LPAR4

UniProt ID: Q99677
Organism: Homo sapiens
Review Status: COMPLETE
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Gene Description

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.

Existing Annotations Review

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.
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 receptorโ€“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.

Core Functions

Acts at the plasma membrane as an LPA-activated GPCR. Recombinant human LPAR4 directly produces cytosolic calcium elevation, Gs-mediated stimulation of adenylyl cyclase and cAMP, and a G12/13-Rho signaling branch. Pharmacological and G-protein mini-gene experiments additionally resolve Gq- and pertussis-toxin-sensitive Gi contributions to calcium mobilization. A separate B103 transfectant study detected Gq/11 and G12/13 but neither Gi/o coupling nor an adenylyl-cyclase response, showing that coupling weights and detectable branch outputs vary with cellular context without negating the activating Gs/cAMP capability. Context-opposing migration, invasion, developmental, and repair phenotypes are downstream outputs rather than additional core molecular activities.

Molecular Function:
lysophosphatidic acid receptor activity
Directly Involved In:
G protein-coupled receptor signaling pathway adenylate cyclase-activating G protein-coupled receptor signaling pathway positive regulation of cytosolic calcium ion concentration positive regulation of Rho protein signal transduction
Cellular Locations:
plasma membrane
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.
  • PMID:17166850
    Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s).
  • PMID:17172642
    In B103 cells stably expressing LPA(4), we observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity.
  • PMID:17172642
    Thus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o).

Directly binds lysophosphatidic acid as the extracellular ligand-recognition step of LPAR4 signaling. Recombinant membrane binding establishes nanomolar 1-oleoyl-LPA binding and a defined potency order among acyl and ether LPA species. This specific activity does not imply nucleotide receptor activity or indiscriminate binding to unrelated lipid classes.

Molecular Function:
lysophosphatidic acid binding
Cellular Locations:
plasma membrane
Supporting Evidence:
  • PMID:12724320
    Membrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm.
  • PMID:12724320
    Competition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA.

References

GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000052
Gene Ontology annotation based on curation of immunofluorescence data
GO_REF:0000107
Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
PMID:9073069
Cloning and chromosomal mapping of four putative novel human G-protein-coupled receptor genes.
  • GPR23, the gene later deorphanized as LPAR4, was initially isolated as a human putative GPCR and mapped to Xq13-q21.1.
    "This resulted in the isolation of genes GPR21, GPR22 and GPR23."
  • The initial human GPR23 study established chromosomal position but did not identify a ligand or signaling function.
    "Fluorescence in situ hybridization (FISH) was used to map GPR20 to chromosome 8q, region 24.3-24.2, GPR21 to chromosome 9, region q33, GPR22 to chromosome 7, region q22-q31.1, and GPR23 to chromosome X, region q13-q21.1."
PMID:9223435
Cloning of a human heptahelical receptor closely related to the P2Y5 receptor.
  • The human receptor historically called P2Y5-like/P2Y9 was cloned as a 370-aa heptahelical GPCR.
    "Starting from this partial sequence, we have isolated a complete clone and identified a 1113 base pair open reading frame encoding a new G-coupled receptor that we have called P2Y5-like."
  • Forty tested nucleotides and nucleosides failed to activate the receptor in four signaling assays, arguing against interpreting the historical P2Y name as demonstrated purinergic activity.
    "None of the 40 nucleotides and nucleosides tested was able to elicit a response in any of four functional assays: inositol phosphate formation, stimulation or inhibition of cAMP formation, and extracellular acidification measured with a microphysiometer."
PMID:12724320
Identification of p2y9/GPR23 as a novel G protein-coupled receptor for lysophosphatidic acid, structurally distant from the Edg family.
  • Human p2y9/GPR23 was deorphanized as LPA4 by direct, saturable 1-oleoyl-LPA binding in membranes of receptor-expressing cells.
    "Membrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm."
  • LPAR4 discriminates among LPA molecular species, preferring 1-oleoyl-LPA in the reported rank order.
    "Competition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA."
  • In heterologous CHO cells, LPA-bound LPAR4 elevated intracellular calcium and stimulated adenylyl cyclase.
    "In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity."
  • The original human tissue survey found LPAR4 transcript most abundant in ovary.
    "Quantitative real-time PCR demonstrated that mRNA of p2y9/GPR23 was significantly abundant in ovary compared with other tissues."
PMID:17172642
LPA4/p2y9/GPR23 mediates rho-dependent morphological changes in a rat neuronal cell line.
  • LPAR4 drove Gq/11-dependent calcium mobilization in rat B103 neuroblastoma transfectants but did not alter adenylyl cyclase in that system.
    "In B103 cells stably expressing LPA(4), we observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity."
  • LPA induced Rho-dependent neurite retraction, aggregation, and cadherin-dependent adhesion through transfected LPAR4.
    "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."
  • The study inferred LPAR4 coupling to Gq/11 and G12/13 but not Gi/o in this rat neuronal-cell context.
    "Thus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o)."
PMID:17166850
LPA(4)/GPR23 is a lysophosphatidic acid (LPA) receptor utilizing G(s)-, G(q)/G(i)-mediated calcium signaling and G(12/13)-mediated Rho activation.
  • Epitope-tagged LPAR4 showed concentration-dependent LPA responses and specific membrane binding, independently confirming it as an LPA receptor.
    "Using epitope-tagged LPA(4), pharmacological intervention, and G protein mini-genes, we provide independent confirmatory evidence that supports LPA(4) as a fourth LPA receptor, including LPA concentration-dependent responses and specific membrane binding."
  • The study attributed LPAR4-dependent cAMP increases to Gs, calcium mobilization to Gq and a pertussis-toxin-sensitive pathway, and Rho-dependent cytoskeletal outputs to G12/13.
    "Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s)."
  • LPA stimulation induced internalization of epitope-tagged LPAR4 in the study system.
    "Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s)."
PMID:18843048
Role of LPA4/p2y9/GPR23 in negative regulation of cell motility.
  • Mouse Lpar4 loss enhanced LPA-driven fibroblast migration and shifted Akt/Rac/Rho signaling, whereas receptor reconstitution reduced motility.
    "Consistent with negative modulation of the phosphatidylinositol 3 kinase pathway by LPA(4), LPA(4) deficiency potentiated Akt and Rac but decreased Rho activation induced by LPA. Reconstitution of LPA(4) converted LPA(4)-negative cells into a less motile phenotype."
  • Ectopic LPAR4 inhibited migration and invasion in human cancer cells and antagonized LPA1-driven motility in receptor-null B103 cells.
    "In support of the biological relevance of these observations, ectopic expression of LPA(4) strongly inhibited migration and invasion of human cancer cells. When coexpressed with LPA(1) in B103 neuroblastoma cells devoid of endogenous LPA receptors, LPA(4) attenuated LPA(1)-driven migration and invasion, indicating functional antagonism between the two subtypes of LPA receptors."
  • The first targeted mouse Lpar4 knockout had no gross apparent phenotype, bounding strong developmental claims from this model.
    "Although LPA(4)-deficient mice displayed no apparent abnormalities, LPA(4)-deficient mouse embryonic fibroblasts (MEFs) were hypersensitive to LPA-induced cell migration."
PMID:20484039
Autotaxin promotes cancer invasion via the lysophosphatidic acid receptor 4: participation of the cyclic AMP/EPAC/Rac1 signaling pathway in invadopodia formation.
  • In fibrosarcoma cells, autotaxin-derived LPA signaled through LPAR4 to promote invadopodia through a cAMP-EPAC-Rac1 pathway.
    "We further provide evidence that LPA(4) signaling in fibrosarcoma cells regulates invadopodia formation downstream of ATX, a process mediated through the activation of EPAC by cyclic AMP and subsequent Rac1 activation."
  • LPAR4 knockdown supported a requirement for the receptor in invasion and in vivo metastasis formation in this cancer model.
    "Results using LPA(4) shRNA support the requirement of the LPA(4) receptor for cell invasion and in vivo metastasis formation."
PMID:20482379
Strategy for the identification of GPR23/LPA4 receptor agonists and inverse agonists.
  • Human GPR23 expressed in CHO cells reproduced LPA-dependent cAMP and calcium responses.
    "In Chinese hamster ovary cells expressing the human GPR23, LPA induced an increase in cellular cyclic adenosine monophosphate (cAMP) and calcium levels."
  • Orthogonal reporter and radioligand-binding screens yielded LPAR4 agonists, inverse agonists, and a negative modulator.
    "Here we report the identification of novel GPR23 agonists, inverse agonists, and a negative modulator from 2 high-throughput screens, a beta-lactamase reporter screen, and a [3H]LPA-binding screen."
  • Candidate mechanisms and selectivity were tested with reporter, cAMP, and other LPA-receptor assays.
    "Several screening hits were selected for mechanism of action studies using the beta-lactamase reporter assay and a cAMP assay. An evaluation of their selectivity at the other LPA receptors was also conducted."
PMID:21050927
Development of a GPR23 cell-based ฮฒ-lactamase reporter assay.
  • An inducible reporter assay detected constitutive GPR23/LPAR4 activity.
    "This report describes how a tetracycline-inducible system was utilized in conjunction with a sensitive ฮฒ-lactamase reporter gene to develop an assay in which constitutive activity of the receptor could be monitored."
  • Screening with the reporter identified the first reported small-molecule inverse agonists for GPR23.
    "This assay was then utilized to screen a 1.1 million compound library to identify the first small molecule inverse agonists for the receptor."
PMID:33160074
Lysophosphatidic Acid Receptor 4 Is Transiently Expressed during Cardiac Differentiation and Critical for Repair of the Damaged Heart.
  • LPAR4 expression transiently peaks in cardiac progenitors during both mouse and human PSC differentiation.
    "During inย vitro differentiation of mouse and human PSCs toward cardiac lineage, LPAR4 expression peaked after 3-7ย days of differentiation in cardiac progenitors and then declined."
  • Sequential LPAR4 stimulation and downstream p38 inhibition enhanced cardiac differentiation in mouse and human PSC cultures.
    "Sequential stimulation and inhibition of LPAR4 using these agents enhanced the inย vitro efficiency of cardiac differentiation from mouse and human PSCs."
  • The in vivo cardiac-repair result was demonstrated in mice and should not be transferred directly to human physiology.
    "Importantly, inย vivo, this sequential stimulation and inhibition of LPAR4 reduced the infarct size and rescued heart dysfunction in mice."
PMID:39083597
Multiplexed mapping of the interactome of GPCRs with receptor activity-modifying proteins.
  • The RAMP interactome screen tested every GPCR-RAMP pair by ectopic coexpression and membrane-solubilization assays.
    "All potential GPCR-RAMP interacting pairs were expressed ectopically, solubilized and analyzed using the multiplexed suspension bead array (SBA) strategy."
  • The library screen used one biological replicate per GPCR-only or GPCR-RAMP sample, with separate detection schemes supplying the two replicates.
    "For the DuET libraryโ€“based GPCR-RAMP interactome screen, we used one biological replicate of each of the four unique GPCR-containing samples (each GPCR alone and each GPCR with each of the three RAMPs) in two replicates. Each replicate represented one detection scheme."
Reactome:R-HSA-379048
Liganded Gq/11-activating GPCRs act as GEFs for Gq/11
  • Reactome places Gq/11-linked GPCRs upstream of PLC, IP3, and calcium mobilization.
    "PLC hydrolyzes phosphatidylinositol (PIP2) to diacyl glycerol (DAG) and inositol triphosphate (IP3)."
Reactome:R-HSA-417820
P2Y9 receptor can bind to LPA
  • Reactome identifies human GPR23/P2Y9 as LPAR4 and models LPA binding with signaling through multiple G proteins.
    "P2Y9 has been reported to bind lysophosphatidic acid (LPA) as a ligand and elicit numerous effects via multiple G proteins (Lee CW et al, 2007)."
Reactome:R-HSA-749448
Liganded Gq-activating GPCRs bind inactive heterotrimeric Gq
  • Reactome models the generic binding step between liganded Gq-coupled GPCRs and inactive heterotrimeric Gq.
    "Numerous functionally unrelated GPCRs couple with the Gq G-protein subtype."
Reactome:R-HSA-749452
The Ligand:GPCR:Gq complex dissociates
  • Reactome models dissociation of activated Gq alpha from beta-gamma after receptor activation.
    "Activated G alpha (q) and the beta:gamma dimer then participate in separate signaling cascades."

Suggested Questions for Experts

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?

Suggested Experiments

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

Knowledge Gaps

What is not known โ€” 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.

๐Ÿ“š Additional Documentation

Notes

(LPAR4-notes.md)

LPAR4 literature notes

Research provenance

  • Project deep research was attempted with just deep-research-perplexity human LPAR4 --timeout 120 on 2026-08-11. The provider returned HTTP 401 insufficient_quota ("You exceeded your current quota"), all providers failed, and no LPAR4-deep-research-perplexity.md file was produced. This notes file records the manual primary-literature audit; no provider output was fabricated.
  • Literature was identified from the reviewed UniProt Q99677 record, GOA/Reactome provenance, PubMed/Europe PMC searches, and citation chaining. Publication text was fetched with the project's fetch-pmid tooling and all quotations below were checked against the local caches.

Identity and nomenclature

  • The gene product is the reviewed 370-aa human lysophosphatidic acid receptor 4, Q99677. Historical names include GPR23, P2Y9, and P2Y5-like; these names predate its LPA deorphanization and should not be read as evidence of nucleotide receptor activity.
  • The first GPR23 report established identity and genomic location: โ€œThis resulted in the isolation of genes GPR21, GPR22 and GPR23.โ€ It mapped GPR23 to โ€œchromosome X, region q13-q21.1.โ€ [PMID:9073069, abstract]
  • The independent P2Y5-like cloning study isolated โ€œa complete clone and identified a 1113 base pair open reading frame encoding a new G-coupled receptor that we have called P2Y5-like.โ€ Crucially, โ€œNone of the 40 nucleotides and nucleosides tested was able to elicit a response in any of four functional assays: inositol phosphate formation, stimulation or inhibition of cAMP formation, and extracellular acidification measured with a microphysiometer.โ€ [PMID:9223435, abstract] This is a direct boundary against treating historical P2Y9 nomenclature as demonstrated purinergic function.

Direct human receptor and ligand evidence

  • The decisive deorphanization result was direct membrane binding: โ€œMembrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm.โ€ [PMID:12724320, abstract]
  • Ligand pharmacology was not generic lipid binding: โ€œCompetition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA.โ€ [PMID:12724320, abstract]
  • In a separate engineered human-receptor system, โ€œIn Chinese hamster ovary cells expressing the human GPR23, LPA induced an increase in cellular cyclic adenosine monophosphate (cAMP) and calcium levels.โ€ [PMID:20482379, abstract]
  • The 2010 pharmacology screen used both signaling and binding formats: โ€œHere we report the identification of novel GPR23 agonists, inverse agonists, and a negative modulator from 2 high-throughput screens, a beta-lactamase reporter screen, and a [3H]LPA-binding screen.โ€ [PMID:20482379, abstract]
  • A related assay paper established engineered constitutive activity rather than native basal signaling: โ€œThis report describes how a tetracycline-inducible system was utilized in conjunction with a sensitive ฮฒ-lactamase reporter gene to develop an assay in which constitutive activity of the receptor could be monitored.โ€ [PMID:21050927, abstract]

Coupling is assay- and cell-context dependent

  • In the founding CHO-cell experiment, โ€œ1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity.โ€ [PMID:12724320, abstract] This supports calcium and Gs/adenylyl-cyclase outputs for recombinant human LPAR4 but does not establish their quantitative importance in every native human cell.
  • Lee et al. independently confirmed receptor identity with โ€œLPA concentration-dependent responses and specific membrane binding.โ€ Their mechanistic study reported โ€œG(q) protein and pertussis toxin-sensitive calcium mobilizationโ€ together with โ€œcAMP increases mediated by G(s)โ€ and โ€œG(12/13)- and Rho-mediated neurite retraction and stress fiber formationโ€. [PMID:17166850, abstract] Pertussis-toxin sensitivity supports a Gi/o contribution to calcium mobilization in that tagged-receptor system; it does not by itself establish a universal endogenous-human Gi/o output.
  • The same study reported โ€œreceptor internalizationโ€ among the LPA-dependent activities of epitope-tagged LPAR4. [PMID:17166850, abstract] This establishes internalization capability in that experimental system, while leaving the endogenous trafficking mechanism, kinetics, and regulators unresolved.
  • In rat B103 neuroblastoma transfectants, โ€œwe observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity.โ€ [PMID:17172642, abstract] The difference from CHO cells is positive evidence that cyclic-nucleotide direction and coupling weights are context-sensitive.
  • The same B103 study concluded: โ€œThus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o).โ€ [PMID:17172642, abstract] This conflicts with the pertussis-toxin-sensitive calcium component reported by PMID:17166850 and is therefore system-specific heterologous evidence, not a direct native-human Gi/o exclusion.
  • LPA also produced โ€œneurite retraction, cell aggregation, and cadherin-dependent cell adhesion, which involved Rho-mediated signaling pathwaysโ€ in those transfectants. [PMID:17172642, abstract] These cell behaviors support a G12/13-Rho branch in that system; the proposed neurodevelopmental role remains an inference from a rat cell line.
  • Reactome R-HSA-417820 cites Lee et al. 2007 (PMID:17166850), not the separate Yanagida et al. B103 study (PMID:17172642), for its statement that P2Y9/LPAR4 acts through multiple G proteins.

Motility and developmental boundaries

  • In mouse embryonic fibroblasts, Lpar4 loss shifted LPA signaling and motility: โ€œLPA(4) deficiency potentiated Akt and Rac but decreased Rho activation induced by LPA. Reconstitution of LPA(4) converted LPA(4)-negative cells into a less motile phenotype.โ€ [PMID:18843048, abstract]
  • Human evidence in that paper was ectopic and disease-cell based: โ€œectopic expression of LPA(4) strongly inhibited migration and invasion of human cancer cells.โ€ [PMID:18843048, abstract] It supports receptor capacity to restrain motility but is not proof of a universal normal-tissue role.
  • A separate fibrosarcoma model gave the opposite directional output: โ€œWe further provide evidence that LPA(4) signaling in fibrosarcoma cells regulates invadopodia formation downstream of ATX, a process mediated through the activation of EPAC by cyclic AMP and subsequent Rac1 activation.โ€ [PMID:20484039, abstract] LPAR4 knockdown also supported a role in invasion and metastasis in that model. Together, these cancer-cell studies show that LPAR4 effects on motility are strongly context dependent rather than universally suppressive.
  • The knockout boundary matters: โ€œAlthough LPA(4)-deficient mice displayed no apparent abnormalities, LPA(4)-deficient mouse embryonic fibroblasts (MEFs) were hypersensitive to LPA-induced cell migration.โ€ [PMID:18843048, abstract] Later tissue-specific or sensitized phenotypes should not be rewritten as a gross constitutive developmental requirement from this first model.
  • Cardiac differentiation evidence spans human and mouse PSC cultures: โ€œDuring inย vitro differentiation of mouse and human PSCs toward cardiac lineage, LPAR4 expression peaked after 3-7ย days of differentiation in cardiac progenitors and then declined.โ€ [PMID:33160074, abstract]
  • The functional differentiation result was also culture-based: โ€œSequential stimulation and inhibition of LPAR4 using these agents enhanced the inย vitro efficiency of cardiac differentiation from mouse and human PSCs.โ€ The injury-repair result was explicitly murine: โ€œImportantly, inย vivo, this sequential stimulation and inhibition of LPAR4 reduced the infarct size and rescued heart dysfunction in mice.โ€ [PMID:33160074, abstract] These data support a transient progenitor context and a mouse repair model, not a constitutive core function of adult human LPAR4.

Tissue, interactions, trafficking, isoforms, and variants

  • The founding study reported: โ€œQuantitative real-time PCR demonstrated that mRNA of p2y9/GPR23 was significantly abundant in ovary compared with other tissues.โ€ [PMID:12724320, abstract] The reviewed UniProt record summarizes โ€œHigh expression in ovaryโ€ and non-detection in the surveyed thalamus, putamen, caudate, frontal cortex, pons, hypothalamus, and hippocampus. This is assay- and tissue-panel context, not proof of absence from every neural cell state.
  • GOA/IntAct contains three partner-level IPI rows from PMID:39083597: RAMP1 (O60894), RAMP2 (O60895), and RAMP3 (O60896). The screen context is explicit: โ€œAll potential GPCR-RAMP interacting pairs were expressed ectopically, solubilized and analyzed using the multiplexed suspension bead array (SBA) strategy.โ€ [PMID:39083597, introduction]
  • The library screen also used โ€œone biological replicate of each of the four unique GPCR-containing samples (each GPCR alone and each GPCR with each of the three RAMPs) in two replicates. Each replicate represented one detection scheme.โ€ [PMID:39083597, results] Thus the LPAR4-RAMP rows support screen-level complex detection, but the audited evidence does not show an endogenous LPAR4-RAMP complex or a functional effect on LPAR4 trafficking, surface delivery, ligand selectivity, or signaling.
  • No direct LPAR4 internalization, desensitization, or ฮฒ-arrestin trafficking study was identified in the audited primary set. RAMP proteins can regulate trafficking for some GPCRs, but that general property must not be transferred to LPAR4 without receptor-specific functional evidence.
  • The reviewed UniProt Q99677 record contains no ALTERNATIVE PRODUCTS section, so no reviewed protein isoform is established here. Its three CONFLICT features are sequence-source discrepanciesโ€”โ€œV -> A (in Ref. 8; AAH69996)โ€, โ€œF -> L (in Ref. 3; AAB66322)โ€, and โ€œI -> V (in Ref. 8; AAH95538)โ€โ€”not demonstrated functional alleles or isoform-specific biology.

Reference-level conclusions for later annotation review

  • Core evidence supports an LPA-binding plasma-membrane GPCR with direct ligand binding and recombinant calcium/adenylyl-cyclase responses.
  • Gq/11, G12/13-Rho, and Gs/cAMP capability are supported in heterologous systems. Gi/o evidence differs between pertussis-toxin-sensitive calcium in PMID:17166850 and the B103 no-Gi/o conclusion in PMID:17172642; neither should be treated as a universal native-human coupling profile.
  • Motility suppression, cardiac progenitor differentiation, and mouse injury/development phenotypes are important bounded outputs, not additional receptor molecular activities.
  • RAMP1/2/3 are high-throughput interaction candidates. Functional consequences and endogenous human complexes remain open questions.
  • No reviewed isoform-specific function or experimentally reconciled functional variant was found; historical nucleotide-receptor names and sequence conflicts should not be overinterpreted.

๐Ÿ“„ View Raw YAML

 
id: Q99677
gene_symbol: LPAR4
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  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.
existing_annotations:
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Accepted as the core membrane location of LPAR4 receptor activity.
    action: ACCEPT
    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
      source_entities:
      - source_id: MGI:MGI:1298208
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: MGI:MGI:1925384
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: MGI:MGI:1929509
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: PANTHER:PTN000660850
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: RGD:2586
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P25116
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P43657
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P46093
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P55085
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q86VZ1
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q8IYL9
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q96RI0
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q99677
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q99678
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q9BXC1
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q9HC97
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q9Y2T6
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ZFIN:ZDB-GENE-061013-343
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - MGI:MGI:1298208
  - MGI:MGI:1925384
  - MGI:MGI:1929509
  - PANTHER:PTN000660850
  - RGD:2586
  - UniProtKB:P25116
  - UniProtKB:P43657
  - UniProtKB:P46093
  - UniProtKB:P55085
  - UniProtKB:Q86VZ1
  - UniProtKB:Q8IYL9
  - UniProtKB:Q96RI0
  - UniProtKB:Q99677
  - UniProtKB:Q99678
  - UniProtKB:Q9BXC1
  - UniProtKB:Q9HC97
  - UniProtKB:Q9Y2T6
  - ZFIN:ZDB-GENE-061013-343
- term:
    id: GO:0007186
    label: G protein-coupled receptor signaling pathway
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: Accepted as the core signaling process initiated by LPA-bound LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: MGI:MGI:101802
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: MGI:MGI:101910
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: MGI:MGI:1298208
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: MGI:MGI:2441992
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: PANTHER:PTN000660850
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: RGD:2586
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P25116
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P46093
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:P55085
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q86VZ1
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q8IYL9
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q9HC97
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB:Q9Y2T6
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ZFIN:ZDB-GENE-061013-343
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ZFIN:ZDB-GENE-101202-1
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - MGI:MGI:101802
  - MGI:MGI:101910
  - MGI:MGI:1298208
  - MGI:MGI:2441992
  - PANTHER:PTN000660850
  - RGD:2586
  - UniProtKB:P25116
  - UniProtKB:P46093
  - UniProtKB:P55085
  - UniProtKB:Q86VZ1
  - UniProtKB:Q8IYL9
  - UniProtKB:Q9HC97
  - UniProtKB:Q9Y2T6
  - ZFIN:ZDB-GENE-061013-343
  - ZFIN:ZDB-GENE-101202-1
- term:
    id: GO:0070915
    label: lysophosphatidic acid receptor activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Accepted as the ligand-specific core molecular function of LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: MGI:MGI:1925384
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: PANTHER:PTN002796593
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - MGI:MGI:1925384
  - PANTHER:PTN002796593
- term:
    id: GO:0004930
    label: G protein-coupled receptor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: Generic GPCR activity is valid but less informative than the established ligand-specific receptor activity.
    action: MODIFY
    reason: The InterPro GPCR mapping is correct, while direct human receptor evidence supports the more precise lysophosphatidic acid receptor activity term.
    proposed_replacement_terms:
    - id: GO:0070915
      label: lysophosphatidic acid receptor activity
    propagation_review:
      root_cause: TERM_SCOPING_PROBLEM
      failure_modes:
      - GRANULARITY_MISMATCH
      source_entities:
      - source_id: InterPro:IPR000276
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement.
  supporting_entities:
  - InterPro:IPR000276
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: Accepted as the core membrane location of LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: UniProtKB:Q8BLG2
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ensembl:ENSMUSP00000053986
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: UniProtKB-SubCell:SL-0039
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - UniProtKB:Q8BLG2
  - ensembl:ENSMUSP00000053986
  - UniProtKB-SubCell:SL-0039
- term:
    id: GO:0007186
    label: G protein-coupled receptor signaling pathway
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: Accepted as the core signaling process initiated by LPA-bound LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: InterPro:IPR000276
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - InterPro:IPR000276
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: located_in
  review:
    summary: The generic membrane term should be replaced by the established plasma-membrane location.
    action: MODIFY
    reason: LPAR4 is an integral membrane receptor, but the reviewed record and direct localization support the more informative plasma membrane term.
    proposed_replacement_terms:
    - id: GO:0005886
      label: plasma membrane
    propagation_review:
      root_cause: TERM_SCOPING_PROBLEM
      failure_modes:
      - GRANULARITY_MISMATCH
      source_entities:
      - source_id: InterPro:IPR000276
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement.
      - source_id: InterPro:IPR017452
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; it supports the broad concept, while direct human LPAR4 evidence supports the more precise replacement.
  supporting_entities:
  - InterPro:IPR000276
  - InterPro:IPR017452
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:39083597
  qualifier: enables
  review:
    summary: RAMP interactions are experimentally detected, but generic protein binding is an overbroad functional label.
    action: MARK_AS_OVER_ANNOTATED
    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.
  supporting_entities:
  - UniProtKB:O60894
  - UniProtKB:O60895
  - UniProtKB:O60896
  extensions:
  - predicate: BFO:0000066
    term:
      id: GO:0005886
      label: plasma membrane
- term:
    id: GO:0035727
    label: lysophosphatidic acid binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: Accepted as the specific extracellular ligand-binding activity of LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: UniProtKB:Q8BLG2
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ensembl:ENSMUSP00000053986
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - UniProtKB:Q8BLG2
  - ensembl:ENSMUSP00000053986
- term:
    id: GO:0070915
    label: lysophosphatidic acid receptor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: enables
  review:
    summary: Accepted as the ligand-specific core receptor activity of LPAR4.
    action: ACCEPT
    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
      source_entities:
      - source_id: UniProtKB:Q8BLG2
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
      - source_id: ensembl:ENSMUSP00000053986
        source_status: SUPPORTS_TRANSFER
        comment: Exact WITH/FROM source; the propagated assignment agrees with established human LPAR4 biology.
  supporting_entities:
  - UniProtKB:Q8BLG2
  - ensembl:ENSMUSP00000053986
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Accepted as direct protein-level plasma-membrane localization evidence.
    action: ACCEPT
    reason: The Human Protein Atlas immunofluorescence mapping agrees with the reviewed UniProt cell-membrane assignment and receptor topology.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-379048
  qualifier: located_in
  review:
    summary: Accepted as the plasma-membrane location used in curated proximal Gq/11 signaling.
    action: ACCEPT
    reason: Reactome places LPAR4 in the plasma-membrane GPCR step that activates Gq/11, consistent with direct receptor-mediated calcium signaling.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-417820
  qualifier: located_in
  review:
    summary: Accepted as the plasma-membrane location of the curated LPAR4 ligand-binding event.
    action: ACCEPT
    reason: Reactome identifies P2Y9/LPAR4 as an LPA-binding receptor at the plasma membrane, consistent with direct human receptor characterization.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-749448
  qualifier: located_in
  review:
    summary: Accepted as the plasma-membrane location used in curated Gq binding.
    action: ACCEPT
    reason: Reactome places liganded LPAR4 at the plasma membrane when binding inactive heterotrimeric Gq, consistent with its receptor topology and calcium response.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-749452
  qualifier: located_in
  review:
    summary: Accepted as the plasma-membrane location used in the curated Gq activation cycle.
    action: ACCEPT
    reason: "Reactome models dissociation of the liganded receptor\u2013Gq complex at the plasma membrane; this is pathway context consistent with established LPAR4 signaling."
- term:
    id: GO:0007204
    label: positive regulation of cytosolic calcium ion concentration
  evidence_type: IDA
  original_reference_id: PMID:12724320
  qualifier: involved_in
  review:
    summary: Proposed as a new annotation for the directly demonstrated LPAR4-dependent calcium response.
    action: NEW
    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.
    supported_by:
    - reference_id: PMID:12724320
      supporting_text: In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity.
    additional_reference_ids:
    - PMID:17166850
    - PMID:17172642
- term:
    id: GO:0007189
    label: adenylate cyclase-activating G protein-coupled receptor signaling pathway
  evidence_type: IDA
  original_reference_id: PMID:12724320
  qualifier: involved_in
  review:
    summary: Proposed as a new annotation for directly demonstrated LPAR4-dependent stimulation of adenylyl cyclase.
    action: NEW
    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.
    supported_by:
    - reference_id: PMID:12724320
      supporting_text: In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity.
    additional_reference_ids:
    - PMID:20482379
    - PMID:17166850
    - PMID:17172642
- term:
    id: GO:0035025
    label: positive regulation of Rho protein signal transduction
  evidence_type: IDA
  original_reference_id: PMID:17172642
  qualifier: involved_in
  review:
    summary: Proposed as a new annotation for the directly supported G12/13-Rho branch of LPAR4 signaling.
    action: NEW
    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.
    supported_by:
    - reference_id: PMID:17172642
      supporting_text: 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.
    additional_reference_ids:
    - PMID:17166850
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Standard InterPro-to-GO mapping provenance for broad GPCR, signaling, and membrane annotations; it is not direct experimental evidence for human LPAR4.
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: PANTHER/GO phylogenetic-inference provenance for conserved receptor activity, signaling, and plasma-membrane annotations; transfer quality must be assessed against direct LPAR4 evidence.
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Human Protein Atlas immunofluorescence-curation provenance for plasma-membrane localization; useful for location but not receptor mechanism.
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Ensembl Compara orthology-transfer provenance from mouse Lpar4; species and tissue context must not be silently transferred to human LPAR4.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Combined automated location provenance; supports a plasma-membrane assignment but adds no LPAR4-specific mechanism.
- id: PMID:9073069
  title: Cloning and chromosomal mapping of four putative novel human G-protein-coupled receptor genes.
  findings:
  - statement: GPR23, the gene later deorphanized as LPAR4, was initially isolated as a human putative GPCR and mapped to Xq13-q21.1.
    supporting_text: This resulted in the isolation of genes GPR21, GPR22 and GPR23.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The initial human GPR23 study established chromosomal position but did not identify a ligand or signaling function.
    supporting_text: Fluorescence in situ hybridization (FISH) was used to map GPR20 to chromosome 8q, region 24.3-24.2, GPR21 to chromosome 9, region q33, GPR22 to chromosome 7, region q22-q31.1, and GPR23 to chromosome X, region q13-q21.1.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Primary human cloning and mapping paper for GPR23 identity; it predates LPA deorphanization and does not establish receptor function.
- id: PMID:9223435
  title: Cloning of a human heptahelical receptor closely related to the P2Y5 receptor.
  findings:
  - statement: The human receptor historically called P2Y5-like/P2Y9 was cloned as a 370-aa heptahelical GPCR.
    supporting_text: Starting from this partial sequence, we have isolated a complete clone and identified a 1113 base pair open reading frame encoding a new G-coupled receptor that we have called P2Y5-like.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Forty tested nucleotides and nucleosides failed to activate the receptor in four signaling assays, arguing against interpreting the historical P2Y name as demonstrated purinergic activity.
    supporting_text: 'None of the 40 nucleotides and nucleosides tested was able to elicit a response in any of four functional assays: inositol phosphate formation, stimulation or inhibition of cAMP formation, and extracellular acidification measured with a microphysiometer.'
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary human cloning and negative ligand-screen evidence; important for the historical P2Y9 nomenclature boundary but superseded functionally by LPA deorphanization.
- id: PMID:12724320
  title: Identification of p2y9/GPR23 as a novel G protein-coupled receptor for lysophosphatidic acid, structurally distant from the Edg family.
  findings:
  - statement: Human p2y9/GPR23 was deorphanized as LPA4 by direct, saturable 1-oleoyl-LPA binding in membranes of receptor-expressing cells.
    supporting_text: Membrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: LPAR4 discriminates among LPA molecular species, preferring 1-oleoyl-LPA in the reported rank order.
    supporting_text: Competition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: In heterologous CHO cells, LPA-bound LPAR4 elevated intracellular calcium and stimulated adenylyl cyclase.
    supporting_text: In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The original human tissue survey found LPAR4 transcript most abundant in ovary.
    supporting_text: Quantitative real-time PCR demonstrated that mRNA of p2y9/GPR23 was significantly abundant in ovary compared with other tissues.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Decisive primary human receptor study with direct ligand binding, ligand rank order, calcium and adenylyl-cyclase outputs, and tissue-expression context; signaling was measured in heterologous rodent cells.
- id: PMID:17172642
  title: LPA4/p2y9/GPR23 mediates rho-dependent morphological changes in a rat neuronal cell line.
  findings:
  - statement: LPAR4 drove Gq/11-dependent calcium mobilization in rat B103 neuroblastoma transfectants but did not alter adenylyl cyclase in that system.
    supporting_text: In B103 cells stably expressing LPA(4), we observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: LPA induced Rho-dependent neurite retraction, aggregation, and cadherin-dependent adhesion through transfected LPAR4.
    supporting_text: 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.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The study inferred LPAR4 coupling to Gq/11 and G12/13 but not Gi/o in this rat neuronal-cell context.
    supporting_text: Thus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o).
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Direct mechanistic LPAR4 signaling study in a rat neuroblastoma transfection system; valuable for coupling and Rho biology but not proof of native human neuronal function.
- id: PMID:17166850
  title: LPA(4)/GPR23 is a lysophosphatidic acid (LPA) receptor utilizing G(s)-, G(q)/G(i)-mediated calcium signaling and G(12/13)-mediated Rho activation.
  findings:
  - statement: Epitope-tagged LPAR4 showed concentration-dependent LPA responses and specific membrane binding, independently confirming it as an LPA receptor.
    supporting_text: Using epitope-tagged LPA(4), pharmacological intervention, and G protein mini-genes, we provide independent confirmatory evidence that supports LPA(4) as a fourth LPA receptor, including LPA concentration-dependent responses and specific membrane binding.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: The study attributed LPAR4-dependent cAMP increases to Gs, calcium mobilization to Gq and a pertussis-toxin-sensitive pathway, and Rho-dependent cytoskeletal outputs to G12/13.
    supporting_text: 'Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s).'
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: LPA stimulation induced internalization of epitope-tagged LPAR4 in the study system.
    supporting_text: 'Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s).'
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary mechanistic LPAR4 study using tagged receptor, pharmacological perturbation, and G-protein mini-genes. It directly informs Gs, Gq/pertussis-toxin-sensitive calcium, G12/13-Rho, and internalization capability, but the abstract does not establish endogenous native-human coupling weights.
- id: PMID:18843048
  title: Role of LPA4/p2y9/GPR23 in negative regulation of cell motility.
  findings:
  - statement: Mouse Lpar4 loss enhanced LPA-driven fibroblast migration and shifted Akt/Rac/Rho signaling, whereas receptor reconstitution reduced motility.
    supporting_text: Consistent with negative modulation of the phosphatidylinositol 3 kinase pathway by LPA(4), LPA(4) deficiency potentiated Akt and Rac but decreased Rho activation induced by LPA. Reconstitution of LPA(4) converted LPA(4)-negative cells into a less motile phenotype.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  - statement: Ectopic LPAR4 inhibited migration and invasion in human cancer cells and antagonized LPA1-driven motility in receptor-null B103 cells.
    supporting_text: In support of the biological relevance of these observations, ectopic expression of LPA(4) strongly inhibited migration and invasion of human cancer cells. When coexpressed with LPA(1) in B103 neuroblastoma cells devoid of endogenous LPA receptors, LPA(4) attenuated LPA(1)-driven migration and invasion, indicating functional antagonism between the two subtypes of LPA receptors.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  - statement: The first targeted mouse Lpar4 knockout had no gross apparent phenotype, bounding strong developmental claims from this model.
    supporting_text: Although LPA(4)-deficient mice displayed no apparent abnormalities, LPA(4)-deficient mouse embryonic fibroblasts (MEFs) were hypersensitive to LPA-induced cell migration.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary genetic and cell-biological evidence for negative regulation of motility; combines mouse knockout/MEF data with ectopic human cancer-cell assays, so neither context should be generalized to all native human tissues.
- id: PMID:20484039
  title: 'Autotaxin promotes cancer invasion via the lysophosphatidic acid receptor 4: participation of the cyclic AMP/EPAC/Rac1 signaling pathway in invadopodia formation.'
  findings:
  - statement: In fibrosarcoma cells, autotaxin-derived LPA signaled through LPAR4 to promote invadopodia through a cAMP-EPAC-Rac1 pathway.
    supporting_text: We further provide evidence that LPA(4) signaling in fibrosarcoma cells regulates invadopodia formation downstream of ATX, a process mediated through the activation of EPAC by cyclic AMP and subsequent Rac1 activation.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: LPAR4 knockdown supported a requirement for the receptor in invasion and in vivo metastasis formation in this cancer model.
    supporting_text: Results using LPA(4) shRNA support the requirement of the LPA(4) receptor for cell invasion and in vivo metastasis formation.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary disease-model evidence for an LPAR4-cAMP-EPAC-Rac1 invasion pathway. The abstract does not establish a normal-tissue role or identify the fibrosarcoma cells as a native human LPAR4 context, so the finding is kept explicitly model-bounded.
- id: PMID:20482379
  title: Strategy for the identification of GPR23/LPA4 receptor agonists and inverse agonists.
  findings:
  - statement: Human GPR23 expressed in CHO cells reproduced LPA-dependent cAMP and calcium responses.
    supporting_text: In Chinese hamster ovary cells expressing the human GPR23, LPA induced an increase in cellular cyclic adenosine monophosphate (cAMP) and calcium levels.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Orthogonal reporter and radioligand-binding screens yielded LPAR4 agonists, inverse agonists, and a negative modulator.
    supporting_text: Here we report the identification of novel GPR23 agonists, inverse agonists, and a negative modulator from 2 high-throughput screens, a beta-lactamase reporter screen, and a [3H]LPA-binding screen.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Candidate mechanisms and selectivity were tested with reporter, cAMP, and other LPA-receptor assays.
    supporting_text: Several screening hits were selected for mechanism of action studies using the beta-lactamase reporter assay and a cAMP assay. An evaluation of their selectivity at the other LPA receptors was also conducted.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary human-receptor pharmacology in engineered CHO cells; supports constitutive activity and ligand-tool development, not endogenous physiological responses.
- id: PMID:21050927
  title: Development of a GPR23 cell-based ฮฒ-lactamase reporter assay.
  findings:
  - statement: An inducible reporter assay detected constitutive GPR23/LPAR4 activity.
    supporting_text: This report describes how a tetracycline-inducible system was utilized in conjunction with a sensitive ฮฒ-lactamase reporter gene to develop an assay in which constitutive activity of the receptor could be monitored.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  - statement: Screening with the reporter identified the first reported small-molecule inverse agonists for GPR23.
    supporting_text: This assay was then utilized to screen a 1.1 million compound library to identify the first small molecule inverse agonists for the receptor.
    full_text_unavailable: true
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Methods-focused primary pharmacology paper establishing assay-detectable constitutive activity and inverse agonists in an engineered system.
- id: PMID:33160074
  title: Lysophosphatidic Acid Receptor 4 Is Transiently Expressed during Cardiac Differentiation and Critical for Repair of the Damaged Heart.
  findings:
  - statement: LPAR4 expression transiently peaks in cardiac progenitors during both mouse and human PSC differentiation.
    supporting_text: During inย vitro differentiation of mouse and human PSCs toward cardiac lineage, LPAR4 expression peaked after 3-7ย days of differentiation in cardiac progenitors and then declined.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  - statement: Sequential LPAR4 stimulation and downstream p38 inhibition enhanced cardiac differentiation in mouse and human PSC cultures.
    supporting_text: Sequential stimulation and inhibition of LPAR4 using these agents enhanced the inย vitro efficiency of cardiac differentiation from mouse and human PSCs.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  - statement: The in vivo cardiac-repair result was demonstrated in mice and should not be transferred directly to human physiology.
    supporting_text: Importantly, inย vivo, this sequential stimulation and inhibition of LPAR4 reduced the infarct size and rescued heart dysfunction in mice.
    full_text_unavailable: false
    reference_section_type: ABSTRACT
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Mixed human/mouse PSC differentiation and mouse injury study; informs developmental context and an LPAR4-responsive progenitor state but not the receptor's universal core function or native adult-human cardiac mechanism.
- id: PMID:39083597
  title: Multiplexed mapping of the interactome of GPCRs with receptor activity-modifying
    proteins.
  findings:
  - statement: The RAMP interactome screen tested every GPCR-RAMP pair by ectopic coexpression and membrane-solubilization assays.
    supporting_text: All potential GPCR-RAMP interacting pairs were expressed ectopically, solubilized and analyzed using the multiplexed suspension bead array (SBA) strategy.
    full_text_unavailable: false
    reference_section_type: INTRODUCTION
  - statement: The library screen used one biological replicate per GPCR-only or GPCR-RAMP sample, with separate detection schemes supplying the two replicates.
    supporting_text: For the DuET libraryโ€“based GPCR-RAMP interactome screen, we used one biological replicate of each of the four unique GPCR-containing samples (each GPCR alone and each GPCR with each of the three RAMPs) in two replicates. Each replicate represented one detection scheme.
    full_text_unavailable: false
    reference_section_type: RESULTS
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: IntAct/GOA reports LPAR4 partner rows for RAMP1, RAMP2, and RAMP3 from this multiplexed screen. The paper supports screen-level complex detection in engineered cells, not an endogenous LPAR4 complex or a demonstrated RAMP effect on LPAR4 trafficking or pharmacology.
- id: Reactome:R-HSA-379048
  title: Liganded Gq/11-activating GPCRs act as GEFs for Gq/11
  findings:
  - statement: Reactome places Gq/11-linked GPCRs upstream of PLC, IP3, and calcium mobilization.
    supporting_text: PLC hydrolyzes phosphatidylinositol (PIP2) to diacyl glycerol (DAG) and inositol triphosphate (IP3).
    full_text_unavailable: false
    reference_section_type: DATABASE_ENTRY
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Curated human Gq/11 pathway context consistent with LPAR4 calcium signaling; the cached summary is generic rather than direct LPAR4 experimental evidence.
- id: Reactome:R-HSA-417820
  title: P2Y9 receptor can bind to LPA
  findings:
  - statement: Reactome identifies human GPR23/P2Y9 as LPAR4 and models LPA binding with signaling through multiple G proteins.
    supporting_text: P2Y9 has been reported to bind lysophosphatidic acid (LPA) as a ligand and elicit numerous effects via multiple G proteins (Lee CW et al, 2007).
    full_text_unavailable: false
    reference_section_type: DATABASE_ENTRY
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Directly relevant curated human pathway entry; its multiple-G-protein statement cites the epitope-tagged LPAR4 coupling study PMID:17166850, so experimental context remains important.
- id: Reactome:R-HSA-749448
  title: Liganded Gq-activating GPCRs bind inactive heterotrimeric Gq
  findings:
  - statement: Reactome models the generic binding step between liganded Gq-coupled GPCRs and inactive heterotrimeric Gq.
    supporting_text: Numerous functionally unrelated GPCRs couple with the Gq G-protein subtype.
    full_text_unavailable: false
    reference_section_type: DATABASE_ENTRY
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Curated pathway-step provenance for the LPAR4-Gq model; the cached text is generic and is not direct physical-interaction evidence for LPAR4.
- id: Reactome:R-HSA-749452
  title: The Ligand:GPCR:Gq complex dissociates
  findings:
  - statement: Reactome models dissociation of activated Gq alpha from beta-gamma after receptor activation.
    supporting_text: Activated G alpha (q) and the beta:gamma dimer then participate in separate signaling cascades.
    full_text_unavailable: false
    reference_section_type: DATABASE_ENTRY
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Generic curated Gq-cycle context used for an LPAR4 pathway step; it does not independently demonstrate this molecular event for LPAR4.
core_functions:
- description: >-
    Acts at the plasma membrane as an LPA-activated GPCR. Recombinant human LPAR4
    directly produces cytosolic calcium elevation, Gs-mediated stimulation of
    adenylyl cyclase and cAMP, and a G12/13-Rho signaling branch. Pharmacological and
    G-protein mini-gene experiments additionally resolve Gq- and
    pertussis-toxin-sensitive Gi contributions to calcium mobilization. A separate
    B103 transfectant study detected Gq/11 and G12/13 but neither Gi/o coupling nor
    an adenylyl-cyclase response, showing that coupling weights and detectable branch
    outputs vary with cellular context without negating the activating Gs/cAMP
    capability. Context-opposing migration, invasion, developmental, and repair
    phenotypes are downstream outputs rather than additional core molecular
    activities.
  molecular_function:
    id: GO:0070915
    label: lysophosphatidic acid receptor activity
  directly_involved_in:
  - id: GO:0007186
    label: G protein-coupled receptor signaling pathway
  - id: GO:0007189
    label: adenylate cyclase-activating G protein-coupled receptor signaling pathway
  - id: GO:0007204
    label: positive regulation of cytosolic calcium ion concentration
  - id: GO:0035025
    label: positive regulation of Rho protein signal transduction
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - reference_id: PMID:12724320
    supporting_text: In Chinese hamster ovary cells expressing p2y9/GPR23, 1-oleoyl-LPA induced an increase in intracellular Ca2+ concentration and stimulated adenylyl cyclase activity.
  - reference_id: PMID:17166850
    supporting_text: 'Importantly, we further demonstrate new LPA-dependent activities of LPA(4) that include the following: receptor internalization; G(12/13)- and Rho-mediated neurite retraction and stress fiber formation; G(q) protein and pertussis toxin-sensitive calcium mobilization and activation of a nonselective cation conductance; and cAMP increases mediated by G(s).'
  - reference_id: PMID:17172642
    supporting_text: In B103 cells stably expressing LPA(4), we observed G(q/11)-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity.
  - reference_id: PMID:17172642
    supporting_text: Thus, our results demonstrated that LPA(4) as well as LPA(1) couple to G(q/11) and G(12/13), whereas LPA(4) differs from LPA(1) in that it does not couple to G(i/o).
- description: >-
    Directly binds lysophosphatidic acid as the extracellular ligand-recognition
    step of LPAR4 signaling. Recombinant membrane binding establishes nanomolar
    1-oleoyl-LPA binding and a defined potency order among acyl and ether LPA
    species. This specific activity does not imply nucleotide receptor activity or
    indiscriminate binding to unrelated lipid classes.
  molecular_function:
    id: GO:0035727
    label: lysophosphatidic acid binding
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - reference_id: PMID:12724320
    supporting_text: Membrane fractions of RH7777 cells transiently expressing p2y9/GPR23 displayed a specific binding for 1-oleoyl-LPA with a Kd value of around 45 nm.
  - reference_id: PMID:12724320
    supporting_text: Competition binding and reporter gene assays showed that p2y9/GPR23 preferred structural analogs of LPA with a rank order of 1-oleoyl- > 1-stearoyl- > 1-palmitoyl- > 1-myristoyl- > 1-alkyl- > 1-alkenyl-LPA.
knowledge_gaps:
- gap_statement: >-
    The quantitative Gq/11, G12/13, Gs, Gi/o, and beta-arrestin coupling profile of
    endogenous human LPAR4 is unresolved across native cell types.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    Coupling weights determine whether LPAR4 changes calcium, cAMP, Rho, or arrestin
    signaling and may explain opposing phenotypes across tissues.
  resolution: >-
    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_statement: >-
    Native LPAR4 affinity and efficacy across physiological LPA molecular species
    remain incompletely defined.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    Molecular-species selectivity could shape tissue responses and therapeutic
    pharmacology.
  resolution: >-
    Quantify binding and branch-resolved efficacy for defined LPA species in native
    human membranes and cells using LPAR4 knockout and genomic rescue controls.
- gap_statement: >-
    Why LPAR4 suppresses motility in some cancer-cell models but promotes
    invadopodia, invasion, and metastasis in fibrosarcoma is unresolved.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    Resolving the switch would distinguish receptor-intrinsic coupling bias from
    differences in receptor abundance, LPA production, and signaling-network state.
  resolution: >-
    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_statement: >-
    The extent to which mouse Lpar4 and stem-cell-culture developmental phenotypes
    predict normal human tissue functions remains unclear.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  status: OPEN
  significance: >-
    Careful conservation testing is needed before translating transient progenitor
    and mouse injury-repair effects into human physiology or therapy.
  resolution: >-
    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_statement: >-
    The physiological consequences of screen-detected LPAR4 interactions with
    RAMP1, RAMP2, and RAMP3 are unknown.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  dark_aspect: RESIDUAL_SUBGAP
  status: OPEN
  significance: >-
    RAMP-dependent modulation could explain cell-context differences, but
    screen-level contacts must not be treated as constitutive receptor components.
  resolution: >-
    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_statement: >-
    LPAR4 structural determinants, receptor-specific trafficking regulation, and
    possible endogenous proteoform variation remain largely uncharacterized.
  boundary: >-
    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.
  gap_kind:
  - BIOLOGY
  - CURATION
  status: OPEN
  significance: >-
    These missing data limit mechanistic interpretation of ligand selectivity,
    coupling bias, surface residence, and sequence-dependent functional variation.
  resolution: >-
    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.
proposed_new_terms: []
suggested_questions:
- question: >-
    What are the quantitative Gq/11, G12/13, Gs, Gi/o, and beta-arrestin coupling
    profiles of endogenous LPAR4 in physiologically relevant human cells?
- question: >-
    Which physiological LPA molecular species bind and activate native human LPAR4
    most effectively?
- question: >-
    Which signaling-network features determine whether LPAR4 suppresses or promotes
    cancer-cell motility and invasion?
- question: >-
    Which transient cardiac-progenitor and mouse injury-repair effects of LPAR4 are
    conserved in genetically controlled human cardiac models?
- question: >-
    Do RAMP1-3 alter endogenous LPAR4 surface delivery, ligand pharmacology,
    coupling bias, or trafficking?
- question: >-
    What structural features control LPA recognition and coupling by LPAR4, and how
    is the receptor phosphorylated, internalized, recycled, or desensitized?
suggested_experiments:
- hypothesis: >-
    Native LPAR4 coupling weights vary by human cell type and differ from both CHO
    and B103 heterologous assays.
  description: >-
    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.
  experiment_type: endogenous multiplexed GPCR coupling analysis
- hypothesis: >-
    Native LPAR4 retains a measurable preference for 1-oleoyl-LPA but the complete
    LPA-species rank order differs from recombinant membrane assays.
  description: >-
    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.
  experiment_type: receptor-specific native ligand pharmacology
- 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.
  description: >-
    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.
  experiment_type: context-resolved motility signaling epistasis
- hypothesis: >-
    Transient LPAR4 activity has a stage-specific role in human cardiac progenitors
    but is dispensable or different in mature cardiomyocytes.
  description: >-
    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.
  experiment_type: stage-resolved human cardiac differentiation
- hypothesis: >-
    At least one RAMP modifies LPAR4 only in human cells with native coexpression.
  description: >-
    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.
  experiment_type: endogenous RAMP genetic epistasis and receptor pharmacology
- hypothesis: >-
    Ligand-specific conformations and intracellular phosphorylation sites jointly
    determine LPAR4 coupling and surface residence.
  description: >-
    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.
  experiment_type: integrated receptor structural biology and trafficking