Recombinant Human Taste receptor type 2 member 31 (TAS2R31)

What is TAS2R31 and what is its role in bitter taste perception?

TAS2R31 (also formerly known as T2R31 or T2R44) is a G protein-coupled receptor that belongs to the TAS2R family of bitter taste receptors. It functions by responding to bitter tastants, triggering depolarization of taste bud cells, which initiates the sensation of bitterness . This receptor is particularly notable for its role in mediating the bitter aftertaste of artificial sweeteners, including saccharin and acesulfame potassium . TAS2R31 represents an important component of the approximately 25 different bitter taste receptors in humans that collectively enable the detection of thousands of structurally diverse bitter compounds.

The mechanism of action involves ligand binding to the receptor, which triggers a signal transduction cascade that ultimately leads to taste perception. Interestingly, TAS2R31 demonstrates overlapping ligand specificity with other TAS2R receptors, suggesting an integrated network of bitter taste perception rather than isolated receptor functions .

How can researchers express and characterize TAS2R31 in laboratory settings?

Researchers typically employ heterologous expression systems to study TAS2R31 function. The most common approach involves:

• Cloning the TAS2R31 coding sequence into an expression vector

• Transfecting the construct into appropriate cell lines (commonly HEK293T cells)

• Co-expressing chimeric G proteins to enhance coupling efficiency

• Measuring receptor activation through calcium imaging or other suitable functional assays

For optimal expression, researchers often use codon optimization and add epitope tags that don't interfere with receptor function. The receptor's response can be assessed by measuring intracellular calcium release following application of known agonists such as saccharin or acesulfame K . Dosage-response curves should be generated to fully characterize receptor functionality, with EC50 values determined to quantify sensitivity to specific compounds.

When studying receptor variants, site-directed mutagenesis techniques can be employed to generate specific amino acid substitutions of interest, followed by comparative functional analyses to assess their impact on receptor function .

What are the known agonists and antagonists of TAS2R31?

TAS2R31 responds to several agonists, most notably the artificial sweeteners saccharin and acesulfame potassium, which produce bitter aftertastes in humans . The receptor's activation by these compounds explains their characteristic bitter off-taste that some individuals perceive more strongly than others.

Regarding antagonists, research has identified 3β-hydroxypelenolide as a compound that can block TAS2R31 activation . This molecule demonstrates the interesting complexity of bitter taste modulation, as it blocks TAS2R31 while simultaneously acting as an agonist for other TAS2R receptors. This dual functionality as both agonist and antagonist for different receptors within the same family highlights the intricate nature of bitter taste perception mechanisms.

The identification of both agonists and antagonists provides valuable tools for researchers to probe receptor function and potentially develop taste-modifying compounds for various applications in food science and pharmacology.

How does genetic variation in TAS2R31 influence bitter taste perception across populations?

Genetic variation in TAS2R31 has significant implications for bitter taste perception across global populations. Comprehensive sequencing studies have revealed extensive diversity in TAS2R genes, including TAS2R31 . One particularly notable variant is the TAS2R31-R35W substitution, which has been demonstrated to have strong effects on receptor function and is associated with altered taste responses to the bitter off-tastes of saccharin and acesulfame potassium .

The complexity of genetic influence is further illustrated by the TAS2R31-D45H substitution, which affects receptor function only when combined with the W35 allele, demonstrating the importance of considering haplotypes rather than individual polymorphisms in isolation . This epistatic interaction exemplifies why simplistic genotype-phenotype correlations may be insufficient for understanding taste variation.

Global population studies have cataloged numerous variants across TAS2R genes, with a comprehensive analysis of the 1000 Genomes Project data identifying hundreds of nonsynonymous variants in the TAS2R family . Specifically for TAS2R31, multiple potentially functionally impactful variants have been identified, though many occur at low frequencies in populations, suggesting their effects may be limited to small subsets of individuals.

What experimental approaches are most effective for studying TAS2R31 function in relation to its genetic variants?

The most effective experimental paradigm for studying TAS2R31 genetic variants employs a complementary approach combining:

• In vitro functional characterization:

  • Heterologous expression of receptor variants in cell culture systems

  • Calcium mobilization assays to quantify receptor activation

  • Dose-response measurements to determine EC50 and Emax values for various agonists

  • Competition assays with known antagonists to characterize binding properties

• In silico structural modeling:

  • Homology modeling of TAS2R31 based on other GPCRs

  • Molecular docking simulations to predict ligand interactions

  • Analysis of how amino acid substitutions might affect protein structure

• Human psychophysical studies:

  • Genotyping participants for known TAS2R31 variants

  • Threshold testing to determine bitter sensitivity to TAS2R31 agonists

  • Scaling methods to quantify perceived intensity

This integrated approach enables researchers to establish direct links between genetic variants, receptor function, and perceptual outcomes. For example, Roudnitzky et al. successfully employed this strategy to demonstrate how the TAS2R31-R35W substitution affects both receptor function in vitro and taste responses in human subjects .

What is known about the linkage disequilibrium (LD) structure surrounding TAS2R31 and how does it impact genetic association studies?

Linkage disequilibrium (LD) patterns in the genomic region containing TAS2R31 present significant challenges for genetic association studies. TAS2R31 belongs to a five-member subfamily (TAS2R30-46) that spans approximately 140 kb and contains considerable sequence variation . High LD spanning functionally distinct TAS2R loci can result in correlated bitter taste responses to many compounds even when they are mediated by different genes .

Research has demonstrated that apparent associations between taste responses and specific TAS2R variants can sometimes be spurious due to LD with functional variants in neighboring genes. For example, Roudnitzky et al. found that many markers initially associated with saccharin and acesulfame K perception were actually in high LD (r > 0.95) with functional variants in TAS2R31, rather than being causal themselves .

This complex LD structure necessitates careful experimental design for genetic association studies:

• Comprehensive haplotype analyses rather than single-SNP approaches

• Functional validation of putative causal variants

• Consideration of population-specific LD patterns, as these can vary substantially across ethnic groups

The presence of high LD in this region predicts that bitter taste responses to many compounds will be strongly correlated even when they are mediated by different genes, complicating attempts to isolate the effects of individual receptors .

What computational tools are recommended for predicting the functional impact of TAS2R31 variants?

For researchers investigating novel or uncharacterized TAS2R31 variants, several computational approaches can provide preliminary insights into potential functional consequences:

• SIFT and PolyPhen-2:
  These widely used tools have proven valuable for predicting the functional impact of nonsynonymous variants in TAS2R genes . In comprehensive analyses of TAS2R variants, there was substantial agreement between these tools, with 131 SNPs predicted to be both "Possibly or Probably Damaging" by PolyPhen-2 and "Deleterious" by SIFT across the TAS2R family .

• Population genetics metrics:
  Analyzing nucleotide diversity (π), Tajima's D, and population differentiation (FST) can provide insights into selective pressures on TAS2R31 . These metrics can help determine whether variants are under neutral, positive, or balancing selection.

• Structural modeling with molecular dynamics:
  Given the challenges of crystallizing GPCRs, computational models can predict how specific variants might alter receptor structure, stability, or ligand binding.

It's important to note that while these computational approaches provide valuable insights, they should be considered preliminary until validated with experimental functional assays. For instance, studies of TAS2R variants have shown that computational predictions are largely in agreement with experimental findings, but there are cases where the tools disagree or fail to capture complex interactions between multiple amino acid substitutions .

What are the best approaches for investigating TAS2R31 expression in non-gustatory tissues?

TAS2R31, like other bitter taste receptors, is expressed in various extra-oral tissues beyond the taste buds, suggesting broader physiological roles. To investigate these expressions, researchers should consider:

• Transcript detection methods:

  • Quantitative RT-PCR with appropriate reference genes

  • RNA-Seq for broader transcriptome analysis

  • Single-cell RNA sequencing to identify specific cell types expressing TAS2R31

• Protein detection methods:

  • Immunohistochemistry with validated antibodies specific to TAS2R31

  • Western blotting with appropriate controls

  • Proximity ligation assays to investigate protein-protein interactions

• Functional characterization:

  • Calcium imaging in identified cell types

  • Tissue-specific knockout models

  • Ex vivo tissue preparations with agonist/antagonist application

Particular attention should be paid to tissues where TAS2R expression has been previously documented, including cells in gut and bronchial smooth muscle that respond to ingested and inhaled compounds . Researchers should recognize that expression levels may be substantially lower than in taste tissues, necessitating highly sensitive detection methods and appropriate controls.

The investigation of TAS2R31 in non-gustatory tissues opens avenues for understanding how this receptor may contribute to diverse physiological responses, including endocrine signaling, immune function, and gastrointestinal responses .

How should researchers design psychophysical studies to investigate TAS2R31-mediated taste perception?

Designing effective psychophysical studies to investigate TAS2R31-mediated taste perception requires careful consideration of multiple factors:

• Participant selection and characterization:

  • Genotype participants for known functional TAS2R31 variants, particularly R35W

  • Screen for other factors affecting taste perception (age, smoking status, medications)

  • Consider population diversity to capture genetic variation

• Stimulus preparation:

  • Use pharmaceutical-grade compounds (saccharin, acesulfame K)

  • Prepare solutions with deionized water under controlled conditions

  • Verify concentrations using appropriate analytical methods

• Testing protocols:

  • Employ multiple psychophysical methods (detection thresholds, recognition thresholds, intensity scaling)

  • Use appropriate control stimuli and blinding procedures

  • Incorporate replicate testing to assess reliability

• Data analysis recommendations:

  • Analyze relationships between genotype and phenotype using appropriate statistical models

  • Account for potential confounding variables

  • Consider haplotype analysis rather than single SNP associations

When specifically studying TAS2R31, researchers should focus on known agonists like saccharin and acesulfame K, which have been demonstrated to activate this receptor and produce bitter aftertastes that vary based on TAS2R31 genotype . The approach used by Roudnitzky et al., combining in vitro and psychophysical analysis, represents a robust model for investigating genotype-phenotype relationships in bitter taste perception .

What techniques are most effective for studying TAS2R31 in the context of its contribution to extra-oral physiological functions?

Investigating TAS2R31's contribution to extra-oral functions requires specialized approaches to address the unique challenges of studying taste receptors outside the gustatory system:

• Tissue-specific functional assays:

  • Ex vivo preparations of respiratory or gastrointestinal tissues

  • Measurement of physiological responses (muscle contraction, secretion)

  • Calcium imaging in primary cell cultures from relevant tissues

• Molecular tools for mechanistic studies:

  • CRISPR-Cas9 gene editing to create tissue-specific knockouts

  • Inducible expression systems to control receptor levels

  • Fluorescently tagged receptors to track localization

• In vivo approaches:

  • Conditional knockout models

  • Administration of specific agonists/antagonists

  • Measurement of relevant physiological parameters

Recent research suggests TAS2R31 may have significant extra-oral functions. Evidence indicates that bitter taste receptors in the gut can trigger endocrine responses and affect gastric emptying, while those in the airways can mediate responses to compounds entering the lungs . Understanding TAS2R31's role in these processes may have implications for conditions ranging from glucose regulation to respiratory infections.

Researchers should be particularly attentive to potential interactions between TAS2R31 and other signaling systems in these tissues, as the receptor may function within complex networks rather than in isolation. The growing appreciation of TAS2Rs' roles beyond taste perception represents an important frontier in understanding these receptors' full physiological significance .

How does TAS2R31 function intersect with broader health implications and dietary behaviors?

The role of TAS2R31 extends beyond basic taste perception to influence dietary behaviors and potentially impact health outcomes. Research in this area should consider:

• Dietary preference studies:

  • Correlating TAS2R31 genotypes with food preferences, particularly for items containing relevant bitter compounds

  • Longitudinal studies of diet selection based on taste receptor variants

  • Intervention studies examining modification of food preferences

• Health outcome associations:

  • Investigating correlations between TAS2R31 variants and BMI or other metabolic parameters

  • Examining potential relationships with conditions like type 2 diabetes

  • Considering how perception of artificial sweeteners might influence consumption patterns and consequent health outcomes

Research into these broader implications should employ interdisciplinary approaches combining genetics, sensory science, nutrition, and epidemiology to develop a comprehensive understanding of how variation in TAS2R31 may contribute to individual differences in dietary behaviors and health outcomes.

What is known about the evolutionary history of TAS2R31 and how does it inform current functional diversity?

The evolutionary history of TAS2R31 provides important context for understanding its current functional diversity:

• Evolutionary selective pressures:

  • Evidence suggests TAS2R genes may have undergone different selective pressures during human evolution

  • Some TAS2Rs show evidence of balancing selection, maintaining diversity

  • Population differentiation metrics (FST) can indicate local adaptation

• Comparative genomics insights:

  • TAS2R31 belongs to a five-member subfamily (TAS2R30-46) that has undergone expansion in primates

  • Comparison with orthologs in other species can reveal conserved functional domains

  • Patterns of selection at specific codons can highlight functionally important residues

The extensive variation observed in TAS2R31 and other TAS2R genes could reflect their value in evaluating potentially toxic compounds in novel environments encountered during human migration and population expansion . The global patterns of TAS2R diversity documented in studies of the 1000 Genomes Project data reveal hundreds of nonsynonymous variants, many predicted to alter receptor function .

Understanding this evolutionary context helps researchers interpret current patterns of variation and may guide investigations into the functional significance of specific variants. The distribution of potentially functionally impactful (PHI) variants is particularly informative, with most such variants occurring at low frequencies but a few reaching moderate population frequencies, suggesting different selective pressures on different sites within the receptor .

What emerging technologies might advance our understanding of TAS2R31 structure and function?

Several emerging technologies hold promise for advancing TAS2R31 research:

• Cryo-electron microscopy (cryo-EM):
  While GPCRs have traditionally been challenging to crystallize, advances in cryo-EM may enable structural determination of TAS2R31 in different conformational states, providing unprecedented insights into ligand binding and activation mechanisms.

• AlphaFold and other AI-based structural prediction:
  As AI-based protein structure prediction tools continue to improve, they may provide increasingly accurate models of TAS2R31, particularly when combined with experimental validation techniques.

• High-throughput functional screening:
  Development of cell-based assays compatible with high-throughput screening could enable rapid testing of large compound libraries to identify novel agonists and antagonists.

• Single-cell transcriptomics:
  Applying single-cell RNA sequencing to taste tissues and other TAS2R31-expressing tissues could reveal cell-specific expression patterns and co-expression with other signaling components.

• Organoid models:
  Development of taste bud organoids or other relevant tissue models could provide more physiologically relevant systems for studying TAS2R31 function than current heterologous expression systems.

These technologies, particularly when used in combination, have the potential to overcome current limitations in TAS2R research and provide more comprehensive understanding of receptor structure, function, and physiological roles.

What are the key unresolved questions regarding TAS2R31 function and regulation?

Despite significant advances, several important questions about TAS2R31 remain unresolved:

• Structural determinants of ligand specificity:

  • Which specific amino acid residues are critical for binding different agonists?

  • How does the three-dimensional structure of TAS2R31 differ from other TAS2Rs?

  • What structural features determine whether a compound acts as an agonist or antagonist?

• Signaling mechanisms beyond gustatory tissues:

  • Does TAS2R31 couple to the same G proteins in all tissues where it's expressed?

  • What are the downstream signaling cascades in non-gustatory tissues?

  • How is TAS2R31 signaling regulated in different physiological contexts?

• Developmental and environmental regulation:

  • How is TAS2R31 expression regulated during development?

  • Do environmental factors influence receptor expression or function?

  • Are there epigenetic mechanisms controlling TAS2R31 expression?

• Clinical relevance:

  • Could TAS2R31 variants predict response to certain medications?

  • Are there associations between TAS2R31 variants and specific health conditions?

  • Could targeting TAS2R31 have therapeutic applications?

Addressing these questions will require integrative approaches combining structural biology, molecular pharmacology, genetics, and clinical research. The answers promise to enhance our understanding not only of taste perception but potentially also of broader physiological processes involving this multifunctional receptor.