Recombinant Mouse Taste receptor type 2 member 123 (Tas2r123)

What is Recombinant Mouse Taste receptor type 2 member 123 (Tas2r123)?

Tas2r123 belongs to the taste receptor type 2 (Tas2r) family in mice, which are G protein-coupled receptors responsible for bitter taste perception. The recombinant form refers to the protein expressed from cloned Tas2r123 DNA in a heterologous expression system. As part of the mouse bitter taste receptor repertoire, Tas2r123 is one of 35 putatively functional Tas2r genes that collectively enable mice to recognize numerous bitter compounds. Like other Tas2r proteins, it likely contains seven transmembrane domains characteristic of G protein-coupled receptors and functions within the taste transduction pathway .

Where is Tas2r123 expressed in mouse tissues?

While specific expression data for Tas2r123 is not directly provided in the search results, we can infer from studies of other mouse Tas2r genes that Tas2r123 is likely expressed in gustatory tissue, particularly in the posterior papillae of the tongue. Quantitative RT-PCR analysis has demonstrated that all mouse Tas2r genes are expressed in the epithelium of the posterior tongue, though with varying expression levels. Some receptors show high expression (reaching ~20% of α-gustducin mRNA levels), while others are barely detectable . Additionally, Tas2r genes have been detected in non-gustatory tissues such as testis and heart, suggesting Tas2r123 may potentially be expressed in these tissues as well .

How does Tas2r123 function in bitter taste perception?

As a member of the Tas2r family, Tas2r123 is involved in the detection of bitter compounds, which often signal potentially harmful substances. The receptor likely functions through G protein-coupled signaling pathways, specifically coupling with gustducin or related G proteins in taste cells. When activated by bitter ligands, Tas2r123 would initiate a signaling cascade resulting in calcium release and eventual nerve impulse generation, leading to the perception of bitter taste .

Mouse Tas2r receptors vary considerably in their response profiles, with some being highly selective for specific compounds (specialists) and others responding to multiple bitter substances (generalists). The specific bitter compounds that activate Tas2r123 would need to be determined experimentally through deorphanization studies similar to those conducted for other Tas2r receptors .

What experimental methods are used to study Tas2r123 expression?

Based on approaches used for other mouse Tas2r receptors, the following methods are commonly employed to study Tas2r expression:

• Quantitative RT-PCR (qRT-PCR): This method quantifies the relative expression levels of Tas2r genes in taste tissues, with expression typically normalized to control genes like α-gustducin .

• In situ hybridization: This technique visualizes Tas2r mRNA expression at the cellular level in tissue sections, revealing both the number of cells expressing the gene and the signal intensity .

• Immunohistochemistry: Using antibodies specific to Tas2r123, this method can localize the protein in tissue sections.

• Single-cell RNA sequencing: This advanced approach can provide insights into the co-expression patterns of Tas2r123 with other genes in individual taste cells.

These methods collectively provide a comprehensive picture of where, when, and at what levels Tas2r123 is expressed in mouse tissues .

How is Tas2r123 related to other mouse taste receptors?

Tas2r123 is one of the 35 putatively functional Taste receptor type 2 (Tas2r) genes in mice. These receptors form a family with varying degrees of sequence similarity and likely evolved through gene duplication events to enable the recognition of diverse bitter compounds. The mouse Tas2r gene family exhibits significant variation in expression levels, with some receptors being highly abundant in taste cells while others show much lower expression .

Mouse Tas2r receptors also vary considerably in their tuning breadth, with some functioning as generalists that recognize many compounds and others as specialists with narrower response profiles. In mice, only a single Tas2r (Tas2r105) has been identified as an extreme generalist, responding to over 30% of tested bitter compounds . The relationship of Tas2r123 to this spectrum of specificity would need to be determined experimentally.

What are the optimal conditions for heterologous expression of Tas2r123?

Based on successful expression systems used for other mouse Tas2r receptors, the following conditions are recommended for heterologous expression of Tas2r123:

Expression System Parameters:

The search results indicate that the choice of G protein significantly affects assay sensitivity. For example, Tas2r105 showed responses to multiple compounds when coupled with Gα16gust44 but appeared highly selective when using Gα15 . Therefore, Gα16gust44 is strongly recommended for Tas2r123 studies to avoid false negatives, particularly when screening for receptor agonists .

What bitter compounds are known to activate Tas2r123?

While the search results don't specifically identify agonists for Tas2r123, deorphanization of this receptor would require a systematic approach similar to that used for other mouse Tas2r receptors:

Methodology for Agonist Identification:

• Express Tas2r123 in HEK293T cells with Gα16gust44

• Screen with a diverse library of bitter compounds (similar to the 128 compound library mentioned)

• Measure activation using calcium imaging or other functional assays

• Validate hits with dose-response curves to determine EC50 values

Based on patterns observed with other mouse Tas2r receptors, common activators might include:

The search results indicate that most bitter compounds activate several mouse Tas2r receptors. Quinine and sucralose each activated seven different Tas2r receptors, while PROP and diphenidol activated six receptors each . Therefore, it's likely that Tas2r123 shares some agonists with other mouse bitter taste receptors, though it may also have unique response characteristics.

How does Tas2r123 compare functionally to its human ortholog?

While the search results don't specifically discuss Tas2r123's human ortholog, they provide insights about functional comparisons between mouse and human bitter taste receptors:

Comparative Characteristics of Mouse vs. Human Bitter Receptors:

The search results reveal that functional differences exist among mouse and human bitter taste receptor orthologs, requiring some "adjustment of firm beliefs in light of these data" . This suggests that even if a human ortholog for Tas2r123 can be identified through sequence homology, its functional properties may differ significantly.

To establish functional orthology, comparative pharmacological profiling would be necessary, testing both Tas2r123 and candidate human orthologs against the same panel of bitter compounds under identical experimental conditions .

How can I design experiments to determine the binding sites of Tas2r123?

Designing experiments to identify the binding sites of Tas2r123 requires a multi-faceted approach:

Experimental Strategy for Binding Site Identification:

• Computational Approaches:

  • Homology modeling based on GPCR structures

  • Molecular docking simulations with known or predicted agonists

  • Sequence alignment with other Tas2r receptors to identify conserved motifs

• Site-Directed Mutagenesis Strategy:

  • Target conserved residues in transmembrane domains

  • Focus on TM3, TM5, and TM6, which often form the binding pocket in GPCRs

  • Create alanine scanning mutations followed by targeted substitutions

  Mutation Type	Target Selection	Analysis Method
  Alanine Scanning	Conserved residues in TMs	Calcium imaging with dose-response curves
  Conservative Substitutions	Residues affecting function in alanine scan	Calculation of EC50 shifts
  Radical Substitutions	Key binding residues	Analysis of both efficacy and potency changes

• Chimeric Receptor Approach:

  • Create domain swaps between Tas2r123 and functionally characterized Tas2r receptors

  • Test with multiple agonists to identify regions responsible for ligand specificity

  • Narrow focus to specific residues through subsequent mutagenesis

• Validation Through Structure-Activity Relationships:

  • Test structurally related compounds with varying potencies

  • Correlate structural features of ligands with binding site mutations

  • Build a pharmacophore model that explains structure-activity relationships

This systematic approach would provide comprehensive insights into the molecular determinants of ligand binding to Tas2r123, contributing to the broader understanding of bitter taste receptor pharmacology .

What are the challenges in studying receptor-ligand interactions for Tas2r123?

Based on challenges reported for other Tas2r receptors, several technical and biological hurdles likely affect Tas2r123 research:

Technical Challenges:

• Expression System Limitations:

  • Poor membrane trafficking in heterologous systems

  • Differences in signaling efficiency between expression systems

  • Variable results depending on G protein co-expression (as observed with Tas2r105)

• Assay Considerations:

  • Many bitter compounds have limited solubility and may precipitate at higher concentrations

  • Bitter compounds may exhibit non-specific effects at high concentrations

  • Some compounds may be autofluorescent, interfering with calcium imaging assays

Biological Challenges:

• Promiscuity and Selectivity:

  • Most bitter compounds activate multiple Tas2r receptors (e.g., quinine activates seven mouse Tas2rs)

  • Difficult to distinguish physiologically relevant interactions from in vitro artifacts

  • Potential for receptor cooperativity or dimerization affecting ligand responses

• Structural Complexity:

  • Lack of crystal structures for any Tas2r receptor

  • Limited homology with GPCRs of known structure

  • Multiple potential binding sites for structurally diverse bitter compounds

Methodological Solutions:

How can contradictory data in Tas2r123 activation assays be analyzed and explained?

When confronted with contradictory results in Tas2r123 activation studies, a systematic approach to reconciliation is essential:

Framework for Analyzing Contradictory Data:

• Methodological Comparison:

  Experimental Factor	Potential Impact	Resolution Approach
  G Protein Coupling	Different G proteins (Gα15 vs. Gα16gust44) dramatically affect sensitivity	Direct comparison using identical conditions
  Expression Level	Variable receptor expression affects response magnitude	Normalize to surface expression
  Assay Sensitivity	Different detection methods have varying thresholds	Use multiple orthogonal assays
  Compound Preparation	Solubilization methods affect compound availability	Standardize preparation protocols

• Statistical Analysis Framework:

  • Apply appropriate statistical tests to determine if differences are significant

  • Calculate confidence intervals for EC50 and efficacy values

  • Consider meta-analysis approaches when comparing across multiple studies

• Biological Explanations to Consider:

  • Receptor states and conformations may differ between assay systems

  • Allosteric modulators may be present in some experimental contexts

  • Post-translational modifications may affect receptor function

  • Species or strain differences in receptor sequence or expression

What are the best experimental designs for studying Tas2r123 in vivo?

Based on the experimental design principles in the search results and the specific characteristics of taste receptor research, several robust approaches are recommended for studying Tas2r123 in vivo:

True Experimental Designs:

Specialized Behavioral Paradigms:

Physiological Approaches:

• Neural Recording:

  • Record from gustatory nerves (chorda tympani, glossopharyngeal)

  • Compare responses to bitter compounds between wild-type and Tas2r123-modified mice

  • Correlate with behavioral responses

• Calcium Imaging in Taste Cells:

  • Isolate taste cells or use ex vivo preparations

  • Measure responses to bitter compounds

  • Compare cells from wild-type and Tas2r123-modified mice

Genetic Approaches:

• Conventional Knockout:
  Traditional gene deletion to eliminate Tas2r123 function.

• Conditional Knockout:
  Using Cre-loxP system for taste-cell-specific or inducible deletion.

• Knockin Models:
  Replace mouse Tas2r123 with human ortholog to study functional differences.

The search results emphasize that the selection of appropriate experimental design is crucial for valid scientific inference, as different designs control for different threats to validity . For Tas2r123 research, combining behavioral, physiological, and genetic approaches provides the most comprehensive understanding of receptor function in vivo.