Recombinant Macaca fascicularis G-protein coupled receptor 15 (GPR15)

What is GPR15 and what is its significance in immunological research?

GPR15 is a G-protein coupled receptor with structural similarity to other members of the chemoattractant receptor family. Initially identified as a co-receptor for HIV and SIV, GPR15 has gained significant research attention for its involvement in immune homeostasis and inflammatory disorders .

GPR15 functions primarily as a colon-homing receptor for T cells, establishing GPR15-C10orf99 (GPR15L) as a novel signaling axis that controls intestinal homeostasis and inflammation through immune cell migration . The receptor lacks cysteines in the NH2-terminal region and third extracellular loop (typically required for optimal ligand binding in many GPCRs), but contains several Tyr and acidic residues - features shared with multiple chemokine receptors .

Methodologically, GPR15 research requires careful consideration of species differences, as expression patterns vary between humans and mice, affecting experimental design and interpretation of results.

How is recombinant Macaca fascicularis GPR15 protein produced and characterized?

Recombinant Macaca fascicularis GPR15 protein is typically expressed in mammalian cell expression systems to ensure proper folding and post-translational modifications. According to available product information, commercially available recombinant rhesus monkey GPR15 proteins are often His-tagged for purification purposes and expressed as either full-length or partial-length proteins .

The production methodology involves:

• Gene synthesis or cloning of the GPR15 coding sequence (NM_001042647) from Macaca fascicularis

• Insertion into appropriate expression vectors

• Transfection and expression in mammalian cell lines

• Purification using affinity chromatography (leveraging the His-tag)

• Quality control testing including purity assessment (>80%), endotoxin testing (<1.0 EU per μg), and functional validation

The purified protein is typically stored in PBS buffer and maintained at +4°C for short-term storage or -20°C to -80°C for long-term storage .

What are the key structural features of GPR15 that influence its function?

GPR15 exhibits several distinctive structural features that influence its function:

Understanding these structural features is essential when designing expression constructs and interpreting functional assays using recombinant GPR15 proteins.

What experimental systems are most suitable for studying GPR15 signaling?

Several experimental systems have proven effective for studying GPR15 signaling:

• Cell-based assays: HEK293 cells stably expressing GPR15 (hGPR15-HEK293A) represent a reliable system for studying receptor signaling. These cells can be used in conjunction with various readouts including calcium mobilization, cAMP inhibition, and IP1 accumulation assays .

• BRET assays: Bioluminescence resonance energy transfer (BRET) assays using Venus-Gβγ and masGRK3ct-Nluc constructs provide a sensitive method for detecting G-protein activation following GPR15 stimulation .

• HTRF-based assays: Homogenous time-resolved FRET (HTRF)-based cAMP and IP1 assays are effective for measuring downstream signaling of GPR15 through Gi/o, Gs, and Gq/11 pathways .

• Calcium imaging: For neuronal activation studies, calcium imaging using indicators like GCaMP6 has been employed to measure GPR15L-induced responses in sensory neurons .

• Whole-cell patch-clamp recording: This technique has been used to confirm GPR15L-induced action potentials in neurons, providing direct electrophysiological evidence of receptor activation .

When designing experiments, researchers should consider the use of pertussis toxin (PTX) at 100 ng/ml for 24 hours before experiments to inhibit Gi/o signaling, allowing for isolation of specific signaling pathways .

How can single-cell RNA sequencing be optimally applied to investigate GPR15 expression in tissue samples?

Single-cell RNA sequencing (scRNA-seq) offers powerful insights into cell-type-specific expression of GPR15 and its ligand in complex tissues. Based on published methodologies:

Analytical workflow for GPR15 scRNA-seq studies:

• Sample preparation: Single-cell suspensions should be prepared from fresh tissue samples (e.g., colorectal tumors and adjacent non-malignant tissue) with minimal processing time to preserve RNA integrity.

• Data preprocessing: Following sequencing, expression matrices should be log-normalized and scaled as demonstrated in studies of colorectal tumors .

• Variable gene identification: Approximately 2,000 variable genes should be identified using the variance stabilizing transformation (vst) method for downstream analysis .

• Dimensionality reduction: Principal component analysis (PCA) followed by nonlinear dimensionality reduction (t-SNE and UMAP) should be performed. The optimal number of principal components can be determined using the PCElbowPlot and JackStrawPlot functions .

• Cell clustering and annotation: FindClusters method with different resolution parameters provides flexibility in identifying cell populations, while cell annotation can leverage established ontology-based labels .

• GPR15 expression analysis: After identifying cell clusters, GPR15 and GPR15L expression can be examined across different cell types, enabling the identification of key cellular sources and potential interactions.

This approach has successfully revealed distinct expression patterns of GPR15 in immune cell populations within the tumor microenvironment, providing insights into its role in cancer biology .

What are the crucial considerations for designing functional assays to evaluate GPR15L-GPR15 interactions?

When designing functional assays to evaluate GPR15L-GPR15 interactions, researchers should consider:

• Ligand selection: GPR15L has multiple active forms. The full-length ligand demonstrates superior activation potency compared to truncated forms, but the C-terminal 11 amino acids (GPR15L C11) retain significant activity and are suitable for many assays . The GPR15L(25-81) and GPR15L(71-81) fragments have been successfully used as pharmacological tool compounds to probe GPR15 receptor signaling .

• Cell systems: GPR15 activation studies require appropriate cellular contexts. HEK293 cells stably expressing GPR15 receptor have been established through selection with G418 (500 μg/ml) and provide a reliable system for signaling studies .

• Mutagenesis approaches: Site-directed mutagenesis targeting key residues including W89, K92, I113, R172, Y182, C183, E185, K187, F257, A291, and F292 can provide insights into the molecular determinants of receptor activation. These residues have been shown to markedly reduce GPR15 activation when mutated .

• G-protein coupling analysis: Examining GPR15 coupling to different Gα proteins (representing Gi/o, Gs, Gq/11 families) is essential for a comprehensive understanding of signaling pathways. BRET-based assays using Venus-Gβγ and masGRK3ct-Nluc constructs offer sensitive detection of G-protein activation .

• Functional readouts: Multiple complementary readouts should be employed:

  • HTRF-based cAMP assays for Gi/o and Gs pathways

  • IP1 accumulation assays for Gq/11 pathways

  • Calcium mobilization assays for rapid signaling events

  • Migration assays to assess chemotactic responses relevant to GPR15's role in immune cell trafficking

• Specificity controls: Experiments should include controls to confirm specificity, such as PTX treatment (100 ng/ml, 24h pre-treatment) to inhibit Gi/o signaling .

How do species differences in GPR15 biology impact translational research?

Species differences in GPR15 biology present significant challenges for translational research:

• Expression pattern divergence: Studies have revealed distinct expression patterns of GPR15 between humans and mice. While initial reports suggested GPR15 is expressed by effector T cells (Teff) in humans and regulatory T cells (Treg) in mice, subsequent investigations have shown that human Tregs in peripheral blood express GPR15 at similar or even higher levels than Teff cells .

• Context-dependent expression: Expression patterns differ not only between species but also between tissue contexts. For example, in human colon tissues, GPR15 expression frequency can be lower in Teff cells compared to Treg cells, contradicting the simplified model of species differences .

• Disease-specific variations: In ulcerative colitis, GPR15 expression increases in both Treg and Teff cells from uninflamed regions but not from inflamed regions in human patients, highlighting context-dependent regulation .

• Cancer biology differences: In colorectal cancer, the frequency of GPR15+ Tregs (but not GPR15+ Teffs) is significantly higher in tumor sites compared to non-tumor sites in humans. Genetic deletion of GPR15 in mice reduces tumor-associated Treg infiltration and tumor development, suggesting conserved but complex roles across species .

Methodological approaches to address species differences:

• Parallel studies in multiple species: Conduct experiments with recombinant GPR15 from different species (human, mouse, and non-human primates like Macaca fascicularis) to identify conserved and divergent signaling mechanisms.

• Humanized mouse models: Consider using mice with humanized immune systems to better model human GPR15 biology.

• Cross-species validation: Validate findings from animal models in human tissues whenever possible, particularly for inflammatory and immune-related processes.

• Context-specific analysis: Analyze GPR15 expression and function in multiple tissue contexts and disease states rather than generalizing from a single experimental system.

What methodological approaches can be used to investigate GPR15 coupling selectivity toward different Gα proteins?

Investigating GPR15 coupling selectivity toward different Gα proteins requires sophisticated methodological approaches:

• BRET-based G protein activation assays: This technique measures the interaction between Venus-tagged Gβγ subunits and Nluc-tagged masGRK3ct when released from the heterotrimer following receptor activation. This approach allows for real-time monitoring of G protein activation in living cells .

• Comprehensive G protein panel testing: Studies have examined GPR15 coupling selectivity toward 11 Gα proteins representing the four families (Gi1, Gi2, Gi3, GoA, GoB, Gz, Gs, and others). This comprehensive approach reveals the coupling preference of GPR15 .

• Signaling pathway-specific assays:

  • cAMP assays: For measuring Gi/o (inhibition) and Gs (stimulation) pathway activation

  • IP1 accumulation assays: For assessing Gq/11 pathway activation

  • These HTRF-based assays provide quantitative measurements of downstream signaling events

• Pharmacological inhibition: Using G protein-specific inhibitors like pertussis toxin (PTX, 100 ng/ml, 24h pre-treatment) to selectively block Gi/o signaling, allowing for isolation of specific signaling pathways .

• Dose-response analysis: Determining potency (EC50 values) of GPR15L peptides (such as GPR15L(25-81) and GPR15L(71-81)) across different signaling pathways provides insights into biased signaling properties .

• Mutational analysis: Creating receptor mutants with altered G protein coupling interfaces to map the structural determinants of coupling selectivity.

These approaches collectively provide a comprehensive profile of GPR15 coupling preferences, which is essential for understanding its signaling mechanisms and for drug discovery efforts targeting this receptor.

How can GPR15 activation be quantitatively measured in different experimental contexts?

Quantitative measurement of GPR15 activation can be achieved through multiple complementary approaches:

• Calcium mobilization assays:

  • GCaMP6-based imaging: In neuronal studies, dissociated dorsal root ganglion (DRG) neurons expressing GCaMP6 calcium indicators have been used to detect GPR15L-induced calcium mobilization. This allows for single-cell resolution of activation responses .

  • FLIPR calcium assays: Fluorescence imaging plate reader (FLIPR) calcium assays in HEK cells heterologously expressing GPR15 provide a high-throughput method for screening activators and measuring dose-dependent responses .

• Electrophysiological recordings:

  • Whole-cell patch-clamp: This technique directly measures GPR15L-induced action potentials in neurons, providing high-resolution temporal data on receptor activation .

• G protein activation assays:

  • BRET assays: Measuring bioluminescence resonance energy transfer between Venus-tagged Gβγ and masGRK3ct-Nluc constructs provides a sensitive readout of G protein activation following receptor stimulation .

• Second messenger assays:

  • cAMP inhibition: Since GPR15 couples to Gi/o proteins, measuring the inhibition of forskolin-stimulated cAMP production provides a quantitative readout of receptor activation.

  • IP1 accumulation: For assessment of Gq/11 pathway activation .

• Dose-response relationship analysis:

  • EC50 determination for various ligands: Comparing potencies of full-length GPR15L versus fragments like GPR15L(25-81) and GPR15L(71-81) .

  • Structure-activity relationship studies: Testing engineered variants of GPR15L to identify key determinants of activation.

• Functional cellular assays:

  • Migration assays: Measuring the chemotactic responses of immune cells expressing GPR15 in response to concentration gradients of GPR15L.

  • T cell trafficking: In vivo assessment of GPR15-dependent homing of T cells to intestinal tissues.

Each method provides complementary information, and combining multiple approaches yields a comprehensive understanding of GPR15 activation mechanisms.

What are the key considerations when designing and interpreting GPR15 knockout studies in disease models?

When designing and interpreting GPR15 knockout studies in disease models, researchers should consider:

• Knockout strategy selection:

  • Global vs. conditional knockouts: Global GPR15 knockouts may have developmental effects that confound disease model interpretation. Conditional knockouts using Cre-loxP systems allow for tissue- or cell-specific deletion and temporal control of gene inactivation.

  • Complete vs. partial deletion: Confirm complete deletion of functional GPR15 through genomic, transcript, and protein analyses.

• Immune cell phenotyping:

  • T cell subset analysis: Given GPR15's role in T cell trafficking, comprehensive analysis of T cell subsets (particularly Tregs) is essential. Studies have shown that GPR15 knockout mice have increased proportions of IFN-γ and IL-17A producing cells in the lamina propria of the large intestine, indicating inflammation .

  • Trafficking assays: Assess changes in immune cell trafficking to relevant tissues, particularly the colon. When GPR15+ cells and control cells were mixed at a 1:1 ratio and then transferred into mice, a 10-fold enrichment for GPR15+ cells was observed in the large intestine, highlighting its critical role in intestinal homing .

• Disease model selection:

  • Inflammatory models: Models of inflammatory bowel disease are particularly relevant given GPR15's role in intestinal immunity. Infection with Citrobacter rodentium has been used to study GPR15 function in intestinal immune responses .

  • Cancer models: In colorectal cancer models, GPR15 deletion significantly decreases tumor-associated Treg infiltration, reduces the Treg/CD8+ T cell ratio, and diminishes tumor development .

  • Infection models: Given GPR15's role as a co-receptor for HIV/SIV, appropriate infection models may reveal additional functions.

• Data interpretation challenges:

  • Compensatory mechanisms: Absence of GPR15 may trigger compensatory upregulation of other chemokine receptors or immune pathways.

  • Species differences: Findings in mouse models may not directly translate to humans due to differences in GPR15 expression patterns between species .

  • Context-dependent effects: GPR15's roles may differ between homeostatic conditions and various disease states.

• Validation strategies:

  • Rescue experiments: Reintroduction of GPR15 should restore normal phenotypes if effects are directly attributable to GPR15 deletion.

  • Human tissue correlation: Validation of key findings in human patient samples provides translational relevance.

  • Pharmacological confirmation: Use of GPR15 antagonists should phenocopy genetic deletion if effects are receptor-specific.

What techniques can be used to study the role of tyrosine sulfation in GPR15-GPR15L interactions?

Tyrosine sulfation is critical for optimal binding of GPR15 to its ligand GPR15L. The following techniques can be employed to study this post-translational modification:

• Site-directed mutagenesis:

  • Generate tyrosine-to-phenylalanine mutations at potential sulfation sites in the N-terminal region of GPR15.

  • Compare binding affinity and signaling responses between wild-type and mutant receptors to identify critical sulfated residues.

• Mass spectrometry analysis:

  • Employ liquid chromatography-tandem mass spectrometry (LC-MS/MS) to directly identify and characterize sulfated tyrosine residues in purified recombinant GPR15.

  • Use precursor ion scanning for the marker ions of tyrosine sulfate (m/z 79 and 97) to enhance detection sensitivity.

• Sulfation inhibitors:

  • Treat GPR15-expressing cells with sodium chlorate (a competitive inhibitor of PAPS synthesis, required for sulfation) to reduce tyrosine sulfation.

  • Measure the impact on GPR15L binding and downstream signaling.

• Anti-sulfotyrosine antibodies:

  • Use anti-sulfotyrosine antibodies in Western blotting and immunoprecipitation to detect and quantify sulfated GPR15.

  • Compare sulfation levels across different cell types and experimental conditions.

• In vitro sulfation assays:

  • Express and purify recombinant tyrosylprotein sulfotransferases (TPSTs).

  • Conduct in vitro sulfation of synthetic peptides corresponding to the N-terminal domain of GPR15.

  • Analyze sulfation patterns and efficiency using mass spectrometry.

• Binding assays with differentially sulfated receptors:

  • Compare binding affinity of GPR15L to fully sulfated, partially sulfated, and non-sulfated GPR15 using surface plasmon resonance (SPR) or bioluminescence resonance energy transfer (BRET).

  • Determine EC50 values for receptor activation using functional assays.

• Co-expression with tyrosylprotein sulfotransferases:

  • Co-express GPR15 with TPST-1 and/or TPST-2 to enhance sulfation.

  • Compare receptor function with and without enhanced sulfation.

The sulfated tyrosine residues in the GPR15 N-terminus have been shown to be required for optimal binding to GPR15L, similar to their role in facilitating receptor binding of HIV/SIV . Understanding this modification is essential for accurately characterizing recombinant GPR15 proteins and interpreting functional studies.

How can molecular dynamics simulations be leveraged to understand GPR15 structure-function relationships?

Molecular dynamics (MD) simulations offer powerful approaches to understand GPR15 structure-function relationships, particularly given recent advances in structural biology:

• Simulation setup based on cryo-EM structures:

  • Use the recently determined 2.9 Å resolution cryo-EM structure of the GPR15-Gi complex bound to GPR15L C11 as a starting point for simulations .

  • Embed the receptor in a lipid bilayer that mimics the composition of cell membranes where GPR15 is naturally expressed.

  • Include explicit water molecules and appropriate ion concentrations to simulate physiological conditions.

• Ligand binding dynamics:

  • Simulate the binding process of GPR15L to GPR15, focusing on the unique V-shaped conformation adopted by the ligand within the binding pocket .

  • Investigate how key residues (W89, K92, I113, R172, Y182, C183, E185, K187, F257, A291, and F292) contribute to ligand recognition and receptor activation .

  • Compare binding dynamics of full-length versus truncated ligands to understand the molecular basis for their different potencies.

• G protein coupling simulations:

  • Model the dynamics of GPR15 coupling to different Gα proteins to understand coupling selectivity.

  • Identify key residues at the receptor-G protein interface that determine coupling preferences.

  • Simulate the conformational changes that occur during G protein activation.

• Allosteric modulation:

  • Identify potential allosteric binding sites that could be targeted for drug development.

  • Simulate the effects of virtual compounds binding to these sites on receptor conformation and function.

• Post-translational modifications:

  • Incorporate sulfated tyrosine residues in the N-terminal domain to understand how this modification affects ligand binding.

  • Compare dynamics of sulfated versus non-sulfated receptor to identify mechanism of enhanced binding.

• Membrane environment effects:

  • Investigate how different lipid compositions affect receptor conformation and dynamics.

  • Simulate potential receptor clustering or dimerization in membrane microdomains.

• Analysis techniques:

  • Root mean square deviation (RMSD) and fluctuation (RMSF) analyses to identify stable regions and dynamic elements.

  • Principal component analysis (PCA) to identify dominant modes of motion.

  • Free energy calculations to quantify binding affinities and activation barriers.

  • Hydrogen bond and salt bridge analyses to identify key stabilizing interactions.

• Integration with experimental data:

  • Validate simulation predictions with mutagenesis experiments targeting key residues identified in silico.

  • Use simulation-derived hypotheses to guide the design of modified ligands with optimized properties.

By combining MD simulations with experimental approaches, researchers can gain atomic-level insights into GPR15 function that would be difficult to obtain through experimental methods alone.