LPAR4 is a class-A seven-transmembrane G protein-coupled receptor that binds extracellular lysophosphatidic acid at the plasma membrane. Human P2Y9/GPR23 binding and reporter assays establish preferential recognition of LPA molecular species in the order 1-oleoyl, 1-stearoyl, 1-palmitoyl, 1-myristoyl, 1-alkyl, and 1-alkenyl LPA. In heterologous cells, LPAR4 activates Gs to increase cAMP, uses Gq and a pertussis-toxin-sensitive Gi component to mobilize calcium, and activates a G12/13-Rho branch that drives neurite retraction and stress-fiber formation. Another B103 transfectant system detected Gq/11 and G12/13 signaling but neither Gi/o coupling nor an adenylyl-cyclase response, showing that the relative use and observable output of these branches depend on cellular context. LPA also induces receptor internalization. LPAR4 mRNA is enriched in human ovary, consistent with a tissue-biased physiological role, while developmental expression and functional studies implicate the receptor transiently in cardiac progenitors. LPAR4 can suppress migration and invasion in some cancer-cell settings, yet promotes cAMP-EPAC-Rac1-dependent invadopodia and metastasis in fibrosarcoma, reflecting context-dependent integration of its signaling branches. Mouse Lpar4-null fibroblast motility and infarct-repair phenotypes provide organism-level evidence for roles in cell movement and cardiac responses but do not define additional human receptor molecular activities.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the core membrane location of LPAR4 receptor activity. Reason: LPAR4 is an integral seven-transmembrane receptor that recognizes extracellular lysophosphatidic acid and signals at the plasma membrane; the reviewed UniProt record and direct localization evidence concur. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:1298208 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. MGI:MGI:1925384 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. MGI:MGI:1929509 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. PANTHER:PTN000660850 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. RGD:2586 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P25116 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P43657 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P46093 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P55085 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q86VZ1 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q8IYL9 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q96RI0 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q99677 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q99678 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q9BXC1 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q9HC97 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q9Y2T6 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ZFIN:ZDB-GENE-061013-343 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0007186 G protein-coupled receptor signaling pathway | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the core signaling process initiated by LPA-bound LPAR4. Reason: Direct human receptor characterization shows LPA-dependent calcium elevation and stimulation of adenylyl cyclase, consistent with heterotrimeric G-protein signaling. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:101802 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. MGI:MGI:101910 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. MGI:MGI:1298208 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. MGI:MGI:2441992 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. PANTHER:PTN000660850 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. RGD:2586 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P25116 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P46093 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:P55085 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q86VZ1 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q8IYL9 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q9HC97 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB:Q9Y2T6 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ZFIN:ZDB-GENE-061013-343 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ZFIN:ZDB-GENE-101202-1 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0070915 lysophosphatidic acid receptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: Accepted as the ligand-specific core molecular function of LPAR4. Reason: Human LPAR4/P2Y9 is directly characterized as a receptor for lysophosphatidic acid with a defined rank order among LPA molecular species. Propagation Review Root cause: NO FAILURE CORE Sources checked: MGI:MGI:1925384 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. PANTHER:PTN002796593 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0004930 G protein-coupled receptor activity | IEA GO_REF:0000002 | MODIFY | Summary: Generic GPCR activity is valid but less informative than the established ligand-specific receptor activity. Reason: The InterPro GPCR mapping is correct, while direct human receptor evidence supports the more precise lysophosphatidic acid receptor activity term. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: InterPro:IPR000276 SUPPORTS TRANSFER Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement. Proposed replacements: lysophosphatidic acid receptor activity |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Accepted as the core membrane location of LPAR4. Reason: The combined orthology and UniProt location mapping agrees with direct plasma-membrane localization and the topology of an integral seven-transmembrane receptor. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q8BLG2 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ensembl:ENSMUSP00000053986 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. UniProtKB-SubCell:SL-0039 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0007186 G protein-coupled receptor signaling pathway | IEA GO_REF:0000002 | ACCEPT | Summary: Accepted as the core signaling process initiated by LPA-bound LPAR4. Reason: The rhodopsin-like GPCR family mapping is consistent with direct human LPAR4 calcium and adenylyl-cyclase responses. Propagation Review Root cause: NO FAILURE CORE Sources checked: InterPro:IPR000276 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: The generic membrane term should be replaced by the established plasma-membrane location. Reason: LPAR4 is an integral membrane receptor, but the reviewed record and direct localization support the more informative plasma membrane term. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: InterPro:IPR000276 SUPPORTS TRANSFER Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement. InterPro:IPR017452 SUPPORTS TRANSFER Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement. Proposed replacements: plasma membrane |
| GO:0005515 protein binding | IPI PMID:39083597 Multiplexed mapping of the interactome of GPCRs with recepto... | MARK AS OVER ANNOTATED | Summary: RAMP interactions are experimentally detected, but generic protein binding is an overbroad functional label. Reason: The multiplexed pairwise screen detects LPAR4 with RAMP1, RAMP2, and RAMP3 at the plasma membrane (PMID:39083597). It does not establish a stable physiological complex or an LPAR4-specific effect on ligand recognition, trafficking, or signaling. |
| GO:0035727 lysophosphatidic acid binding | IEA GO_REF:0000107 | ACCEPT | Summary: Accepted as the specific extracellular ligand-binding activity of LPAR4. Reason: The mouse orthology transfer agrees with direct human evidence that LPAR4 responds to LPA and distinguishes the potency of multiple LPA molecular species. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q8BLG2 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ensembl:ENSMUSP00000053986 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0070915 lysophosphatidic acid receptor activity | IEA GO_REF:0000107 | ACCEPT | Summary: Accepted as the ligand-specific core receptor activity of LPAR4. Reason: The mouse orthology transfer is supported independently by direct human characterization of P2Y9/LPAR4 as an LPA-responsive receptor. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q8BLG2 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. ensembl:ENSMUSP00000053986 SUPPORTS TRANSFER Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology. |
| GO:0005886 plasma membrane | IDA GO_REF:0000052 | ACCEPT | Summary: Accepted as direct protein-level plasma-membrane localization evidence. Reason: The Human Protein Atlas immunofluorescence mapping agrees with the reviewed UniProt cell-membrane assignment and receptor topology. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-379048 | ACCEPT | Summary: Accepted as the plasma-membrane location used in curated proximal Gq/11 signaling. Reason: Reactome places LPAR4 in the plasma-membrane GPCR step that activates Gq/11, consistent with direct receptor-mediated calcium signaling. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-417820 | ACCEPT | Summary: Accepted as the plasma-membrane location of the curated LPAR4 ligand-binding event. Reason: Reactome identifies P2Y9/LPAR4 as an LPA-binding receptor at the plasma membrane, consistent with direct human receptor characterization. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-749448 | ACCEPT | Summary: Accepted as the plasma-membrane location used in curated Gq binding. Reason: Reactome places liganded LPAR4 at the plasma membrane when binding inactive heterotrimeric Gq, consistent with its receptor topology and calcium response. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-749452 | ACCEPT | Summary: Accepted as the plasma-membrane location used in the curated Gq activation cycle. Reason: Reactome models dissociation of the liganded receptor–Gq complex at the plasma membrane; this is pathway context consistent with established LPAR4 signaling. |
| GO:0007204 positive regulation of cytosolic calcium ion concentration | IDA PMID:12724320 Identification of p2y9/GPR23 as a novel G protein-coupled re... | NEW | Summary: Proposed as a new annotation for the directly demonstrated LPAR4-dependent calcium response. Reason: Human P2Y9/GPR23 expressed in CHO cells increases intracellular calcium after 1-oleoyl-LPA stimulation, and an independent B103 transfectant study resolves a Gq/11-dependent calcium response. The process is receptor proximal, while the heterologous-cell context remains explicit. Supporting Evidence: PMID:12724320 In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity. |
| GO:0007189 adenylate cyclase-activating G protein-coupled receptor signaling pathway | IDA PMID:12724320 Identification of p2y9/GPR23 as a novel G protein-coupled re... | NEW | Summary: Proposed as a new annotation for directly demonstrated LPAR4-dependent stimulation of adenylyl cyclase. Reason: LPA stimulates adenylyl cyclase and raises cAMP in CHO cells expressing human LPAR4, and independent pharmacological work confirms LPAR4-dependent cAMP elevation. A separate B103 system showed no adenylyl-cyclase response, indicating context-dependent branch engagement without negating the directly demonstrated activating capability. Supporting Evidence: PMID:12724320 In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity. |
| GO:0035025 positive regulation of Rho protein signal transduction | IDA PMID:17172642 LPA4/p2y9/GPR23 mediates rho-dependent morphological changes... | NEW | Summary: Proposed as a new annotation for the directly supported G12/13-Rho branch of LPAR4 signaling. Reason: Human LPAR4 expressed in B103 cells couples to G12/13 and produces Rho-dependent neurite retraction, aggregation, and cadherin-dependent adhesion. This supports a proximal Rho signaling branch but does not generalize the cell-behavior outputs to native human tissues. Supporting Evidence: PMID:17172642 In LPA(4) transfectants, LPA induced dramatic morphological changes, i.e. neurite retraction, cell aggregation, and cadherin-dependent cell adhesion, which involved Rho-mediated signaling pathways. |
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Download this section (compressed HTML)Q: What are the quantitative Gq/11, G12/13, Gs, Gi/o, and beta-arrestin coupling profiles of endogenous LPAR4 in physiologically relevant human cells?
Q: Which physiological LPA molecular species bind and activate native human LPAR4 most effectively?
Q: Which signaling-network features determine whether LPAR4 suppresses or promotes cancer-cell motility and invasion?
Q: Which transient cardiac-progenitor and mouse injury-repair effects of LPAR4 are conserved in genetically controlled human cardiac models?
Q: Do RAMP1-3 alter endogenous LPAR4 surface delivery, ligand pharmacology, coupling bias, or trafficking?
Q: What structural features control LPA recognition and coupling by LPAR4, and how is the receptor phosphorylated, internalized, recycled, or desensitized?
Experiment: CRISPR-tag endogenous LPAR4 in ovarian and additional native-expression human cell models and measure Gq/11, G12/13, Gs, Gi/o, beta-arrestin, calcium, cAMP, and Rho responses in parallel. Resolve dose and time courses with selective G-alpha knockout and rescue.
Hypothesis: Native LPAR4 coupling weights vary by human cell type and differ from both CHO and B103 heterologous assays.
Type: endogenous multiplexed GPCR coupling analysis
Experiment: Measure direct binding and branch-resolved efficacy for acyl, alkyl, and alkenyl LPA species in wild-type, LPAR4-knockout, and genomically rescued human cells while controlling expression of other LPA receptors.
Hypothesis: Native LPAR4 retains a measurable preference for 1-oleoyl-LPA but the complete LPA-species rank order differs from recombinant membrane assays.
Type: receptor-specific native ligand pharmacology
Experiment: Compare endogenous LPAR4 knockout and rescue across fibrosarcoma, other cancer, and non-transformed cells under matched LPA production. Quantify proximal coupling, EPAC-Rac1, Rho, Akt, focal structures, migration, and invasion.
Hypothesis: The direction of LPAR4-dependent motility is set by relative cAMP-EPAC-Rac1, Rho, and Akt pathway engagement rather than by one universal receptor output.
Type: context-resolved motility signaling epistasis
Experiment: Introduce inducible LPAR4 loss and rescue at defined stages of human pluripotent stem-cell cardiac differentiation, measure lineage trajectories and receptor signaling, and compare mature engineered heart tissues under injury-like stress.
Hypothesis: Transient LPAR4 activity has a stage-specific role in human cardiac progenitors but is dispensable or different in mature cardiomyocytes.
Type: stage-resolved human cardiac differentiation
Experiment: Verify endogenous LPAR4-RAMP proximity, perturb each RAMP singly and in combination, and quantify receptor surface abundance, LPA affinity, coupling bias, internalization, and resensitization with interaction-defective add-back.
Hypothesis: At least one RAMP modifies LPAR4 only in human cells with native coexpression.
Type: endogenous RAMP genetic epistasis and receptor pharmacology
Experiment: Determine structures of LPAR4 bound to representative LPA species and G-protein partners, map agonist-dependent phosphosites and arrestin recruitment, and test precise endogenous mutants for internalization, recycling, desensitization, and signaling bias.
Hypothesis: Ligand-specific conformations and intracellular phosphorylation sites jointly determine LPAR4 coupling and surface residence.
Type: integrated receptor structural biology and trafficking
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The quantitative Gq/11, G12/13, Gs, Gi/o, and beta-arrestin coupling profile of endogenous human LPAR4 is unresolved across native cell types.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Human LPAR4 constructs support Gs-mediated cAMP elevation, Gq- and pertussis-toxin-sensitive Gi contributions to calcium, and G12/13-Rho activation. A separate B103 transfectant study shows Gq/11 calcium and G12/13-Rho responses without an adenylyl-cyclase effect and is interpreted as lacking Gi/o coupling. These heterologous results establish an activating Gs/cAMP capability and cell-context-dependent branch use, not a fixed native-human coupling hierarchy or universal Gi/o exclusion.
Significance: Coupling weights determine whether LPAR4 changes calcium, cAMP, Rho, or arrestin signaling and may explain opposing phenotypes across tissues.
What would resolve it: Measure endogenous LPAR4 with parallel Gq/11, G12/13, Gs, Gi/o, beta-arrestin, calcium, cAMP, and Rho biosensors across matched LPA dose and time-course conditions in relevant human cells.
Gap: Native LPAR4 affinity and efficacy across physiological LPA molecular species remain incompletely defined.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Recombinant membrane binding and reporter assays define an LPA-species potency order and nanomolar 1-oleoyl-LPA binding, but do not establish whether the same ranking holds at endogenous receptor abundance in human tissues.
Significance: Molecular-species selectivity could shape tissue responses and therapeutic pharmacology.
What would resolve it: Quantify binding and branch-resolved efficacy for defined LPA species in native human membranes and cells using LPAR4 knockout and genomic rescue controls.
Gap: Why LPAR4 suppresses motility in some cancer-cell models but promotes invadopodia, invasion, and metastasis in fibrosarcoma is unresolved.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Ectopic LPAR4 suppresses migration and invasion in some human cancer cells, whereas fibrosarcoma studies identify an LPAR4-cAMP-EPAC-Rac1 invasion pathway. These opposing disease-cell outputs cannot be generalized into a single normal human motility function.
Significance: Resolving the switch would distinguish receptor-intrinsic coupling bias from differences in receptor abundance, LPA production, and signaling-network state.
What would resolve it: Compare endogenous LPAR4 perturbation across matched cancer and non-transformed human cells while measuring G-protein branches, EPAC-Rac1, Rho, Akt, migration, invadopodia, and invasion.
Gap: The extent to which mouse Lpar4 and stem-cell-culture developmental phenotypes predict normal human tissue functions remains unclear.
OPEN BIOLOGY
What is known: Initial Lpar4-null mice show no gross abnormality despite altered fibroblast motility. Sequential LPAR4 stimulation and inhibition improves mouse and human pluripotent-stem-cell cardiac differentiation in vitro, while infarct reduction and cardiac rescue are demonstrated in mice. These findings do not establish a constitutive adult-human cardiac or developmental core function.
Significance: Careful conservation testing is needed before translating transient progenitor and mouse injury-repair effects into human physiology or therapy.
What would resolve it: Define endogenous LPAR4 timing and necessity during human cardiac-progenitor differentiation, then test mature human cardiac models with receptor-selective genetic controls before extrapolating the mouse repair phenotype.
Gap: The physiological consequences of screen-detected LPAR4 interactions with RAMP1, RAMP2, and RAMP3 are unknown.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: All three contacts were detected after ectopic pairwise expression and membrane solubilization. The audited evidence does not establish endogenous coexpression, a stable complex, or an LPAR4-specific effect on surface delivery, ligand selectivity, coupling, or trafficking.
Significance: RAMP-dependent modulation could explain cell-context differences, but screen-level contacts must not be treated as constitutive receptor components.
What would resolve it: Validate endogenous proximity in human cells that naturally coexpress LPAR4 and each RAMP, then use knockout and add-back to measure receptor pharmacology, surface abundance, signaling bias, and trafficking.
Gap: LPAR4 structural determinants, receptor-specific trafficking regulation, and possible endogenous proteoform variation remain largely uncharacterized.
OPEN BIOLOGYCURATION
What is known: Agonist-induced internalization is demonstrated for epitope-tagged LPAR4, but the route, phosphorylation code, arrestin dependence, recycling, degradation, and endogenous-tissue regulation remain unresolved. No experimentally determined LPAR4 structure or established alternative protein product was identified, and recorded sequence conflicts are not validated functional isoforms or alleles.
Significance: These missing data limit mechanistic interpretation of ligand selectivity, coupling bias, surface residence, and sequence-dependent functional variation.
What would resolve it: Determine ligand-bound LPAR4 structures with relevant G proteins, map agonist-dependent phosphorylation and arrestin recruitment at endogenous abundance, and reconcile candidate transcripts or variants before assigning isoform-specific function.
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