Recombinant Mouse Gastrin/cholecystokinin type B receptor (Cckbr)

What is the Gastrin/cholecystokinin type B receptor (Cckbr) and what are its primary functions?

The Gastrin/cholecystokinin type B receptor (Cckbr), also known as CCK-B or CCK2, is a G protein-coupled receptor encoded by the CCKBR gene. It functions as a receptor for both gastrin and cholecystokinin (CCK), which are regulatory peptides found in the central nervous system and gastrointestinal tract. This receptor has high affinity for both sulfated and nonsulfated CCK analogs .

In the gastrointestinal system, Cckbr plays a crucial role in regulating gastric acid secretion and gastric mucosal cell proliferation. Studies with Cckbr-deficient mice have demonstrated that this receptor is essential for physiological cell growth of stomach mucosal cells . In the central nervous system, Cckbr significantly influences neurotransmission in the brain, regulating numerous functions including anxiety, feeding behaviors, and locomotion .

How does Cckbr expression differ between tissues in mice?

Mouse Cckbr shows distinctive tissue-specific expression patterns that reflect its diverse physiological roles. The receptor is abundantly expressed in the cerebral cortex, brain basal ganglion, and stomach of wild-type mice, as confirmed by RNA analysis . Within the gastrointestinal tract, Cckbr is prominently expressed in parietal cells and enterochromaffin-like (ECL) cells, where it mediates gastrin-stimulated acid secretion and cell proliferation .

The expression of Cckbr does not appear to affect the expression levels of the related CCK-A receptor, as demonstrated in Cckbr knockout mice which showed normal CCK-A receptor mRNA levels in tissues that typically express both receptors . This tissue-specific expression pattern is critical for understanding the receptor's differential roles in various physiological processes and for designing targeted experimental approaches.

What phenotypic changes are observed in Cckbr knockout mice?

Cckbr knockout mice (CCKBR-/-) exhibit several distinctive phenotypic changes despite appearing normal in their general appearance compared to wild-type littermates. The most notable changes include:

• Hypochlorhydria: CCKBR-/- mice show significantly reduced basal acid output (3.8 ± 2.1 mEq/hr) compared to wild-type mice (13.1 ± 3.3 mEq/hr), indicating impaired gastric acid secretion .

• Hypergastrinemia: Serum gastrin levels in CCKBR-/- mice (1582 ± 257 pg/ml) are approximately five times higher than those in wild-type mice (293 ± 60.6 pg/ml) . This elevation is likely a consequence of reduced gastric acidity, as luminal acidity normally inhibits gastrin release.

• Gastric mucosal atrophy: CCKBR-/- mice display macroscopically observable atrophy of the gastric mucosa, characterized by reduced proliferation of parietal and chromogranin A-positive ECL cells .

• Resistance to PPI-induced mucosal hypertrophy: Unlike wild-type mice, CCKBR-/- mice do not develop gastric mucosal hypertrophy in response to proton pump inhibitor (PPI) treatment, demonstrating that this receptor is essential for the PPI-induced trophic response .

These phenotypic changes clearly demonstrate the critical role of Cckbr in gastric physiology, particularly in acid secretion and mucosal cell proliferation.

How does ligand-induced internalization of Cckbr differ from that of CCK-A receptors?

Ligand-induced internalization is a key regulatory mechanism for G protein-coupled receptors, including Cckbr. Research has revealed significant differences in the internalization patterns between Cckbr and its related receptor, CCKAR. While both receptors undergo internalization following ligand binding, their structural determinants and kinetic profiles differ substantially.

Studies using radioligand stripping techniques have demonstrated that wild-type CCKBR undergoes rapid internalization following CCK-8 binding, with approximately 89% of the bound radioligand becoming resistant to stripping after 120 minutes of incubation . In contrast, mutations affecting the receptor's structure dramatically alter its internalization capacity.

The carboxyl-terminal domain of Cckbr plays a critical role in its internalization. When this receptor is truncated (CCKBR Tr408) or when its serine/threonine residues are mutated to alanines (CCKBR ΔS/T), internalization is profoundly attenuated. After 120 minutes of incubation, cells transfected with these mutant receptors showed only 26 ± 0.8% and 39 ± 5.3% internalization for CCKBR Tr408 and CCKBR ΔS/T, respectively, compared to wild-type Cckbr .

Interestingly, similar mutations in the CCKAR produce markedly different effects. Truncation of CCKAR (Tr399) significantly reduced receptor internalization, whereas mutation of serine/threonine residues to alanines (CCKAR ΔS/T) had minimal impact on internalization kinetics . These differences highlight the distinct structural requirements for internalization between these related receptor subtypes.

What role does Cckbr play in dopaminergic neurotransmission and neurological disorders?

Cckbr significantly influences dopaminergic neurotransmission in the brain, with implications for various neurological disorders. The receptor exhibits complex, location-dependent effects on dopamine activity:

• General inhibitory action: Cckbr activation generally inhibits dopamine activity in the brain, opposing the dopamine-enhancing effects of CCK-A receptors .

• Regional variation: In the rat striatum, Cckbr antagonism enhances dopamine release, while in the anterior nucleus accumbens, activation enhances GABA release, indirectly modulating dopamine activity .

• Drug response modulation: Cckbr influences individual variability in response to amphetamine and modulates the development of tolerance to opioids. Activation of Cckbr decreases amphetamine-induced dopamine release .

• Addiction and substance abuse: In rat models, Cckbr antagonism prevents stress-induced reactivation of cocaine-induced conditioned place preference and blocks the reinstatement of morphine-induced conditioned place preference, suggesting its potential role in addiction mechanisms .

• Parkinson's disease: Cckbr may have a modulatory role in Parkinson's disease. Blockade of Cckbr in dopamine-depleted squirrel monkeys enhances the locomotor response to L-DOPA. Additionally, visual hallucinations in Parkinson's disease have been associated with cholecystokinin polymorphisms .

These findings highlight the complex interplay between Cckbr and dopaminergic systems, suggesting potential therapeutic targets for neurological disorders involving dopamine dysregulation.

What structural elements of Cckbr are critical for receptor function and how do mutations affect its signaling properties?

The structural elements of Cckbr play crucial roles in determining its binding affinity, internalization dynamics, and signaling properties. Several key structural regions have been identified as critical for receptor function:

• Ligand binding domain: Cckbr has high affinity for both sulfated and nonsulfated CCK analogs, with IC50 values ranging from 0.42 ± 0.05 to 0.53 ± 0.05 nM for different receptor constructs . The binding domain maintains similar affinity across various receptor mutants, suggesting its structural integrity is preserved even with significant C-terminal modifications.

• Carboxyl-terminal domain: The C-terminal region of Cckbr is essential for receptor internalization. Truncation of this domain (CCKBR Tr408) dramatically reduces the receptor's capacity for internalization following ligand binding . This suggests that the C-terminal tail contains critical determinants for interaction with the endocytic machinery.

• Serine/threonine residues: Mutation of all serine and threonine residues in the C-terminus to alanines (CCKBR ΔS/T) significantly impairs receptor internalization . This indicates that phosphorylation of these residues likely plays a role in the receptor's regulatory mechanisms.

• G-protein coupling regions: As a G protein-coupled receptor, Cckbr contains intracellular domains that interact with G proteins to initiate downstream signaling cascades. These regions are essential for the receptor's ability to transmit mitogenic signals in a ligand-dependent manner .

Mutations in these structural elements can have profound effects on receptor function. For example, truncation or modification of the C-terminal domain significantly impairs internalization without affecting ligand binding affinity . In the context of disease, misspliced transcript variants including an intron have been observed in cells from colorectal and pancreatic tumors , potentially altering receptor function and contributing to pathological processes.

What are the recommended methods for studying Cckbr in transgenic mouse models?

When investigating Cckbr function using transgenic mouse models, researchers should consider several methodological approaches that have been successfully employed in previous studies:

• Gene targeting and confirmation of knockout: The generation of Cckbr-deficient mice through gene targeting in embryonic stem cells has proven valuable for investigating receptor function in vivo. Confirmation of gene disruption should include both RNA analysis (to verify absence of receptor mRNA) and radioligand binding assays (to confirm lack of functional protein) .

• Physiological assessment of gastric function: Given Cckbr's important role in gastric physiology, assessment of acid secretion is crucial. Basal acid output can be measured and compared between knockout and wild-type mice. In previous studies, CCKBR-/- mice showed significantly reduced acid output (3.8 ± 2.1 mEq/hr) compared to wild-type mice (13.1 ± 3.3 mEq/hr) .

• Serum gastrin measurement: Radioimmunoassay for gastrin is essential for evaluating the hypergastrinemia that typically results from Cckbr deficiency. Studies have shown that serum gastrin levels in CCKBR-/- mice (1582 ± 257 pg/ml) are approximately five times higher than those in wild-type mice (293 ± 60.6 pg/ml) .

• Histological and immunohistochemical analysis: Examination of gastric mucosa through histological staining and immunohistochemistry for markers such as H+,K+-ATPase (for parietal cells) and chromogranin A (for ECL cells) provides valuable insights into morphological changes associated with Cckbr deficiency .

• Pharmacological challenge studies: Administration of proton pump inhibitors (PPIs) or other drugs that affect gastric physiology can be used to investigate the receptor's role in drug-induced responses. For example, PPI treatment induces hypergastrinemia and gastric mucosal hypertrophy in wild-type mice but not in CCKBR-/- mice .

• Age-dependent phenotyping: Long-term studies tracking mice up to 24 months of age are recommended to identify any age-dependent phenotypes associated with Cckbr deficiency .

These methodological approaches provide a comprehensive framework for investigating the physiological and pathological roles of Cckbr in vivo.

What techniques are most effective for studying Cckbr internalization and trafficking?

Several specialized techniques have proven effective for investigating Cckbr internalization and trafficking in cell culture systems:

• Radioligand binding and stripping assays: This approach involves incubating cells with radiolabeled ligands (e.g., 125I-BH-CCK-8) followed by stripping of surface-bound ligand using KSCN. The remaining cell-associated radioactivity represents internalized receptor-ligand complexes. This technique allows for quantitative assessment of internalization kinetics over time .

• Cell cloning and receptor expression standardization: Selecting cell clones with similar receptor densities is crucial for comparative studies. Previous research has utilized clones with receptor densities ranging from 26 × 103 to 291 × 103 receptors/cell . Standardizing receptor expression levels allows for direct comparison between wild-type and mutant receptors.

• Competition binding assays: These assays help determine the binding affinity of receptors for various ligands. For Cckbr, competition curves using 125I-BH-CCK-8 and unlabeled CCK-8 have demonstrated specific high-affinity binding with IC50 values between 0.42 ± 0.05 and 0.53 ± 0.05 nM .

• Mutant receptor construction: Site-directed mutagenesis techniques can be employed to create receptor variants with specific modifications, such as truncations or substitution of serine/threonine residues with alanines. These mutants can then be used to investigate the structural determinants of receptor internalization .

• Fluorescence-based trafficking assays: While not explicitly mentioned in the search results, fluorescent tagging of receptors (e.g., with GFP) combined with confocal microscopy is widely used to visualize receptor trafficking in real-time.

• Biochemical fractionation: Subcellular fractionation techniques can be used to isolate different cellular compartments and quantify the distribution of receptors between membrane and intracellular locations following ligand stimulation.

When designing experiments to study Cckbr internalization, researchers should consider the significant differences in internalization properties between Cckbr and related receptors like CCKAR, as well as the impact of structural modifications on these processes.

How can researchers effectively measure and analyze Cckbr signaling pathways?

Effective measurement and analysis of Cckbr signaling pathways require specialized techniques targeting various aspects of receptor function:

• G protein coupling assays: As a G protein-coupled receptor, Cckbr initiates signaling through interaction with G proteins. Techniques such as [35S]GTPγS binding assays can be used to measure G protein activation following receptor stimulation, providing direct evidence of functional coupling.

• Downstream second messenger analysis: Depending on the G protein subtype coupled to Cckbr, various second messengers may be activated. Measurement of intracellular calcium mobilization, cAMP production, or phosphoinositide hydrolysis can provide insights into the receptor's signaling mechanisms.

• Mitogenic activity assessment: Cckbr has been shown to transmit mitogenic signals in a ligand-dependent manner . Proliferation assays using techniques such as BrdU incorporation, Ki-67 staining, or cell counting can be used to assess the receptor's role in cell growth regulation.

• Gene expression analysis: The effects of Cckbr activation on target gene expression can be assessed using techniques such as RT-PCR, RNA sequencing, or microarray analysis. For example, the expression of genes such as H+,K+-ATPase and histidine decarboxylase (HDC) in the stomach is influenced by Cckbr signaling .

• Signaling inhibitor studies: The use of specific inhibitors targeting different components of potential signaling pathways can help delineate the exact mechanisms through which Cckbr exerts its effects. This approach can be particularly valuable for identifying the pathways involved in Cckbr-mediated cell proliferation.

• Receptor phosphorylation analysis: Phosphorylation of Cckbr, particularly at serine/threonine residues in the C-terminal domain, may play a role in receptor regulation. Western blotting with phospho-specific antibodies or mass spectrometry-based approaches can be used to detect and quantify receptor phosphorylation.

• Receptor-protein interaction studies: Techniques such as co-immunoprecipitation, proximity ligation assays, or yeast two-hybrid screens can identify proteins that interact with Cckbr, providing insights into its signaling and regulatory mechanisms.

By combining these approaches, researchers can develop a comprehensive understanding of the complex signaling networks activated by Cckbr in different cellular contexts.

What is the connection between Cckbr and gastrointestinal cancers?

Cckbr has been implicated in several gastrointestinal cancers, with research highlighting its potential roles in cancer development and progression:

The connection between Cckbr and gastrointestinal cancers is an active area of research, with potential implications for both understanding cancer biology and developing targeted therapies.

How does Cckbr modulate neurological functions and what are its implications for neurological disorders?

Cckbr plays diverse roles in modulating neurological functions, with significant implications for various neurological disorders:

• Anxiety and depression: CCK-B expression may correlate with anxiety and depression phenotypes in humans, suggesting a potential role for this receptor in mood disorders . The receptor's abundant expression in the cerebral cortex and brain basal ganglion supports its involvement in higher brain functions .

• Dopamine regulation: Cckbr exhibits complex effects on dopaminergic neurotransmission, generally exerting an inhibitory action on dopamine activity in the brain. This regulation varies by brain region, with CCK-B antagonism enhancing dopamine release in the rat striatum while receptor activation enhances GABA release in the anterior nucleus accumbens .

• Addiction and substance abuse: Cckbr plays a role in addiction mechanisms, with antagonism preventing stress-induced reactivation of cocaine-induced conditioned place preference and blocking the maintenance and reinstatement of morphine-induced conditioned place preference in rats. Blockade of CCK-B potentiates cocaine-induced dopamine overflow in rat striatum .

• Parkinson's disease: Cckbr may have a modulatory role in Parkinson's disease. Blockade of CCK-B in dopamine-depleted squirrel monkeys enhances the locomotor response to L-DOPA, suggesting a potential therapeutic target for improving dopaminergic therapy. Visual hallucinations in Parkinson's disease have been associated with cholecystokinin polymorphisms .

• Feeding behavior: As a receptor for cholecystokinin, which is known to regulate appetite and satiety, Cckbr likely plays a role in the central regulation of feeding behavior .

These diverse neurological functions highlight Cckbr as a potential therapeutic target for various neurological and psychiatric disorders, particularly those involving dopaminergic dysregulation or anxiety.

What are the potential therapeutic applications of targeting Cckbr in various diseases?

Based on its diverse physiological roles, targeting Cckbr presents several promising therapeutic applications:

• Gastric disorders: Given Cckbr's critical role in gastric acid secretion and mucosal cell proliferation , modulating its activity could be beneficial for treating conditions such as peptic ulcer disease, gastroesophageal reflux disease (GERD), or atrophic gastritis.

• Gastrointestinal cancers: The association of CCKBR with pancreatic and stomach cancers , along with the observation of misspliced variants in colorectal and pancreatic tumors , suggests that targeting this receptor might be a strategy for cancer treatment or prevention.

• Parkinson's disease: CCK-B antagonism enhances the locomotor response to L-DOPA in animal models of Parkinson's disease , suggesting that Cckbr antagonists could potentially be used as adjunctive therapy to improve the efficacy of dopaminergic treatments.

• Addiction and substance abuse: The role of Cckbr in modulating dopamine release and influencing addiction-related behaviors points to potential applications in treating drug addiction, particularly for substances like cocaine and opioids.

• Anxiety and mood disorders: The correlation between CCK-B expression and anxiety/depression phenotypes suggests that Cckbr ligands could be developed as novel anxiolytic or antidepressant agents.

• Feeding disorders and obesity: As Cckbr is involved in regulating feeding behaviors , targeting this receptor might offer approaches for managing appetite and weight in obesity or eating disorders.