Recombinant Human G-protein coupled receptor 62 (GPR62)

What is GPR62 and how is it classified among GPCRs?

GPR62 (Probable G-protein coupled receptor 62) is a member of the G protein-coupled receptor (GPCR) family, specifically classified as an orphan GPCR whose endogenous ligand remains unknown. It is a protein encoded by the GPR62 gene in humans and consists of 368 amino acids with the characteristic 7 transmembrane domain structure common to all GPCRs . GPR62 is also known as G-protein coupled receptor GPCR8 or hGPCR8 . As part of the largest membrane protein class in the human genome, GPR62 belongs to the subset of more than 140 orphan GPCRs that still require functional elucidation .

What expression patterns have been documented for GPR62?

GPR62 has been documented to be expressed in the rat and human brain . Additionally, studies have demonstrated that GPR62 is expressed in male germ cells in mice, with expression levels increasing progressively during sperm differentiation . This specific expression pattern suggests potential roles in neural functions and reproductive processes. For researchers interested in expression studies, quantitative PCR and immunohistochemistry techniques optimized for neural tissues and reproductive organs would be appropriate methodological approaches to further characterize GPR62 expression patterns across different developmental stages and physiological conditions.

How can the constitutive activity of GPR62 be experimentally measured?

The constitutive activity of GPR62 can be measured through several experimental approaches:

• cAMP Assay: Expression of increasing amounts of GPR62 has been shown to increase cAMP levels in a dose-dependent manner, indicating constitutive activity through the Gs/cAMP pathway . Researchers should use sensitive cAMP detection methods such as ELISA, FRET-based sensors, or radioimmunoassay.

• Inositol Phosphate (IP1) Measurement: Studies have demonstrated that expression of HA-tagged GPR62 in HEK293T cells leads to increased IP1 production in a dose-dependent manner (up to 40% increase), suggesting constitutive activity through the Gq/IP1 pathway . This can be measured using IP1 accumulation assays.

• BRET-based G Protein Coupling Assay: Bioluminescence Resonance Energy Transfer (BRET) assays have been used to detect the interaction between GPR62 and various G proteins. In this approach, the formation of GDP-sensitive complexes between the receptor and G proteins in permeabilized cells can be measured when apyrase is used to hydrolyze residual guanine nucleotides .

For robust experimental design, researchers should include appropriate positive controls (known constitutively active GPCRs) and negative controls (empty vector transfection) when assessing GPR62 activity.

What approaches can be used to identify potential ligands for orphan GPR62?

As GPR62 is an orphan receptor, identifying its endogenous ligand(s) remains a significant research challenge. Several methodological approaches can be employed:

• High-throughput Screening: Utilizing cell-based assays that measure downstream signaling (cAMP production, calcium mobilization, or β-arrestin recruitment) to screen compound libraries.

• Reverse Pharmacology: Expressing GPR62 in cell lines and testing candidate ligands based on phylogenetic relationships with other GPCRs or tissue expression patterns.

• Transcriptional Reporter Systems: Similar to the approach described for β2-adrenergic receptor characterization, barcoded transcriptional reporters can be used to detect GPCR activation in multiplex formats . This approach is particularly useful for orphan GPCRs where transcriptional reporters for their signaling pathways exist.

• Structure-based Virtual Screening: With advances in structural biology and computational modeling, virtual screening based on predicted binding pockets could identify potential ligands.

• Metabolomics Approaches: Analyzing biological samples from tissues with high GPR62 expression to identify molecules that activate the receptor.

Researchers should employ multiple complementary approaches and validate hits through dose-response studies and specificity testing against related GPCRs.

What is known about the role of GPR62 in oligodendrocyte development and myelination?

Recent research has identified GPR62 as a regulator of oligodendrocyte maturation and myelination processes. Key findings include:

• While GPR62 has been shown to be unnecessary for developmental myelination, it plays a significant role in remyelination following injury .

• GPR62 regulates process formation during remyelination by Gli1-expressing neural stem cells from the subventricular zone of the adult mouse brain .

• Mechanistically, GPR62 signals through the cAMP pathway to downregulate oligodendrocyte maturation .

For researchers investigating GPR62 in oligodendrocyte biology, appropriate experimental models would include primary oligodendrocyte precursor cell (OPC) cultures, organotypic slice cultures, and in vivo demyelination models such as cuprizone or lysolecithin-induced demyelination. Analysis techniques should assess both morphological changes in oligodendrocytes (process extension and complexity) and molecular markers of oligodendrocyte maturation (MBP, PLP, CNP expression).

How does GPR62 signaling affect remyelination processes?

GPR62 has been identified as a key regulator in remyelination processes following injury. The receptor signals through the cAMP pathway to downregulate oligodendrocyte maturation . This finding suggests that inhibition of GPR62 might potentially enhance remyelination capacity. Specifically, GPR62 regulates process formation in remyelination by Gli1-expressing neural stem cells from the subventricular zone of the adult mouse brain .

Researchers investigating this aspect should consider:

• Genetic Approaches: Using GPR62 knockout or conditional knockout models to assess remyelination efficiency in demyelination models.

• Pharmacological Modulation: Developing or applying inverse agonists (similar to those developed for related orphan receptors like GPR61 ) to inhibit GPR62's constitutive activity.

• Cell-specific Effects: Distinguishing between GPR62's effects on oligodendrocyte precursor cells versus mature oligodendrocytes, and its impact on other glial cells and neurons.

• Temporal Considerations: Determining whether GPR62 modulation is most effective at specific timepoints during the remyelination process.

Methodologically, combining in vivo demyelination models with cell-type-specific manipulations of GPR62 expression would provide the most informative results.

How can deep mutational scanning be applied to characterize GPR62 structure-function relationships?

Deep mutational scanning (DMS) represents a powerful approach to characterize structure-function relationships in GPCRs, and could be applied to GPR62 following methodologies similar to those used for other GPCRs like the β2-adrenergic receptor . The approach would involve:

• Library Construction: Creating a comprehensive library of GPR62 variants with single amino acid substitutions throughout the protein sequence.

• Stable Expression System: Establishing a cell line stably expressing the variant library at controlled copy numbers to avoid expression-related artifacts .

• Barcoded Transcriptional Reporter: Implementing a transcriptional reporter system that responds to GPR62-mediated signaling, with each variant linked to a unique barcode for identification .

• Multiplexed Functional Assessment: Using RNA-seq to simultaneously profile the activity of all variants in the library under basal conditions (to assess constitutive activity) and potentially in response to any identified ligands .

• Structure-Function Analysis: Mapping the functional consequences of mutations onto structural models of GPR62 to identify key regions for receptor stability, G protein coupling, and potential ligand binding sites.

This approach would be particularly valuable for GPR62 as an orphan receptor where direct structural information may be unavailable . The resulting comprehensive mutational data could work in concert with other structural biology techniques to enhance our understanding of GPR62's structure-function relationships.

What are the optimal expression systems and purification strategies for recombinant GPR62 production?

For researchers aiming to produce recombinant GPR62 for structural or biochemical studies, several methodological considerations are important:

• Expression Systems:

  • Mammalian Cell Lines: HEK293T or CHO cells are preferred for functional studies as they provide appropriate post-translational modifications and trafficking.

  • Insect Cell Systems: Sf9 or High Five cells using baculovirus expression systems are often optimal for structural biology applications, offering higher protein yields while maintaining proper folding.

  • Microbial Systems: While E. coli systems typically struggle with proper folding of GPCRs, specialized strains and fusion partners may be used for specific applications.

• Construct Design:

  • Fusion Partners: Incorporation of stabilizing proteins like BRIL (apocytochrome b562 RIL) between TM5 and TM6 can improve expression and stability .

  • Thermostabilizing Mutations: Introduction of specific mutations that enhance receptor stability without affecting function.

  • Epitope Tags: Addition of purification tags (His, FLAG, etc.) at N- or C-terminus to facilitate purification.

• Purification Strategy:

  • Detergent Selection: Careful screening of detergents (DDM, LMNG, etc.) to effectively solubilize GPR62 while preserving its functional state.

  • Lipid Supplementation: Addition of specific lipids during purification to maintain receptor stability.

  • Affinity Chromatography: Utilizing epitope tags for initial capture, followed by size exclusion chromatography for final purification.

• Quality Control:

  • Binding Assays: If ligands are identified, radioligand or fluorescent ligand binding assays to verify functionality.

  • G Protein Coupling Assays: In vitro assays with purified G proteins to confirm functional integrity.

  • Thermal Stability Assays: Differential scanning fluorimetry to assess protein stability.

The specific approach would need to be optimized empirically for GPR62, as each GPCR has unique properties affecting expression and stability.

What structural biology techniques are most promising for elucidating GPR62 structure?

Several cutting-edge structural biology techniques hold promise for elucidating the structure of GPR62:

• Cryo-Electron Microscopy (Cryo-EM):

  • Currently the most successful approach for GPCR structure determination

  • Particularly effective for active-state structures where GPR62 would be complexed with a G protein (Gs, Gi1, or G15)

  • For inactive-state structures, strategies similar to those used for other GPCRs could be employed, such as rigid fusion of stabilizing proteins (e.g., BRIL) between TM5 and TM6

• X-ray Crystallography:

  • Traditionally used for GPCR structures but requires highly stable, well-expressing constructs

  • May require identification of a high-affinity ligand (agonist, antagonist, or inverse agonist) to stabilize a particular conformation

  • Thermostabilizing mutations often necessary, which should be identified through systematic mutagenesis

• AlphaFold and Computational Modeling:

  • AlphaFold-driven approaches have shown success in GPCR construct design for structural studies

  • Homology modeling based on related GPCRs with known structures

  • Molecular dynamics simulations to predict conformational states

• NMR Spectroscopy:

  • Solution NMR for studying dynamics and ligand binding

  • Solid-state NMR for structural studies in more native-like lipid environments

• Combined Approaches:

  • Integration of experimental structural data with computational modeling

  • Deep mutational scanning data to validate and refine structural models

Each approach has specific advantages and challenges, and the optimal strategy would depend on the specific research questions and available resources.

What is the potential therapeutic relevance of targeting GPR62 in demyelinating disorders?

Given GPR62's role in downregulating oligodendrocyte maturation through cAMP signaling , it represents a potential therapeutic target for demyelinating disorders such as multiple sclerosis (MS). The therapeutic potential includes:

• Remyelination Enhancement: As GPR62 negatively regulates oligodendrocyte maturation, inhibiting GPR62 activity (via inverse agonists or antagonists) might promote remyelination in diseases characterized by myelin loss .

• Neural Stem Cell Modulation: GPR62 regulates process formation in remyelination by Gli1-expressing neural stem cells from the subventricular zone . Targeting GPR62 could potentially enhance endogenous repair mechanisms by modulating these neural stem cell populations.

• Combination Therapies: GPR62 modulation could complement existing immunomodulatory therapies for MS by addressing the remyelination aspect of the disease.

Methodologically, researchers investigating this therapeutic potential should:

• Test GPR62 modulators in established preclinical models of demyelination/remyelination (cuprizone model, EAE model, lysolecithin-induced focal demyelination)

• Evaluate both preventive and therapeutic treatment paradigms

• Assess long-term functional outcomes through electrophysiological and behavioral measurements

• Investigate potential side effects, particularly in reproductive tissues where GPR62 is also expressed

How can inverse agonists for GPR62 be developed and characterized?

Development of inverse agonists for orphan GPCRs like GPR62 presents unique challenges but can follow approaches similar to those used for related receptors like GPR61 :

• High-throughput Screening Strategy:

  • Develop cell-based assays measuring reduction of GPR62's constitutive activity (decreased baseline cAMP or IP1 levels)

  • Screen diverse chemical libraries against cells expressing GPR62

  • Counter-screen hits against related GPCRs to establish selectivity

• Structure-based Design Approach:

  • Utilize computational models or experimental structures of GPR62

  • Focus on potential allosteric binding sites, which are common targets for inverse agonists

  • Apply molecular docking to identify compounds that stabilize the inactive conformation

• Lead Optimization Process:

  • Establish structure-activity relationships through systematic chemical modifications

  • Optimize for potency, selectivity, and drug-like properties

  • Develop robust pharmacological characterization assays

• Characterization of Binding Mode and Mechanism:

  • Employ site-directed mutagenesis to identify key residues involved in inverse agonist binding

  • Use biophysical techniques (e.g., HDX-MS, NMR) to characterize conformational changes

  • Determine if inverse agonists act through orthosteric or allosteric mechanisms

• In Vivo Validation:

  • Establish appropriate pharmacokinetic properties

  • Validate effects in disease-relevant animal models

  • Assess target engagement in relevant tissues (brain penetration would be critical)

The discovery of selective inverse agonists would not only provide therapeutic candidates but also valuable chemical tools to better understand GPR62 biology.

What future research directions would advance our understanding of GPR62 biology?

Several key research directions would significantly advance our understanding of GPR62 biology:

• Identification of Endogenous Ligand(s):

  • Tissue extract fractionation and testing on GPR62-expressing cells

  • Targeted metabolomics of tissues with high GPR62 expression

  • In silico prediction based on phylogenetic relationships and binding pocket analysis

• Comprehensive Signaling Pathway Mapping:

  • Characterization of signaling beyond cAMP and IP1 pathways

  • Investigation of β-arrestin recruitment and biased signaling properties

  • Elucidation of downstream effectors in relevant cell types

• Physiological Function in Specific Cell Types:

  • Development of conditional knockout models for cell-type-specific GPR62 deletion

  • Single-cell transcriptomics to identify co-expression patterns with other signaling molecules

  • Functional studies in primary cell cultures from tissues with high GPR62 expression

• Developmental and Disease-related Expression Patterns:

  • Systematic analysis of GPR62 expression during development

  • Examination of expression changes in various pathological conditions

  • Correlation with clinical outcomes in demyelinating disorders

• Structural Biology Investigations:

  • Determination of GPR62 structure in both active and inactive conformations

  • Identification of key residues for constitutive activity

  • Investigation of potential homo- or heterodimerization with other GPCRs

• Translational Research:

  • Development of PET ligands for GPR62 to enable in vivo imaging

  • Exploration of genetic variations in human populations and their functional consequences

  • Investigation of GPR62 as a biomarker in relevant diseases

Each of these research directions would require specific methodological approaches and would collectively provide a comprehensive understanding of GPR62 biology.