Recombinant Praomys natalensis Gastrin/cholecystokinin type B receptor (CCKBR)

What is the molecular structure of Praomys natalensis CCKBR?

Praomys natalensis CCKBR is a G-protein coupled receptor consisting of 450 amino acids. The full amino acid sequence includes characteristic transmembrane domains with the expression region spanning positions 1-450. The protein contains several functional domains including extracellular N-terminal domain, seven transmembrane helices, and intracellular loops that participate in G-protein coupling. The sequence shows typical features of Class A GPCRs including conserved motifs critical for receptor activation and signal transduction . Analysis of the amino acid sequence reveals structural similarities with human and other mammalian CCKBR proteins, with especially high sequence identity to mouse (96%) and rat (96%) orthologs .

How does CCKBR differ from CCK1R at the molecular level?

CCKBR (also designated as CCK2R according to IUPHAR nomenclature) differs from CCK1R in several key aspects:

This differential binding profile makes CCKBR particularly important in gastrin-mediated physiological processes throughout various tissues .

What are the primary physiological functions of CCKBR?

CCKBR functions as a receptor for both gastrin and cholecystokinin, which are regulatory peptides found in the brain and gastrointestinal tract. Physiologically, CCKBR plays crucial roles in:

• Regulation of gastric acid secretion via parietal cells

• Modulation of smooth muscle contractions

• Neural signaling as the predominant brain CCK receptor

• Cellular growth and proliferation in various tissues including the kidney

• Involvement in neuroendocrine tumor development, particularly type 1 gastric neuroendocrine tumors (gNETs) in conditions of hypergastrinemia

Recent research has expanded our understanding of CCKBR's role beyond traditional gastric functions to include renal physiology, where it may act as a growth factor influencing proximal tubular cell proliferation .

What are the optimal conditions for storing recombinant Praomys natalensis CCKBR?

For optimal preservation of recombinant Praomys natalensis CCKBR activity:

• Store at -20°C for routine use

• For extended storage, maintain at -20°C or -80°C

• Use storage buffer containing Tris-based components with 50% glycerol optimized for protein stability

• Avoid repeated freeze-thaw cycles as this significantly reduces protein integrity

• For working experiments, prepare small aliquots and store at 4°C for up to one week

These storage recommendations ensure maintenance of protein conformation and functional activity for research applications . Deterioration of CCKBR under suboptimal storage conditions may manifest as reduced binding capacity in experimental assays.

How can researchers validate the functional activity of recombinant CCKBR?

Validation of recombinant CCKBR functional activity can be performed through multiple complementary approaches:

• Radioligand binding assays: Using [125I]-BH-CCK and performing displacement experiments with various agonists and antagonists to confirm typical CCKBR pharmacology

• Calcium mobilization assays: Measuring intracellular Ca2+ changes in response to CCK or gastrin stimulation in cells expressing the recombinant receptor

• G-protein activation assays: Assessing downstream signaling through measurement of inositol 1,4,5-trisphosphate generation

• Functional cell-based assays: Examining cell proliferation responses in appropriate cell lines (e.g., MCT cells) following receptor activation

• Competitive binding analysis: Using receptor antagonists like netazepide or YM022 (at 100 nmol/L) to verify specific binding characteristics

Researchers should include positive controls with known CCK2R activity and negative controls with unrelated receptors to ensure specificity of experimental observations.

What techniques can be used to detect CCKBR expression in tissue samples?

Multiple techniques are available for detecting CCKBR expression in tissue samples:

Research has successfully employed these techniques to identify CCKBR expression in various tissues including kidney, where it was localized to proximal tubules, distal collecting ducts, and mesangium cells using immunohistochemistry with polyclonal antibodies . For optimal results in immunodetection, pre-incubation of antibodies with protein control fragments (using a 100× molar excess) for 30 minutes at room temperature can be used for blocking experiments to verify specificity .

How is CCKBR involved in the pathophysiology of type 1 gastric neuroendocrine tumors?

CCKBR plays a critical role in the development of type 1 gastric neuroendocrine tumors (gNETs) through the following mechanisms:

• In patients with autoimmune atrophic gastritis and achlorhydria, hypergastrinemia occurs as a compensatory mechanism

• Sustained hypergastrinemia leads to hyperproliferation of enterochromaffin-like (ECL) cells in the gastric mucosa through CCKBR activation

• CCKBR signaling activates downstream pathways that promote cell growth, survival, and neoplastic transformation

• The receptor mediates increased expression of pappalysin 2 (PAPPA2), which cleaves insulin-like growth factor binding protein-3 (IGFBP-3), increasing IGF bioavailability

• Enhanced IGF signaling contributes to cellular migration, structural remodeling, and tumor development

This understanding has led to therapeutic approaches using CCKBR antagonists like netazepide (YF476), which has shown efficacy in eradicating some type 1 gNETs during 12 months of treatment .

What experimental models are available for studying CCKBR function?

Several experimental models have been developed for investigating CCKBR function:

• Cell line models:

  • AGS-GR cells: Human gastric adenocarcinoma cell line stably expressing human CCK2R

  • MCT cells: Proximal tubular cells expressing CCKBR, suitable for studying renal functions

  • MMC cells: Mesangial cells with CCKBR expression

• Primary culture systems:

  • Mouse gastric organoids (gastroids): Three-dimensional culture systems that recapitulate in vivo gastric epithelial cell organization and function

• Animal models:

  • INS-GAS transgenic mice: Exhibit hypergastrinemia and develop altered gastric corpus histology

  • African cotton rats: Develop spontaneous type 1 gNETs in response to hypergastrinemia

  • FVB/N mice: Used as normogastrinemic controls in comparative studies

• Human tissue samples:

  • Gastric corpus biopsies from patients with hypergastrinemia and type 1 gNETs

  • Kidney tissue samples for studying renal CCKBR expression

These models enable comprehensive investigation of CCKBR biology from molecular mechanisms to physiological outcomes in various tissues and disease states .

How do CCKBR antagonists affect gastric neuroendocrine tumor growth?

CCKBR antagonists demonstrate significant effects on gastric neuroendocrine tumor growth through several mechanisms:

• Direct inhibition of proliferative signaling: Antagonists like netazepide (YF476) and YM022 block gastrin-induced cell proliferation by inhibiting CCKBR activation

• Suppression of PAPPA2 expression: CCKBR antagonists (at 100 nmol/L) completely reverse gastrin-induced increases in PAPPA2 expression, a key mediator in the growth-promoting pathway

• Regulation of IGF bioavailability: By suppressing PAPPA2 expression, CCKBR antagonists reduce cleavage of IGFBP-3, limiting IGF bioavailability and its mitogenic effects

• Inhibition of cellular migration: Treatment with CCKBR antagonists significantly reduces gastrin-induced cellular migration and structural remodeling

• Tumor regression in clinical settings: In patients with type 1 gNETs, 12 months of treatment with netazepide has successfully eradicated tumors, demonstrating translational efficacy

These findings highlight the therapeutic potential of CCKBR antagonists in treating neuroendocrine tumors associated with hypergastrinemia .

How can researchers investigate the CCKBR signaling pathway in kidney cells?

To investigate CCKBR signaling in kidney cells, researchers can employ a multi-faceted approach:

• Cell type-specific expression analysis:

  • Use RT-PCR to identify CCKBR transcript expression in different renal cell types (proximal tubules, mesangium cells, etc.)

  • Compare expression levels between kidney compartments (tubules > glomeruli > interstitium) to identify primary sites of action

• Pharmacological characterization:

  • Perform displacement experiments using [125I]-BH-CCK and various agonists/antagonists

  • Identify binding sites with typical CCKBR pharmacology to confirm receptor identity

• Functional growth assays:

  • Assess cell proliferation in MCT cells (proximal tubular cells) treated with gastrin 17-1

  • Measure [3H]-thymidine incorporation to quantify DNA synthesis

  • Compare treated cells with controls to establish growth factor activity (approximately 40% increase has been observed)

• Immunohistochemical localization:

  • Use polyclonal antibodies against CCKBR to visualize receptor distribution

  • Examine tissue sections to establish precise cellular localization in proximal tubules, distal collecting ducts, and mesangium cells

• Signaling pathway analysis:

  • Investigate downstream effectors of CCKBR activation

  • Measure calcium mobilization, phospholipase C activation, and MAP kinase phosphorylation

This comprehensive approach has revealed that CCKBR is expressed in selected areas of the kidney and likely functions as a growth factor in this organ .

What are the methodological challenges in studying species differences in CCKBR pharmacology?

Investigating species differences in CCKBR pharmacology presents several methodological challenges:

• Binding affinity variations:

  • Different species show varying affinities for ligands and antagonists

  • Canine CCKBR exhibits atypical binding to the CCK1R antagonist L-364,718 (19 nM) compared to the CCK2R antagonist L-365,260 (130 nM)

  • These differences necessitate species-specific validation of pharmacological tools

• Expression system considerations:

  • Heterologous expression in cells like COS-7 may not recapitulate native receptor environments

  • Membrane composition differences can affect receptor conformation and signaling

  • Cell-specific post-translational modifications may alter pharmacological properties

• Experimental design requirements:

  • Comparative studies require parallel testing with standardized conditions

  • Concentration-response curves must be established across a broad range

  • Multiple antagonists should be tested to establish comprehensive pharmacological profiles

• Data interpretation complexities:

  • IC50 values must be converted to Ki values accounting for species-specific parameters

  • Allosteric interactions may vary between species, affecting antagonist efficacy

  • Functional readouts may not directly correlate with binding parameters

• Technical considerations:

  • Radioligand binding assays require consistent specific activity across experiments

  • Functional assays need calibration for species-specific baseline responses

  • Recombinant protein quality and modification status must be carefully controlled

These challenges highlight the importance of comprehensive characterization when translating findings between species or when selecting model systems for CCKBR research .

How can gene expression profiling be utilized to understand CCKBR-mediated tumor regression?

Gene expression profiling offers powerful insights into CCKBR-mediated tumor regression mechanisms:

• Experimental design approach:

  • Obtain gastric corpus biopsy specimens from patients with hypergastrinemia and type 1 gNETs before, during, and after treatment with CCKBR antagonists

  • Extract total RNA and prepare amplified and biotinylated sense-strand DNA targets

  • Analyze using comprehensive platforms such as Affymetrix Human Gene 2.0 ST microarrays

  • Identify differentially expressed genes across treatment timepoints

• Validation strategy:

  • Confirm key findings in multiple models:

    • Human AGS-GR gastric adenocarcinoma cell line expressing human CCK2R

    • Primary mouse gastroids

    • Transgenic hypergastrinemic INS-GAS mice

    • Patient samples

  • Use qPCR, Western blot, and immunohistochemistry for multi-level validation

• Target identification and characterization:

  • PAPPA2 has been identified as a key mediator of gastrin/CCKBR effects

  • Expression increases dose-dependently (maximal at 10 nmol/L gastrin) and time-dependently

  • CCKBR antagonists (YM022 or netazepide at 100 nmol/L) completely reverse gastrin-induced PAPPA2 expression

• Mechanistic investigation:

  • Examine PAPPA2's role in cleaving IGFBP-3

  • Assess IGF bioavailability using IGF-1-receptor inhibitors (e.g., AG1024)

  • Investigate effects on cellular migration and structural remodeling

  • Evaluate dose-response relationships across multiple endpoints

This integrated approach has revealed that CCKBR antagonists inhibit gastrin-induced PAPPA2 expression, reducing IGF bioavailability and thereby suppressing cellular proliferation, migration, and tumor growth .

What are the implications of CCKBR splice variants in disease pathogenesis?

The presence of CCKBR splice variants has significant implications for disease pathogenesis that merit further investigation:

• Cancer association: A misspliced transcript variant including an intron has been observed specifically in colorectal and pancreatic tumors, suggesting potential roles in carcinogenesis

• Differential signaling properties: Splice variants may exhibit altered ligand binding affinities, coupling to different G proteins, or modified downstream signaling cascades

• Tissue-specific expression patterns: Various splice variants might show differential expression across tissues, potentially explaining tissue-specific responses to gastrin and CCK

• Therapeutic resistance mechanisms: Splice variants could contribute to variable responses to CCKBR antagonists in clinical settings, potentially explaining treatment resistance in some patients

• Biomarker potential: Detection of specific splice variants might serve as diagnostic or prognostic biomarkers for various malignancies

Future research should focus on comprehensive characterization of CCKBR splice variants across different tissues and disease states, their functional properties, and potential as therapeutic targets or biomarkers.

How might comparative studies between human and Praomys natalensis CCKBR advance therapeutic development?

Comparative studies between human and Praomys natalensis CCKBR could significantly advance therapeutic development through:

• Evolutionary insights:

  • Identification of conserved domains critical for receptor function

  • Recognition of species-specific variations that might inform drug design

  • Understanding of selective pressures that have shaped receptor pharmacology

• Structural biology applications:

  • Comparison of binding pocket architectures to optimize antagonist design

  • Identification of species-specific conformational states that affect drug efficacy

  • Exploration of allosteric modulation sites that might be therapeutically targetable

• Translational models:

  • Development of improved animal models for testing CCKBR-targeted therapeutics

  • Better prediction of human responses to novel compounds

  • Understanding of species-specific adverse effects

• Pharmacological optimization:

  • Design of species-selective compounds for experimental purposes

  • Development of broadly effective antagonists that work across species

  • Identification of compounds with improved pharmacokinetic properties

The high sequence identity between human and rodent CCKBR orthologs (96% with mouse and rat) provides a strong foundation for translational research, though species-specific differences in pharmacology must be carefully considered .

What novel methodologies might improve detection and quantification of CCKBR in research settings?

Emerging methodologies hold promise for enhanced detection and quantification of CCKBR:

• Advanced imaging techniques:

  • Super-resolution microscopy for precise subcellular localization

  • FRET-based approaches to study receptor dimerization and protein interactions

  • Live-cell imaging with fluorescent ligands to track receptor dynamics

• Proteomics approaches:

  • Targeted mass spectrometry for absolute quantification of receptor proteins

  • Phosphoproteomics to map receptor activation states

  • Cross-linking mass spectrometry to identify protein interaction networks

• Single-cell technologies:

  • Single-cell RNA sequencing to identify cell populations expressing CCKBR

  • Spatial transcriptomics to map receptor expression in tissue contexts

  • CyTOF/mass cytometry for multiparameter analysis of signaling pathways

• Biosensor development:

  • BRET/FRET-based sensors to monitor receptor conformational changes

  • Genetically encoded calcium indicators for real-time signaling analysis

  • CRISPR-based reporters for endogenous receptor monitoring

• Computational approaches:

  • Machine learning algorithms for image analysis and receptor quantification

  • Molecular dynamics simulations to predict ligand binding and receptor activation

  • Systems biology models of receptor signaling networks

Implementation of these advanced methodologies would significantly enhance our ability to study CCKBR biology in both physiological and pathological contexts.

What are the current gaps in CCKBR research that require further investigation?

Despite significant advances, several important knowledge gaps remain in CCKBR research:

• Tissue-specific signaling mechanisms: How CCKBR activation leads to different outcomes in various tissues (brain, stomach, kidney) remains incompletely understood

• Receptor regulation: Mechanisms controlling receptor expression, trafficking, and desensitization in different physiological and pathological states need further elucidation

• Cross-talk with other signaling pathways: Interactions between CCKBR and other receptors/pathways (e.g., IGF system) require more detailed characterization

• Genetic variations: The impact of genetic polymorphisms on receptor function and disease susceptibility remains largely unexplored

• Long-term safety of receptor antagonists: Comprehensive evaluation of prolonged CCKBR blockade on various physiological systems is needed

• Receptor structure-function relationships: Detailed structural insights into ligand binding, receptor activation, and G-protein coupling would facilitate better drug design

Addressing these gaps would significantly advance our understanding of CCKBR biology and support development of targeted therapeutic approaches for CCKBR-associated pathologies.

What standardized protocols should researchers consider when working with recombinant CCKBR?

Researchers working with recombinant CCKBR should consider the following standardized protocols:

• Quality control assessments:

  • Protein purity verification via SDS-PAGE

  • Mass spectrometry confirmation of intact protein

  • Circular dichroism to verify proper folding

  • Functional binding assays to confirm activity

• Storage and handling:

  • Prepare small working aliquots to avoid freeze-thaw cycles

  • Store at -20°C for routine use or -80°C for long-term storage

  • Maintain in Tris-based buffer with 50% glycerol

  • Establish quality control checkpoints throughout experimental timelines

• Experimental design considerations:

  • Include appropriate positive and negative controls

  • Perform concentration-response curves to establish optimal working ranges

  • Account for species-specific pharmacological differences

  • Validate findings across multiple experimental approaches

• Data reporting standards:

  • Document complete methodological details including protein source, concentration, and buffer composition

  • Report receptor binding parameters (Kd, Bmax) with appropriate statistical analysis

  • Include raw data representations alongside processed results

  • Clearly state any limitations of experimental approaches

Adherence to these standardized protocols would enhance reproducibility and facilitate cross-study comparisons in CCKBR research .

How might interdisciplinary approaches advance our understanding of CCKBR biology?

Interdisciplinary approaches offer powerful opportunities to advance CCKBR research:

• Integrating structural biology with medicinal chemistry:

  • Crystal structure determination of CCKBR in different activation states

  • Structure-based drug design for improved receptor antagonists

  • Computational modeling of ligand-receptor interactions

• Combining systems biology with physiological studies:

  • Network analysis of CCKBR signaling pathways

  • Integration of multi-omics data to identify novel regulatory mechanisms

  • Mathematical modeling of receptor dynamics in different tissues

• Merging clinical research with basic science:

  • Translational studies correlating receptor polymorphisms with clinical outcomes

  • Patient-derived organoids for personalized drug response testing

  • Biomarker development for stratifying patients for CCKBR-targeted therapies

• Incorporating bioengineering approaches:

  • Development of novel biosensors for real-time monitoring of receptor activity

  • Microfluidic platforms for high-throughput screening of receptor modulators

  • Tissue engineering to study CCKBR function in complex 3D environments

• Leveraging artificial intelligence and machine learning:

  • Prediction of novel CCKBR ligands through virtual screening

  • Pattern recognition in large-scale gene expression datasets

  • Automated image analysis for receptor localization studies