Recombinant Mouse Integral membrane protein GPR137 (Gpr137)

What is GPR137 and what are its basic molecular characteristics?

GPR137 (G protein-coupled receptor 137) is an orphan receptor belonging to the G protein-coupled receptor family. Mouse GPR137 is a 396-amino acid integral membrane protein with multiple transmembrane domains characteristic of GPCRs . It is ubiquitously expressed, including significant expression in the central nervous system (CNS) . Current research indicates that GPR137 plays important roles in cellular proliferation and differentiation, particularly in neuronal cells, and has been implicated in several cancer types .

The protein has a molecular weight of approximately 45 kDa and contains several transmembrane domains that anchor it within the cell membrane. The full amino acid sequence of mouse GPR137 has been determined and is available for research applications, with specific functional domains still being characterized through knockout studies .

How does GPR137 expression vary across normal and disease states?

Research indicates that GPR137 expression patterns differ significantly between normal and pathological tissues. In bladder cancer studies, GPR137 mRNA and protein expression levels in tumor tissues (1.83 ± 0.33) were significantly higher than those in adjacent normal tissues (1 ± 0.09) . This differential expression appears to correlate with disease progression, as GPR137 levels at stages I-II and III-IV were significantly elevated compared to adjacent normal tissues, although no significant difference was observed between early and late-stage tumors .

In normal tissues, GPR137 demonstrates ubiquitous expression with important functional roles in the central nervous system, where it appears to regulate the balance between neuronal proliferation and differentiation . Immunohistochemical analyses have shown that in bladder cancer tissue sections, GPR137 expression was strongly positive in approximately 80.9% of samples, compared to weak expression in only 30.9% of normal tissue samples .

Gender-based analysis has revealed that male bladder cancer patients exhibit significantly higher GPR137 expression than female patients (p = 0.023), suggesting potential hormonal or sex-linked regulation of this protein .

What expression systems are most effective for producing recombinant GPR137?

Based on published research, E. coli has been successfully employed as an expression system for producing recombinant mouse GPR137 protein . When selecting an expression system, researchers should consider several important factors:

For bacterial expression:

• E. coli systems have demonstrated success with full-length mouse GPR137 (1-396 amino acids)

• N-terminal His-tagging facilitates efficient purification via affinity chromatography

• Expression in E. coli yields protein suitable for structural studies and antibody production

For mammalian expression (functional studies):

• Vectors incorporating strong promoters like EF1-alpha provide robust expression

• Lipofectamine-mediated transfection has been effective for introducing GPR137 constructs

• Stable cell lines can be established through limited dilution of transfected cells

The choice between expression systems should be guided by research objectives. For studies requiring properly folded, functionally active protein, mammalian expression systems may be preferable despite lower yields. For applications where higher quantities of protein are needed (antibody production, structural studies), bacterial expression systems offer advantages in terms of scalability and cost-effectiveness.

What are the optimal protocols for generating and validating GPR137 knockout models?

CRISPR/Cas9 technology has emerged as the method of choice for generating GPR137 knockout models. Based on published protocols, the following approach is recommended :

Guide RNA Design and Validation:

• Design GPR137-specific guide RNAs targeting exonic regions. Previously validated sequences include:

  • gRNA No.1: Forward: 5′-CCGGCTCTGGCCGACGCTTCGCCT-3′; Reverse: 5′-AAACAGGCGAAGCGTCGGCCAGAG-3′ (PAM sequence: TGG)

  • gRNA No.2: Forward: 5′-CCGGAGGCATCTAGCCGGCTCCGA-3′; Reverse: 5′-AAACTCGGAGCCGGCTAGATGCCT-3′ (PAM sequence: GGG)

• Use CRISPR design tools to minimize off-target effects

Knockout Generation and Validation:

• Transfect target cells with CRISPR/Cas9 constructs containing validated gRNAs

• Isolate single-cell clones through limited dilution to establish monoclonal knockout lines

• Confirm knockout using multiple complementary methods:

  • Genomic DNA sequencing to verify indels or frameshift mutations

  • RT-PCR with primers flanking the targeted region to confirm absence of wild-type mRNA

  • Western blot analysis to verify absence of GPR137 protein expression

Rescue Experiments:
To confirm that observed phenotypes are specifically due to GPR137 deletion, rescue experiments are essential:

• Amplify the full open reading frame of mouse GPR137 cDNA using PCR with appropriate primers (e.g., Forward: 5′-GAGGAAGAAGCCTCCCAATC-3′ and Reverse: 5′-CACCTGGGAGAAGAGCAGAG-3′)

• Clone the amplified sequence into an expression vector with a strong promoter (e.g., pEF6/V5-His vector)

• Transfect the construct into knockout cells and select stable transfectants

• Verify GPR137 re-expression via Western blot analysis

This comprehensive approach ensures the generation of reliable knockout models and enables accurate attribution of observed phenotypes to GPR137 deletion.

What are the recommended storage and handling protocols for recombinant GPR137?

For optimal stability and experimental reproducibility, recombinant GPR137 requires specific storage and handling conditions. The following guidelines are based on established protocols :

Receipt and Initial Storage:

• Upon receipt, store lyophilized GPR137 powder at -20°C to -80°C

• Briefly centrifuge the vial before opening to ensure all material is at the bottom

Reconstitution Procedure:

• Reconstitute the protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• For long-term storage, add glycerol to a final concentration of 5-50% (50% is recommended as the default)

• Divide into small working aliquots to minimize freeze-thaw cycles

Storage Recommendations:

• For long-term storage: Keep aliquots at -20°C or preferably -80°C

• For short-term use (up to one week): Working aliquots can be stored at 4°C

• Avoid repeated freeze-thaw cycles as they significantly reduce protein activity

Buffer Conditions:

• Optimal storage buffer: Tris/PBS-based buffer containing 6% Trehalose, pH 8.0

• Trehalose functions as a cryoprotectant and stabilizing agent

Handling for Experiments:

• Allow protein to equilibrate to room temperature before opening tubes

• Maintain sterile conditions to prevent microbial contamination

• For dilutions, use buffers compatible with downstream applications

Following these guidelines will help ensure that recombinant GPR137 maintains its structural integrity and functional properties throughout experimental procedures.

How does GPR137 regulate neuronal differentiation and proliferation?

GPR137 plays a crucial role in the balance between neuronal cell proliferation and differentiation, acting as a molecular switch that promotes cell cycle exit and neuronal maturation. Research using GPR137 knockout neuro2A cell models has revealed several key mechanisms :

Cell Proliferation Regulation:
GPR137 functions as a negative regulator of cell proliferation, as evidenced by:

• Increased cell numbers in GPR137 knockout cells compared to wild-type controls in MTT assays

• Elevated expression of PHH3 (phosphorylated histone H3), a marker of proliferation, in knockout cells

• Upregulation of cyclin D1, a key cell cycle regulator, following GPR137 deletion

Neuronal Differentiation Promotion:
GPR137 positively regulates neuronal differentiation through multiple mechanisms:

• GPR137 knockout cells exhibit decreased neurite outgrowth, a morphological indicator of neuronal differentiation

• Reduced expression of NeuroD1, a critical neuronal differentiation marker, in knockout cells

• Lower expression levels of neurite outgrowth markers STAT3 and GAP43 in the absence of GPR137

Molecular Pathway Integration:
GPR137 appears to integrate multiple signaling pathways to coordinate the proliferation-differentiation balance:

• It modulates cell cycle machinery through regulation of cyclin D1 expression

• GPR137 influences neuronal-specific transcription factors like NeuroD1

• It activates cytoskeletal remodeling pathways necessary for neurite extension

• The receptor potentially regulates STAT3 signaling, which is critical for neuronal development

Importantly, rescue experiments involving re-expression of GPR137 in knockout cells reversed all these phenotypes, confirming that these effects are specifically mediated by GPR137 . These findings suggest that GPR137 may play an important role in brain development by promoting the transition from neural progenitor proliferation to post-mitotic neuronal differentiation.

What is the significance of GPR137 as a biomarker in cancer research?

GPR137 has emerged as a promising biomarker in cancer research, with particularly strong evidence for its prognostic value in bladder cancer. Multiple lines of investigation have established its significance :

Differential Expression in Cancer:
Quantitative analyses demonstrate substantial GPR137 upregulation in cancer tissues:

• qRT-PCR shows 1.83-fold higher expression in bladder cancer tissues compared to adjacent normal tissues (p < 0.001)

• Immunohistochemistry reveals strong GPR137 positivity in 80.9% of bladder cancer samples versus only 30.9% of normal tissues

• Western blot analysis confirms significant protein-level overexpression in tumor samples

Correlation with Clinical Parameters:
GPR137 expression associates with several clinicopathological features:

• Tumor size (p = 0.006): Higher expression correlates with larger tumors

• TNM stage (p = 0.012): Advanced stages show increased expression

• Gender differences (p = 0.023): Male patients exhibit higher GPR137 expression than females

Prognostic Significance:
Most importantly, GPR137 expression strongly predicts patient outcomes:

The table below summarizes the relationship between GPR137 expression and clinicopathological features in bladder cancer:

These findings collectively establish GPR137 as both a biomarker for cancer detection and a prognostic indicator for patient outcomes. The consistent association with tumor aggressiveness suggests GPR137 may play functional roles in cancer progression, making it a potential therapeutic target worthy of further investigation .

What molecular techniques are most effective for studying GPR137 signaling pathways?

Investigating GPR137 signaling pathways requires a multifaceted approach utilizing complementary molecular techniques. Based on published research, the following methodologies are particularly effective:

Gene Manipulation Techniques:

• CRISPR/Cas9-mediated knockout:

  • Enables complete elimination of GPR137 expression

  • Allows identification of downstream effects via comparative analyses

  • Can target specific domains to dissect structure-function relationships

• RNA interference (RNAi):

  • siRNA or shRNA targeting GPR137 for transient or stable knockdown

  • Permits dose-dependent reduction in expression to identify threshold effects

  • Useful for time-course studies of GPR137 depletion

Protein Expression and Interaction Analysis:

• Western blotting with phospho-specific antibodies:

  • Monitors activation of downstream signaling components (e.g., STAT3)

  • Can detect changes in cell cycle regulators like cyclin D1

  • Effective for analyzing expression of differentiation markers like NeuroD1

• Co-immunoprecipitation:

  • Identifies direct protein interaction partners of GPR137

  • Can be coupled with mass spectrometry for unbiased interactome analysis

  • Helps establish direct versus indirect signaling connections

Functional Assays:

• Proliferation assays:

  • MTT assay has been successfully used to quantify GPR137's effects on cell proliferation

  • PHH3 immunostaining provides a marker of actively dividing cells

• Differentiation assessments:

  • Neurite outgrowth measurements to quantify neuronal differentiation

  • Immunostaining for differentiation markers (NeuroD1, GAP43)

Rescue Experiments:
These are essential for confirming specificity of observed phenotypes:

• Re-expression of wild-type GPR137 in knockout cells

• Domain-specific mutants to map functional regions

• Point mutations to disrupt specific interactions

Transcriptomic and Proteomic Analyses:

• RNA-seq after GPR137 manipulation:

  • Identifies global transcriptional changes

  • Helps establish direct and indirect targets

  • Enables pathway enrichment analysis

• Phosphoproteomics:

  • Maps kinase activation patterns downstream of GPR137

  • Identifies key nodes in signaling networks

  • Reveals potential therapeutic targets

By integrating these complementary approaches, researchers can build comprehensive models of GPR137 signaling networks and their functional consequences in different cellular contexts.

How should researchers interpret conflicting results in GPR137 studies?

When faced with conflicting results in GPR137 research, scientists should employ a systematic approach to data interpretation that considers multiple contextual factors:

Methodological Differences:
Experimental approach variations often underlie apparently contradictory findings:

• Cell/tissue specificity: GPR137 function appears to be context-dependent, with potentially different roles in neuronal versus cancer cells

• Knockout strategies: Complete deletion versus partial knockdown may produce different phenotypes

• Expression levels: Overexpression may trigger different signaling pathways than physiological expression

Temporal Considerations:
The timing of manipulation and analysis can significantly impact results:

• Acute versus chronic GPR137 manipulation may reveal different aspects of its function

• Developmental stage differences may account for varied outcomes in similar models

• Compensatory mechanisms may mask phenotypes in long-term studies

Reconciliation Strategies:
To resolve apparent conflicts in the literature, researchers should:

• Perform comprehensive rescue experiments, as demonstrated in GPR137 knockout studies where re-expression restored normal proliferation and differentiation phenotypes

• Use multiple complementary approaches to assess the same endpoint (e.g., proliferation measured by MTT assay, PHH3 expression, and cyclin D1 levels)

• Conduct dose-response studies to identify potential threshold effects

• Consider biphasic effects where GPR137 may promote opposite outcomes at different expression levels

Integration Framework:
A useful approach is to develop an integrated model that can accommodate seemingly contradictory findings:

• GPR137 may function differently in normal development versus pathological contexts

• Different downstream effectors may predominate in different cell types

• The receptor may engage in cross-talk with tissue-specific signaling networks

By carefully analyzing methodological differences and integrating findings across studies, researchers can develop a more nuanced understanding of GPR137 function that accommodates apparently conflicting results.

What statistical approaches should be used when analyzing GPR137 expression data?

For Comparing Expression Levels:

• Two-sample comparisons:

  • Student's t-test is appropriate for comparing GPR137 expression between two groups (e.g., tumor vs. normal tissue)

  • Report results with exact p-values and include measures of variation (SEM)

• Multiple group comparisons:

  • One-way ANOVA with post-hoc tests (Newman-Keuls has been successfully employed)

  • Two-way ANOVA followed by Tukey's test for experimental designs with two independent variables

For Survival Analysis:

Data Reporting Standards:

• Express data as mean ± SEM for clarity and consistency with published literature

• Consider p-values < 0.05 as statistically significant, following biomedical research conventions

• Use Graph Pad Prism or similar statistical software for analysis and visualization

Sample Size and Power Considerations:

• For clinical studies: The bladder cancer study examining GPR137 included 110 patients, providing sufficient power to detect significant associations

• For cell-based studies: Perform experiments in triplicate and repeat independently at least three times

Advanced Statistical Approaches:

• Consider multivariate analysis when examining GPR137 alongside other biomarkers

• Employ receiver operating characteristic (ROC) curves to determine optimal cut-off values for high versus low GPR137 expression

• Use hierarchical clustering for pattern identification in complex datasets

Adhering to these statistical approaches ensures robust analysis of GPR137 expression data, facilitating reliable interpretation and comparison with existing literature.

How can researchers differentiate between direct and indirect effects of GPR137?

Distinguishing between direct and indirect effects of GPR137 is crucial for understanding its precise molecular functions. Researchers should employ a multi-faceted approach:

Rescue Experiments:
The gold standard for establishing direct causality is performing rescue experiments:

• Re-expression of GPR137 in knockout cells should reverse phenotypes directly caused by GPR137 deletion

• Research has demonstrated that re-expression of GPR137 in knockout neuro2A cells restored normal proliferation and differentiation, confirming these as direct effects

• Domain-specific mutants can help map functional regions responsible for specific effects

Temporal Analysis:
The timing of molecular events following GPR137 manipulation provides valuable information:

• Immediate responses (minutes to hours) are more likely to represent direct effects

• Delayed changes (days) typically reflect secondary or tertiary consequences

• Time-course experiments can help establish the sequence of events and identify primary versus secondary effects

Molecular Proximity and Interaction Studies:
Direct physical interactions provide strong evidence for direct effects:

• Co-immunoprecipitation to identify direct binding partners

• Proximity ligation assays to detect close associations in intact cells

• FRET/BRET approaches to measure protein-protein interactions in real-time

Signaling Pathway Dissection:
Pharmacological and genetic approaches can help delineate signaling relationships:

• Specific inhibitors of suspected downstream pathways can help determine if effects are mediated through those pathways

• Combinatorial knockdown/knockout experiments (GPR137 plus potential mediators) can reveal epistatic relationships

• Phosphorylation status of putative downstream targets following GPR137 activation/inhibition helps establish directness of connections

Transcriptomic Analysis:
Gene expression changes following GPR137 manipulation can be categorized:

• Primary response genes (early changes, often independent of protein synthesis)

• Secondary response genes (requiring protein synthesis, often regulated by primary response genes)

• Bioinformatic analysis of promoter regions can identify enrichment of specific transcription factor binding sites

By integrating these complementary approaches, researchers can construct well-supported models of which cellular processes are directly regulated by GPR137 versus those that represent downstream consequences of its activity.

What are the most promising therapeutic applications of GPR137 research?

Based on current understanding of GPR137 biology, several promising therapeutic applications are emerging:

Cancer Therapeutics:
Given GPR137's role as a prognostic marker in bladder cancer, it presents several therapeutic opportunities:

• Development of small molecule antagonists to inhibit GPR137 signaling in tumors with overexpression

• Use of GPR137 expression levels for patient stratification in clinical trials

• Combination therapies targeting GPR137 alongside established cancer treatments

• Potential for developing GPR137-targeted antibody-drug conjugates for precise tumor targeting

Neurodevelopmental Applications:
GPR137's role in neuronal differentiation suggests applications in neurodevelopmental disorders:

• Modulation of GPR137 activity to promote neuronal differentiation in conditions characterized by defective neuronal development

• Potential therapeutic target for promoting neuronal regeneration after injury

• Application in stem cell-based therapies to direct differentiation toward neuronal lineages

Diagnostic and Prognostic Tools:
GPR137 expression has significant potential as a clinical biomarker:

• Development of immunohistochemical assays for tumor prognostication

• Inclusion in multi-biomarker panels for improved cancer detection and classification

• Liquid biopsy applications for non-invasive detection of GPR137 expression in circulating tumor cells or exosomes

Drug Discovery Platform:
Identification of GPR137's endogenous ligand would enable:

• Development of high-throughput screening assays for novel modulators

• Structure-based drug design targeting specific receptor conformations

• Allosteric modulators that could fine-tune rather than completely activate or inhibit receptor function

These therapeutic applications represent promising avenues for translating GPR137 basic research into clinical benefits. As our understanding of GPR137 biology continues to expand, additional therapeutic opportunities are likely to emerge.

What key methodological advances would accelerate GPR137 research?

Several methodological advances would significantly enhance our understanding of GPR137 biology and accelerate its research applications:

Ligand Identification Technologies:
Deorphanizing GPR137 represents a critical research priority:

• Development of more sensitive screening approaches for identifying endogenous ligands

• Application of computational methods to predict potential ligands based on receptor structure

• Creation of biosensor cell lines with real-time GPR137 activation readouts for high-throughput screening

• Metabolomic approaches to identify natural ligands in tissues where GPR137 functions

Advanced Structural Biology Techniques:
Determining GPR137's three-dimensional structure would enable structure-based drug design:

• Cryo-electron microscopy of GPR137 in various conformational states

• X-ray crystallography of stabilized receptor constructs

• NMR-based approaches to study GPR137 dynamics and ligand interactions

• Computational modeling to predict binding sites and conformational changes

Improved Genetic Models:
More sophisticated in vivo models would facilitate physiological studies:

• Conditional and inducible GPR137 knockout mice for tissue-specific and temporal studies

• Knockin models expressing tagged GPR137 for in vivo localization studies

• Patient-derived xenograft models expressing varying levels of GPR137

• CRISPR-based precise editing to introduce specific mutations identified in disease states

Single-Cell Analysis Technologies:
Understanding cell-specific GPR137 functions requires advanced single-cell approaches:

• Single-cell RNA-seq to identify cell populations with differential GPR137 expression

• Single-cell signaling analysis to measure GPR137 activity at the individual cell level

• Spatial transcriptomics to map GPR137 expression within complex tissues

• Live-cell imaging of GPR137 trafficking and signaling with improved resolution

Translational Research Tools:
Bridging basic and clinical research requires specialized methodologies:

• Development of standardized GPR137 assays for patient sample analysis

• Creation of patient-derived organoids to study GPR137 function in human disease contexts

• Improved biomarker detection methods for minimally invasive GPR137 assessment

• Systems biology approaches to integrate GPR137 data across multiple biological levels

These methodological advances would collectively accelerate GPR137 research by providing new tools to address current knowledge gaps and explore emerging therapeutic applications.