Recombinant Bovine N-arachidonyl glycine receptor (GPR18)

What is GPR18 and how is it classified within the GPCR family?

GPR18 is a G-protein-coupled receptor that belongs to the orphan class A family. Despite sharing low sequence homology with cannabinoid receptors CB1R and CB2R, research suggests it may be functionally related to the endocannabinoid system. This relationship is supported by GPR18's ability to interact with certain cannabinoid ligands and its potential to form heteromers with cannabinoid receptors . GPR18 has attracted research interest for potential therapeutic applications including regulation of intraocular pressure, cancer treatment, and immune system modulation .

Where is GPR18 predominantly expressed in mammalian tissues?

GPR18 exhibits a tissue-specific expression pattern with highest mRNA levels detected in:

• Testis

• Spleen

• Lymph nodes

• Peripheral blood leukocytes

At the cellular level, GPR18 shows particularly strong expression in:

• CD8αα intraepithelial lymphocytes (high expression)

• CD8+ T cells (considerable expression)

• CD4+ T cells (moderate expression)

• Human T-Cell lymphotrophic virus-transformed cell lines

• Phytohaemagglutinin-activated CD4+ T-cells (very high expression)

Within T cell populations, GPR18 expression is maintained in both CD44hi CD62Llo effector memory and CD44hi CD62Lhi central memory CD8 T cells . Some research has suggested microglial expression, but this finding has been inconsistently replicated across studies .

What is the current evidence regarding N-arachidonyl glycine as a potential endogenous ligand for GPR18?

The status of N-arachidonyl glycine (NAGly) as an endogenous ligand for GPR18 remains controversial with contradictory experimental findings:

Supporting evidence:

• NAGly (10 μM) was identified as a hit in a bioactive lipid screen using cells stably expressing human GPR18, where it induced calcium flux

Contradictory evidence:

• A high-throughput β-arrestin-based screen failed to detect activation of GPR18 by NAGly

• Studies in rat sympathetic neurons expressing GPR18 suggest "NAGly is not an agonist for GPR18 or that GPR18 signaling involves noncanonical pathways not examined in these studies"

• Human glioblastoma cell lines endogenously expressing GPR18 showed no response to 10 μM NAGly treatment, despite responding to cannabinoid receptor ligands

These contradictions highlight the complexity of GPR18 pharmacology and suggest that: (1) NAGly may activate GPR18 only in specific cellular contexts, (2) GPR18 might signal through pathways not captured in certain experimental systems, or (3) NAGly may not be a universal GPR18 ligand.

What experimental systems are most appropriate for studying GPR18 function?

The search for reliable experimental systems to study GPR18 has proven challenging. Several approaches have been employed with varying success:

Heterologous expression systems:

• HEK cells: Commonly used but with inconsistent results; some researchers reported "disappointing inability to reproduce published studies on HEK cells expressing hGPR18"

• Rat sympathetic neurons: Used to investigate GPR18 in "a native neuronal system with endogenous signaling pathways and effectors"

• L929 cells: Mouse connective tissue cell line used to create stable lines expressing human GPR18

Endogenously expressing cells:

• Primary human glioblastoma cell lines (NZB11 and NZB19): Express GPR18 mRNA alongside CB1 (but not CB2 or GPR55)

• Mouse microglial BV-2 cell line and human endometrial HEC-1b cell line: Reported to express functional GPR18 in some studies, but GPR18 mRNA was undetectable in others

In vivo models:

• GPR18 knockout (Gpr18−/−) mice: Exhibit phenotypes particularly in CD8+ T cell populations, providing a valuable system for studying GPR18 function

• Competitive bone marrow chimeras: Created by reconstituting mice with mixtures of wild-type and GPR18-deficient bone marrow to determine cell-intrinsic effects

The inconsistent results across different experimental systems highlight the importance of using multiple approaches and carefully selecting appropriate models for GPR18 research.

How can researchers validate potential GPR18 ligands given the conflicting literature?

Due to conflicting reports about GPR18 pharmacology, a comprehensive multi-faceted approach is recommended for ligand validation:

• Multiple functional readouts:

  • Calcium flux assays

  • MAP kinase and Akt phosphorylation

  • N-type calcium channel modulation

  • β-arrestin recruitment (though this has produced negative results for some ligands)

  • Effects on inflammatory mediator production (e.g., reactive oxygen intermediates, nitric oxide, TNF-α)

• Pathway verification:

  • Pertussis toxin (PTX) sensitivity: GPR18 appears to couple to Gi/o proteins, so PTX blockade of effects supports GPR18 involvement

  • Examination of downstream effectors specific to GPR18 signaling

• Genetic approaches:

  • Comparison of ligand effects in wild-type versus GPR18-knockout systems

  • Rescue experiments with GPR18 re-expression in knockout backgrounds

• Structure-guided methods:

  • Virtual screening using validated homology models of GPR18

  • Structure-activity relationship (SAR) studies of potential ligands

• Cross-validation in multiple systems:

  • Testing in both heterologous expression systems and cells naturally expressing GPR18

  • Comparison of in vitro and in vivo effects

The contradictory nature of GPR18 pharmacology necessitates this comprehensive approach to confidently identify and validate ligands.

What signaling pathways have been associated with GPR18 activation?

GPR18 signaling remains incompletely characterized, with evidence suggesting involvement of several pathways:

G protein coupling:

• Evidence suggests coupling to Gαi/o proteins, as responses to putative GPR18 ligands show sensitivity to pertussis toxin

Downstream effectors:

• Phosphorylation of p44/42 mitogen-activated protein (MAP) kinase

• Activation of protein kinase B/Akt

• Calcium mobilization (in some but not all studies)

Functional outcomes in specific cell types:

• In endothelial cells: Activation of pathways involved in cell migration

• In immune cells: Modulation of inflammatory responses, including:

  • Suppression of reactive oxygen intermediate production

  • Inhibition of LPS-induced nitric oxide production

  • Reduction of TNF-α production

The possibility of non-canonical signaling has been proposed to explain some conflicting experimental results, suggesting GPR18 may utilize pathways beyond those typically examined in GPCR screening assays .

What is the functional significance of GPR18 in the immune system?

GPR18 plays a critical role in shaping immune cell populations, with particularly strong effects on CD8+ T cells:

Effects on CD8+ T cell populations:

• GPR18 knockout mice exhibit reduced frequencies of CD44hi CD62Llo effector memory CD8 T cells

• Particularly strong deficiency in KLRG1+ cells within the CD8 effector memory compartment

• This phenotype becomes more pronounced with age (more severe in 6-month-old vs. 2-month-old mice)

Cell-intrinsic mechanism:

• Competitive bone marrow chimera experiments demonstrate that GPR18-deficient CD8 T cells fail to generate normal frequencies of KLRG1+ effector memory cells even when developing alongside wild-type cells

• Retroviral transduction of GPR18 into GPR18-deficient bone marrow cells restores normal levels of CD8 effector memory and KLRG1+ cells

Molecular markers affected:

• GPR18-deficient CD8 effector memory cells show reduced expression of Granzyme B, a cytotoxic mediator

• T-bet (Tbx21) expression is maintained in the remaining KLRG1+ cells, suggesting GPR18 acts downstream of or parallel to T-bet

Specificity of immune effects:

• No significant alterations in CD4 T cell populations

• Naive and central memory CD8 T cell frequencies are unaffected

Table 1: Key Phenotypes in GPR18-Deficient Mice

These findings establish GPR18 as an important regulator of CD8 effector T cell populations, particularly the KLRG1+ subset with cytotoxic potential.

What approaches have been most successful for modeling the structure of GPR18?

Due to challenges in obtaining experimental structures of GPCRs, computational modeling has been crucial for understanding GPR18 structure. A comprehensive approach has employed multiple complementary methods:

Modeling approaches employed:

• Template-based homology modeling (the "classical method")

• Threading methods (identifying structural templates based on fold recognition)

• Ab initio modeling using RoseTTAFold and trRosetta

• AlphaFold2-generated models as reference standards

Model selection and refinement:

• From a larger initial set, 15 homologous models were selected for optimization:

  • 3 structures from classical homology modeling

  • 4 models generated using threading methods

  • 7 models from ab initio approaches

  • 1 AlphaFold2 reference model

• Selected models underwent geometric and energy optimization processes

Evaluation criteria:

• Numerical quality parameters (C-score, Ramachandran plot statistics)

• Visual assessment of structural features

• Functional evaluation through enrichment tests measuring ability to recognize known active ligands

The most reliable GPR18 models were those that performed well across all evaluation metrics, particularly those demonstrating the ability to recognize ligands in virtual screening assays. The inclusion of AlphaFold2 models represents the cutting edge of structure prediction approaches for this challenging receptor.

How can structural models of GPR18 be leveraged for drug discovery?

Validated structural models of GPR18 provide valuable tools for structure-based drug discovery approaches:

Applications of GPR18 structural models:

• Virtual screening campaigns:

  • Docking of compound libraries to identify potential ligands

  • Enrichment of active compounds from diverse chemical libraries

• Binding site characterization:

  • Identification of key residues involved in ligand recognition

  • Understanding the molecular basis for ligand selectivity

• Structure-activity relationship studies:

  • Rational design of derivatives based on known ligands

  • Optimization of ligand properties (potency, selectivity, physicochemical characteristics)

• Allosteric modulator discovery:

  • Identification of binding sites distinct from the orthosteric site

  • Design of compounds that modify receptor function without competing with endogenous ligands

• Investigation of receptor dynamics:

  • Molecular dynamics simulations to understand conformational changes

  • Prediction of activation mechanisms

For maximum reliability, drug discovery campaigns should utilize consensus results from multiple high-quality models rather than relying on a single structural prediction, particularly given the challenges in modeling GPCRs accurately.

How can researchers address the contradictory findings regarding GPR18 pharmacology?

The contradictory findings in GPR18 research represent a significant challenge requiring systematic investigation:

Strategies to address contradictions:

• Standardized experimental protocols:

  • Development of consensus assay systems and conditions

  • Detailed reporting of methodological parameters

  • Multi-laboratory validation studies

• Comprehensive pharmacological profiling:

  • Testing putative ligands across multiple functional assays

  • Dose-response relationships rather than single-concentration tests

  • Examination of receptor residence time and signaling kinetics

• Consideration of cellular context:

  • Investigation of required cofactors or interacting proteins

  • Evaluation of receptor expression levels and localization

  • Assessment of signaling machinery present in different cell types

• Genetic approaches:

  • CRISPR-based receptor knockout and rescue experiments

  • Point mutations to identify critical residues for ligand interaction

  • Chimeric receptor studies to delineate functional domains

• Investigation of receptor heterogeneity:

  • Post-translational modifications affecting ligand binding or signaling

  • Alternative splicing variants with different pharmacological profiles

  • Species differences in receptor structure and function

By systematically addressing these factors, researchers may resolve current contradictions and develop a more coherent understanding of GPR18 pharmacology.

What are the most promising therapeutic applications for GPR18 modulation?

Based on current knowledge of GPR18 biology, several therapeutic avenues warrant investigation:

Potential therapeutic applications:

• Immune system modulation:

  • The critical role of GPR18 in CD8+ effector memory T cell populations suggests potential for immunotherapeutic applications

  • GPR18 ligands might enhance cytotoxic T cell responses in cancer immunotherapy

  • Alternatively, GPR18 antagonists might dampen inappropriate CD8+ T cell responses in autoimmunity

• Anti-inflammatory applications:

  • GPR18 activation has been shown to suppress production of inflammatory mediators including:

    • Reactive oxygen intermediates

    • Nitric oxide

    • TNF-α

  • This suggests potential utility in inflammatory conditions

• Regulation of intraocular pressure:

  • GPR18 has been implicated in the regulation of intraocular pressure

  • This suggests potential applications in glaucoma treatment

• Cancer therapies:

  • GPR18 has been identified as a subject of research interest for cancer applications

  • The specific mechanisms warrant further investigation

• Endothelial cell functions:

  • GPR18 activation stimulates pathways involved in endothelial cell migration

  • This suggests potential roles in angiogenesis, wound healing, or vascular repair

Advancing these therapeutic applications will require resolving current contradictions in GPR18 pharmacology and developing reliable, selective modulators of receptor function.