Identification of endogenous agonists of extra-oral bitter taste receptors

In humans and other mammals, the detection and signal transduction of the taste qualities sweet, umami and bitter is mediated by G protein-coupled receptors. Besides the oral cavity, where they are responsible for a first evaluation of ingested food, their expression was further proven in several non-gustatory tissues like the gastrointestinal tract, the heart, and the respiratory tract. This knowledge hints to further physiological functions of taste receptors beside taste perception. However, nowadays only a few of these functions, and the involved agonists, are known.

As taste receptors are rather insensitive and in the case of bitter taste receptors, called TAS2Rs, the ingestion of high concentrations of bitter substances is avoided by their bitter taste, it is questionable whether compounds that are ingested by food will frequently reach non-gustatory TAS2Rs in concentrations sufficient for activation. Therefore, the question arises by which substances extra-oral bitter taste receptors are activated. One aim of this thesis was the elucidation of the potential of endogenous substances or metabolites to activate bitter taste receptors.

Nowadays, bioinformatics is an important tool in research. Modeling studies can be an appropriate alternative assisting in-vitro experiments to speed up the identification of potent endogenous agonists. Nevertheless, knowledge of receptor structures is a key requirement for accurate modeling. Until recently, no experimental structure of bitter taste receptors was elucidated. Therefore, receptor models were based on homology modeling with related G protein-coupled receptors, even though sequence identities are not completely overlapping. To contribute to improvement of receptor models, amino acid positions that are important for agonist binding and receptor activation were summarized in a review.

Furthermore, the conservation of receptor responses was investigated. In evolution, individuals, which have an advantage over their conspecifics, are selected. Therefore, conservation of agonists in species that divided millions of years ago can hint to the importance of these agonists. In recent studies, the activation of human TAS2R7 by di- and trivalent metal ions was shown. The expression of the one-to-one orthologous receptor of the vampire bat in a cell culture model showed the conservation of metal ion response in this species. This information may indicate that the excessive consumption of metal ions has a harmful effect on mammals and should therefore be avoided. Nevertheless, metal ions are involved in several processes in the body. For example, they act as cofactors in enzymes or as second messengers in cellular signaling cascades. Therefore, high metal ion concentrations can be present locally, which may result in activation of the TAS2R7 in non-gustatory tissues.

The bitter taste of the body fluid bile has long been known. To elucidate, which human bitter taste receptors are addressed by the compound class of bile acids, 25 functional human TAS2Rs were expressed in cell culture models and their activation was measured after application of several bile acids. In doing so, five of these receptors were identified as bile acid receptors. By measuring dose-response relationships, an overlap of endogenous bile acid concentrations and threshold concentrations for bitter taste receptor activation was detected. As already mentioned, conservation of biological processes can hint to the relevance of these processes for the well-being of animals. For this reason, this experimental design was applied to 34 mouse bitter taste receptors. Thereby, six receptors that responded to bile acids were deciphered.

Finally, this thesis shows that non-gustatory bitter taste receptors are activated by essential endogenous substances, which are present in the human body in concentrations that suffice to activate TAS2Rs. The task for future researchers will be to use this knowledge to elucidate biological effects of extra-oral bitter taste receptor activation. This information will be useful for the food to develop dietary supplements for human well-being and for the pharma industry to identify new pharmacological targets to address diseases.