The taste of D- and L-amino acids: In vitro binding assays with cloned human bitter (TAS2Rs) and sweet (TAS1R2/TAS1R3) receptors

Beyond the Bottle: Exploring Health-Promoting Compounds in Wine and Wine-Related Products-Extraction, Detection, Quantification, Aroma Properties, and Terroir Effects

Health-promoting compounds in wine and wine-related products are important due to their potential benefits to human health. Through an extensive literature review, this study explores the presence of these compounds in wine and wine-related products, examining their relationship with terroir and their impact on the aromatic and flavor properties that are perceived orally: sunlight exposure, rainfall patterns, and soil composition impact grapevines' synthesis and accumulation of health-promoting compounds. Enzymes, pH, and the oral microbiome are crucial in sensory evaluation and perception of health promotion. Moreover, their analysis of health-promoting compounds in wine and wine-related products relies on considerations such as the specific target compound, selectivity, sensitivity, and the complexity of the matrix.

Faba Bean (Vicia faba L. minor) Bitterness: An Untargeted Metabolomic Approach to Highlight the Impact of the Non-Volatile Fraction

In the context of climate change, faba beans are an interesting alternative to animal proteins but are characterised by off-notes and bitterness that decrease consumer acceptability. However, research on pulse bitterness is often limited to soybeans and peas. This study aimed to highlight potential bitter non-volatile compounds in faba beans. First, the bitterness of flours and air-classified fractions (starch and protein) of three faba bean cultivars was evaluated by a trained panel. The fractions from the high-alkaloid cultivars and the protein fractions exhibited higher bitter intensity. Second, an untargeted metabolomic approach using ultra-high-performance liquid chromatography-diode array detector-tandem-high resolution mass spectrometry (UHPLC-DAD-HRMS) was correlated with the bitter perception of the fractions. Third, 42 tentatively identified non-volatile compounds were associated with faba bean bitterness by correlated sensory and metabolomic data. These compounds mainly belonged to different chemical classes such as alkaloids, amino acids, phenolic compounds, organic acids, and terpenoids. This research provided a better understanding of the molecules responsible for bitterness in faba beans and the impact of cultivar and air-classification on the bitter content. The bitter character of these highlighted compounds needs to be confirmed by sensory and/or cellular analyses to identify removal or masking strategies.

Exploring the correlation of metabolites changes and microbial succession in solid-state fermentation of Sichuan Sun-dried vinegar

BACKGROUND

The traditional Sichuan Sun-dried vinegar (SSV) with unique flavor and taste is believed to be generated by the solid-state fermentation craft. However, how microorganisms and their metabolites change along with fermentation has not yet been explored.

RESULTS

In this study, our results demonstrated that the middle and late stages of SSV fermentation were the periods showing the largest accumulation of organic acids and amino acids. Furthermore, in the bacterial community, the highest average relative abundance was Lactobacillus (ranging from 37.55 to 92.50%) in all fermentation stages, while Acetobacters ranked second position (ranging from 20.15 to 0.55%). The number of culturable lactic acid bacteria is also increased during fermentation process (ranging from 3.93 to 8.31 CFU/g). In fungal community, Alternaria (29.42%), Issatchenkia (37.56%) and Zygosaccharomyces (69.24%) were most abundant in different fermentation stages, respectively. Interestingly, Zygosaccharomyces, Schwanniomyces and Issatchenkia were first noticed as the dominant yeast genera in vinegar fermentation process. Additionally, spearman correlation coefficients exhibited that Lactobacillus, Zygosaccharomyces and Schwanniomyces were significant correlation with most metabolites during the fermentation, implying that these microorganisms might make a significant contribution to the flavor formation of SSV.

CONCLUSION

The unique flavor of SSV is mainly produced by the core microorganisms (Lactobacillus, Zygosaccharomyces and Schwanniomyces) during fermentation. This study will provide detailed information related to the structure of microorganism and correlation between changes in metabolites and microbial succession in SSV. And it will be very helpful for proposing a potential approach to monitor the traditional fermentation process.

Interkingdom Detection of Bacterial Quorum-Sensing Molecules by Mammalian Taste Receptors

Bitter and sweet taste G protein-coupled receptors (known as T2Rs and T1Rs, respectively) were originally identified in type II taste cells on the tongue, where they signal perception of bitter and sweet tastes, respectively. Over the past ~15 years, taste receptors have been identified in cells all over the body, demonstrating a more general chemosensory role beyond taste. Bitter and sweet taste receptors regulate gut epithelial function, pancreatic β cell secretion, thyroid hormone secretion, adipocyte function, and many other processes. Emerging data from a variety of tissues suggest that taste receptors are also used by mammalian cells to "eavesdrop" on bacterial communications. These receptors are activated by several quorum-sensing molecules, including acyl-homoserine lactones and quinolones from Gram-negative bacteria such as Pseudomonas aeruginosa, competence stimulating peptides from Streptococcus mutans, and D-amino acids from Staphylococcus aureus. Taste receptors are an arm of immune surveillance similar to Toll-like receptors and other pattern recognition receptors. Because they are activated by quorum-sensing molecules, taste receptors report information about microbial population density based on the chemical composition of the extracellular environment. This review summarizes current knowledge of bacterial activation of taste receptors and identifies important questions remaining in this field.

Aspartame consumption during pregnancy impairs placenta growth in mice through sweet taste receptor-reactive oxygen species-dependent pathway

The prevalence of obesity has risen dramatically over recent years, and so has the prevalence of adverse obesity-associated pregnancy outcomes. To combat obesity, the calorie contents of many foods and beverages may be reduced by the use of artificial sweeteners, such as aspartame. However, animal studies suggest that aspartame and its metabolites may exhibit toxicity, and the effects of aspartame on pregnancy are largely unknown. In this study, we treated pregnant mice with aspartame by oral gavage and found that the treatment decreased fasting blood glucose level, whereas systolic blood pressure was elevated. Importantly, the aspartame-treated animals also had low placenta and fetus weights, as well as reduced thickness of the placenta decidua layer. Moreover, aspartame decreased the expression of epithelial-mesenchymal transition proteins and manganese superoxide dismutase (MnSOD) in mouse placentae. In order to clarify the mechanisms though which aspartame affects placenta, we performed experiments on 3A-sub-E trophoblasts. In the cells, aspartame treatments induced cell cycle arrest and reduced the proliferation rate, epithelial-mesenchymal transition, migration activity and invasion activity. We also found that aspartame increased reactive oxygen species (ROS) levels to hyper-activate Akt and downregulate MnSOD expression. Pretreatment with antioxidants or sweet taste receptor inhibitors reversed the effects of aspartame on trophoblast function. We also found that the aspartame metabolite phenylalanine similarly induced ROS production and affected proliferation of trophoblasts. Taken together, our data suggest that aspartame consumption during pregnancy may impact the structure, growth and function of the placenta via sweet taste receptor-mediated stimulation of oxidative stress.

Effect of T1R3 Taste Receptor Gene Deletion on Dextran Sulfate Sodium-Induced Colitis in Mice

Taste receptor type 1 member 3 (T1R3) recognize umami or sweet tastes and also contributes type 2 immunity and autophagy in small intestine and muscle cells, respectively. Since imbalance of type 1 and type 2 immunity and autophagy affect intestinal bowel disease (IBD), we hypothesized that T1R3 have a potential role in the incidence and progression of colitis. In the present study, we investigated whether genetic deletion of T1R3 impacted aggravation of DSS-induced colitis in mice. We found that T1R3-KO mice showed reduction in colon damage, including reduced inflammation and colon shrinking relative to those of WT mice following DSS treatment. mRNA expression of tight junction components, particularly claudin1 was significantly lower in T1R3-KO mice with trend to lower inflammation related gene mRNA expression in colon. Other parameters, such as response to microbial stimuli in splenic lymphocytes and peritoneal macrophages, gut microbiota composition, and expression of autophagy-related proteins, were similar between WT and KO mice. Together, these results indicated that deletion of T1R3 has a minor role in intestinal inflammation induced by DSS-induced acute colitis in mice.

Current Advances and Future Aspects of Sweetener Synergy: Properties, Evaluation Methods and Molecular Mechanisms

Sweetener synergy is the phenomenon in which certain combinations of sweeteners work more effectively than the theoretical sum of the effects of each components. It provides benefits in reducing sweetener dosages and improving their sweetness. Many mixtures of sweeteners with synergistic effects have been reported up to now. Both artificial high-intensity sweeteners and natural sweeteners are popularly used in sweetener mixtures for synergism, although the former seem to display more potential to exhibit synergy than the latter. Furthermore, several evaluation methods to investigate sweetener synergy have been applied, which could lead to discrepancies in results. Moreover, structurally dissimilar sweeteners could cooperatively bind at the different sites in the sweet taste receptor T1R2/T1R3 to activate the receptor, and their hydration characters/packing characteristics in solvents could affect their interaction with the receptor, providing the preliminary explanations for the molecular basis of sweetener synergy. In this article, we firstly present a systematic review, analysis and comment on the properties, evaluation methods and molecular mechanisms of sweetener synergy. Secondly, challenges of sweetener synergy in both theory and practice and possible strategies to overcome these limitations are comprehensively discussed. Finally, future perspectives for this important performance in human sweet taste perception are proposed.

A Debittered Complex of Glucose-Phenylalanine Amadori Rearrangement Products with β-Cyclodextrin: Structure, Molecular Docking and Thermal Degradation Kinetic Study

Non-volatile flavor precursors could be used to overcome the flavor loss problems of volatile flavor enhancers during long-term storage. Glu- and Phe-derived Amadori rearrangement products (ARPs) produce pleasant aroma tones thermally but are bitter. We used β-cyclodextrin (β-CD) for debittering Glu-Phe ARPs. ITC analysis indicated that CD-ARP complexes with 1:1 stoichiometry were obtained. NMR analysis indicated that the aromatic ring of Glu-Phe ARPs was embedded in the β-CD cavity. Molecular docking simulations of the bitter taste receptor hT2R1 showed that CD-ARP complex was inactive compared to Glu-Phe ARPs. Complexation with β-CD resulted in the thermal stabilization of Glu-Phe ARPs and a decrease in the degradation rate constant. Compared to Glu-Phe ARPs, the CD-ARP complex in the thermally treated food system slowed down the formation of browning compounds but didn’t inhibit flavor compound formation. The CD-ARP complex is a promising flavor enhancer for applications in flavored and heated foods.

A Dynamic Mass Redistribution Assay for the Human Sweet Taste Receptor Uncovers G-Protein Dependent Biased Ligands

The sweet taste receptor is rather unique, recognizing a diverse repertoire of natural or synthetic ligands, with a surprisingly large structural diversity, and with potencies stretching over more than six orders of magnitude. Yet, it is not clear if different cell-based assays can faithfully report the relative potencies and efficacies of these molecules. Indeed, up to now, sweet taste receptor agonists have been almost exclusively characterized using cell-based assays developed with overexpressed and promiscuous G proteins. This non-physiological coupling has allowed the quantification of receptor activity via phospholipase C activation and calcium mobilization measurements in heterologous cells on a FLIPR system, for example. Here, we developed a novel assay for the human sweet taste receptor where endogenous G proteins and signaling pathways are recruited by the activated receptor. The effects of several sweet taste receptor agonists and other types of modulators were recorded by measuring changes in dynamic mass redistribution (DMR) using an Epic^® reader. Potency and efficacy values obtained in the DMR assay were compared to those results obtained with the classical FLIPR assay. Results demonstrate that for some ligands, the two assay systems provide similar information. However, a clear bias for the FLIPR assay was observed for one third of the agonists evaluated, suggesting that the use of non-physiological coupling may influence the potency and efficacy of sweet taste receptor ligands. Replacing the promiscuous G protein with a chimeric G protein containing the C-terminal tail 25 residues of the physiologically relevant G protein subunit Gα_gustducin reduced or abrogated bias.

Exploring the Effects of Magnesium Deficiency on the Quality Constituents of Hydroponic-Cultivated Tea (Camellia sinensis L.) Leaves

Magnesium (Mg) plays important roles in photosynthesis, sucrose partitioning, and biomass allocation in plants. However, the specific mechanisms of tea plant response to Mg deficiency remain unclear. In this study, we investigated the effects of Mg deficiency on the quality constituents of tea leaves. Our results showed that the short-term (7 days) Mg deficiency partially elevated the concentrations of polyphenols, free amino acids, and caffeine but decreased the contents of chlorophyll and Mg. However, long-term (30 days) Mg-deficient tea displayed decreased contents of these constituents. Particularly, Mg deficiency increased the index of catechins' bitter taste and the ratio of total polyphenols to total free amino acids. Moreover, the transcription of key genes involved in the biosynthesis of flavonoid, caffeine, and theanine was differentially affected by Mg deficiency. Additionally, short-term Mg deficiency induced global transcriptome change in tea leaves, in which a total of 2522 differentially expressed genes were identified involved in secondary metabolism, amino acid metabolism, and chlorophyll metabolism. These results may help to elucidate why short-term Mg deficiency partially improves the quality constituents of tea, while long-term Mg-deficient tea may taste more bitter, more astringent, and less umami.

Chemoinformatics View on Bitter Taste Receptor Agonists in Food

Food compounds with a bitter taste have a role in human health, both for their capability to influence food choice and preferences and for their possible systemic effect due to the modulation of extra-oral bitter taste receptors (TAS2Rs). Investigating the interaction of bitter food compounds with TAS2Rs is a key step to unravel their complex effects on health and to pave the way to rationally design new additives for food formulation or drugs. Here, we propose a collection of food bitter compounds, for which in vitro activity data against TAS2Rs are available. The patterns of TAS2R subtype-specific agonists were analyzed using scaffold decomposition and chemical space analysis, providing a detailed characterization of the associations between food bitter tastants and TAS2Rs.

Fermentation characteristics of Pixian broad bean paste in closed system of gradient steady-state temperature field

A closed system of gradient steady-state temperature field (GSTF) was constructed to ferment Pixian broad bean paste (PBP). The contents of physicochemical factors and organic acids in the fermentation under GSTF (FG) were closer to those in the traditional fermentation (TF). The taste intensities of 8 free amino acids in the FG were higher than those in the constant temperature fermentation (CTF), but 14 in the TF showed the highest among the processes of FG, CTF and TF. The FG product had the most volatiles with 87, and its flavor properties were more stable. The FG produced great effects on the microbe evolutions especially improved the fungal diversity. Bacillus were identified as the core microbes in the FG while the roles of Staphylococcus, Lactobacillus and Pantoea were strengthened. The results indicated that the fermentation characteristics in the FG had been further improved compared with the CTF.

To Detect and Reject, Parallel Roles for Taste and Immunity

PURPOSE OF REVIEW

From single cells to entire organisms, biological entities are in constant communication with their surroundings, deciding what to 'allow' in, and what to reject. In very different ways, the immune and taste systems both fulfill this function, with growing evidence suggesting a relationship between the two, through shared signaling pathways, receptors, and feedback loops. The purpose of this review was to explore recent reports on taste and immunity in model animals and in humans to explore our understanding of the interplay between these systems.

RECENT FINDINGS

Acute infections in the upper airway, as with SARS-CoV-2, are associated with a proinflammatory state, and blunted taste perception. Further, recent findings highlight taste receptors working as immune sentinels throughout the body. Work in humans and mice also points to inflammation from obesity impacting taste, altering taste bud abundance and composition. There is accumulating evidence that taste cells, and particularly their receptors, play a role in airway and gut immunity, responsive to invading organisms. Inflammation itself may further act on taste buds and other taste receptor expressing cells throughout the body as a form of homeostatic control.

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

Denatonium as a Bitter Taste Receptor Agonist Modifies Transcriptomic Profile and Functions of Acute Myeloid Leukemia Cells

The contribution of cell-extrinsic factors in Acute Myeloid Leukemia (AML) generation and persistence has gained interest. Bitter taste receptors (TAS2Rs) are G protein-coupled receptors known for their primary role as a central warning signal to induce aversion toward noxious or harmful substances. Nevertheless, the increasing amount of evidence about their extra-oral localization has suggested a wider function in sensing microenvironment, also in cancer settings. In this study, we found that AML cells express functional TAS2Rs. We also highlighted a significant association between the modulation of some TAS2Rs and the poor-prognosis AML groups, i.e., TP53- and TET2-mutated, supporting a potential role of TAS2Rs in AML cell biology. Gene expression profile analysis showed that TAS2R activation with the prototypical agonist, denatonium benzoate, significantly modulated a number of genes involved in relevant AML cellular processes. Functional assay substantiated molecular data and indicated that denatonium reduced AML cell proliferation by inducing cell cycle arrest in G0/G1 phase or induced apoptosis via caspase cascade activation. Moreover, denatonium exposure impaired AML cell motility and migratory capacity, and inhibited cellular respiration by decreasing glucose uptake and oxidative phosphorylation. In conclusion, our results in AML cells expand the observation of cancer TAS2R expression to the setting of hematological neoplasms and shed light on a role of TAS2Rs in the extrinsic regulation of leukemia cell functions.

Bitter-Tasting Amino Acids l-Arginine and l-Isoleucine Differentially Regulate Proton Secretion via T2R1 Signaling in Human Parietal Cells in Culture

This study aimed at identifying whether the bitter-tasting amino acids l-arginine (l-ARG) and l-isoleucine (l-ILE) differentially regulate mechanisms of gastric acid secretion in human parietal cells (HGT-1 cells) via activation of bitter taste sensing receptors (T2Rs). In a first set of experiments, involvement of T2Rs in l-ARG and l-ILE-modulated proton secretion was demonstrated by co-treatment of HGT-1 cells with T2R antagonists. Subsequent whole genome screenings by means of cDNA arrays revealed T2R1 as a prominent target for both amino acids. Next, the functional role of T2R1 was verified by means of a T2R1 CRISPR-Cas9 knock-out approach. Here, the effect of l-ARG on proton secretion decreased by 65.7 ± 21.9% and the effect of l-ILE increased by 93.2 ± 24.1% in HGT-1 T2R1 ko versus HGT-1 wt cells (p < 0.05). Overall, our results indicate differential effects of l-ARG and l-ILE on proton secretion in HGT-1 cells and our molecular docking studies predict distinct binding for these amino acids in the binding site of T2R1. Further studies will elucidate whether the mechanism of differential effects involves structure-specific ligand-biased signaling of T2R1 or additional cellular targets.

Identification of d-amino acids in tea leaves

During manufacturing processes and in the storage period of tea, amino acids may undergo enantiomeric isomerization, converting their l- to d-forms. To examine the hypothesis, a method was developed for the analysis of the enantiomers in tea leaves. After enriched by ion-exchange solid-phase extraction, the enantiomeric pairs were separated by a chiral high performance liquid chromatography (HPLC) and subsequently detected and identified by using a high resolution quadrupole time-of-flight mass spectrometry (QTOF MS). Only l-forms of amino acids were found in fresh tea leaves. A total of 11 d-amino acids were found in 19 tea samples, ranging from trace amount to 43 µg/g. The results indicated that the enantioisomerization of amino acids occurred in post-harvest tea leaves, and affected by process conditions and storage time.

D-amino acids in foods

With the only exception of glycine, all amino acids exist in two specular structures which are mirror images of each other, called D-(dextro) and L-(levo) enantiomers. During evolution, L-amino acids were preferred for protein synthesis and main metabolism; however, the D-amino acids (D-AAs) acquired different and specific functions in different organisms (from playing a structural role in the peptidoglycan of the bacterial cell wall to modulating neurotransmission in mammalian brain). With the advent of sophisticated and sensitive analytical techniques, it was established during the past few decades that many foods contain considerable amounts of D-AAs: we consume more than 100 mg of D-AAs every day. D-AAs are present in a variety of foodstuffs, where they fulfill a relevant role in producing differences in taste and flavor and in their antimicrobial and antiaging properties from the corresponding L-enantiomers. In this review, we report on the derivation of D-AAs in foods, mainly originating from the starting materials, fermentation processes, racemization during food processing, or contamination. We then focus on leading-edge methods to identify and quantify D-AAs in foods. Finally, current knowledge concerning the effect of D-AAs on the nutritional state and human health is summarized, highlighting some positive and negative effects. Notwithstanding recent progress in D-AA research, the relationships between presence and nutritional value of D-AAs in foods represent a main scientific issue with interesting economic impact in the near future.

Bioelectronic tongue: Current status and perspectives

In the course of evolution, nature has endowed humans with systems for the recognition of a wide range of tastes with a sensitivity and selectivity which are indispensable for the evaluation of edibility and flavour attributes. Inspiration by a biological sense of taste has become a basis for the design of instruments, operation principles and parameters enabling to mimic the unique properties of their biological precursors. In response to the demand for fast, sensitive and selective techniques of flavouring analysis, devices belonging to the group of bioelectronic tongues (B-ETs) have been designed. They combine achievements of chemometric analysis employed for many years in electronic tongues (ETs), with unique properties of bio-inspired materials, such as natural taste receptors (TRs) regarding receptor/ligand affinity. Investigations of the efficiency of the prototype devices create new application possibilities and suggest successful implementation in real applications. With advances in the field of biotechnology, microfluidics and nanotechnologies, many exciting developments have been made in the design of B-ETs in the last five years or so. The presented characteristics of the recent design solutions, application possibilities, critical evaluation of potentialities and limitations as well as the outline of further development prospects related to B-ETs should contribute to the systematisation and expansion of our knowledge.

A conserved odorant binding protein is required for essential amino acid detection in Drosophila

Animals need to detect in the food essential amino acids that they cannot synthesize. We found that the odorant binding protein OBP19b, which is highly expressed in Drosophila melanogaster taste sensilla, is necessary for the detection of several amino acids including the essential l-phenylalanine. The recombinant OBP19b protein was produced and characterized for its binding properties: it stereoselectively binds to several amino acids. Using a feeding-choice assay, we found that OBP19b is necessary for detecting l-phenylalanine and l-glutamine, but not l-alanine or D-phenylalanine. We mapped the cells expressing OBP19b and compared the electrophysiological responses of a single taste sensillum to several amino acids: OBP19b mutant flies showed a reduced response compared to control flies when tested to preferred amino acids, but not to the other ones. OBP19b is well conserved in phylogenetically distant species suggesting that this protein is necessary for detection of specific amino acids in insects.

Taste Perception of Nutrients Found in Nutritional Supplements: A Review

Nutritional supplements are prescribed when one’s nutritional status is not conducive to good health. These foodstuffs constitute concentrated sources of nutrients such as vitamins, minerals, amino acids, and fatty acids. For nutritional supplements to be effective, patients must consume the amount that has been prescribed for the recommended period of time. Therefore, special attention must be given to the sensory attributes of these products. Indeed, the presence of active compounds can cause an off-taste or aftertaste. These negative sensations can lead to a reduction in the consumption of nutritional supplements and reduce the effectiveness of the treatment. In this manuscript, we provide an overview of the sensory characteristics and the sensing receptor mechanism of the main compounds present in oral nutritional supplements, such as amino acids, minerals, fatty acids, and vitamins. Part of this article is devoted to the development of new masking strategies and the corresponding potential influence at the industrial level.

Taste Receptors in Upper Airway Innate Immunity

Taste receptors, first identified on the tongue, are best known for their role in guiding our dietary preferences. The expression of taste receptors for umami, sweet, and bitter have been demonstrated in tissues outside of the oral cavity, including in the airway, brain, gastrointestinal tract, and reproductive organs. The extra-oral taste receptor chemosensory pathways and the endogenous taste receptor ligands are generally unknown, but there is increasing data suggesting that taste receptors are involved in regulating some aspects of innate immunity, and may potentially control the composition of the nasal microbiome in healthy individuals or patients with upper respiratory diseases like chronic rhinosinusitis (CRS). For this reason, taste receptors may serve as potential therapeutic targets, providing alternatives to conventional antibiotics. This review focuses on the physiology of sweet (T1R) and bitter (T2R) taste receptors in the airway and their activation by secreted bacterial products. There is particular focus on T2R38 in sinonasal ciliated cells, as well as the sweet and bitter receptors found on specialized sinonasal solitary chemosensory cells. Additionally, this review explores the impact of genetic variations in these receptors on the differential susceptibility of patients to upper airway infections, such as CRS.

The Origin of Unpleasant Aftertastes in Synthetic Sweeteners: A Hypothesis

Most sweeteners are plagued with unwanted unpleasant aftertastes. Here we examined the possibility that one of the main reasons for this is the similarity of sweet and umami receptors. We performed docking calculations on models of sweet and umami receptors using as template the recently determined solid state structure of the first taste receptor, the medaka fish T1R2-T1R3 receptor. Our results show convincingly that sweeteners can be recognized also by the T1R1-T1R3 umami receptor, owing to the similarity of its architecture to that of the sweet receptor. We hypothesize that the T1R1-T1R3 receptor plays a key role in modulating the quality of sweet tastants, hinting at a simple explanation of their aftertaste. The prevailing ideas on taste coding favor strict labeling of taste cells, which would exclude that umami receptors can recognize other taste sensations. If some cross-talk based on the combinatorial model of taste is accepted, some sweet ligands can exert a bitter sensation. However, even if cross-talk is not admitted, direct stimulation of the umami receptor is bound to cause an aftertaste incompatible with good sweet quality.

Identification of Bitter-Taste Intensity and Molecular Weight as Amino Acid Determinants for the Stimulating Mechanisms of Gastric Acid Secretion in Human Parietal Cells in Culture

Secretion of gastric acid, aimed at preventing bacterial growth and aiding the digestion of foods in the stomach, is chiefly stimulated by dietary intake of protein and amino acids (AAs). However, AAs' key structural determinants responsible for their effects on mechanisms regulating gastric acid secretion (GAS) have not been identified yet. In this study, AAs have been tested in the parietal cell model HGT-1 on GAS and on mRNA expression of genes regulating GAS. AAs' taste intensities from 0 (not bitter at all) to 10 (very bitter) were assessed in a sensory study, in which ARG (l: 6.42 ± 0.41; d: 4.62 ± 0.59) and ILE (l: 4.21 ± 0.43; d: 2.28 ± 0.33) were identified as bitter-tasting candidates in both isomeric forms. Pearson correlation showed that GAS in HGT-1 cells is directly associated with the bitter taste quality ( r: -0.654) in combination with the molecular weight of l-AA ( r: -0.685).

Human Sweet Receptor T1R3 is Functional in Human Gastric Parietal Tumor Cells (HGT-1) and Modulates Cyclamate and Acesulfame K-Induced Mechanisms of Gastric Acid Secretion

The noncaloric sweeteners (NCSs) cyclamate (Cycl) and acesulfame K (AceK) are widely added to foods and beverages. Little is known about their impact on gastric acid secretion (GAS), which is stimulated by dietary protein and bitter-tasting compounds. Since Cycl and AceK have a bitter off taste in addition to their sweet taste, we hypothesized they modulate mechanisms of GAS in human gastric parietal cells (HGT-1). HGT-1 cells were exposed to sweet tastants (50 mM of glucose, d-threonine, Cycl, or AceK) and analyzed for their intracellular pH index (IPX), as an indicator of proton secretion by means of a pH-sensitive dye, and for mRNA levels of GAS-associated genes by RT-qPCR. Since the NCSs act via the sweet taste-sensing receptor T1R2/T1R3, mRNA expression of the corresponding genes was analyzed in addition to immunocytochemical localization of the T1R2 and T1R3 receptor proteins. Exposure of HGT-1 cells to AceK or d-threonine increased the IPX to 0.60 ± 0.05 and 0.80 ± 0.04 ( P ≤ 0.05), respectively, thereby indicating a reduced secretion of protons, whereas Cycl demonstrated the opposite effect with IPX values of -0.69 ± 0.08 ( P ≤ 0.05) compared to controls (IPX = 0). Cotreatment with the T1R3-inhibitor lactisole as well as a TAS1R3 siRNA knock-down approach reduced the impact of Cycl, AceK, and d-thr on proton release ( P ≤ 0.05), whereas cotreatment with 10 mM glucose enhanced the NCS-induced effect ( P ≤ 0.05). Overall, we demonstrated Cycl and AceK as modulators of proton secretion in HGT-1 cells and identified T1R3 as a key element in this response.

Emerging Role of D-Amino Acid Metabolism in the Innate Defense

Mammalian innate and adaptive immune systems use the pattern recognition receptors, such as toll-like receptors, to detect conserved bacterial and viral components. Bacteria synthesize diverse D-amino acids while eukaryotes and archaea generally produce two D-amino acids, raising the possibility that many of bacterial D-amino acids are bacteria-specific metabolites. Although D-amino acids have not been identified to bind to any known pattern recognition receptors, D-amino acids are enantioselectively recognized by some other receptors and enzymes including a flavoenzyme D-amino acid oxidase (DAO) in mammals. At host-microbe interfaces in the neutrophils and intestinal mucosa, DAO catalyzes oxidation of bacterial D-amino acids, such as D-alanine, and generates H₂O₂, which is linked to antimicrobial activity. Intestinal DAO also modifies the composition of microbiota through modulation of growth for some bacteria that are dependent on host nutrition. Furthermore, regulation and recognition of D-amino acids in mammals have additional meanings at various host-microbe interfaces; D-phenylalanine and D-tryptophan regulate chemotaxis of neutrophils through a G-coupled protein receptor, D-serine has a bacteriostatic role in the urinary tract, D-phenylalanine and D-leucine inhibit innate immunity through the sweet taste receptor in the upper airway, and D-tryptophan modulates immune tolerance in the lower airway. This mini-review highlights recent evidence supporting the hypothesis that D-amino acids are utilized as inter-kingdom communication at host-microbe interface to modulate bacterial colonization and host defense.

Bitter and sweet tasting molecules: It's complicated

"Bitter" and "sweet" are frequently framed in opposition, both functionally and metaphorically, in regard to affective responses, emotion, and nutrition. This oppositional relationship is complicated by the fact that some molecules are simultaneously bitter and sweet. In some cases, a small chemical modification, or a chirality switch, flips the taste from sweet to bitter. Molecules humans describe as bitter are recognized by a 25-member subfamily of class A G-protein coupled receptors (GPCRs) known as TAS2Rs. Molecules humans describe as sweet are recognized by a TAS1R2/TAS1R3 heterodimer of class C GPCRs. Here we characterize the chemical space of bitter and sweet molecules: the majority of bitter compounds show higher hydrophobicity compared to sweet compounds, while sweet molecules have a wider range of sizes. Importantly, recent evidence indicates that TAS1Rs and TAS2Rs are not limited to the oral cavity; moreover, some bitterants are pharmacologically promiscuous, with the hERG potassium channel, cytochrome P450 enzymes, and carbonic anhydrases as common off-targets. Further focus on polypharmacology may unravel new physiological roles for tastant molecules.

Taste receptors in the upper airway

Taste receptors were named for their originally-identified expression on the tongue and role in the sensation of taste (gustation). They are now known to be involved in many chemosensory processes outside the tongue. Expression of the receptors for bitter, sweet, and umami was recently identified in many organs, including the brain, airway, gastrointestinal tract, and reproductive systems. We do not yet know the full roles of these receptors in all of these tissues, nor do we know all of the endogenous ligands that activate them. However, taste receptors are emerging as potentially important therapeutic targets. Moreover, they may mediate some off target effects of drugs, as many medications in common clinical use are known to be bitter. The focus of this review is on recent basic and clinical data describing the expression of bitter (T2R) and sweet (T1R) receptors in the airway and their activation by secreted bacterial compounds. These receptors play important roles in innate immune nitric oxide production and antimicrobial peptide secretion, and may be useful targets for stimulating immune responses in the upper respiratory tract via topical therapies. Moreover, genetic variation in these receptors may play a role in the differential susceptibility of patients to certain types of respiratory infections as well as to differential outcomes in patients with chronic rhinosinusitis (CRS). CRS is a syndrome of chronic upper respiratory infection and inflammation and has a significant detrimental impact on patient quality of life. CRS treatment accounts for approximately 20% of adult antibiotic prescriptions and is thus a large driver of the public health crisis of antibiotic resistance. Taste receptors represent a novel class of therapeutic target to potentially stimulate endogenous immune responses and treat CRS patients without conventional antibiotics.

Activation of bitter taste receptors (tas2rs) relaxes detrusor smooth muscle and suppresses overactive bladder symptoms

Bitter taste receptors (TAS2Rs) are traditionally thought to be expressed exclusively on the taste buds of the tongue. However, accumulating evidence has indicated that this receptor family performs non-gustatory functions outside the mouth in addition to taste. Here, we examined the role of TAS2Rs in human and mouse detrusor smooth muscle (DSM). We showed that mRNA for various TAS2R subtypes was expressed in both human and mouse detrusor smooth muscle (DSM) at distinct levels. Chloroquine (CLQ), an agonist for TAS2Rs, concentration-dependently relaxed carbachol- and KCl-induced contractions of human DSM strips. Moreover, 100 μM of CLQ significantly inhibited spontaneous and electrical field stimulation (EFS)-induced contractions of human DSM strips. After a slight contraction, CLQ (1 mM) entirely relaxed carbachol-induced contraction of mouse DSM strips. Furthermore, denatonium and quinine concentration-dependently decreased carbachol-induced contractions of mouse DSM strips. Finally, we demonstrated that CLQ treatment significantly suppressed the overactive bladder (OAB) symptoms of mice with partial bladder outlet obstruction (PBOO). In conclusion, we for the first time provide evidence of the existence of TAS2Rs in the urinary DSM and demonstrate that TAS2Rs may represent a potential target for OAB. These findings open a new approach to develop drugs for OAB in the future.

d-Leucine: Evaluation in an epilepsy model

BACKGROUND

Current medicines do not provide sufficient seizure control for nearly one-third of patients with epilepsy. New options are needed to address this treatment gap. We recently found that the atypical amino acid d-leucine protected against acutely-induced seizures in mice, but its effect in chronic seizures has not been explored. We hypothesized that d-leucine would protect against spontaneous recurrent seizures. We also investigated whether mice lacking a previously-described d-leucine receptor (Tas1R2/R3) would be protected against acutely-induced seizures.

METHODS

Male FVB/NJ mice were subjected to kainic acid-induced status epilepticus and monitored by video-electroencephalography (EEG) (surgically implanted electrodes) for 4weeks before, during, and after treatment with d-leucine. Tas1R2/R3 knockout mice and controls underwent the maximal electroshock threshold (MES-T) and 6-Hz tests.

RESULTS

There was no difference in number of calendar days with seizures or seizure frequency with d-leucine treatment. In an exploratory analysis, mice treated with d-leucine had a lower number of dark cycles with seizures. Tas1R2/R3 knockout mice had elevated seizure thresholds in the MES-T test but not the 6-Hz test.

CONCLUSIONS

d-Leucine treatment was ineffective against chronic seizures after kainic acid-induced status epilepticus, but there was some efficacy during the dark cycle. Because d-leucine is highly concentrated in the pineal gland, these data suggest that d-leucine may be useful as a tool for studying circadian patterns in epilepsy. Deletion of the Tas1R2/R3 receptor protected against seizures in the MES-T test and, therefore, may be a novel target for treating seizures.

Bacterial d-amino acids suppress sinonasal innate immunity through sweet taste receptors in solitary chemosensory cells

In the upper respiratory epithelium, bitter and sweet taste receptors present in solitary chemosensory cells influence antimicrobial innate immune defense responses. Whereas activation of bitter taste receptors (T2Rs) stimulates surrounding epithelial cells to release antimicrobial peptides, activation of the sweet taste receptor (T1R) in the same cells inhibits this response. This mechanism is thought to control the magnitude of antimicrobial peptide release based on the sugar content of airway surface liquid. We hypothesized that d-amino acids, which are produced by various bacteria and activate T1R in taste receptor cells in the mouth, may also activate T1R in the airway. We showed that both the T1R2 and T1R3 subunits of the sweet taste receptor (T1R2/3) were present in the same chemosensory cells of primary human sinonasal epithelial cultures. Respiratory isolates of Staphylococcus species, but not Pseudomonas aeruginosa, produced at least two d-amino acids that activate the sweet taste receptor. In addition to inhibiting P. aeruginosa biofilm formation, d-amino acids derived from Staphylococcus inhibited T2R-mediated signaling and defensin secretion in sinonasal cells by activating T1R2/3. d-Amino acid-mediated activation of T1R2/3 also enhanced epithelial cell death during challenge with Staphylococcus aureus in the presence of the bitter receptor-activating compound denatonium benzoate. These data establish a potential mechanism for interkingdom signaling in the airway mediated by bacterial d-amino acids and the mammalian sweet taste receptor in airway chemosensory cells.

Assessment of swallowability and palatability of oral dosage forms in children: Report from an M-CERSI pediatric formulation workshop

The acceptability of pediatric pharmaceutical products to patients and their caregivers can have a profound impact on the resulting therapeutic outcome. However, existing methodology and approaches used for acceptability assessments for pediatric products is fragmented, making robust and consistent product evaluations difficult. A pediatric formulation development workshop took place in Washington, DC in June 2016 through the University of Maryland's Center of Excellence in Regulatory Science and Innovation (M-CERSI). A session at the workshop was dedicated to acceptability assessments and focused on two major elements that affect the overall acceptability of oral medicines, namely swallowability and palatability. The session started with presentations to provide an overview of literature, background and current state on swallowability and palatability assessments. Five parallel breakout discussions followed the presentations on each element, focusing on three overarching themes, risk-based approaches, methodology and product factors. This article reports the key outcomes of the workshop related to swallowability and palatability assessments.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

Genetics of Amino Acid Taste and Appetite

The consumption of amino acids by animals is controlled by both oral and postoral mechanisms. We used a genetic approach to investigate these mechanisms. Our studies have shown that inbred mouse strains differ in voluntary amino acid consumption, and these differences depend on sensory and nutritive properties of amino acids. Like humans, mice perceive some amino acids as having a sweet (sucrose-like) taste and others as having an umami (glutamate-like) taste. Mouse strain differences in the consumption of some sweet-tasting amino acids (d-phenylalanine, d-tryptophan, and l-proline) are associated with polymorphisms of a taste receptor, type 1, member 3 gene (Tas1r3), and involve differential peripheral taste responsiveness. Strain differences in the consumption of some other sweet-tasting amino acids (glycine, l-alanine, l-glutamine, and l-threonine) do not depend on Tas1r3 polymorphisms and so must be due to allelic variation in other, as yet unknown, genes involved in sweet taste. Strain differences in the consumption of l-glutamate may depend on postingestive rather than taste mechanisms. Thus, genes and physiologic mechanisms responsible for strain differences in the consumption of each amino acid depend on the nature of its taste and postingestive properties. Overall, mouse strain differences in amino acid taste and appetite have a complex genetic architecture. In addition to the Tas1r3 gene, these differences depend on other genes likely involved in determining the taste and postingestive effects of amino acids. The identification of these genes may lead to the discovery of novel mechanisms that regulate amino acid taste and appetite.

Phytochemicals from Ruta graveolens Activate TAS2R Bitter Taste Receptors and TRP Channels Involved in Gustation and Nociception

Ruta graveolens (rue) is a spontaneous plant in the Mediterranean area with a strong aroma and a very intense bitter taste, used in gastronomy and in folk medicine. From the leaves, stems and fruits of rue, we isolated rutin, rutamarin, three furanocoumarins, two quinolinic alkaloids, a dicoumarin and two long chain ketones. Bitter taste and chemesthetic properties have been evaluated by in vitro assays with twenty receptors of the TAS2R family and four TRP ion channels involved in gustation and nociception. Among the alkaloids, skimmianine was active as a specific agonist of T2R14, whereas kokusaginin did not activate any of the tested receptors. The furanocoumarins activates TAS2R10, 14, and 49 with different degrees of selectivity, as well as the TRPA1 somatosensory ion channel. Rutamarin is an agonist of TRPM5 and TRPV1 and a strong antagonist of TRPM8 ion channels.

Dried bonito dashi: taste qualities evaluated using conditioned taste aversion methods in wild-type and T1R1 knockout mice

The primary taste of dried bonito dashi is thought to be umami, elicited by inosine 5'-monphosphate (IMP) and L-amino acids. The present study compared the taste qualities of 25% dashi with 5 basic tastes and amino acids using conditioned taste aversion methods. Although wild-type C57BL/6J mice with compromised olfactory systems generalized an aversion of dashi to all 5 basic tastes, generalization was greater to sucrose (sweet), citric acid (sour), and quinine (bitter) than to NaCl (salty) or monosodium L-glutamate (umami) with amiloride. At neutral pH (6.5-6.9), the aversion generalized to l-histidine, L-alanine, L-proline, glycine, L-aspartic acid, L-serine, and monosodium L-glutamate, all mixed with IMP. Lowering pH of the test solutions to 5.7-5.8 (matching dashi) with HCl decreased generalization to some amino acids. However, adding lactic acid to test solutions with the same pH increased generalization to 5'-inosine monophosphate, L-leucine, L-phenylalanine, L-valine, L-arginine, and taurine but eliminated generalization to L-histidine. T1R1 knockout mice readily learned the aversion to dashi and generalized the aversion to sucrose, citric acid, and quinine but not to NaCl, glutamate, or any amino acid. These results suggest that dashi elicits a complex taste in mice that is more than umami, and deleting T1R1 receptor altered but did not eliminate their ability to taste dashi. In addition, lactic acid may alter or modulate taste transduction or cell-to-cell signaling.

Potent anti-seizure effects of D-leucine

There are no effective treatments for millions of patients with intractable epilepsy. High-fat ketogenic diets may provide significant clinical benefit but are challenging to implement. Low carbohydrate levels appear to be essential for the ketogenic diet to work, but the active ingredients in dietary interventions remain elusive, and a role for ketogenesis has been challenged. A potential antiseizure role of dietary protein or of individual amino acids in the ketogenic diet is understudied. We investigated the two exclusively ketogenic amino acids, L-leucine and L-lysine, and found that only L-leucine potently protects mice when administered prior to the onset of seizures induced by kainic acid injection, but not by inducing ketosis. Unexpectedly, the D-enantiomer of leucine, which is found in trace amounts in the brain, worked as well or better than L-leucine against both kainic acid and 6Hz electroshock-induced seizures. However, unlike L-leucine, D-leucine potently terminated seizures even after the onset of seizure activity. Furthermore, D-leucine, but not L-leucine, reduced long-term potentiation but had no effect on basal synaptic transmission in vitro. In a screen of candidate neuronal receptors, D-leucine failed to compete for binding by cognate ligands, potentially suggesting a novel target. Even at low doses, D-leucine suppressed ongoing seizures at least as effectively as diazepam but without sedative effects. These studies raise the possibility that D-leucine may represent a new class of anti-seizure agents, and that D-leucine may have a previously unknown function in eukaryotes.