Recombinant Mouse Taste receptor type 2 member 136 (Tas2r136)

What is Tas2r136 and how is it classified within the mouse bitter taste receptor family?

Tas2r136 is a member of the taste receptor type 2 (Tas2r) family in mice, which functions as a G protein-coupled receptor responsible for bitter taste sensation. It belongs to the broader collection of approximately 35 putatively functional Tas2r genes in mice that enable the recognition of numerous bitter chemicals . Mouse Tas2r136 is part of the genetic repertoire that allows for detection of potentially harmful substances through bitter taste perception. Within the classification system, Tas2r136 is taxonomically positioned among other bitter taste receptors, which vary in their tuning breadth (ability to respond to multiple compounds) and expression patterns .

What is known about the expression profile of Tas2r136 in gustatory and non-gustatory tissues?

Research indicates that Tas2r136, like other members of the Tas2r family, is expressed in gustatory tissue, confirming its role in bitter taste perception. Quantitative RT-PCR (qRT-PCR) expression analyses have demonstrated that all mouse Tas2r genes, including Tas2r136, are expressed in the epithelium of the posterior tongue, though at varying levels . While some receptor mRNAs like Tas2r108, Tas2r118, Tas2r126, Tas2r135, and Tas2r137 show abundant expression (reaching ~20% of α-gustducin mRNA levels), others like Tas2r114, Tas2r122, and Tas2r140 are barely detectable .

Beyond gustatory tissues, Tas2r receptors have been identified in non-gustatory tissues including the choroid plexus, the epithelium, and the vascular system . Though specific data for Tas2r136 in non-gustatory tissues is limited in the provided search results, research suggests that bitter taste receptors, including possibly Tas2r136, may have functional roles beyond taste perception .

How does the expression of Tas2r136 compare with other Tas2r family members?

Expression data from the limited information available suggests that Tas2r136 shows expression levels of 2.3, 2.5, and 1.8 in different conditions, though the exact context of these measurements is not fully clear from the search results . Generally, mouse Tas2r family members exhibit considerable variation in expression levels. Some receptors like Tas2r108, Tas2r118, Tas2r126, Tas2r135, and Tas2r137 show high expression levels, while others like Tas2r114, Tas2r122, and Tas2r140 show much lower expression . The majority of Tas2r genes exhibit intermediate expression levels .

What are the functional characteristics of Tas2r136 compared to other members of the mouse Tas2r family?

While specific functional data for Tas2r136 is limited in the provided search results, insights from research on other mouse Tas2r receptors suggest considerable variation in functional characteristics among family members. Mouse Tas2r receptors vary significantly in their tuning breadth, with some functioning as generalists capable of recognizing many different bitter compounds, while others act as specialists responding to only a few specific compounds .

For instance, Tas2r105 functions as a generalist with an extremely broad agonist profile, recognizing over 30% of tested bitter compounds, while most other Tas2r receptors show more selective response profiles . Without specific functional characterization data for Tas2r136, researchers should consider heterologous expression analysis similar to that performed for other Tas2r receptors to determine its specific agonist profile and response characteristics. Such analysis would likely involve expressing the receptor in cell lines like HEK293T cells expressing appropriate G-protein alpha subunits (such as Gα16gust44), followed by calcium mobilization assays with a library of bitter compounds .

How do hormonal changes affect the expression of Tas2r136 and other taste receptors?

Evidence from studies on the choroid plexus (CP) suggests that female sex hormones may regulate certain taste receptors. In silico analysis of cDNA microarray data revealed that declining hormone levels in female rats upon ovariectomy induced up-regulation of several T2Rs, including Tas2r109, Tas2r124, Tas2r134, and Tas2r144, as well as downstream effector molecules Plcb2 and Trpm5 . Additionally, Tas2r109 and Tas2r144 showed sexual dimorphism in expression, with higher levels in males .

Though specific data for Tas2r136 regulation by hormones is not explicitly mentioned in the search results, the observed hormonal regulation of other Tas2r family members suggests that Tas2r136 might also be subject to similar regulatory mechanisms. Researchers interested in hormonal regulation of Tas2r136 should consider experimental designs that compare expression levels between males and females, and between normal and hormone-depleted animals (e.g., ovariectomized females) or tissue cultures treated with hormones like estradiol (E2) and progesterone (P4) .

How does Tas2r136 contribute to bitter taste perception in the context of other co-expressed receptors?

For instance, seven mouse receptors (Tas2r105, Tas2r108, Tas2r115, Tas2r126, Tas2r137, Tas2r140, and Tas2r144) are activated by quinine at similar concentrations between 3.0 and 10 μM . In contrast, saccharin activates four different receptors (Tas2r135, Tas2r105, Tas2r109, and Tas2r144) with staggered threshold concentrations ranging from 0.1 mM to 10 mM, potentially resulting in a graded bitter response involving progressively more receptors as concentration increases .

Understanding Tas2r136's specific role would require determining its expression pattern, identifying its agonists, and characterizing its response profiles in comparison to co-expressed receptors.

What heterologous expression systems are most effective for functional characterization of Tas2r136?

For functional characterization of Tas2r136, researchers should consider the following heterologous expression systems that have proven effective for other Tas2r receptors:

• HEK293T cells expressing Gα16gust44: This system has shown higher sensitivity than Gα15-based assays for detecting Tas2r responses . The Gα16gust44 chimeric G-protein, which contains the last 44 amino acids of gustducin, more efficiently couples bitter taste receptors to the phospholipase C signaling cascade, resulting in calcium release that can be measured with calcium-sensitive fluorescent dyes.

• Recommended assay protocol:

  • Transfect HEK293T cells with Tas2r136 expression construct and Gα16gust44

  • Load transfected cells with calcium-sensitive dye (e.g., Fluo-4)

  • Apply test compounds and monitor calcium responses using fluorescence imaging or plate reader

  • Include positive controls (known bitter compounds) and negative controls (vehicle only)

  • For compounds showing activity, determine dose-response relationships

• Validation approaches: Responses should be validated using:

  • Receptor antagonists where available

  • Comparison with non-transfected cells or cells expressing other Tas2r receptors

  • Mutation of key residues in the receptor to confirm specificity

The choice between different G-protein partners (Gα15 vs. Gα16gust44) is critical, as demonstrated by the discovery that low-efficacy activators of Tas2r105 showed diminished or absent responses in Gα15-expressing cells compared to Gα16gust44-expressing cells .

What strategies can be employed to identify specific agonists and antagonists for Tas2r136?

To identify specific agonists and antagonists for Tas2r136, researchers should consider the following systematic approach:

• Compound library screening:

  • Start with a diverse library of bitter compounds (similar to the 128 substances used for other Tas2r receptors)

  • Include natural bitter compounds, synthetic bitter chemicals, and pharmaceuticals known to have bitter taste

  • Test compounds at multiple concentrations to determine threshold response levels

  • Compare responses to those of other Tas2r receptors to identify selective agonists

• Structure-activity relationship (SAR) studies:

  • For compounds showing activity, test structural analogs to determine molecular features required for activation

  • Use computational modeling based on receptor structure predictions to guide compound selection

  • Develop potency and efficacy profiles for active compounds

• Antagonist identification:

  • Screen for compounds that inhibit responses to known agonists

  • Test general bitter taste antagonists like Probenecid, which has been shown to block T2R responses in choroid plexus cells

  • Characterize antagonist specificity across multiple Tas2r receptors

• Validation in native tissue:

  • Confirm findings from heterologous expression systems in native tissue preparations where possible

  • Use calcium imaging in taste cells from transgenic mice or tissue preparations expressing Tas2r136

  • Correlate receptor activation with behavioral responses in mice

What is the significance of Tas2r136 expression in non-gustatory tissues?

The expression of bitter taste receptors, including potentially Tas2r136, in non-gustatory tissues suggests important physiological roles beyond taste perception. Based on research on other T2Rs, these non-gustatory functions might include:

• Epithelial function: Bitter taste receptors in the epithelium may serve as chemical sensors that detect potentially harmful compounds and trigger protective responses . The expression of bitter taste receptors in the choroid plexus suggests they may play a role in sensing compounds in the cerebrospinal fluid (CSF) and/or blood .

• Vascular system function: Bitter taste receptors in the vascular system may contribute to vascular regulation. Research has shown that bitter taste receptors serve functions in both the epithelium and the vascular system in non-gustatory tissues .

• Potential clinical implications: Understanding the role of Tas2r136 in non-gustatory tissues could have implications for various conditions. For instance, research has shown upregulation of bitter taste receptors in severe asthmatics , suggesting potential involvement in respiratory function or pathology.

Research on the choroid plexus has demonstrated that bitter stimuli can elicit calcium responses in CP cells, and these responses are diminished in the presence of the T2R blocker Probenecid, suggesting functional bitter taste sensing in this tissue . Similar approaches could be used to investigate the functional significance of Tas2r136 in non-gustatory tissues.

How does the agonist profile of Tas2r136 compare between mouse models and human orthologs?

While specific information about Tas2r136's agonist profile and human orthologs is not provided in the search results, general comparisons between mouse and human bitter taste receptors reveal several important patterns:

• Species-specific differences in receptor tuning:

  • Mouse and human bitter taste receptors often show different agonist profiles, even for orthologous pairs

  • Some bitter compounds activate similar numbers of receptors in both species (e.g., quinine activates seven mouse and nine human receptors)

  • Other compounds show species-specific patterns (e.g., diphenidol activates more than twice as many receptors in humans as in mice, while PROP activates more mouse receptors than human receptors)

• Evolutionary implications:

  • Species-specific expansions of Tas2r genes may have resulted in specialized receptors for bitter compounds of species-specific relevance

  • The differences in receptor tuning may reflect adaptation to different ecological niches and dietary requirements

• Translational research considerations:

  • Researchers should be cautious when extrapolating findings from mouse Tas2r136 to human bitter taste receptors

  • Parallel testing of mouse Tas2r136 and potential human orthologs with the same compound libraries would be necessary to establish functional equivalence

To properly characterize Tas2r136 in comparison to human orthologs, researchers would need to conduct comparative pharmacological profiling using the same methodologies and compound libraries.

What is the current understanding of Tas2r136 regulation by hormonal and environmental factors?

While specific information about Tas2r136 regulation is limited in the search results, studies on other bitter taste receptors provide insights into potential regulatory mechanisms:

• Hormonal regulation:

  • Sex hormones appear to regulate the expression of several bitter taste receptors in the choroid plexus

  • Ovariectomy in female rats induces up-regulation of multiple T2Rs (Tas2r109, Tas2r124, Tas2r134, and Tas2r144)

  • Some bitter taste receptors show sexual dimorphism in expression, with higher levels in males (Tas2r109 and Tas2r144)

• Developmental regulation:

  • The expression patterns of bitter taste receptors may change during development

  • Studies have used choroid plexus explants from newborn rats to investigate hormonal regulation of taste-related genes

• Tissue-specific regulation:

  • The regulation of Tas2r genes appears to be tissue-specific

  • For example, receptors with low expression in gustatory tissue (e.g., Tas2r114) may show robust expression in other tissues like testis

To specifically investigate Tas2r136 regulation, researchers could:

• Compare expression levels between male and female animals

• Examine expression changes in hormone-depleted animals (e.g., ovariectomized females)

• Treat cultured cells or tissue explants expressing Tas2r136 with hormones like estradiol and progesterone

• Investigate developmental changes in expression patterns

What are the promising approaches for studying the structure-function relationship of Tas2r136?

Given the limited structural information available for bitter taste receptors, including Tas2r136, several approaches could advance our understanding of structure-function relationships:

• Homology modeling and molecular dynamics simulations:

  • Develop structural models of Tas2r136 based on available GPCR structures

  • Use computational approaches to predict ligand binding sites

  • Simulate receptor-ligand interactions to understand binding modes and activation mechanisms

• Site-directed mutagenesis:

  • Identify conserved and variable residues across the Tas2r family

  • Create point mutations in predicted ligand-binding sites or activation domains

  • Test mutant receptors in functional assays to correlate structural features with agonist specificity

• Chimeric receptor approaches:

  • Create chimeric receptors between Tas2r136 and other Tas2r receptors with known agonist profiles

  • Map domains responsible for agonist specificity and receptor activation

  • Identify structural determinants of species-specific differences in receptor function

• Advanced structural biology techniques:

  • Apply emerging techniques like cryo-electron microscopy or X-ray crystallography to determine the three-dimensional structure of Tas2r136

  • Use biophysical methods to study conformational changes upon agonist binding

These approaches would provide valuable insights into how Tas2r136 recognizes specific bitter compounds and how structural features contribute to its unique functional properties.

What technological advances would facilitate better characterization of Tas2r136 in native tissues?

Several technological advances could enhance our ability to study Tas2r136 in its native cellular context:

• Single-cell transcriptomics:

  • Apply single-cell RNA sequencing to taste bud cells to characterize the co-expression patterns of Tas2r136 with other taste receptors and signaling components

  • Identify distinct cell populations expressing Tas2r136 in gustatory and non-gustatory tissues

• CRISPR-Cas9 genome editing:

  • Generate Tas2r136 knockout mice to study the physiological consequences of receptor deletion

  • Create knock-in mice with tagged Tas2r136 to facilitate protein detection and localization studies

  • Introduce reporter genes under the control of the endogenous Tas2r136 promoter to monitor expression patterns

• Advanced imaging techniques:

  • Apply super-resolution microscopy to visualize Tas2r136 distribution in native tissues

  • Use calcium imaging in acute tissue preparations to monitor receptor activation in real-time

  • Develop biosensors to detect Tas2r136 activation in living animals

• Organoid models:

  • Develop taste bud organoids expressing Tas2r136 to study receptor function in a more physiologically relevant context

  • Create organoid models of non-gustatory tissues expressing Tas2r136 to investigate its roles beyond taste perception

These technological advances would provide more comprehensive and physiologically relevant insights into Tas2r136 function and regulation.