Recombinant Rat Gastrin-releasing peptide receptor (Grpr)
Description
Fundamental Characteristics of Gastrin-releasing Peptide Receptor
Gastrin-releasing peptide receptor (Grpr) is a member of the bombesin receptor family, specifically classified as the BB2 receptor subtype. This receptor is a glycosylated, seven-transmembrane G-protein coupled receptor that primarily activates the phospholipase C signaling pathway . Grpr serves as the primary receptor for gastrin-releasing peptide (GRP), a mammalian bombesin-like peptide that regulates numerous functions in both gastrointestinal and central nervous systems .
The receptor demonstrates high binding affinity for its natural ligand GRP, with studies indicating a K₀.₅ of 3.5 nM, which aligns well with the reported Kd of 3.1 ± 1.4 nM for Grpr expressed in Balb 3T3 cells . This remarkable affinity ensures sensitive detection of physiological GRP levels, enabling precise regulation of multiple downstream effects including hormone secretion, smooth muscle contraction, and cell proliferation.
Molecular Structure and Expression
Rat Grpr shares significant homology with other mammalian Grpr proteins, featuring the characteristic seven-transmembrane domain structure typical of G-protein coupled receptors. The receptor contains specialized binding domains for GRP and related peptides, alongside specific interaction sites for downstream signaling molecules. The recombinant version of rat Grpr is typically engineered with affinity tags (such as histidine or GST) to facilitate purification and detection in experimental settings.
Physiological Distribution and Function
In native tissues, Grpr exhibits widespread distribution throughout the rat nervous system and peripheral organs. The receptor plays particularly important roles in the central nervous system, where it contributes to regulation of food intake, circadian rhythms, fear memory, and sensory processing . In the paraventricular thalamic nucleus, Grpr activation induces sustained membrane depolarizations that trigger rhythmic burst discharges or tonic firing in neurons .
Signaling Mechanisms of Rat Grpr
Rat Grpr employs sophisticated signaling cascades that begin with selective G-protein coupling. Experimental evidence demonstrates that Grpr functionally couples to Gαq but not to the pertussis toxin-sensitive Gαi/o or retinal-specific Gαt . This selective coupling profile determines the downstream signaling events and cellular responses mediated by receptor activation.
G-protein Coupling and Signal Transduction
When GRP binds to Grpr, the receptor catalyzes the exchange of GDP for GTPγS on the Gαq subunit, a process dependent on both agonist presence and βγ subunits . This exchange represents the initial step in signal transduction, leading to activation of phospholipase C and subsequent generation of second messengers that modulate cellular functions. The GDP/GTP exchange rate on Gαq provides a quantitative measure of receptor activation, with the Km for squid retinal Gαq reported at 90 nM .
Ion Channel Modulation
In rat paraventricular thalamic neurons, Grpr activation produces complex electrophysiological effects through simultaneous modulation of multiple ion conductances. Specifically, receptor stimulation suppresses a barium-sensitive inward rectifier potassium conductance while activating a non-selective cation conductance with transient receptor potential vanilloid 1 (TRPV1)-like properties . These combined actions result in membrane depolarization that can trigger various firing patterns depending on the cellular context.
The table below summarizes the key signaling pathways associated with rat Grpr:
| Pathway | Associated Proteins | Physiological Outcome |
|---|---|---|
| Calcium signaling | Gαq, Phospholipase C, IP₃ | Intracellular calcium mobilization |
| Neuroactive ligand-receptor interaction | GRIN2D, CCKBR, LHCGR, GHSR | Neuronal excitability modulation |
| Ion channel regulation | TRPV1-like channels, K⁺ inward rectifiers | Membrane potential changes |
Production of Recombinant Rat Grpr
Recombinant rat Grpr is typically produced through molecular cloning and heterologous expression systems. The process begins with isolation of rat Grpr cDNA, which is then inserted into appropriate expression vectors containing promoter elements, selection markers, and fusion tags to facilitate expression and purification.
Expression Systems
Multiple expression platforms can be employed for producing recombinant rat Grpr, each with distinct advantages:
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Mammalian cell lines (e.g., HEK293 cells) provide an environment that supports proper folding and post-translational modifications of the receptor, crucial for maintaining native-like functionality .
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Insect cell systems using baculovirus vectors offer higher protein yields while still providing essential post-translational processing capabilities.
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Bacterial systems (E. coli) provide cost-effective production but may require additional refolding steps due to the membrane protein nature of Grpr.
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Cell-free expression systems offer rapid production for initial screening studies.
Purification Strategies
Purification of recombinant rat Grpr typically employs affinity chromatography targeting engineered tags (His, GST, or FLAG), followed by additional purification steps such as size exclusion or ion exchange chromatography to achieve high purity. The membrane-bound nature of Grpr necessitates careful consideration of detergent selection to maintain protein stability and functionality throughout the purification process.
Research Applications of Recombinant Rat Grpr
Recombinant rat Grpr serves as an invaluable tool across multiple research domains, enabling detailed investigation of receptor pharmacology, structure-function relationships, and potential therapeutic applications.
Pharmacological Characterization
Recombinant rat Grpr facilitates high-throughput screening of potential agonists and antagonists, allowing for detailed characterization of binding affinities, activation kinetics, and signaling outcomes. These studies provide critical insights into structure-activity relationships that guide drug development targeting the Grpr system.
Functional assays using recombinant rat Grpr have demonstrated that the receptor-catalyzed binding of GTPγS is selective for Gαq, highlighting the specificity of this signaling pathway . Such assays enable precise measurement of ligand potency and efficacy, providing quantitative parameters for compound optimization.
Pathophysiological Relevance and Therapeutic Potential
The Grpr system has significant implications in various pathological conditions, positioning recombinant rat Grpr as a valuable tool for both basic research and drug discovery efforts.
Oncological Significance
Grpr is aberrantly expressed in numerous cancers including those of the lung, colon, and prostate . This altered expression pattern suggests roles in tumor growth, invasion, and metastasis. Recombinant rat Grpr enables the development of screening assays for identifying compounds that could block receptor signaling, potentially leading to novel anti-cancer therapeutics.
Neurological Applications
In the central nervous system, Grpr mediates diverse functions including regulation of food intake, thermoregulation, metabolism, and behavior . Dysregulation of Grpr signaling may contribute to various neurological and psychiatric conditions. Studies using recombinant rat Grpr have elucidated the electrophysiological mechanisms underlying Grpr-mediated neuronal excitation, showing that receptor activation produces membrane depolarization through concurrent modulation of potassium channels and non-selective cation conductances .
Technical Considerations and Challenges
Working with recombinant rat Grpr presents several technical challenges that researchers must address to obtain meaningful results.
Membrane Protein Stability
As a seven-transmembrane receptor, Grpr requires a lipid environment or suitable detergent micelles to maintain native conformation and functionality. Researchers have developed specialized extraction and reconstitution methods to preserve receptor activity, such as the urea extraction procedure that removes endogenous G-proteins while retaining functional Grpr in membrane preparations .
Functional Assay Development
Assessing the functionality of recombinant rat Grpr requires specialized assays that monitor receptor-mediated signaling events. GTPγS binding assays measure G-protein activation, while calcium mobilization assays and electrophysiological recordings provide readouts of downstream signaling effects. These assays must be carefully optimized to ensure sensitivity, specificity, and reproducibility.
Future Directions and Emerging Applications
Research involving recombinant rat Grpr continues to evolve, with several promising directions emerging for future investigation.
Therapeutic Development
The detailed characterization of recombinant rat Grpr provides a foundation for developing targeted therapeutics for Grpr-associated pathologies. Potential applications include cancer diagnostics and treatment, management of gastrointestinal disorders, and intervention in neurological conditions involving abnormal Grpr signaling.
Product Specs
Note: We prioritize shipping the format currently in stock. However, if you have specific format requirements, please indicate them during order placement, and we will accommodate your request.
Note: All proteins are shipped with standard blue ice packs. If dry ice shipping is required, please communicate with us in advance, as additional fees may apply.
Generally, the shelf life of liquid form is 6 months at -20°C/-80°C. The shelf life of lyophilized form is 12 months at -20°C/-80°C.
Target Background
- Peptidergic pathways form the core of a sigh control circuit, integrating physiological and potentially emotional input to transform normal breaths into sighs. PMID: 26855425
- These findings highlight the role of GRPR signaling in sepsis outcome. PMID: 22735756
- Activation in the lumbosacral spinal cord is required for ejaculation. PMID: 22429708
- The mRNA for the primary receptor subtype, GRP-R, was present in fetal and adult rat kidneys. PMID: 15140764
- Data suggest that the memory-impairing effects of gastrin-releasing peptide receptor antagonists might be mediated by inhibiting function and/or expression of neuronal bFGF or diminishing bFGF signaling. PMID: 17406984
Q&A
What is the molecular characterization of recombinant rat Grpr?
Recombinant rat Gastrin-releasing peptide receptor (Grpr) belongs to the superfamily of G protein-coupled receptors with seven transmembrane domains. The receptor functions primarily through G protein signaling pathways, with particular selectivity for Gαq family proteins. When expressed in heterologous systems, Grpr typically displays a molecular weight of approximately 43 kDa, which can vary depending on post-translational modifications and the expression system used. Functional characterization typically involves ligand binding studies using radioligand assays with specific agonists like bombesin (Bn) or gastrin-releasing peptide (GRP) and antagonists such as [d-Tyr6]bombesin(6–13) methyl ester .
Researchers should note that epitope-tagged versions of the receptor (such as c-myc tags) can be created without disrupting normal receptor function. These modified constructs exhibit characteristics indistinguishable from wild-type Grpr in terms of ligand binding, activation of phospholipase C (PLC), internalization, and desensitization patterns .
What are the optimal expression systems for recombinant rat Grpr production?
Mammalian cell expression systems provide the most reliable platform for functional recombinant rat Grpr production. Balb 3T3 mouse fibroblast cells have been successfully used for stable transfection with Grpr cDNA, achieving expression levels of approximately 10^6 receptors per cell . This high expression level makes these cells particularly suitable for membrane preparation and subsequent G protein coupling studies.
How does rat Grpr compare with human and mouse homologs in experimental settings?
Rat Grpr shares high sequence homology with both human and mouse orthologs, making it a valuable model for comparative studies. Research has demonstrated that the pharmacological profiles of rat, mouse, and human Grpr are similar, with nearly identical rank orders of potency for agonists: bombesin ≥ GRP ≫ neuromedin B .
When conducting cross-species studies, researchers should consider that despite these similarities, there may be subtle differences in binding affinities and downstream signaling efficiency. For instance, binding studies with 125I-labeled [d-Tyr6]bombesin(6–13) methyl ester show a single binding site with a Kd = 1.4 nM ± 0.4 in mouse fibroblasts expressing mouse Grpr . Similar binding characteristics are typically observed with rat Grpr, although species-specific variations in post-translational modifications may influence receptor pharmacology.
What are the key considerations for establishing a functional reconstitution assay for rat Grpr?
Developing an in situ reconstitution assay for rat Grpr requires several critical steps:
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Membrane preparation: Use 6 M urea extraction on P2 membrane pellets pretreated with 100 nM bombesin or GRP for 30 minutes at 25°C.
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Verification of receptor integrity: Perform Scatchard analysis to confirm a single high-affinity antagonist binding site. Extracted membranes should show 2-3 fold enrichment of binding sites compared to unextracted preparations (approximately 15-22 pmol of receptor per mg of protein) .
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G protein reconstitution: Combine extracted membranes with purified G protein subunits (particularly Gαq and Gβγ) to measure receptor-catalyzed binding of GTPγS.
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Assay conditions: The standard GDP/GTPγS exchange assay should include:
This reconstitution protocol allows examination of carefully controlled protein interactions between receptor and G protein that would be unavailable when using intact cells .
What methods are most effective for studying rat Grpr signaling pathways?
Studying rat Grpr signaling pathways requires a multi-faceted approach:
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G protein activation assays: The receptor-catalyzed exchange of GDP for GTPγS on Gαq provides a direct measure of receptor activity. This method has revealed that Grpr-catalyzed binding of GTPγS is selective for Gαq, with no detectable receptor-catalyzed exchange when using either Gαi/o or Gαt .
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Dose-response and kinetic analyses: Titration experiments with varying concentrations of agonists (0-10 nM range) can determine optimum doses for receptor activation. Similarly, time-course experiments (0-24 hours) help establish the temporal dynamics of Grpr signaling .
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Gene expression analysis: Quantitative RT-PCR with carefully designed primer pairs can measure changes in gene expression following Grpr activation. All primer pairs should be validated for specificity through gel electrophoresis and sequencing .
The table below summarizes key parameters for Grpr-G protein coupling determined through reconstitution assays:
| Parameter | Value | Notes |
|---|---|---|
| EC50 for GRP | 3.5 nM | Correlates with Kd of 3.1 nM for GRP binding |
| Km for Gαq | 87-90 nM | Using squid retinal Gαq |
| Apparent Kd for Gβγ | 57-60 nM | Using bovine brain Gβγ |
These values provide important benchmarks for researchers developing rat Grpr signaling assays .
How can recombinant rat Grpr be used to study epithelial-mesenchymal transition (EMT) in cancer models?
Recombinant rat Grpr expressed in cancer cell lines provides a powerful system for investigating the receptor's role in epithelial-mesenchymal transition (EMT). This approach has revealed significant insights into Grpr's contribution to cancer progression:
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Experimental design: Researchers should consider stably transfecting cancer cell lines with human or rat Grpr and using both in vitro and in vivo models to comprehensively evaluate Grpr-mediated effects on EMT .
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Gene expression profiling: After Grpr activation by bombesin or GRP, quantitative RT-PCR should be performed to measure changes in EMT marker expression. The optimal dose for studying gene expression changes has been determined to be around 1.0 nM for bombesin .
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Morphological and functional analyses: Changes in cell morphology, migration, and proliferation should be systematically evaluated following Grpr activation. Migration assays and proliferation studies provide quantitative metrics of Grpr's impact on cancer cell behavior .
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In vivo models: Subcutaneous and orthotopic (e.g., intratibial) tumor models in immunocompromised mice enable assessment of Grpr's impact on tumor growth, morphology, and metastatic potential. Bioluminescence imaging provides a non-invasive method for monitoring tumor growth over time .
What are the critical parameters for optimizing G protein coupling studies with rat Grpr?
Optimizing G protein coupling studies with rat Grpr requires careful attention to several parameters:
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Membrane quality: Receptor-containing membranes should be treated with agonist prior to chaotropic extraction to ensure complete uncoupling from endogenous G proteins. This produces a clean system for reconstitution experiments .
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G protein subunit composition: Both Gα and Gβγ subunits are necessary for optimal receptor-catalyzed GDP/GTPγS exchange. The apparent Kd for bovine brain Gβγ is approximately 60 nM, and the Km for squid retinal Gαq is around 90 nM .
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Agonist concentration: For GRP, an EC50 of 3.5 nM has been established, which correlates well with the reported Kd of 3.1 nM for GRP binding to Grpr expressed in mouse fibroblasts .
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Assay temperature and time: These parameters should be optimized based on the specific experimental question. Typically, assays are performed at 25-30°C for periods ranging from 5-60 minutes to capture the linear phase of G protein activation .
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Data analysis: Single-site binding models typically provide good fits for both Gα and Gβγ saturation curves, though increasing receptor concentration can increase the apparent affinity for Gβγ without affecting the Km for Gα .
What are common challenges in recombinant rat Grpr expression and how can they be addressed?
Researchers often encounter several challenges when working with recombinant rat Grpr:
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Low expression levels: To address this issue:
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Receptor misfolding: Strategies to improve proper folding include:
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Expression at lower temperatures (28-30°C instead of 37°C)
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Addition of chemical chaperones to the culture medium
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Co-expression with cellular chaperones
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Heterogeneous glycosylation: This can be addressed by:
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Using expression systems with more uniform glycosylation patterns
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Treatment with glycosidases for analytical purposes
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Site-directed mutagenesis of glycosylation sites if they're not essential for function
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Variable G protein coupling: To ensure consistent functional analyses:
How can binding assays for rat Grpr be optimized for different experimental questions?
Binding assays for rat Grpr can be tailored to address specific research questions:
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For determination of basic pharmacological parameters:
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For G protein coupling studies:
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For drug discovery applications:
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Develop non-radioactive assays using fluorescent or bioluminescent readouts
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Miniaturize assays for higher throughput
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Incorporate appropriate positive controls (known agonists/antagonists) and negative controls
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For determining binding kinetics:
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Perform association and dissociation experiments at different temperatures
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Calculate kon and koff rates
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Correlate binding kinetics with functional responses
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How might structural biology approaches enhance our understanding of rat Grpr function?
Advanced structural biology techniques offer exciting opportunities to deepen our understanding of rat Grpr:
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Cryo-electron microscopy (cryo-EM): This technique could resolve the three-dimensional structure of rat Grpr in different conformational states (inactive, active, G protein-bound). Such structural information would reveal the molecular basis of G protein coupling selectivity, where rat Grpr shows preference for Gαq but not Gαi/o or Gαt .
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X-ray crystallography: Although challenging for GPCRs, crystallography of the rat Grpr ligand-binding domain in complex with various agonists and antagonists could provide crucial insights into the molecular determinants of ligand specificity.
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Hydrogen-deuterium exchange mass spectrometry (HDX-MS): This approach could map conformational changes in rat Grpr upon ligand binding and G protein coupling, offering dynamic structural information complementary to static structural techniques.
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Molecular dynamics simulations: Computational approaches based on structural data could simulate rat Grpr behavior in a lipid bilayer environment, predicting how different ligands affect receptor dynamics and G protein coupling.
These structural approaches would help rationalize the pharmacological data already available, such as the EC50 of 3.5 nM for GRP and apparent Kd values for G protein subunits .
What emerging techniques might enhance the study of rat Grpr signaling in complex biological systems?
Several cutting-edge approaches show promise for advancing rat Grpr research:
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CRISPR-Cas9 genome editing: This technology enables precise modification of the endogenous rat Grpr gene to introduce reporter tags, mutations, or conditional expression systems in cell lines and animal models.
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Optogenetics and chemogenetics: These approaches permit temporal and spatial control of rat Grpr activation in complex tissues, allowing researchers to dissect the receptor's role in specific biological contexts.
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Single-cell transcriptomics: This technique can reveal how Grpr activation influences gene expression programs at the individual cell level, providing insights into cellular heterogeneity in response to receptor signaling.
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Biosensors for live-cell imaging: Development of FRET-based or other fluorescent biosensors that report on rat Grpr activation or downstream signaling events would enable real-time visualization of receptor function in living cells.
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Spatial transcriptomics and proteomics: These methods could map the geographical distribution of Grpr-induced changes across tissues, providing a more holistic view of receptor function in complex biological systems.
These emerging techniques would complement traditional approaches like the in situ reconstitution assay, offering new perspectives on rat Grpr biology beyond what can be learned from isolated membrane preparations .
