Recombinant Bovine Integral membrane protein GPR137 (GPR137)

What is GPR137 and what are its key characteristics?

GPR137, also known as transmembrane 7 superfamily member 1-like protein, C11orf4, or GPR137A, is a 417 amino acid member of the GPR-137 family of membrane proteins. It was initially discovered in 2003 through GenBank genomic database searches . The GPR137 family comprises three alternatively spliced isoforms: GPR137A (commonly referred to simply as GPR137), GPR137B, and GPR137C .

Unlike GPR137B, which has been studied extensively and is known to be upregulated in the kidney during development, GPR137 expression has been primarily detected in the hippocampus, but not in the hypothalamus, midbrain, thalamus, pons, or basal forebrain . It functions as an integral membrane protein and has been implicated in cellular proliferation pathways.

What expression systems are most effective for producing recombinant bovine GPR137?

For recombinant bovine GPR137 production, researchers typically employ mammalian expression systems due to their ability to properly fold and post-translationally modify complex membrane proteins. The methodology involves:

• Vector selection: Vectors containing strong promoters (e.g., CMV) and appropriate selection markers are recommended.

• Cell line selection: HEK293 or CHO cells are preferred for mammalian membrane protein expression.

• Transfection method: Lipofectamine-based transfection has shown good efficiency for GPR137-related constructs, as demonstrated in studies of human GPR137 .

• Purification approach: A two-step purification process incorporating affinity chromatography (typically using His-tag) followed by size exclusion chromatography yields the purest preparations.

For functional studies, stable cell lines expressing bovine GPR137 can be generated using lentiviral vectors. This approach was successfully employed for human GPR137 studies where lentivirus-based shRNA-expressing vectors were constructed and confirmed by DNA sequencing .

What detection methods are most reliable for bovine GPR137?

For qRT-PCR analysis, the 2^(-ΔΔCt) method is typically used for data analysis, as implemented in studies examining GPR137 expression in bladder cancer patients . When analyzing protein expression levels from Western blots, ImageJ software is commonly employed for quantification of band intensity .

How can I verify the functionality of recombinant bovine GPR137?

Functional verification of recombinant bovine GPR137 should employ multiple approaches:

• Cell proliferation assays: MTT assays can measure changes in cell proliferation upon GPR137 expression or knockdown. Studies with human GPR137 demonstrated that knockdown significantly reduced cancer cell proliferation rates .

• Colony formation assays: These assess the ability of cells expressing GPR137 to form colonies. In human gastric cancer cells, GPR137 knockdown dramatically reduced colony formation ability, with nearly no colonies observed in Lv-shGPR137-infected MGC80-3 cells compared to control groups .

• Cell cycle analysis: Flow cytometry can detect alterations in cell cycle distribution. GPR137 depletion in human gastric cancer cells led to abnormal accumulation of cells in the S phase and particularly in the G2/M phase .

• Protein marker analysis: Immunoblotting for proliferation markers like PHH3 and apoptosis markers like caspase-3 can help determine GPR137's effects on cellular processes .

What CRISPR/Cas9 strategies are most effective for bovine GPR137 knockout studies?

Based on successful strategies used for GPR137 knockout in other species, the following approach is recommended:

• gRNA design: Design 2-3 different gRNAs targeting exons of bovine GPR137. In previous studies, effective gRNAs targeted sequences with PAM sites (TGG or GGG). Example sequences used in successful GPR137 knockout studies include:

  • gRNA1: 5′-CCGGCTCTGGCCGACGCTTCGCCT-3′ (Forward)

  • gRNA1: 5′-AAACAGGCGAAGCGTCGGCCAGAG-3′ (Reverse)

  • gRNA2: 5′-CCGGAGGCATCTAGCCGGCTCCGA-3′ (Forward)

  • gRNA2: 5′-AAACTCGGAGCCGGCTAGATGCCT-3′ (Reverse)

• Verification of knockout:

  • At DNA level: Clone and sequence the targeted region

  • At mRNA level: RT-PCR with primers specific to the deleted region

  • At protein level: Western blotting using anti-GPR137 antibodies

• Clone selection: Generate single-cell clones and verify homozygous mutations using sequencing. Successful knockouts typically show frameshift mutations leading to premature termination codons .

• Rescue experiments: For specificity confirmation, construct cells that re-express GPR137 in the knockout background .

In a neuroblastoma cell line model, GPR137 knockout using CRISPR/Cas9 resulted in homozygous mutants with 5- and 37-base deficiencies accompanying frameshifts, with premature terminations observed at amino acid positions 257 and 235 .

How does bovine GPR137 expression impact cellular proliferation and cell cycle regulation?

Current evidence from studies on other mammalian GPR137 suggests that GPR137 plays a role in regulating cellular proliferation, though the exact mechanisms in bovine cells require further investigation. Based on existing research:

In neuroblastoma cells, GPR137 knockout led to increased cell numbers and higher levels of the proliferation marker PHH3, suggesting that GPR137 may actually inhibit cell proliferation in neural contexts . This contrasts with findings in cancer cells, where GPR137 appears to promote proliferation.

In gastric cancer cells, GPR137 knockdown significantly reduced cell proliferation. By day five of culture, the proliferation rate was reduced by 49.2% in AGS cells and 62.2% in MGC80-3 cells compared to control groups . This indicates a tumor-promoting effect of GPR137 in these cell types.

Cell cycle analysis reveals that GPR137 depletion leads to abnormal accumulation of cells in the S phase and G2/M phase . This suggests that GPR137 may regulate the cell cycle by influencing G2/M phase molecules that mediate microtubule and/or spindle activities.

Researchers investigating bovine GPR137 should examine both proliferative and anti-proliferative effects, as the protein's function appears to be context-dependent across different tissue types.

What signaling pathways does bovine GPR137 interact with?

Although bovine-specific pathway data is limited, evidence from other mammalian models suggests GPR137 interacts with several critical signaling pathways:

For bovine GPR137 studies, these pathways should be prioritized for investigation, particularly in the context of proliferation assays. Antibodies used successfully in previous studies include: anti-STAT3 (MAB1799, R&D Systems), anti-p-STAT3 (#9145T, Cell Signaling Technology), anti-AKT (#587F11, Cell Signaling Technology), anti-p-AKT (#9271S, Cell Signaling Technology), anti-ERK (#9102, Cell Signaling Technology), and anti-p-ERK (sc-7383, Santa Cruz) .

What are the technical challenges in expressing and purifying full-length recombinant bovine GPR137?

As a multi-pass transmembrane protein, recombinant bovine GPR137 presents several technical challenges:

• Membrane protein solubilization: Careful detergent selection is critical. Initial screening with a panel of detergents (DDM, LMNG, GDN) is recommended to identify optimal solubilization conditions.

• Protein aggregation: GPCRs tend to aggregate during purification. Strategies to minimize aggregation include:

  • Addition of cholesterol hemisuccinate (CHS) to stabilize the protein

  • Maintaining low temperatures throughout purification

  • Including glycerol (10-15%) in purification buffers

• Protein stability: GPR137 may exhibit conformational instability. Consider using:

  • Nanobodies or antibody fragments as stabilizing partners

  • Ligands (though GPR137 is an orphan receptor without known ligands)

  • Thermostability assays to optimize buffer conditions

• Expression yield: Typically low for membrane proteins. Strategies to improve yield include:

  • Codon optimization for bovine expression

  • Using inducible expression systems

  • Screening multiple cell lines for optimal expression

When designing expression constructs, include affinity tags (His8 or FLAG) and potential cleavage sites (TEV protease) for purification. For functional studies, GFP fusion constructs can aid in localization studies and expression level monitoring .

What strategies can overcome poor expression of recombinant bovine GPR137?

If facing poor expression of recombinant bovine GPR137, consider these methodological adjustments:

• Codon optimization: Adapt the GPR137 coding sequence to bovine codon usage preferences to enhance translation efficiency.

• Expression vector modifications:

  • Test different promoters (CMV, EF1α)

  • Include Kozak sequence optimization

  • Add protein stabilizing sequences (SUMO tag, thioredoxin)

• Culture condition optimization:

  • Lower culture temperature (28-30°C) during expression phase

  • Test protein expression inducers at varying concentrations

  • Supplement media with protein stabilizers (glycerol, specific amino acids)

• Alternative expression systems:

  • Insect cell (Sf9, High Five) using baculovirus

  • Cell-free expression systems specialized for membrane proteins

  • Yeast expression (Pichia pastoris) for higher yields

• Fusion partners that enhance membrane protein expression:

  • GFP (allows visualization and quantification)

  • MBP (enhances solubility)

  • Truncation constructs removing problematic domains

Successful expression of GPR137-related proteins has been achieved using lentiviral expression systems in previous studies , suggesting this approach may be valuable for bovine GPR137 as well.

How can I validate the structural integrity of purified recombinant bovine GPR137?

To ensure your purified recombinant bovine GPR137 maintains its native conformation:

• Circular Dichroism (CD) Spectroscopy: Assess secondary structure elements characteristic of GPCRs (high α-helical content). Compare spectra with other successfully purified GPCRs.

• Size Exclusion Chromatography with Multi-Angle Light Scattering (SEC-MALS): Evaluate monodispersity and molecular weight to confirm proper folding and assembly.

• Thermal Shift Assays: Measure protein stability across different buffer conditions using fluorescent dyes that bind to hydrophobic regions. Well-folded GPR137 should exhibit cooperative unfolding.

• Ligand Binding Assays: Although GPR137 is an orphan receptor, designing assays that test binding to structural analogs of other GPCR ligands may provide insights into functional integrity.

• Limited Proteolysis: Properly folded membrane proteins show distinctive proteolysis patterns compared to unfolded proteins. Compare digestion patterns with other GPCRs.

• Microscale Thermophoresis: Evaluate protein-protein interactions that may indicate proper folding and functionality.

• Negative Stain Electron Microscopy: Visualize protein particles to assess homogeneity and structural features.

These biophysical and biochemical techniques collectively provide a comprehensive assessment of recombinant bovine GPR137 structural integrity.

How can recombinant bovine GPR137 be used to study its role in cellular proliferation?

To investigate bovine GPR137's role in proliferation, implement these methodological approaches:

• Overexpression and Knockdown Systems:

  • Generate stable cell lines with inducible GPR137 expression

  • Create GPR137 knockdown models using shRNA or CRISPR/Cas9 technology

  • Compare proliferation effects in multiple cell types (fibroblasts, epithelial cells)

• Proliferation Assays:

  • MTT or WST-1 assays to quantify proliferation rates over 1-5 days

  • BrdU incorporation to measure DNA synthesis

  • Ki-67 immunostaining to identify proliferating cells

  • Colony formation assays to assess long-term proliferative capacity

• Cell Cycle Analysis:

  • Flow cytometry with propidium iodide staining to determine cell cycle distribution

  • Immunoblotting for cell cycle markers (cyclins, CDKs)

  • Phospho-Histone H3 (PHH3) staining for mitotic cells

• Signaling Pathway Analysis:

  • Western blotting for key proliferation signals (ERK, AKT, STAT3) and their phosphorylated forms

  • Pathway inhibitors to determine which signals are essential for GPR137-mediated effects

  • Co-immunoprecipitation to identify GPR137 interaction partners

Previous studies have shown that GPR137 knockdown in gastric cancer cells reduced proliferation rates by 49.2% in AGS cells and 62.2% in MGC80-3 cells compared to controls after five days of culture , providing a benchmark for expected effect sizes.

What approaches can identify potential ligands or binding partners for orphan receptor bovine GPR137?

As an orphan receptor, identifying GPR137 ligands presents a significant research challenge. Consider these approaches:

• Proximity-Based Interaction Screening:

  • BioID or APEX2 proximity labeling fused to GPR137 to identify proteins in close proximity

  • Split-GFP complementation assays to validate specific interactions

  • BRET/FRET-based screening with potential interaction partners

• High-Throughput Compound Screening:

  • Design functional assays measuring downstream signaling (Ca²⁺ flux, cAMP levels, β-arrestin recruitment)

  • Screen compound libraries against cells expressing bovine GPR137

  • Conduct concentration-response studies for hit validation

• Computational Approaches:

  • Homology modeling based on crystallized GPCRs

  • Molecular docking of virtual compound libraries

  • Analysis of binding pocket characteristics to identify potential ligand classes

• Proteomic Strategies:

  • Co-immunoprecipitation followed by mass spectrometry

  • Protein microarrays to test direct binding

  • Cross-linking mass spectrometry to capture transient interactions

• Transcriptomic Analysis:

  • Compare gene expression profiles of GPR137-expressing vs. knockout cells

  • Identify pathways affected by GPR137 manipulation

  • Infer potential upstream regulators

In designing these experiments, consider the findings that GPR137 may regulate cell cycle progression, particularly at the G2/M phase , suggesting interaction with cell cycle regulatory machinery.

How can I design experiments to investigate the contradictory roles of GPR137 in different tissue contexts?

The contradictory findings that GPR137 promotes proliferation in cancer cells but may inhibit it in neuronal cells warrants careful experimental design:

• Parallel Expression Systems:

  • Express identical bovine GPR137 constructs in multiple cell types (neuronal, epithelial, fibroblast)

  • Ensure equivalent expression levels through inducible promoters

  • Compare phenotypic outcomes under identical conditions

• Chimeric Receptor Approach:

  • Create chimeric receptors swapping domains between GPR137 and related proteins

  • Identify which domains confer tissue-specific functions

  • Test in multiple cell backgrounds

• Transcriptome and Proteome Analysis:

  • Compare GPR137-dependent gene/protein expression changes across cell types

  • Identify tissue-specific signaling partners

  • Use network analysis to map differential pathway activation

• Conditional Knockout Models:

  • Develop tissue-specific GPR137 knockout systems

  • Compare phenotypic effects across tissues

  • Analyze developmental timing of effects

• Signaling Pathway Comparison:

  • Systematically assess GPR137's effects on key signaling pathways (STAT3, AKT, ERK) across cell types

  • Identify tissue-specific pathway coupling

  • Use pathway inhibitors to determine causality

By implementing these approaches, researchers can begin to unravel the context-dependent functions of bovine GPR137 and identify the molecular basis for its apparently contradictory roles.