Frauke Stähler

Human psychometric and taste receptor responses to steviol glycosides.

Steviol glycosides, the sweet principle of Stevia Rebaudiana (Bertoni) Bertoni, have recently been approved as a food additive in the EU. The herbal non-nutritive high-potency sweeteners perfectly meet the rising consumer demand for natural food ingredients in Europe. We have characterized the organoleptic properties of the most common steviol glycosides by an experimental approach combining human sensory studies and cell-based functional taste receptor expression assays. On the basis of their potency to elicit sweet and bitter taste sensations, we identified glycone chain length, pyranose substitution, and the C16 double bond as the structural features giving distinction to the gustatory profile of steviol glycosides. A comprehensive screening of 25 human bitter taste receptors revealed that two receptors, hTAS2R4 and hTAS2R14, mediate the bitter off-taste of steviol glycosides. For some test substances, e.g., stevioside, we observed a decline in sweet intensity at supra-maximum concentrations. This effect did not arise from allosteric modulation of the hTAS1R2/R3 sweet taste receptor but might be explained by intramolecular cross-modal suppression between the sweet and bitter taste component of steviol glycosides. These results might contribute to the production of preferentially sweet and least bitter tasting Stevia extracts by an optimization of breeding and postharvest downstream processing.

Sweet taste receptor interacting protein CIB1 is a general inhibitor of InsP3-dependent Ca2+ release in vivo.

In a search for sweet taste receptor interacting proteins, we have identified the calcium‐ and integrin‐binding protein 1 (CIB1) as specific binding partner of the intracellular carboxyterminal domain of the rat sweet taste receptor subunit Tas1r2. In heterologous human embryonic kidney 293 (HEK293) cells, the G protein chimeras Gα16gust44 and Gα15i3 link the sweet taste receptor dimer TAS1R2/TAS1R3 to an inositol 1,4,5‐trisphosphate (InsP3)‐dependent Ca2+ release pathway. To demonstrate the influence of CIB1 on the cytosolic Ca2+ concentration, we used sweet and umami compounds as well as other InsP3‐generating ligands in FURA‐2‐based Ca2+ assays in wild‐type HEK293 cells and HEK293 cells expressing functional human sweet and umami taste receptor dimers. Stable and transient depletion of CIB1 by short‐hairpin RNA increased the Ca2+ response of HEK293 cells to the InsP3‐generating ligands ATP, UTP and carbachol. Over‐expression of CIB1 had the opposite effect as shown for the sweet ligand saccharin, the umami receptor ligand monosodium glutamate and UTP. The CIB1 effect was dependent on the thapsigargin‐sensitive Ca2+ store of the endoplasmic reticulum (ER) and independent of extracellular Ca2+. The function of CIB1 on InsP3‐evoked Ca2+ release from the ER is most likely mediated by its interaction with the InsP3 receptor. Thus, CIB1 seems to be an inhibitor of InsP3‐dependent Ca2+ release in vivo.

Tastant-Evoked Arc Expression in the Nucleus of the Solitary Tract and Nodose/Petrosal Ganglion of the Mouse Is Specific for Bitter Compounds

Despite long and intense research, some fundamental questions regarding representation of taste information in the brain still remain unanswered. This might in part be due to shortcomings of the established methods that limit the researcher either to thorough characterization of few elements or to analyze the response of the entirety of neurons to only one stimulus. To overcome these restrictions, we evaluate the use of the immediate early gene Arc as a neuronal activity marker in the early neural structures of the taste pathway, the nodose/petrosal ganglion (NPG) and the nucleus of the solitary tract (NTS). Responses of NPG and NTS neurons were limited to substances that taste bitter to humans and are avoided by mice. Arc-expressing cells were concentrated in the rostromedial part of the dorsal NTS suggesting a role in gustatory processing. The use of Arc as a neuronal activity marker has several advantages, primarily the possibility to analyze the response of large numbers of neurons while using more than one stimulus makes Arc an interesting new tool for research in the early stages of taste processing.