Recombinant Human G-protein coupled receptor 35 (GPR35)

What is the basic structure and sequence of recombinant human GPR35?

Recombinant human GPCR GPR35 is a full-length protein spanning amino acids 1-309, typically expressed in systems such as Escherichia coli with purities exceeding 90% . The receptor belongs to the G-protein coupled receptor 1 family and contains the standard seven-transmembrane domain architecture characteristic of GPCRs . The amino acid sequence begins with MNGTYNT and contains critical binding residues in the transmembrane domains, specifically the arginine residue at position 3.36 and tyrosine at position 3.32, which have been identified as key components of the ligand binding pocket through mutagenesis studies . These residues are particularly important for interactions with acidic ligands, a structural feature shared with other GPCRs that bind acidic ligands, such as GPR81 .

What are the known endogenous ligands for GPR35?

GPR35 functions as a receptor for multiple endogenous ligands, demonstrating versatility in its signaling capabilities. The primary endogenous ligands include:

• Kynurenic acid (KYNA) - a tryptophan metabolite that activates GPR35 with high affinity

• Lysophosphatidic acid (LPA) - a bioactive lipid mediator

• 5-hydroxyindoleacetic acid (5-HIAA) - the major serotonin metabolite that acts as a physiological ligand and plays a role in neutrophil recruitment and bacterial clearance

These ligands activate GPR35 leading to rapid and transient activation of numerous intracellular signaling pathways, making the receptor a potential integration point for multiple biological signals .

What are the primary physiological functions of GPR35?

Based on current research, GPR35 demonstrates several important physiological functions:

• Neutrophil recruitment - GPR35 plays a significant role in recruiting neutrophils to sites of inflammation, particularly through activation by 5-HIAA

• Bacterial clearance - The receptor contributes to immune defense mechanisms

• Metabolic regulation - When activated by kynurenic acid, GPR35 stimulates lipid metabolism, thermogenic activity, and anti-inflammatory gene expression in adipose tissue

• Macrophage function - Activation by lysophosphatidic acid promotes GPR35-induced signaling characterized by TNF production through ERK and NF-kappa-B activation, which subsequently induces chemotaxis of macrophages

• Lipid accumulation inhibition - GPR35 activation has been shown to block liver X-receptor-mediated lipid accumulation, suggesting a potential role in metabolic diseases

These diverse functions position GPR35 as a significant target for understanding inflammation, immunity, and metabolic regulation.

What are the optimal experimental models and assays for studying GPR35 activation?

Several experimental systems have been developed to effectively study GPR35 activation and signaling:

When implementing these assays, researchers should consider that β-arrestin-2 recruitment assays provide information independent of G-protein activation, while [35S]GTPγS and ERK1/2 assays provide insights into specific signaling pathways . For comprehensive characterization, a combination of these methods is recommended to capture the full spectrum of GPR35 signaling dynamics.

How can one differentiate between species-specific responses to GPR35 ligands?

Species selectivity of GPR35 ligands presents a significant challenge in GPR35 research. To differentiate between species-specific responses:

• Employ comparative pharmacology using the PathHunter® assay system to measure ligand potency at different species orthologs simultaneously

• Utilize mutagenesis studies targeting key binding pocket residues (such as the arginine at position 3.36 and tyrosine at position 3.32) to determine the molecular basis of species selectivity

• Consider homology modeling and ligand docking studies to predict species-specific interactions

• Use transgenic models expressing human GPR35 in mouse systems to validate in vitro findings in a more complex biological context

It's important to note that compounds like zaprinast and kynurenic acid show significant species selectivity between human and rat orthologs of GPR35 . Similarly, antagonists like CID-2745687 and ML-145 have been found to be ineffective at mouse GPR35 while showing activity at human GPR35 . These differences highlight the importance of selecting appropriate models when studying GPR35 pharmacology.

What are the established G-protein coupling profiles for GPR35 and how do they influence signaling outcomes?

GPR35 demonstrates coupling to multiple G-protein families, leading to diverse signaling outcomes:

• Gαi/o coupling:

  • Confirmed through pertussis toxin sensitivity experiments with pamoic acid and zaprinast

  • Leads to inhibition of adenylyl cyclase and reduction in cAMP levels

  • Associated with ERK1/2 phosphorylation pathways

• Gα12/13 coupling:

  • Contributes to distinct signaling outcomes

  • Potentially linked to cytoskeletal rearrangements and cell migration

The G-protein coupling profile affects experimental outcomes in several ways:

• Different ligands may preferentially activate specific G-protein pathways, potentially inducing signal bias

• The coupling profile influences which experimental readouts will effectively capture GPR35 activation

• Species differences in G-protein coupling may contribute to observed species selectivity of ligands

Current research suggests that no coherent attempt has yet been made to thoroughly assess if different ligands differentially activate these pathways, indicating an important area for future investigation .

How does GPR35 activation influence metabolic pathways, particularly in relation to lipid metabolism?

GPR35 activation has significant effects on metabolic pathways:

• GPR35 activation is capable of blocking liver X-receptor-mediated lipid accumulation, as demonstrated in multiple experimental systems

• In hepatocytes from transgenic mice expressing human GPR35, the GPR35 agonist lodoxamide prevented T0901317-induced lipid accumulation in a concentration-dependent manner

• This effect was confirmed to be GPR35-specific through:

  • Use of genome-edited HepG2 GPR35 knockout cells

  • Reintroduction of human GPR35a into knockout cells

  • Application of selective antagonists like ML-145

The mechanism involves interference with liver X-receptor pathways, which typically promote lipogenesis when activated. This suggests that GPR35 agonists may have therapeutic potential in metabolic disorders characterized by excessive lipid accumulation .

What cellular and molecular techniques are optimal for studying GPR35 in various tissue contexts?

Research into GPR35 employs several specialized techniques across different tissue contexts:

• Cell line models:

  • HepG2 cells (human liver cancer cell line) for metabolic studies

  • CRISPR-Cas9 genome editing to generate GPR35 knockout cell lines for validation studies

  • Real-time cell analysis systems (xCELLigence) to measure cellular impedance as a readout of GPR35 activation

• Primary cell isolation:

  • Isolation of primary hepatocytes from both wild-type and humanized GPR35 transgenic mice

  • Maintenance in specialized media (William's E medium) for up to 7 days

• Transgenic animal models:

  • Generation of knock-in mice expressing HA-tagged human GPR35

  • Genotyping using PCR-based methods to confirm correct genetic modification

• Analytical techniques:

  • Oil Red O staining to quantify lipid accumulation in cells

  • Real-time reverse-transcription PCR to measure gene expression changes

  • Impedance-based cellular monitoring for real-time analysis of receptor activation

These techniques allow for comprehensive characterization of GPR35 function from molecular interactions to physiological outcomes in complex systems.

What pathological conditions have been linked to GPR35 dysfunction or modulation?

GPR35 has been implicated in several pathological conditions:

• Inflammatory disorders - Based on GPR35's role in neutrophil recruitment and macrophage function

• Asthma - Evidence suggests links between GPR35 signaling and asthmatic processes

• Hypertension - GPR35 may play a role in blood pressure regulation mechanisms

• Diabetes - Metabolic functions of GPR35 suggest involvement in glucose homeostasis

• Metabolic disorders - GPR35's ability to suppress lipid accumulation indicates a potential role in conditions characterized by dysregulated lipid metabolism

These associations make GPR35 a promising target for therapeutic intervention across multiple disease states, though much work remains to fully characterize its role in each condition.

How can researchers effectively validate GPR35 as a drug target in their experimental systems?

Validating GPR35 as a drug target requires multi-layered experimental approaches:

• Target engagement confirmation:

  • Use selective agonists (e.g., lodoxamide, bufrolin) and antagonists (e.g., ML-145 for human GPR35) to demonstrate on-target effects

  • Employ receptor knockout models to confirm ligand specificity

  • Perform receptor reintroduction/rescue experiments to definitively link observed effects to GPR35

• Physiological relevance assessment:

  • Demonstrate functional outcomes in relevant cell types (e.g., immune cells for inflammation, hepatocytes for metabolic effects)

  • Use transgenic animal models expressing human GPR35 to translate in vitro findings to more complex systems

  • Establish concentration-response relationships that fall within physiologically relevant ranges

• Mechanistic validation:

  • Determine the G-protein coupling profile relevant to the disease context

  • Map downstream signaling pathways using phospho-ERK1/2 or other appropriate readouts

  • Perform structure-activity relationship studies with modified ligands to understand binding requirements

These approaches collectively provide strong evidence for GPR35 as a viable drug target in specific disease contexts.

What are the key technical challenges in developing recombinant GPR35 for structural and functional studies?

Researchers face several challenges when working with recombinant GPR35:

• Expression system selection:

  • While E. coli systems can produce recombinant GPR35 with >90% purity , GPCRs often require eukaryotic expression systems for proper folding and post-translational modifications

  • Mammalian cell systems may provide more physiologically relevant protein but at lower yields

• Structural stability:

  • As a seven-transmembrane protein, GPR35 requires detergent or lipid environments for stability outside the cell membrane

  • Finding conditions that maintain native conformation while allowing experimental manipulation presents significant challenges

• Functional assay development:

  • The multiple G-protein coupling pathways of GPR35 require diverse assay systems to fully characterize signaling

  • Species differences in pharmacology necessitate careful consideration when translating between model systems

• Ligand specificity:

  • The polypharmacology of GPR35 (responding to multiple endogenous ligands) complicates interpretation of experimental results

  • Distinguishing on-target from off-target effects requires rigorous controls and validation

Addressing these challenges requires a multidisciplinary approach combining expertise in protein biochemistry, molecular pharmacology, and cell biology.

What emerging technologies might advance our understanding of GPR35 structure-function relationships?

Several emerging technologies hold promise for advancing GPR35 research:

• Cryo-electron microscopy (cryo-EM):

  • Could reveal the three-dimensional structure of GPR35 in various activation states

  • May clarify the binding modes of different ligands and explain species selectivity

  • Could illuminate conformational changes associated with coupling to different G-proteins

• Advanced genetic engineering tools:

  • CRISPR-based screening approaches to identify new modulators of GPR35 function

  • Conditional knockout models to assess GPR35 function in specific tissues or developmental stages

  • Base editing technologies for precise modification of key residues without disrupting the entire gene

• Single-cell analysis:

  • Single-cell RNA sequencing to map GPR35 expression across diverse tissue contexts

  • Single-cell proteomics to assess GPR35 signaling pathways in heterogeneous cell populations

  • Live-cell imaging with fluorescent biosensors to monitor GPR35 activation in real-time

• Computational approaches:

  • Molecular dynamics simulations to model ligand binding and receptor dynamics

  • Artificial intelligence-based prediction of novel ligands and binding sites

  • Systems biology approaches to integrate GPR35 into broader signaling networks

These technologies will likely contribute to more detailed understanding of GPR35 biology at molecular, cellular, and physiological levels.

How might researchers design experiments to resolve contradictory findings about GPR35 signaling pathways?

To address contradictions in GPR35 signaling literature, researchers should consider:

• Systematic comparison of experimental systems:

  • Directly compare different cell types and expression systems within the same study

  • Assess the impact of receptor expression levels on signaling outcomes

  • Control for endogenous GPR35 expression that may confound results

• Comprehensive signaling profiling:

  • Employ multiple complementary assays to assess G-protein coupling (e.g., [35S]GTPγS binding, BRET, and downstream effector activation)

  • Measure β-arrestin recruitment in parallel with G-protein activation to assess potential biased signaling

  • Analyze the temporal dynamics of different signaling pathways following receptor activation

• Rigorous pharmacological validation:

  • Use multiple structurally diverse ligands to distinguish receptor-specific from ligand-specific effects

  • Include appropriate positive and negative controls, including receptor knockout conditions

  • Employ selective antagonists where available to confirm on-target effects

• Cross-species considerations:

  • Clearly differentiate between species orthologs in experimental design and reporting

  • Use humanized animal models when translating findings between systems

  • Perform comparative studies with orthologs from multiple species to identify conserved signaling mechanisms

By implementing these approaches, researchers can systematically address contradictions and develop a more coherent understanding of GPR35 signaling.

What strategies can researchers use to develop more selective tools for studying GPR35?

Developing selective tools for GPR35 research requires:

• Structure-guided design:

  • Utilize mutagenesis data on key binding residues (e.g., arginine at position 3.36 and tyrosine at position 3.32) to design ligands with enhanced selectivity

  • Apply homology modeling and docking studies to predict ligand interactions

  • Incorporate species differences in the binding pocket to create tools with defined species selectivity

• High-throughput screening approaches:

  • Screen diverse chemical libraries using β-arrestin recruitment assays, which have demonstrated high signal-to-background ratios for GPR35

  • Employ counter-screening against related GPCRs to identify truly selective compounds

  • Validate hits with orthogonal assay systems to confirm on-target activity

• Advanced molecular tools:

  • Develop GPR35-specific nanobodies or single-domain antibodies as research tools

  • Create engineered GPR35 variants with modified pharmacology for use in chemogenetic approaches

  • Design fluorescent or bioluminescent biosensors specific for GPR35 activation

• Genetic approaches:

  • Generate conditional or inducible GPR35 knockout systems for temporal control of expression

  • Develop transgenic models with reporter systems linked to GPR35 activation

  • Create cell lines with endogenous GPR35 tagged with epitopes or fluorescent proteins using CRISPR knock-in strategies

These approaches would significantly enhance the toolbox available for studying GPR35 biology with improved selectivity and precision.

What are the optimal conditions for working with recombinant human GPR35 in different experimental contexts?

For optimal results when working with recombinant human GPR35:

• Protein expression and purification:

  • Expression in E. coli systems can yield >90% pure protein suitable for SDS-PAGE and biochemical studies

  • For functional studies, mammalian expression systems may better preserve native conformation and post-translational modifications

  • Consider using affinity tags (e.g., His tag) for purification while ensuring they don't interfere with function

• Cell-based assays:

  • HEK293 cells provide a good background for transfection-based studies with GPR35

  • For metabolic studies, hepatocyte models (e.g., HepG2) have been successfully employed

  • Cell density of approximately 80% confluence before experimental manipulations appears optimal

  • For primary hepatocytes, maintenance in William's E medium supplemented with 10% FBS allows viability for up to 7 days

• Ligand handling:

  • Prepare stock solutions of hydrophobic ligands in DMSO, keeping final DMSO concentration below 0.1% in assays

  • For hydrophilic ligands like kynurenic acid, aqueous buffers may be suitable

  • Include vehicle controls in all experiments to account for solvent effects

• Signal detection:

  • For impedance-based assays, seeding at 2 × 10^4 cells/well provides appropriate sensitivity

  • For lipid accumulation studies, treatment periods of 48-72 hours appear necessary to observe GPR35-mediated effects

These conditions provide starting points for experimental design, though optimization may be necessary for specific applications.

How can researchers overcome common technical hurdles in GPR35 functional studies?

Common technical challenges and solutions include:

• Low signal-to-noise ratio:

  • β-arrestin-2 recruitment assays typically provide strong signal-to-background for GPR35

  • Consider using amplified detection systems like the TangoTM system for enhanced sensitivity

  • Optimize receptor expression levels to balance physiological relevance with detection sensitivity

• Species selectivity issues:

  • Clearly identify which species ortholog is being studied

  • Use appropriate positive controls known to be active at the specific ortholog under study

  • Consider humanized animal models when translating between systems

  • Be aware that antagonists like CID-2745687 and ML-145 are ineffective at mouse GPR35

• Assay interference:

  • Include compound-only controls to identify auto-fluorescence or other detection artifacts

  • Use orthogonal assay systems to confirm findings from primary screens

  • Consider potential off-target effects, especially at high compound concentrations

• Reproducibility challenges:

  • Standardize cell culture conditions including passage number and density

  • Develop robust positive controls for assay calibration

  • Document detailed protocols including timing of treatments and measurements

By anticipating these challenges, researchers can design more robust experiments that yield reliable and interpretable results.

What are the recommended experimental controls for validating GPR35-specific effects?

To ensure rigorous validation of GPR35-specific effects, researchers should implement the following controls:

• Genetic controls:

  • GPR35 knockout cells generated through CRISPR-Cas9 or similar techniques

  • Rescue experiments with reintroduction of GPR35 into knockout backgrounds

  • Expression of mutant GPR35 lacking key functional residues (e.g., R3.36A, Y3.32A)

• Pharmacological controls:

  • Selective antagonists (e.g., ML-145 for human GPR35) to block agonist effects

  • Structurally diverse agonists to confirm common receptor-mediated effects

  • Concentration-response relationships to establish potency consistent with receptor affinity

• Specificity controls:

  • Assessment of effects in cells lacking GPR35 expression

  • Evaluation of compound activity at related GPCRs

  • Testing of inactive structural analogs of active compounds

• Technical controls:

  • Vehicle controls (e.g., DMSO-only) to account for solvent effects

  • Positive controls with well-characterized effects (e.g., T0901317 for LXR activation)

  • Time course studies to establish appropriate treatment durations

These controls collectively provide strong evidence for GPR35-specific effects and help distinguish them from non-specific or off-target activities.

How can findings from recombinant GPR35 studies be effectively translated to physiological contexts?

Translating recombinant GPR35 studies to physiological contexts requires:

• Bridging model systems:

  • Progress from purified proteins to cell lines to primary cells to in vivo models

  • Generate transgenic mice expressing human GPR35 to better translate findings from human recombinant systems

  • Validate key findings across multiple experimental systems

• Physiological ligand concentrations:

  • Determine concentration ranges of endogenous ligands (KYNA, LPA, 5-HIAA) in relevant tissues

  • Ensure that experimental ligand concentrations fall within physiologically relevant ranges

  • Consider potential differences in local microenvironment concentrations versus systemic levels

• Contextual signaling analysis:

  • Examine GPR35 signaling in the context of relevant tissue-specific pathways

  • Consider GPR35's multiple G-protein coupling (Gi/o, Gα12/13) when designing physiological readouts

  • Assess temporal aspects of signaling that may be relevant to physiological functions

• Disease-relevant conditions:

  • Study GPR35 function under conditions that mimic relevant pathological states

  • For metabolic studies, include conditions that model lipid accumulation (e.g., T0901317 treatment)

  • For inflammatory contexts, include appropriate inflammatory stimuli when studying neutrophil recruitment

These approaches help ensure that findings from reductionist systems retain relevance when translated to more complex physiological contexts.

What are the key considerations when designing GPR35-targeted therapeutic approaches?

When considering GPR35 as a therapeutic target, researchers should address:

• Target validation:

  • Confirm GPR35 involvement in the disease pathophysiology using genetic and pharmacological approaches

  • Establish clear disease-relevant readouts that respond to GPR35 modulation

  • Validate findings across multiple models and species when possible

• Pharmacological considerations:

  • Address species differences in ligand binding when translating from preclinical to clinical studies

  • Consider signaling bias of different ligands and how this may affect therapeutic outcomes

  • Develop structure-activity relationships to optimize desired properties (efficacy, selectivity, bioavailability)

• Safety assessment:

  • Consider the broad physiological roles of GPR35 in inflammation, metabolism, and other processes

  • Assess potential on-target adverse effects based on known GPR35 functions

  • Evaluate selectivity against related GPCRs to minimize off-target effects

• Therapeutic context:

  • For inflammatory conditions, focus on GPR35's role in neutrophil recruitment and macrophage function

  • For metabolic disorders, leverage GPR35's ability to suppress lipid accumulation

  • Consider combination approaches that may synergize with GPR35-targeted interventions