatic foci formation in xenograft models.\",\n      \"method\": \"siRNA GPR55 knockdown; CID16020046 antagonism; filopodia assay; migration assays; dual-luciferase HBXIP promoter assay; xenograft nude mouse metastasis model\",\n      \"journal\": \"Acta pharmacologica Sinica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockdown plus pharmacological antagonism with multiple mechanistic readouts and in vivo model, single lab\",\n      \"pmids\": [\"29188802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR55 activation in neural stem cells significantly increases proliferation (blocked by ML193 antagonist) and promotes neuronal differentiation; in vivo, hippocampal GPR55 activation increases Ki67+, DCX+, and BrdU+ cells, and GPR55−/− mice show reduced hippocampal neurogenesis.\",\n      \"method\": \"Flow cytometry of hNSC proliferation/differentiation; GPR55−/− mouse hippocampal cannula infusion with osmotic pump; immunohistochemistry (Ki67, DCX, BrdU)\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout plus in vivo pharmacological activation plus in vitro validation, single lab\",\n      \"pmids\": [\"29888782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR55 activation by LPI elicits intracellular Ca2+ signaling in brain microvascular endothelial cells via voltage-gated Ca2+ channels (not PLC), activates Ca2+-activated K+ channels causing membrane hyperpolarization, transiently disrupts tight and adherens junctions and F-actin, reduces electrical resistance of endothelial monolayers, and increases BBB permeability in vivo.\",\n      \"method\": \"Ca2+ imaging; patch-clamp; ECIS (transendothelial resistance); immunofluorescence of junction proteins; Evans Blue BBB permeability assay in rats; ML-193 antagonism\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods in vitro (electrophysiology, Ca2+ imaging, impedance) plus in vivo BBB permeability, comprehensive mechanistic dissection\",\n      \"pmids\": [\"31279825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GPR55 deficiency in mice causes increased adiposity, reduced insulin sensitivity in skeletal muscle, adipose, and liver (reduced PKB/Akt phosphorylation and downstream targets), impaired insulin receptor substrate-1 in muscle and increased PTEN in liver/fat; conversely, GPR55 activation enhances insulin signaling in cultured cells in a receptor-dependent manner.\",\n      \"method\": \"GPR55−/− mice; insulin tolerance tests; Western blotting of insulin signaling intermediates; GPR55 agonist/antagonist/siRNA in cultured L6 myotubes, 3T3-L1 adipocytes, hepatocytes\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout phenotyping combined with pharmacological and genetic manipulation in multiple cell types and tissues\",\n      \"pmids\": [\"30148676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"LPI exacerbates myocardial ischemia/reperfusion injury in a GPR55- and ROCK-dependent manner: LPI induces ROCK and p38 MAPK phosphorylation in cardiomyocytes (prevented by ROCK inhibitor Y-27632 and CBD), and increases infarct size in isolated WT but not GPR55−/− mouse hearts.\",\n      \"method\": \"Dynamic mass redistribution (DMR) assay in mouse and human iPSC cardiomyocytes; ROCK/p38 MAPK phosphorylation (Western blot); isolated heart I/R model in WT and GPR55−/− mice; infarct size measurement; ROCK inhibitor Y-27632 and CBD pharmacology\",\n      \"journal\": \"Pharmacology research & perspectives\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout plus pharmacological dissection in both cell-based and ex vivo organ models, pathway intermediates identified\",\n      \"pmids\": [\"31149342\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"GPR55 signaling promotes pancreatic cancer cell growth in vitro and in vivo; knockdown or pharmacologic inhibition reduces MAPK signaling, cell cycle progression, and ribonucleotide reductase protein levels; p53 regulates GPR55 protein expression through microRNA miR34b-3p; genetic ablation of Gpr55 in KPC mice significantly prolongs survival.\",\n      \"method\": \"Gpr55 CRISPR/genetic ablation in KPC mouse model; siRNA knockdown; pharmacological GPR55 inhibition (CBD); MAPK signaling Western blots; cell cycle analysis; miR34b-3p functional studies; KPC mouse survival analysis\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout in relevant mouse cancer model plus mechanistic pathway dissection plus miRNA regulatory mechanism, multiple orthogonal approaches\",\n      \"pmids\": [\"30061636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GPR55 negatively regulates intraepithelial lymphocyte (IEL) migration speed and epithelial interactions in response to LPI; GPR55−/− IELs migrate faster and interact more extensively with epithelial cells by intravital imaging; GPR55 deficiency or antagonism protects against NSAID-induced intestinal permeability increases.\",\n      \"method\": \"GPR55−/− mice; intravital two-photon imaging of IEL migration; IEL accumulation counts; intestinal permeability assay; short-term GPR55 antagonist treatment\",\n      \"journal\": \"Science immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout combined with intravital imaging and functional permeability assays, clear mechanistic insight\",\n      \"pmids\": [\"29222090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"N-arachidonoyl glycine (NAGly) is an endogenous GPR55 agonist, inducing concentration-dependent Ca2+ mobilization and MAPK activation in GPR55-expressing CHO cells, effects attenuated by the selective GPR55 antagonist ML193.\",\n      \"method\": \"Ca2+ mobilization assay; ERK/MAPK phosphorylation in HAGPR55/CHO cells; ML193 pharmacological antagonism\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, overexpression system, two functional readouts, no genetic control\",\n      \"pmids\": [\"28698140\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Lysophosphatidylcholine (LPC) activates GPR55 to elicit intracellular Ca2+ mobilization in GPR55-expressing PC-3 cells; this response is blocked by GPR55 antagonists CID16020046 and ML193 and by lipid raft disruption; molecular modeling indicates LPC interacts with overlapping residues as LPI.\",\n      \"method\": \"Ca2+ mobilization assay in PC-3 cells; GPR55 antagonist pharmacology; lipid raft disruption with methyl-β-cyclodextrin; molecular docking modeling\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, endogenous receptor not fully validated, modeling not experimentally confirmed by mutagenesis\",\n      \"pmids\": [\"28552522\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CRISPR/Cas9-generated Gpr55 knockout beta cells show abolished O-1602-induced Ca2+ release and insulin secretion, and impaired responses to Abn-CBD, AM251, and PEA; GPR55 antagonism in vivo attenuates glucoregulatory effects of Abn-CBD, establishing GPR55 as a mediator of insulinotropic effects.\",\n      \"method\": \"CRISPR/Cas9 Gpr55 knockout in BRIN-BD11 beta cells; Ca2+ mobilization assay; insulin secretion assay; in vivo HFD diabetic mouse studies with GPR55 antagonism\",\n      \"journal\": \"Peptides\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR genetic knockout plus pharmacological antagonism in vitro and in vivo, multiple ligands tested\",\n      \"pmids\": [\"31923454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GPR55 ligands promote signaling through multiple downstream pathways including Ca2+, ERK1/2 MAPK, NFAT, NF-κB, and CREB with ligand-dependent differences in potency and efficacy (functional selectivity/biased agonism); receptor endocytosis was also observed.\",\n      \"method\": \"Ca2+ signaling; ERK1/2 phosphorylation; NFAT, NF-κB, CREB reporter assays; receptor endocytosis; dynamic mass redistribution optical assay in stable HEK293-GPR55 cells\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal pathway assays in a defined cell system demonstrating biased agonism, single lab\",\n      \"pmids\": [\"20136841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Curcumin activates GPR55 as an agonist; the F190(5.47) residue of GPR55 is important for curcumin interaction; GPR55 activation by curcumin induces SRE/SRF-mediated transcription (blocked by Rho kinase and GPR55 antagonists) and stimulates GLP-1 secretion in GLUTag cells.\",\n      \"method\": \"GPCR expression screening system; SRE/SRF luciferase reporter assays; GPR55 residue mutagenesis (F190A); GLP-1 secretion assay in GLUTag cells; Rho kinase inhibitor and GPR55 antagonist pharmacology\",\n      \"journal\": \"NPJ science of food\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis plus functional reporter plus secretion assays, single lab\",\n      \"pmids\": [\"35031622\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"LPI application to hippocampal slices of GPR55+/+ mice enhances CA1 long-term potentiation (LTP) and increases paired-pulse ratios; these effects are absent in GPR55−/− mice and blocked by the GPR55 antagonist CID16020046, placing GPR55 as a positive modulator of hippocampal synaptic plasticity.\",\n      \"method\": \"Electrophysiology (LTP induction and paired-pulse ratio) in hippocampal slices of GPR55+/+ and GPR55−/− mice; pharmacological antagonism with CID16020046\",\n      \"journal\": \"Hippocampus\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout combined with pharmacological validation in ex vivo electrophysiology, clear mechanistic readout\",\n      \"pmids\": [\"28653801\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GPR55 is a class A GPCR whose primary endogenous ligand is L-α-lysophosphatidylinositol (LPI); upon activation it couples mainly to Gα13 (and also Gq and G12 in a context-dependent manner) to activate RhoA, Cdc42, Rac1, phospholipase C, and IP3R-mediated Ca2+ release from intracellular stores, with downstream induction of ERK1/2 MAPK, NFAT, NF-κB, and CREB transcription factors; GPR55 undergoes β-arrestin-mediated internalization, exhibits biased/functional-selective agonism, and forms heteromers with CB1 and CB2 receptors that alter signaling properties; in physiological contexts it regulates osteoclast function, synaptic release probability at hippocampal CA3–CA1 synapses (via presynaptic Ca2+ stores), colonic motility (through myenteric neurons), intraepithelial lymphocyte migration, neural stem cell proliferation, insulin secretion from islets, insulin sensitivity in peripheral metabolic tissues, retinal axon growth and targeting, and BBB permeability, while in pathological settings it promotes cancer cell proliferation and metastasis via ERK overactivation and exacerbates myocardial ischemia/reperfusion injury through a GPR55/ROCK/p38 MAPK pathway.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GPR55 is a class A G-protein-coupled receptor that transduces lysophospholipid and cannabinoid signals into Rho-family GTPase and Ca2+ signaling to regulate cell migration, synaptic transmission, metabolism, and tumor growth [#1, #0, #3]. Its principal endogenous ligand is L-\\u03b1-lysophosphatidylinositol (LPI), which drives ERK phosphorylation, GTP\\u03b3S binding, and Ca2+ transients selectively in GPR55-expressing cells [#1], while the receptor is also engaged by cannabinoids such as CP55940, anandamide, and \\u03949-THC [#0, #2] and by additional lipid agonists [#29]. GPR55 couples to G\\u03b113 (and Gq/G12 in a context-dependent manner) to activate RhoA, Cdc42, and Rac1 [#0, #2], producing sustained oscillatory Ca2+ release from IP3R-gated intracellular stores and nuclear translocation of NFAT, with parallel induction of ERK1/2, NF-\\u03baB, and CREB [#2, #3, #32]; ligand-dependent differences in pathway engagement define biased agonism, and agonist binding triggers \\u03b2-arrestin recruitment and receptor internalization [#5, #32]. Residue-level mapping places LPI and synthetic agonists at an orthosteric site involving K2.60, E3.29, Y3.32, H170, and F6.55, with a Y3.32/M3.36/F6.48 cluster relaying activation [#21]. GPR55 forms heteromers with CB1 and CB2 receptors in heterologous cells and native brain tissue, reciprocally reshaping transcription factor and ERK signaling [#11, #14, #15]. Physiologically GPR55 governs osteoclast polarization and resorption [#4], presynaptic Ca2+-dependent release probability and LTP at hippocampal CA3\\u2013CA1 synapses [#12, #34], colonic motility through myenteric neurons [#13], intraepithelial lymphocyte migration and intestinal barrier integrity [#28], neural stem cell proliferation and neurogenesis [#23], retinal axon growth cone dynamics and projection targeting [#16], islet insulin secretion and peripheral insulin sensitivity [#20, #25, #31], and endothelial Ca2+ signaling controlling blood-brain barrier permeability [#8, #24]. In disease, an autocrine LPI/GPR55 loop drives proliferation, migration, and metastasis across prostate, ovarian, colon, breast, and pancreatic cancers via ERK overactivation and downstream effectors including ETV4/PEA3 and HBXIP [#6, #7, #18, #19, #22, #27], and LPI/GPR55 signaling through a ROCK/p38 MAPK axis exacerbates myocardial ischemia/reperfusion injury [#26].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Establishing GPR55's ligands and G-protein coupling answered whether this orphan receptor was a functional signaling GPCR and identified its transduction machinery.\",\n      \"evidence\": \"GTP\\u03b3S binding and G-protein antibody blocking with cannabinoid and LPI ligands in GPR55-transfected cells; downstream RhoA/Cdc42/Rac1 and ERK/Ca2+ readouts\",\n      \"pmids\": [\"17876302\", \"17765871\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative physiological importance of cannabinoid vs LPI ligands left unresolved\", \"Endogenous tissue source of agonist not defined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Dissecting the second-messenger cascade established that GPR55 mobilizes Ca2+ from internal stores via Gq/G12-RhoA-PLC-IP3R and modulates ion channels, linking receptor activation to neuronal excitability.\",\n      \"evidence\": \"Ca2+ imaging and M-current electrophysiology in DRG neurons and HEK293 cells with pharmacological pathway dissection; NFAT reporter and internalization assays in stable HEK293-GPR55 lines\",\n      \"pmids\": [\"18263732\", \"18757503\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G-protein selectivity (G13 vs Gq) varied by system and remained context-dependent\", \"Native neuronal coupling not yet shown by genetic deletion\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defining the ligand pharmacology and arrestin behavior clarified which compounds are agonists versus antagonists and revealed \\u03b2-arrestin-dependent trafficking, and the first knockout phenotype assigned GPR55 a role in bone resorption.\",\n      \"evidence\": \"\\u03b2-arrestin-GFP biosensor and PKC\\u03b2II recruitment assays with a ligand panel; GPR55\\u2212/\\u2212 osteoclast cultures with Rho/ERK assays, resorption, and micro-CT\",\n      \"pmids\": [\"19723626\", \"19805329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conflicting agonist/antagonist classifications across ligands\", \"Mechanism linking GPR55 signaling to osteoclast polarization not fully resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identifying a self-sustaining LPI/GPR55 autocrine loop in cancer cells and endothelial Ca2+/channel coupling showed how the receptor drives proliferation and vascular responses.\",\n      \"evidence\": \"siRNA knockdown, cPLA2/ABCC1 pathway dissection, Akt/ERK assays in cancer lines; Ca2+ imaging and patch-clamp with knockdown/overexpression in endothelial cells; biased-agonism reporter panel\",\n      \"pmids\": [\"20838378\", \"20818416\", \"20735417\", \"20136841\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cancer findings from single labs\", \"In vivo contribution of the autocrine loop to tumor growth not yet tested by knockout\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showing G13/RhoA-dependent cytoskeletal remodeling and immune-cell GTPase crosstalk linked GPR55 to morphological control of migration and polarization.\",\n      \"evidence\": \"GPR55 siRNA and C3 toxin RhoA inactivation with neurite morphometry in PC12 cells; migration/degranulation/ROS and Rac2/Cdc42 assays in neutrophils\",\n      \"pmids\": [\"21904624\", \"21467997\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous cannabinoid responsiveness was cell-type dependent (absent in PC12)\", \"Immune crosstalk mechanism single-lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrating GPR55-CB1 and GPR55-CB2 heteromers established that receptor oligomerization reshapes signaling output, providing a mechanism for context-dependent cannabinoid pharmacology.\",\n      \"evidence\": \"Co-IP, BRET, proximity ligation in cells and native striatum; NFAT/SRE/NF-\\u03baB/CREB and ERK1/2 readouts with label-free assays\",\n      \"pmids\": [\"23161546\", \"25048571\", \"24967683\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and tissue distribution of heteromers not quantified\", \"Functional relevance in vivo only inferred from fingerprint\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Genetic deletion in defined neural circuits assigned GPR55 a presynaptic role controlling release probability and a role in colonic motility, moving beyond heterologous systems.\",\n      \"evidence\": \"GPR55\\u2212/\\u2212 mice with two-photon presynaptic Ca2+ imaging and optical quantal analysis in hippocampal slices; muscle strip contraction and in vivo transit with O-1602/CBD\",\n      \"pmids\": [\"23472002\", \"23603203\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the presynaptic phospholipid agonist not pinned down\", \"Coupling between receptor activation and store Ca2+ release at boutons not molecularly resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Residue-level mutagenesis and structural modeling defined the orthosteric binding pocket and the activation-relaying residue cluster, providing the first structural framework for GPR55 ligand recognition.\",\n      \"evidence\": \"Site-directed mutagenesis with SRE/SRF reporters on a \\u03b4-opioid-based homology model\",\n      \"pmids\": [\"28005346\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental structure\", \"Model-based binding mode for diverse lipid ligands not crystallographically confirmed\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Expanding the in vivo physiology through knockouts placed GPR55 in neurogenesis, retinal axon targeting, intraepithelial lymphocyte behavior, synaptic plasticity, and nociception.\",\n      \"evidence\": \"GPR55\\u2212/\\u2212 mice with neurogenesis IHC, growth-cone morphometry and retinal projection analysis, intravital IEL imaging, hippocampal LTP electrophysiology, and PAG Ca2+/behavioral nociception assays\",\n      \"pmids\": [\"29888782\", \"26730399\", \"29222090\", \"28653801\", \"25972448\", \"28698140\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Several phenotypes from single labs\", \"Downstream effectors in each tissue incompletely mapped\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Knockout and CRISPR studies established GPR55 as a regulator of metabolic homeostasis, controlling insulin secretion from islets and systemic insulin sensitivity.\",\n      \"evidence\": \"GPR55\\u2212/\\u2212 and CRISPR knockout beta cells/islets with Ca2+ microfluorimetry and insulin secretion; insulin tolerance tests and insulin-signaling Western blots across muscle, fat, and liver\",\n      \"pmids\": [\"27561953\", \"30148676\", \"31923454\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct ligand driving islet GPR55 in vivo uncertain\", \"Tissue-autonomous vs systemic contributions to insulin resistance not fully separated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Cancer and cardiac knockout/pathway studies defined disease-relevant effector arms, linking GPR55 to ERK-driven metastatic programs and a ROCK/p38 ischemia/reperfusion axis.\",\n      \"evidence\": \"Gpr55 ablation in KPC pancreatic cancer mice and xenografts with MAPK/ETV4/HBXIP dissection; isolated GPR55\\u2212/\\u2212 heart I/R model with ROCK/p38 phosphorylation and CBD/Y-27632 pharmacology\",\n      \"pmids\": [\"30061636\", \"27340777\", \"29188802\", \"26436760\", \"31149342\", \"31279825\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Effector pathways (ETV4, HBXIP, ROCK/p38) characterized in individual labs\", \"Therapeutic tractability of GPR55 antagonism in these diseases untested clinically\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How distinct ligands, G-protein/arrestin coupling states, and CB receptor heteromerization are integrated to select tissue-specific outputs remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimental high-resolution structure of activated GPR55 with ligand and transducer\", \"Rules governing biased agonism across tissues not defined\", \"Endogenous agonist identity in each physiological context often inferred rather than proven\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 21, 29]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [11, 14, 15]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 12, 24]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 2, 3, 32]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [20, 25, 31]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [6, 18, 19, 27, 26]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [12, 13, 34, 16]}\n    ],\n    \"complexes\": [\n      \"GPR55-CB1 heteromer\",\n      \"GPR55-CB2 heteromer\"\n    ],\n    \"partners\": [\n      \"CB1\",\n      \"CB2\",\n      \"GNA13\",\n      \"ARRB2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"loss","faith_supported":7,"faith_total":7,"faith_pct":100.0}}