Recombinant Mouse Integral membrane protein GPR137B (Gpr137b)

What is GPR137B and what is its cellular localization?

GPR137B (G-protein-coupled receptor 137B) is a lysosomal transmembrane protein that functions as a GPCR-like activator of lysosomal Rag and mTORC1 signaling. Studies have confirmed that GPR137B-YFP (yellow fluorescent protein) colocalizes with the lysosomal marker Lamp2 and with mTOR after amino acid stimulation . For robust localization studies, researchers should employ confocal microscopy with lysosomal markers such as Lamp1 or Lamp2, alongside mTOR co-staining to verify proper localization of recombinant GPR137B.

What is the functional role of GPR137B in cellular signaling?

GPR137B serves as a key regulator in amino acid-mTORC1-autophagy signaling pathways. It activates lysosomal Rag GTPases and regulates the dynamic exchange of active Rags at lysosomes . Knockdown studies demonstrate that GPR137B deficiency leads to enhanced autophagy in macrophages and prevents amino acid-induced mTORC1 signaling activation . For experimental studies, amino acid starvation and refeeding protocols (10-12 hours starvation) provide an effective model to observe GPR137B-mediated effects on mTORC1 translocation and downstream signaling events.

What are the known homologs of GPR137B?

GPR137B belongs to a family that includes two main homologs: GPR137 and GPR137C. These proteins are also localized to the lysosome and appear to have similar roles in regulating mTORC1 signaling. Studies have shown that both GPR137 and GPR137C increase mTOR translocation to lysosomes when overexpressed . When designing knockout experiments, researchers should consider potential compensatory mechanisms by these homologs, which may mask phenotypes in single-gene knockout models.

How is GPR137B expression regulated in different cell types?

GPR137B shows differential expression patterns, with notable expression in macrophages, particularly RAW264 cells . It is significantly upregulated in advanced atherosclerotic plaques . For accurate expression analysis, researchers should use:

How can recombinant mouse GPR137B be produced for functional studies?

For functional studies requiring recombinant GPR137B:

• Select an appropriate expression system: mammalian cells (HEK293, CHO) offer proper folding and post-translational modifications essential for membrane proteins

• Design constructs with optimal tags (e.g., FLAG, GFP) positioned to avoid interference with protein function

• Consider using inducible expression systems for proteins that may affect cell viability

• For purification, use mild detergents (DDM, CHAPS) to maintain native conformation

• Validate recombinant protein functionality by testing its ability to restore phenotypes in GPR137B-knockout cells

What experimental techniques are optimal for studying GPR137B-regulated pathways?

The study of GPR137B-regulated pathways requires multiple complementary approaches:

• For mTORC1 signaling:

  • Monitor phosphorylation of downstream targets (S6K, 4EBP1, rpS6) by western blotting

  • Track mTOR translocation using co-localization with lysosomal markers

  • Employ amino acid starvation (10-12h) followed by refeeding protocols

• For autophagy assessment:

  • Quantify LC3-GFP puncta formation (increased in GPR137B knockdown cells)

  • Measure LC3-I to LC3-II conversion via western blotting

  • Use flux assays with lysosomal inhibitors to distinguish between increased formation versus decreased clearance

How does GPR137B interact with the mTORC1 pathway molecularly?

GPR137B regulates mTORC1 signaling through several interconnected mechanisms:

• It affects mTOR translocation to lysosomes, with overexpression increasing translocation even in the absence of amino acids

• It interferes with HSC70 (heat shock cognate 70) binding to G3BP (Ras GTPase-activating protein-binding protein)

• This interference affects the TSC (tuberous sclerosis complex) tethering to lysosomes, which normally suppresses mTORC1 signaling

• GPR137B appears to function upstream of Rag GTPases, which are essential for amino acid-induced mTORC1 activation

For investigating these interactions, researchers should employ co-immunoprecipitation assays, proximity ligation assays, and FRET/BRET approaches to confirm direct protein-protein interactions.

What is the relationship between GPR137B and autophagy regulation?

GPR137B serves as a negative regulator of autophagy:

• Knockdown of GPR137B increases autophagy, as evidenced by increased LC3-GFP puncta formation

• This regulation occurs primarily through mTORC1, which inhibits the ULK1-mATG13-FIP200 complex required for autophagosome formation

• GPR137B deficiency leads to enhanced autophagy in macrophages, potentially through decreased mTORC1 activity

To study this mechanism comprehensively, researchers should:

• Monitor multiple autophagy markers simultaneously (LC3, p62, WIPI1)

• Combine genetic approaches (GPR137B knockout) with pharmacological interventions (rapamycin, torin1)

• Perform epistasis experiments to determine where in the autophagy pathway GPR137B functions

How does the HSC70-G3BP axis mediate GPR137B function?

The interaction between GPR137B, HSC70, and G3BP represents a key regulatory mechanism:

• HSC70 (heat shock cognate 70) is a downstream binding partner of GPR137B

• HSC70 can rescue impaired autophagy induced by GPR137B

• GPR137B interferes with HSC70 binding to G3BP

• G3BP normally tethers the TSC complex to lysosomes, suppressing mTORC1 signaling

• The NTF2 (nuclear transport factor 2) domain of G3BP binds to HSC70

For detailed molecular interaction studies, researchers should utilize:

• Domain mapping experiments to identify specific interaction regions

• Point mutation analyses to identify critical amino acid residues

• Rescue experiments with wild-type versus mutant proteins

How does GPR137B influence macrophage polarization?

GPR137B plays a significant role in macrophage polarization, particularly toward the M2 phenotype:

• It is abundantly expressed in RAW264 macrophages

• It regulates IL-4-responsive genes involved in alternative activation

• It functions as an orphan G-protein-coupled receptor associated with M2 macrophage polarization

To effectively study this relationship:

• Compare wild-type and GPR137B-knockout macrophages after treatment with polarizing stimuli (IL-4 for M2, LPS/IFN-γ for M1)

• Analyze expression of polarization markers at both mRNA and protein levels

• Perform functional assays characteristic of M1/M2 phenotypes (phagocytosis, ROS production, etc.)

What IL-4-responsive genes are regulated by GPR137B in macrophages?

Microarray analysis has identified Gpr137b-dependent IL-4-responsive genes in mouse macrophages . Researchers can access this comprehensive dataset through the NCBI Gene Expression Omnibus database (accession number GSE117578) . When analyzing this data:

• Ensure RNA quality assessment (RINe values >9.2 indicate high-quality samples)

• Use appropriate normalization methods (e.g., Affymetrix Transcriptome Analysis Console)

• Verify key targets through orthogonal techniques like qRT-PCR

• Perform pathway enrichment analysis to identify biological processes affected

How does GPR137B affect atherosclerosis progression through macrophage function?

GPR137B contributes to atherosclerosis progression through several macrophage-mediated mechanisms:

• It is upregulated in advanced atherosclerotic plaques

• GPR137B deficiency leads to reduced atherosclerotic lesions with fewer necrotic cores and less lipid accumulation

• This occurs through enhanced autophagy in macrophages

• High amino acid levels (induced by Western diet) stimulate mTORC1-autophagy defects in macrophages via GPR137B signaling

For in vivo atherosclerosis studies:

• Generate advanced plaques in ApoE-/- mice with cardiac-specific knockout of GPR137B

• Analyze plaque composition, stability, and progression rates

• Isolate plaque macrophages for ex vivo functional studies

How can CRISPR/Cas9 be optimized for GPR137B functional studies?

Effective CRISPR/Cas9 strategies for GPR137B studies include:

• Design multiple gRNAs targeting functional domains of GPR137B

• For RAW264 macrophages, validated CRISPR/Cas9 systems have successfully generated GPR137B knockout clones

• Verify knockouts at genomic (sequencing), mRNA (qPCR), and protein (western blot) levels

• Create knockout pools alongside clonal lines to address potential clonal variability

• Consider generating conditional knockout models for temporal regulation of GPR137B expression

When analyzing knockout phenotypes, be mindful of potential compensation by GPR137 and GPR137C homologs .

What approaches can resolve contradictory findings in GPR137B research?

To reconcile conflicting data about GPR137B function:

• Systematically evaluate methodological differences:

  • Cell types and culture conditions used

  • Knockout/knockdown techniques employed

  • Assay conditions (starvation duration, amino acid concentrations)

  • Species differences (mouse vs. human GPR137B)

• Perform direct comparative experiments:

  • Use multiple cell lines simultaneously

  • Apply several complementary techniques to measure the same outcome

  • Control for expression levels in overexpression studies

  • Consider the impact of tags (size, position) on protein function

• Investigate context-dependent functions:

  • Test function under different nutrient conditions

  • Examine effects in multiple cell types

  • Consider developmental timing in in vivo models

How can researchers effectively study GPR137B protein-protein interactions?

For studying GPR137B interactions with partners like HSC70, G3BP, and components of the mTORC1 pathway:

• Co-immunoprecipitation strategies:

  • Use mild detergents to preserve membrane protein interactions

  • Include appropriate controls (IgG, knockout cells)

  • Consider crosslinking to capture transient interactions

• Proximity-based approaches:

  • BioID or TurboID fusion proteins for identifying proximal proteins

  • APEX2 for spatially restricted proximity labeling

  • Split-protein complementation assays for direct interaction verification

• Imaging techniques:

  • FRET/FLIM for direct protein-protein interactions

  • Fluorescence correlation spectroscopy for complex dynamics

  • Super-resolution microscopy for spatial organization analysis

How should microarray data for GPR137B-dependent gene expression be analyzed?

Robust analysis of GPR137B-related transcriptomic data requires:

• Quality control assessment:

  • Verify RNA integrity (RINe values >9.2 indicate high quality)

  • Perform principal component analysis to identify outliers

• Differential expression analysis:

  • Use appropriate statistical cutoffs (adjusted p-value <0.05, fold change >1.5)

  • Compare wild-type vs. knockout in both baseline and stimulated conditions

• Pathway and network analysis:

  • Perform gene ontology enrichment analysis

  • Map differentially expressed genes to known pathways (KEGG, Reactome)

  • Construct protein-protein interaction networks

• Validation strategies:

  • Verify key targets by qRT-PCR

  • Confirm protein-level changes by western blotting

  • Perform rescue experiments with recombinant GPR137B

What statistical approaches are most appropriate for GPR137B functional studies?

For rigorous statistical analysis of GPR137B experiments:

• For simple comparisons between two groups:

  • Two-tailed Student's t-test when normality assumptions are met

  • Mann-Whitney U test for non-parametric data

• For multiple group comparisons:

  • One-way ANOVA followed by Tukey's post-hoc test

  • Kruskal-Wallis with Dunn's post-test for non-parametric data

• For complex experimental designs:

  • Two-way ANOVA for factorial designs (e.g., genotype × treatment)

  • Mixed-effects models for repeated measures with missing data

  • ANCOVA when controlling for covariates

• Sample size considerations:

  • Perform power analysis based on expected effect sizes

  • Include sufficient biological replicates (minimum n=3, preferably n≥5)

  • Account for multiple testing correction when performing genome-wide analyses

What are promising therapeutic applications targeting GPR137B?

Given GPR137B's role in pathological processes, potential therapeutic applications include:

• For atherosclerosis:

  • Development of GPR137B inhibitors to enhance macrophage autophagy

  • Targeting the GPR137B-HSC70-G3BP interaction to restore autophagy in advanced plaques

  • Combination therapies targeting both GPR137B and mTORC1

• For metabolic disorders:

  • Modulation of GPR137B to regulate amino acid sensing and mTORC1 activity

  • Development of tissue-specific inhibitors to avoid systemic effects

• For other conditions involving autophagy dysfunction:

  • Neurodegenerative diseases

  • Cancer

  • Inflammatory disorders

Therapeutic development should consider the potential redundancy with GPR137 and GPR137C homologs.

What key questions remain unresolved in GPR137B biology?

Despite significant progress, several important questions remain:

• Molecular mechanisms:

  • Does GPR137B function as a classical GPCR with G-protein coupling?

  • What are the natural ligands for this orphan receptor?

  • How is GPR137B itself regulated at transcriptional and post-translational levels?

• Physiological functions:

  • What is the role of GPR137B in tissues beyond macrophages?

  • How does GPR137B contribute to normal development and homeostasis?

  • What are the consequences of long-term GPR137B inhibition?

• Clinical relevance:

  • Are GPR137B levels altered in human atherosclerotic plaques?

  • Do GPR137B polymorphisms correlate with disease susceptibility?

  • Can GPR137B serve as a biomarker for autophagy dysfunction?