Lactisole interacts with the transmembrane domains of human T1R3 to inhibit sweet taste

The sweet taste receptors in Lemuriformes respond to aspartame, a non-nutritive sweetener and critical residues mediating their taste

Aspartame is a high potency artificial sweetener which is popularly used in foods and beverages. The species-dependent sweet taste toward aspartame has not been completely understood. In a recent publication, we reported that the prosimians Lemuriformes species, which are proposed as aspartame nontasters, could taste aspartame based on the sequence and structure analysis. In this study, by mutagenesis, cell-based functional analysis and molecular simulations, we reveal that Lemuriformes species can respond to aspartame at the cell-based receptor activity level. Furthermore, it is proved that the conserved critical residues D142 and S40 mediate the species-dependent sweet taste toward aspartame. This research provides a deeper insight on the species taste, structure-activity relationship and evolution for eliciting the sweetness of this important synthetic sweetener.

Investigating Taste Perception of Maltodextrins Using Lactisole and Acarbose

Previous research has demonstrated that complex carbohydrates (maltodextrins) can be perceived in the oral cavity. However, little research has been conducted to thoroughly investigate complex carbohydrate taste perception and contributing factors. This study explored the effects of the degree of polymerization and the concentration of complex carbohydrates on taste perception. Additionally, the impact of lactisole and acarbose on carbohydrate taste perception was investigated. Using a blinded, Latin Square design, participants (n = 40) received samples (control) or samples with acarbose (5 mM) or lactisole (1.4 mM). Per visit, participants received solutions: (1) short chain maltodextrin (average DP 6) (SCM), (2) long chain maltodextrin (average DP 24) (LCM), (3) maltose, and (4) glucose. Samples were evaluated in duplicate, both at low concentration and high concentration. Participants tasted the samples and rated sweetness, starchiness, and viscosity (mouthfeel) perceived on a 10 cm continuous line scale and perceived intensity on a Labelled Magnitude Scale. There was a significant effect of degree of polymerisation on sweetness (p = 0.001) and intensity (p = 0.001). For low concentration samples, no significant differences were found between LCM and acarbose LCM or SCM and acarbose SCM for sweetness, starchiness, or mouthfeel (all p > 0.05). Significant differences were observed between LCM and lactisole LCM for sweetness (1.1 ± 0.1 vs. 2.5 ± 0.3, p = 0.001), starchiness (1.4 ± 0.2 vs. 2.3 ± 0.3, p = 0.005), and mouthfeel (1.4 ± 0.2 vs. 2.3 ± 0.3, p = 0.013). In conclusion, the taste perception of maltodextrins is influenced by the degree of polymerisation. Furthermore, for this study, the sweet taste receptor was not involved in maltodextrin taste perception. While salivary α-amylase did not appear to influence taste perception with low concentration maltodextrins, further investigation is necessary.

Dipeptidyl-peptidase 4 (DPP4) mediates fatty acid uptake inhibition by glucose via TAS1R3 and GLUT-2 in Caco-2 enterocytes

Both high glucose intake with a high-fat meal and inhibition of dipeptidyl peptidase-4 (DPP4) have been associated with plasma lipid-lowering effects, but mechanistic understanding linking glucose and fat absorption is lacking. We here hypothesized that glucose ameliorates intestinal fatty acid uptake via a pathway involving DPP4. A concentration of 50 mM glucose reduced mean DPP4 activity in differentiated Caco-2 enterocytes by 42.5 % and fatty acid uptake by 66.0 % via nutrient sensing by the sweet taste receptor subunit TAS1R3 and glucose transporter GLUT-2. No effect of the DPP4 substrates GLP-1 and GIP or of the cellular energy status on the reduced uptake of fatty acids was seen, but a direct interaction between DPP4 and fatty acid transporters is suggested. Conclusively we identified DPP4 as a regulator of fatty acid absorption in Caco-2 enterocytes that mediates the inhibition of intestinal fatty acid uptake by glucose via an interplay of GLUT-2 and TAS1R3.

Activation and inhibition of the sweet taste receptor TAS1R2-TAS1R3 differentially affect glucose tolerance in humans

The sweet taste receptor, TAS1R2-TAS1R3, is expressed in taste bud cells, where it conveys sweetness, and also in intestinal enteroendocrine cells, where it may facilitate glucose absorption and assimilation. In the present study, our objective was to determine whether TAS1R2-TAS1R3 influences glucose metabolism bidirectionally via hyperactivation with 5 mM sucralose (n = 12) and inhibition with 2 mM sodium lactisole (n = 10) in mixture with 75 g glucose loads during oral glucose tolerance tests (OGTTs) in healthy humans. Plasma glucose, insulin, and glucagon were measured before, during, and after OGTTs up to 120 minutes post-prandially. We also assessed individual participants' sweet taste responses to sucralose and their sensitivities to lactisole sweetness inhibition. The addition of sucralose to glucose elevated plasma insulin responses to the OGTT (F(1, 11) = 4.55, p = 0.056). Sucralose sweetness ratings were correlated with early increases in plasma glucose (R2 = 0.41, p<0.05), as well as increases in plasma insulin (R2 = 0.38, p<0.05) when sucralose was added to the OGTT (15 minute AUC). Sensitivity to lactisole sweetness inhibition was correlated with decreased plasma glucose (R2 = 0.84, p<0.01) when lactisole was added to the OGTT over the whole test (120 minute AUC). In summary, stimulation and inhibition of the TAS1R2-TAS1R3 receptor demonstrates that TAS1R2-TAS1R3 helps regulate glucose metabolism in humans and may have translational implications for metabolic disease risk.

A review on natural sweeteners, sweet taste modulators and bitter masking compounds: structure-activity strategies for the discovery of novel taste molecules

Growing demand for the tasty and healthy food has driven the development of low-calorie sweeteners, sweet taste modulators, and bitter masking compounds originated from natural sources. With the discovery of human taste receptors, increasing numbers of sweet taste modulators have been identified through human taste response and molecular docking techniques. However, the discovery of novel taste-active molecules in nature can be accelerated by using advanced spectrometry technologies based on structure-activity relationships (SARs). SARs explain why structurally similar compounds can elicit similar taste qualities. Given the characterization of structural information from reported data, strategies employing SAR techniques to find structurally similar compounds become an innovative approach to expand knowledge of sweeteners. This review aims to summarize the structural patterns of known natural non-nutritive sweeteners, sweet taste enhancers, and bitter masking compounds. Innovative SAR-based approaches to explore sweetener derivatives are also discussed. Most sweet-tasting flavonoids belong to either the flavanonols or the dihydrochalcones and known bitter masking molecules are flavanones. Based on SAR findings that structural similarities are related to the sensory properties, innovative methodologies described in this paper can be applied to screen and discover the derivatives of taste-active compounds or potential taste modulators.

Determination of sweetener specificity of horse gut-expressed sweet taste receptor T1R2-T1R3 and its significance for energy provision and hydration

Studies carried out in several species have demonstrated that detection of low-calorie sweeteners in the lumen of the intestine, by the sweet receptor, T1R2-T1R3, initiates a signaling pathway leading to enhanced expression and activity of intestinal Na+/glucose cotransporter 1, SGLT1. This results in an increased gut capacity to absorb glucose, sodium chloride and water, the basis for oral rehydration therapy. Horses express T1R2, T1R3 and downstream signaling elements in the intestinal tissue. As such, the potential of sweetener-stimulation of T1R2-T1R3 leading to upregulation of SGLT1 allows the provision of more glucose (energy) and hydration for horses. This is especially important when the need for glucose increases during strenuous exercise, pregnancy, and lactation. There are significant differences among species in the ability to detect sweeteners. Amino acid substitutions and pseudogenization of taste receptor genes underlie these variations. Nothing is known about the sweetener specificity of horse T1R2-T1R3. Using heterologous expression methodology, we demonstrate that sweeteners sucralose, stevia and neohesperidin dihydrochalcone (NHDC) activate horse T1R2-T1R3, but cyclamate does not. Determination of sweetener specificity of equine sweet receptor is crucial for developing suitable dietary additives to optimize glucose absorption, hydration and avoiding the intestinal disease brought about by microbial fermentation of unabsorbed carbohydrate reaching the large intestine.

Consumption of the nonnutritive sweetener acesulfame potassium increases central precocious puberty risk

The prevalence of precocious puberty and the consumption of nonnutritive sweeteners (NNS) is rapidly growing worldwide. However, the effects of NNSs on precocious puberty remain unclear. We examined the impact of acesulfame potassium (AceK), one of the most widely used NNS, on central precocious puberty (CPP) development using ex vivo and in vitro studies. 884 girls aged 6-12 were enrolled with complete AceK consumption data and CPP outcome assessment in the Taiwan Pubertal Longitudinal Study from 2018 to 2022. After adjustment for confounders, compared with no AceK consumption, AceK consumption at more than the median dose was associated with higher CPP risk in girls (odds ratio = 1.88, 95% confidence interval = 1.16-3.06; p for trend = 0.003). In rats, AceK consumption from in-utero to post-weaning stages accelerated puberty onset, accompanied by increased brain gonadotropin-releasing hormone (GnRH) expression. Intracerebroventricular AceK injection also induced early puberty onset in rats. In N44 hypothalamic neuron cells, AceK treatment increased reactive oxygen species production, which led to protein kinase A (PKA) activation and increased GnRH expression. These findings suggest that prepubertal girls should consume soft drinks or food products containing AceK more cautiously.

Taste of common prebiotic oligosaccharides: impact of molecular structure

Prebiotic oligosaccharides are naturally occurring nondigestible carbohydrates with demonstrated health benefits. They are also a chemically diverse class of nutrients, offering an opportunity to investigate the impact of molecular structure on oligosaccharide taste perception. Accordingly, a relevant question is whether these compounds are detected by the human gustatory system, and if so, whether they elicit sweet or "starchy" taste. Here, in 3 psychophysical experiments, we investigated the taste perception of 3 commercially popular prebiotics [fructooligosaccharides (FOS), galactooligosaccharides (GOS), xylooligosaccharides (XOS)] in highly pure form. Each of these classes of prebiotics differs in the type of glycosyl residue, and position and type of bond between those residues. In experiments I and II, participants were asked to discriminate a total of 9 stimuli [FOS, GOS, XOS; degree of polymerization (DP) of 2, 3, 4] prepared at 75 mM in the presence and absence of lactisole, a sweet receptor antagonist. We found that all 9 compounds were detectable (P < 0.05). We also found that GOS and XOS DP 4 were discriminable even with lactisole, suggesting that their detection was not via the canonical sweet receptor. Accordingly, in experiment III, the taste of GOS and XOS DP 4 were directly compared with that of MOS (maltooligosaccharides) DP 4-6, which has been reported to elicit "starchy" taste. We found that GOS and MOS were perceived similarly although narrowly discriminable, while XOS was easily discriminable from both GOS and MOS. The current findings suggest that the molecular structure of oligosaccharides impacts their taste perception in humans.

Oral glucose sensing in cephalic phase insulin release

Oral stimulation with foods or food components elicits cephalic phase insulin release (CPIR), which limits postprandial hyperglycemia. Despite its physiological importance, the specific gustatory mechanisms that elicit CPIR have not been clearly defined. While most studies point to glucose and glucose-containing saccharides (e.g., sucrose, maltodextrins) as being the most consistent elicitors, it is not apparent whether this is due to the detection of glucose per se, or to the perceived taste cues associated with these stimuli (e.g., sweetness, starchiness). This study investigated potential sensory mechanisms involved with eliciting CPIR in humans, focusing on the role of oral glucose detection and associated taste. Four stimulus conditions possessing different carbohydrate and taste profiles were designed: 1) glucose alone; 2) glucose mixed with lactisole, a sweet taste inhibitor; 3) maltodextrin, which is digested to starchy- and sweet-tasting products during oral processing; and 4) maltodextrin mixed with lactisole and acarbose, an oral digestion inhibitor. Healthy adults (N = 22) attended four sessions where blood samples were drawn before and after oral stimulation with one of the target stimuli. Plasma c-peptide, insulin, and glucose concentrations were then analyzed. Whereas glucose alone elicited CPIR (one-sample t-test, p < 0.05), it did not stimulate the response in the presence of lactisole. Likewise, maltodextrin alone stimulated CPIR (p < 0.05), but maltodextrin with lactisole and acarbose did not. Together, these findings indicate that glucose is an effective CPIR stimulus, but that an associated taste sensation also serves as an important cue for triggering this response in humans.

Modeling the structure of the transmembrane domain of T1R3, a subunit of the sweet taste receptor, with neohesperidin dihydrochalcone using molecular dynamics simulation

Neohesperidin dihydrochalcone (NHDC) is a sweetener, which interacts with the transmembrane domain (TMD) of the T1R3 subunit of the human sweet taste receptor. Although NHDC and a sweet taste inhibitor lactisole share similar structural motifs, they have opposite effects on the receptor. This study involved the creation of an NHDC-docked model of T1R3 TMD through mutational analyses followed by in silico simulations. When certain NHDC derivatives were docked to the model, His7345.44 was demonstrated to play a crucial role in activating T1R3 TMD. The NHDC-docked model was then compared with a lactisole-docked inactive form, several residues were characterized as important for the recognition of NHDC; however, most of them were distinct from those of lactisole. Residues such as His6413.33 and Gln7947.38 were found to be oriented differently. This study provides useful information that will facilitate the design of sweeteners and inhibitors that interact with T1R3 TMD.

Allosteric modulation of the fish taste receptor type 1 (T1R) family by the extracellular chloride ion

Many G protein-coupled receptors (GPCRs) are allosterically modulated by inorganic ions. Although the intraoral ionic composition of the oral cavity varies depending on the living environment and feeding behavior, little is known about whether and how it affects the function of taste receptor type 1 (T1R), a member of the class C GPCR family. Here, we report that chloride ions allosterically modulate the functions of specific fish T1Rs, namely, mfT1R2a/mfT1R3 and zfT1R2a/zfT1R3. Site-directed mutagenesis revealed mfT1R2a K265, which lies in the extracellular domain of mfT1R2a, to be as a critical residue for the modulation of mfT1R2a/mfT1R3 by Cl-. However, this residue is not conserved in zfT1R2a, and the introduction of the key residue at the corresponding site of another T1R, mfT1R2b, did not confer Cl- susceptibility. These results indicate the variability of the determinants of Cl- susceptibility.

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.

Sweet Taste Receptor T1R3 Expressed in Leydig Cells Is Closely Related to Homeostasis of the Steroid Hormone Metabolism Profile

Taste receptor type 1 subunit 3 (T1R3) is initially expressed in mammal tongue for recognition and response of sweet/umami tastants and is critical to nutrient absorption, even endocrine. In this study, down-regulation of related steroidogenic enzymes such as StAR, 3β-HSD, CYP17A1, and 17β-HSD with the decrease of T1R3 expression was found in Leydig cells treated by a T1R3 inhibitor (lactisole). The abundances of progesterone, 17a-hydroxyprogesterone, androstenedione, testosterone, and deoxycorticosterone were down-regulated by 2.3, 3.5, 1.4, 1.6, and 2.2 times, respectively, after T1R3 inhibition. In addition, opposite results were found in saccharin sodium treatment. T1R3 activation contributed to intracellular cyclic adenosine monophosphate (cAMP) accumulation (14.41 ± 0.58 vs 20.21 ± 0.65) and increased testosterone (20.31 ± 3.49 vs 50.01 ± 7.44) and steroidogenic metabolite levels. Coadministration of human chorionic gonadotropin and saccharin sodium resulted in elevating the testosterone and cAMP levels and enhancing the expression levels of steroidogenic-related factors. Similarly, intratesticular injection of lactisole and saccharin sodium further confirmed that T1R3 inhibition/activation affected the expression of related steroidogenic enzymes and the testosterone levels in mice. The above findings suggest that T1R3 plays a role in testicular steroidogenesis.

Gut taste receptor type 1 member 3 is an intrinsic regulator of Western diet-induced intestinal inflammation

BACKGROUND

Long-term intake of a Western diet (WD), characterized by a high-fat content and sugary drinks, is hypothesized to contribute to the development of inflammatory bowel disease (IBD). Despite the identified clinical association, the molecular mechanisms by which dietary changes contribute to IBD development remain unknown. Therefore, we examined the influence of long-term intake of a WD on intestinal inflammation and the mechanisms by which WD intake affects IBD development.

METHODS

Mice fed normal diet or WD for 10 weeks, and bowel inflammation was evaluated through pathohistological and infiltrated inflammatory cell assessments. To understand the role of intestinal taste receptor type 1 member 3 (TAS1R3) in WD-induced intestinal inflammation, cultured enteroendocrine cells harboring TAS1R3, subjected to RNA interference or antagonist treatment, and Tas1r3-deficient mice were used. RNA-sequencing, flow cytometry, 16S metagenomic sequencing, and bioinformatics analyses were performed to examine the involved mechanisms. To demonstrate their clinical relevance, intestinal biopsies from patients with IBD and mice with dextran sulfate sodium-induced colitis were analyzed.

RESULTS

Our study revealed for the first time that intestinal TAS1R3 is a critical mediator of WD-induced intestinal inflammation. WD-fed mice showed marked TAS1R3 overexpression with hallmarks of serious bowel inflammation. Conversely, mice lacking TAS1R3 failed to exhibit inflammatory responses to WD. Mechanistically, intestinal transcriptome analysis revealed that Tas1r3 deficiency suppressed mTOR signaling, significantly increasing the expression of PPARγ (a major mucosal defense enhancer) and upregulating the expression of PPARγ target-gene (tight junction protein and antimicrobial peptide). The gut microbiota of Tas1r3-deficient mice showed expansion of butyrate-producing Clostridia. Moreover, an increased expression of host PPARγ-signaling pathway proteins was positively correlated with butyrate-producing microbes, suggesting that intestinal TAS1R3 regulates the relationship between host metabolism and gut microflora in response to dietary factors. In cultured intestinal cells, regulation of the TAS1R3-mTOR-PPARγ axis was critical for triggering an inflammatory response via proinflammatory cytokine production and secretion. Abnormal regulation of the axis was observed in patients with IBD.

CONCLUSIONS

Our findings suggest that the TAS1R3-mTOR-PPARγ axis in the gut links Western diet consumption with intestinal inflammation and is a potential therapeutic target for IBD.

Prediction of dynamic allostery for the transmembrane domain of the sweet taste receptor subunit, TAS1R3

The sweet taste receptor plays an essential role as an energy sensor by detecting carbohydrates. However, the dynamic mechanisms of receptor activation remain unclear. Here, we describe the interactions between the transmembrane domain of the G protein-coupled sweet receptor subunit, TAS1R3, and allosteric modulators. Molecular dynamics simulations reproduced species-specific sensitivity to ligands. We found that a human-specific sweetener, cyclamate, interacted with the mouse receptor as a negative allosteric modulator. Agonist-induced allostery during receptor activation was found to destabilize the intracellular part of the receptor, which potentially interfaces with the Gα subunit, through ionic lock opening. A common human variant (R757C) of the TAS1R3 exhibited a reduced response to sweet taste, in support of our predictions. Furthermore, histidine residues in the binding site acted as pH-sensitive microswitches to modulate the sensitivity to saccharin. This study provides important insights that may facilitate the prediction of dynamic activation mechanisms for other G protein-coupled receptors.

Sweet taste receptor subunit T1R3 regulates casein secretion and phosphorylation of STAT5 in mammary epithelial cells

During lactation, mammary epithelial cells (MECs) on the apical membrane are in contact with lactose in milk, while MECs on the basolateral membrane are in contact with glucose in blood. Both glucose and lactose are sweeteners that are sensed by a sweet taste receptor. Previously, we have shown that lactose exposure on the basolateral membrane, but not the apical membrane, inhibits casein production and phosphorylation of STAT5 in MECs. However, it remains unclear whether MECs have a sweet taste receptor. In this study, we confirmed that the sweet taste receptor subunit T1R3 existed in both the apical and basolateral membranes of MECs. Subsequently, we investigated the influence of apical and basolateral sucralose as a ligand for the sweet taste receptor using a cell culture model. In this model, upper and lower media were separated by the MEC layer with less-permeable tight junctions. The results showed in the absence of glucose, both apical and basolateral sucralose induced phosphorylation of STAT5, which is a positive transcriptional factor for milk production. In contrast, the T1R3 inhibitor basolateral lactisole reducing phosphorylated STAT5 and secreted caseins in the presence of glucose. Furthermore, exposure of the apical membrane to sucralose in the presence of glucose inhibited the phosphorylation of STAT5. Simultaneously, GLUT1 was partially translocated from the basolateral membrane to the cytoplasm in MECs. These results suggest that T1R3 functions as a sweet receptor and is closely involved in casein production in MECs.

Sweetener System Intervention Shifted Neutrophils from Homeostasis to Priming

BACKGROUND

Non-nutritive sweeteners (NNS) are part of personalized nutrition strategies supporting healthy glycemic control. In contrast, the consumption of non-nutritive sweeteners has been related to person-specific and microbiome-dependent glycemic impairments. Reports on the effects of NNS on our highly individual cellular immune system are sparse. The recent identification of taste receptor expression in a variety of immune cells, however, suggested their immune-modulatory relevance.

METHODS

We studied the influence of a beverage-typical NNS system on the transcriptional profiling of sweetener-cognate taste receptors, selected cytokines and their receptors, and on Ca²⁺ signaling in isolated blood neutrophils. We determined plasma concentrations of saccharin, acesulfame-K, and cyclamate by HPLC-MS/MS, upon ingestion of a soft drink-typical sweetener surrogate. In an open-labeled, randomized intervention study, we determined pre- versus post-intervention transcript levels by RT-qPCR of sweetener-cognate taste receptors and immune factors.

RESULTS

Here we show that the consumption of a food-typical sweetener system modulated the gene expression of cognate taste receptors and induced the transcriptional regulation signatures of early homeostasis- and late receptor/signaling- and inflammation-related genes in blood neutrophils, shifting their transcriptional profile from homeostasis to priming. Notably, sweeteners at postprandial plasma concentrations facilitated fMLF (N-formyl-Met-Leu-Phe)-induced Ca²⁺ signaling.

CONCLUSIONS

Our results support the notion of sweeteners priming neutrophils to higher alertness towards their adequate stimuli.

Systematic analysis reveals novel insight into the molecular determinants of function, diversity and evolution of sweet taste receptors T1R2/T1R3 in primates

Sweet taste is a primary sensation for the preference and adaption of primates to diet, which is crucial for their survival and fitness. It is clear now that the sweet perception is mediated by a G protein-coupled receptor (GPCR)-sweet taste receptor T1R2/T1R3, and many behavioral or physiological experiments have described the diverse sweet taste sensitivities in primates. However, the structure-function relationship of T1R2s/T1R3s in primates, especially the molecular basis for their species-dependent sweet taste, has not been well understood until now. In this study, we performed a comprehensive sequence, structural and functional analysis of sweet taste receptors in primates to elucidate the molecular determinants mediating their species-dependent sweet taste recognition. Our results reveal distinct taxonomic distribution and significant characteristics (interaction, coevolution and epistasis) of specific key function-related residues, which could partly account for the previously reported behavioral results of taste perception in primates. Moreover, the prosimians Lemuriformes species, which were reported to have no sensitivity to aspartame, could be proposed to be aspartame tasters based on the present analysis. Collectively, our study provides new insights and promotes a better understanding for the diversity, function and evolution of sweet taste receptors in primates.

Utilizing the Off-Target Effects of T1R3 Antagonist Lactisole to Enhance Nitric Oxide Production in Basal Airway Epithelial Cells

Human airway sweet (T1R2 + T1R3), umami (T1R1 + T1R3), and bitter taste receptors (T2Rs) are critical components of the innate immune system, acting as sensors to monitor pathogenic growth. T2Rs detect bacterial products or bitter compounds to drive nitric oxide (NO) production in both healthy and diseased epithelial cell models. The NO enhances ciliary beating and also directly kills pathogens. Both sweet and umami receptors have been characterized to repress bitter taste receptor signaling in healthy and disease models. We hypothesized that the sweet/umami T1R3 antagonist lactisole may be used to alleviate bitter taste receptor repression in airway basal epithelial cells and enhance NO production. Here, we show that lactisole activates cAMP generation, though this occurs through a pathway independent of T1R3. This cAMP most likely signals through EPAC to increase ER Ca²⁺ efflux. Stimulation with denatonium benzoate, a bitter taste receptor agonist which activates largely nuclear and mitochondrial Ca²⁺ responses, resulted in a dramatically increased cytosolic Ca²⁺ response in cells treated with lactisole. This cytosolic Ca²⁺ signaling activated NO production in the presence of lactisole. Thus, lactisole may be useful coupled with bitter compounds as a therapeutic nasal rinse or spray to enhance beneficial antibacterial NO production in patients suffering from chronic inflammatory diseases such as chronic rhinosinusitis.

Anti-HIV drugs lopinavir/ritonavir activate bitter taste receptors

Lopinavir and ritonavir (LPV/r) are the primary anti-human immunodeficiency virus (HIV) drugs recommended by the World Health Organization for treating children aged 3 years and above who are infected with the HIV. These drugs are typically available in liquid formulations to aid in dosing for children who cannot swallow tablets. However, the strong bitter taste associated with these medications can be a significant obstacle to adherence, particularly in young children, and can jeopardize the effectiveness of the treatment. Studies have shown that poor palatability can affect the survival rate of HIV-infected children. Therefore, developing more child-friendly protease inhibitor formulations, particularly those with improved taste, is critical for children with HIV. The molecular mechanism by which lopinavir and ritonavir activate bitter taste receptors, TAS2Rs, is not yet clear. In this study, we utilized a calcium mobilization assay to characterize the activation of bitter taste receptors by lopinavir and ritonavir. We discovered that lopinavir activates TAS2R1 and TAS2R13, while ritonavir activates TAS2R1, TAS2R8, TAS2R13, and TAS2R14. The development of bitter taste blockers that target these receptors with a safe profile would be highly desirable in eliminating the unpleasant bitter taste of these anti-HIV drugs.

The T1R3 subunit of the sweet taste receptor is activated by D2O in transmembrane domain-dependent manner

Deuterium oxide (D2O) is water in which the heavier and rare isotope deuterium replaces both hydrogens. We have previously shown that D2O has a distinctly sweet taste, mediated by the T1R2/T1R3 sweet taste receptor. Here, we explore the effect of heavy water on T1R2 and T1R3 subunits. We show that D2O activates T1R3-transfected HEK293T cells similarly to T1R2/T1R3-transfected cells. The response to glucose dissolved in D2O is higher than in water. Mutations of phenylalanine at position 7305.40 in the transmembrane domain of T1R3 to alanine, leucine, or tyrosine impair or diminish activation by D2O, suggesting a critical role for T1R3 TMD domain in relaying the heavy water signal.

Genetic variation in sweet taste receptors and a mechanistic perspective on sweet and fat taste sensation in the context of obesity

Taste sensation enables humans to make nutritionally important decisions such as food preference and consumption. It functions as deterministic factors for unpropitious eating behavior, leading to overweight and obesity. The hedonistic feeling on consumption of fat and sugar-rich meals, in particular, has a negative influence on health. In addition, impairment in the taste receptors alters the downstream signaling of taste transduction pathway. Hence, genetic polymorphism in typical taste receptors is a predictor of taste sensitivity variance across individuals. The present review summarizes the effect of a single nucleotide polymorphism (SNP) in sweet taste receptors (T1R2/T1R3) on taste perception among individuals of various body mass index (BMI). Furthermore, in the context of obesity, we discussed the possibility of crosstalk between fat and sweet receptors as well as taste dysfunction in diseased individuals. In overall, a greater understanding of the physiological relationship between taste receptors, altered taste sensitivity, and genetic polymorphisms should lead to more effective obesity prevention approaches.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

The Role of D-allulose and Erythritol on the Activity of the Gut Sweet Taste Receptor and Gastrointestinal Satiation Hormone Release in Humans: A Randomized, Controlled Trial

BACKGROUND

Glucose induces the release of gastrointestinal (GI) satiation hormones, such as glucagon-like peptide 1 (GLP-1) and peptide tyrosine tyrosine (PYY), in part via the activation of the gut sweet taste receptor (T1R2/T1R3).

OBJECTIVES

The primary objective was to investigate the importance of T1R2/T1R3 for the release of cholecystokinin (CCK), GLP-1, and PYY in response to D-allulose and erythritol by assessing the effect of the T1R2/T1R3 antagonist lactisole on these responses and as secondary objectives to study the effect of the T1R2/T1R3 blockade on gastric emptying, appetite-related sensations, and GI symptoms.

METHODS

In this randomized, controlled, double-blind, crossover study, 18 participants (5 men) with a mean ± SD BMI (in kg/m2) of 21.9 ± 1.7 and aged 24 ± 4 y received an intragastric administration of 25 g D-allulose, 50 g erythritol, or tap water, with or without 450 parts per million (ppm) lactisole, respectively, in 6 different sessions. 13C-sodium acetate was added to all solutions to determine gastric emptying. At fixed time intervals, blood and breath samples were collected, and appetite-related sensations and GI symptoms were assessed. Data were analyzed with linear mixed-model analysis.

RESULTS

D-allulose and erythritol induced a significant release of CCK, GLP-1, and PYY compared with tap water (all PHolm < 0.0001, dz >1). Lactisole did not affect the D-allulose- and erythritol-induced release of CCK, GLP-1, and PYY (all PHolm > 0.1). Erythritol significantly delayed gastric emptying, increased fullness, and decreased prospective food consumption compared with tap water (PHolm = 0.0002, dz = -1.05; PHolm = 0.0190, dz = 0.69; and PHolm = 0.0442, dz = -0.62, respectively).

CONCLUSIONS

D-allulose and erythritol stimulate the secretion of GI satiation hormones in humans. Lactisole had no effect on CCK, GLP-1, and PYY release, indicating that D-allulose- and erythritol-induced GI satiation hormone release is not mediated via T1R2/T1R3 in the gut.

OUP accepted manuscript

Oligosaccharides, a subclass of complex carbohydrates, occur both naturally in foods and as a result of oral starch digestion. We have previously shown that humans can taste maltooligosaccharides (MOS) and that their detection is independent of the canonical sweet taste receptor. While MOSs most commonly occur in a linear form, they can also exist in cyclic structures, referred to as cyclodextrins (CD). The aim of this study was to investigate how the structure of the MOS backbone (i.e. cyclic form) and the size (i.e. degree of polymerization; DP) affect their taste perception. We tested taste detection of cyclodextrins with DP of 6, 7, and 8 (i.e. α-, β-, and γ-CD, respectively) in the presence and absence of lactisole, a sweet receptor antagonist. We found that subjects could detect the taste of cyclodextrins in aqueous solutions at a significant level (P < 0.05), but were not able to detect them in the presence of lactisole (P > 0.05). These findings suggest that the cyclodextrins, unlike their linear analogs, are ligands of the human sweet taste receptor, hT1R2/hT1R3. Study findings are discussed in terms of how chemical structures may contribute to tastes of saccharides.

Biophysical and functional characterization of the human TAS1R2 sweet taste receptor overexpressed in a HEK293S inducible cell line

Sweet taste perception is mediated by a heterodimeric receptor formed by the assembly of the TAS1R2 and TAS1R3 subunits. TAS1R2 and TAS1R3 are class C G-protein-coupled receptors whose members share a common topology, including a large extracellular N-terminal domain (NTD) linked to a seven transmembrane domain (TMD) by a cysteine-rich domain. TAS1R2-NTD contains the primary binding site for sweet compounds, including natural sugars and high-potency sweeteners, whereas the TAS1R2-TMD has been shown to bind a limited number of sweet tasting compounds. To understand the molecular mechanisms governing receptor–ligand interactions, we overexpressed the human TAS1R2 (hTAS1R2) in a stable tetracycline-inducible HEK293S cell line and purified the detergent-solubilized receptor. Circular dichroism spectroscopic studies revealed that hTAS1R2 was properly folded with evidence of secondary structures. Using size exclusion chromatography coupled to light scattering, we found that the hTAS1R2 subunit is a dimer. Ligand binding properties were quantified by intrinsic tryptophan fluorescence. Due to technical limitations, natural sugars have not been tested. However, we showed that hTAS1R2 is capable of binding high potency sweeteners with K_d values that are in agreement with physiological detection. This study offers a new experimental strategy to identify new sweeteners or taste modulators that act on the hTAS1R2 and is a prerequisite for structural query and biophysical studies.

Saccharin and Sucralose Protect the Glomerular Microvasculature In Vitro against VEGF-Induced Permeability

Diabetic kidney disease (DKD) has become a global health concern, with about 40% of people living with type 1 and type 2 diabetes mellitus developing DKD. Upregulation of vascular endothelial growth factor (VEGF) in the kidney is a significant pathology of DKD associated with increased glomerular vascular permeability. To date, however, current anti-VEGF therapies have demonstrated limited success in treating DKD. Recent studies have shown that artificial sweeteners exhibit anti-VEGF potential. The aim of this study was therefore to assess the effects of aspartame, saccharin, and sucralose on VEGF-induced leak using an in vitro model of the glomerular endothelium. Saccharin and sucralose but not aspartame protected against VEGF-induced permeability. Whilst the sweeteners had no effect on traditional VEGF signalling, GC-MS analysis demonstrated that the sweetener sucralose was not able to enter the glomerular endothelial cell to exert the protective effect. Chemical and molecular inhibition studies demonstrated that sweetener-mediated protection of the glomerular endothelium against VEGF is dependent on the sweet taste receptor, T1R3. These studies demonstrate the potential for sweeteners to exert a protective effect against VEGF-induced increased permeability to maintain a healthy endothelium and protect against vascular leak in the glomerulus in settings of DKD.

Drinking Water with Saccharin Sodium Alters the Microbiota-Gut-Hypothalamus Axis in Guinea Pig

The effects of saccharin, as a type of sweetener additive, on the metabolism and development of mammals are still controversial. Our previous research revealed that saccharin sodium (SS) promoted the feed intake and growth of guinea pigs. In this experiment, we used the guinea pig model to study the physiological effect of SS in the microbiota-gut-hypothalamus axis. Adding 1.5 mM SS to drinking water increased the serum level of glucose, followed by the improvement in the morphology and barrier function of the ileal villus, such as SS supplementation which increased the villus height and villus height/crypt depth ratio. Saccharin sodium (SS) treatment activated the sweet receptor signaling in the ileum and altered GHRP hormone secretion. In the hypothalamus of SS and control (CN) group, RNA-seq identified 1370 differently expressed genes (796 upregulated, 574 downregulated), enriching into the taste signaling transduction, and neuroactive ligand-receptor interaction. LEfSe analysis suggested that Lactobacillaceae-Lactobacillus was the microbe with significantly increased abundance of ileum microorganisms in the SS-treated group, while Brevinema-Andersonii and Erysipelotrichaceae-Ilebacterium were the microbes with significantly increased abundance of the control. Furthermore, SS treatment significantly enhanced the functions of chemoheterotrophy and fermentation of ileal microflora compared to the CN group. Accordingly, SS treatment increased levels of lactic acid and short-chain fatty acids (acetic acid, propionic acid and N-valeric acid) in the ileal digesta. In summary, drinking water with 1.5 mM SS activated sweet receptor signaling in the gut and altered GHRP hormone secretion, followed by the taste signaling transduction in the hypothalamus.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Ibuprofen, a Nonsteroidal Anti-Inflammatory Drug, is a Potent Inhibitor of the Human Sweet Taste Receptor

A sweet taste receptor is composed of heterodimeric G-protein-coupled receptors T1R2 and T1R3. Although there are many sweet tastants, only a few compounds have been reported as negative allosteric modulators (NAMs), such as lactisole, its structural derivative 2,4-DP, and gymnemic acid. In this study, candidates for NAMs of the sweet taste receptor were explored, focusing on the structural motif of lactisole. Ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), has an α-methylacetic acid moiety, and this structure is also shared by lactisole and 2,4-DP. When ibuprofen was applied together with 1 mM aspartame to the cells that stably expressed the sweet taste receptor, it inhibited the receptor activity in a dose-dependent manner. The IC50 value of ibuprofen against the human sweet taste receptor was calculated as approximately 12 μM, and it was almost equal to that of 2,4-DP, which is known as the most potent NAM for the receptor to date. On the other hand, when the inhibitory activities of other profens were examined, naproxen also showed relatively potent NAM activity against the receptor. The results from both mutant analysis for the transmembrane domain (TMD) of T1R3 and docking simulation strongly suggest that ibuprofen and naproxen interact with T1R3-TMD, similar to lactisole and 2,4-DP. However, although 2,4-DP and ibuprofen had almost the same inhibitory activities, these activities were acquired by filling different spaces of the ligand pocket of T1R3-TMD; this knowledge could lead to the rational design of a novel NAM against the sweet taste receptor.

Sensory-Guided Multidimensional Exploration of Antisweet Principles from Gymnema sylvestre (Retz) Schult

We report on activity-guided investigation of the key antisweet principles of Gymnema sylvestre. Orosensory-guided fractionation by means of solid phase extraction, preparative 2D-LC, and semipreparative HPLC followed by accurate MS and 1D/2D NMR experiments revealed six known and three previously unknown gymnemic acids as the key constituents of seven highly sensory-active fractions. Localized via a modified comparative taste dilution analysis (cTDA) and taste modulation probability (TMP) based screening techniques, a strong intrinsic bitterness was also observed for gymnemic acids. In addition, the suppressive effects of the most abundant acids on the response of the human sweet taste receptor to sucrose were verified by means of a functional hTAS1R2/hTAS1R3 sweet taste receptor assay. This in vitro screening revealed large differences in antisweet activity among the isolated compounds, where gymnemic acids XV and XIX showed the highest sweet suppressing activity. This broad-based molecular characterization of the sweet taste inhibiting activity of Gymnema sylvestre will enable further insight into the molecular basis of sweet taste modulation at the receptor level.

Sweet taste of heavy water

Hydrogen to deuterium isotopic substitution has only a minor effect on physical and chemical properties of water and, as such, is not supposed to influence its neutral taste. Here we conclusively demonstrate that humans are, nevertheless, able to distinguish D₂O from H₂O by taste. Indeed, highly purified heavy water has a distinctly sweeter taste than same-purity normal water and can add to perceived sweetness of sweeteners. In contrast, mice do not prefer D₂O over H₂O, indicating that they are not likely to perceive heavy water as sweet. HEK 293T cells transfected with the TAS1R2/TAS1R3 heterodimer and chimeric G-proteins are activated by D₂O but not by H₂O. Lactisole, which is a known sweetness inhibitor acting via the TAS1R3 monomer of the TAS1R2/TAS1R3, suppresses the sweetness of D₂O in human sensory tests, as well as the calcium release elicited by D₂O in sweet taste receptor-expressing cells. The present multifaceted experimental study, complemented by homology modelling and molecular dynamics simulations, resolves a long-standing controversy about the taste of heavy water, shows that its sweet taste is mediated by the human TAS1R2/TAS1R3 taste receptor, and opens way to future studies of the detailed mechanism of action.

Ben Abu, Mason and colleagues use molecular dynamics, cell-based experiments, mouse models, and human subjects to determine that, unlike ordinary water, heavy water tastes sweet to humans, but not mice. Mechanistically, this effect is mediated by the human TAS1R/TAS1R3 sweet taste receptor.

Sweet taste of heavy water

Hydrogen to deuterium isotopic substitution has only a minor effect on physical and chemical properties of water and, as such, is not supposed to influence its neutral taste. Here we conclusively demonstrate that humans are, nevertheless, able to distinguish D₂O from H₂O by taste. Indeed, highly purified heavy water has a distinctly sweeter taste than same-purity normal water and can add to perceived sweetness of sweeteners. In contrast, mice do not prefer D₂O over H₂O, indicating that they are not likely to perceive heavy water as sweet. HEK 293T cells transfected with the TAS1R2/TAS1R3 heterodimer and chimeric G-proteins are activated by D₂O but not by H₂O. Lactisole, which is a known sweetness inhibitor acting via the TAS1R3 monomer of the TAS1R2/TAS1R3, suppresses the sweetness of D₂O in human sensory tests, as well as the calcium release elicited by D₂O in sweet taste receptor-expressing cells. The present multifaceted experimental study, complemented by homology modelling and molecular dynamics simulations, resolves a long-standing controversy about the taste of heavy water, shows that its sweet taste is mediated by the human TAS1R2/TAS1R3 taste receptor, and opens way to future studies of the detailed mechanism of action.

From receptors to the brain: Psychophysical clues to taste physiology

To understand human taste requires not only physiological studies ranging from receptor mechanisms to brain circuitry, but also psychophysical studies that quantitatively describe the perceptual output of the system. As obvious as this requirement is, differences in research approaches, methodologies, and objectives complicate the ability to meet it. Discussed here is an example of how the discovery two decades ago of a perceptual taste illusion (thermal taste) has led to physiological and psychophysical research on both peripheral and central mechanisms of taste, including most recently a psychophysical study of the heat sensitivity of the human sweet taste receptor TAS1R2/T1R3, and an fMRI study of a possible central gain mechanism that may underlie, in part, differences in human taste sensitivity. In addition to the new data and hypotheses these studies have generated, they illustrate instances of research on taste motivated by evidence derived from different approaches and levels of analysis.

Pharmacology of TAS1R2/TAS1R3 Receptors and Sweet Taste

The detection of energy-rich sweet food items has been important for our survival during evolution, however, in light of the changing lifestyles in industrialized and developing countries our natural sweet preference is causing considerable problems. Hence, it is even more important to understand how our sense of sweetness works, and perhaps even, how we may deceive it for our own benefit. This chapter summarizes current knowledge about sweet tastants and sweet taste modulators on the compound side as well as insights into the structure and function of the sweet taste receptor and the transduction of sweet signals. Moreover, methods to assess the activity of sweet substances in vivo and in vitro are compared and discussed.

Non-nutritive sweetener activation of the pig sweet taste receptor T1R2-T1R3 in vitro mirrors sweetener stimulation of the gut-expressed receptor in vivo

The perception of sweet is mediated by the sweet taste receptor T1R2-T1R3 expressed in taste cells of the lingual epithelium. This receptor is also expressed in intestinal enteroendocrine cells and is required for sensing luminal sugars and sweeteners to regulate expression of intestinal Na+-glucose cotransporter 1 (SGLT1). There are some notable differences amongst species in the ability to detect certain non-nutritive (artificial) sweeteners. Amino acid substitutions and pseudogenization of taste receptor genes are responsible for these disparities. Using heterologous expression, we demonstrate that the commonly used non-nutritive sweeteners sucralose, saccharin and acesulfame K activate pig T1R2-T1R3, but that aspartame and cyclamate do not. Furthermore, we show that in vitro sweetener activation of pig T1R2-T1R3 mirrors the sweetener stimulation of the gut-expressed receptor in vivo. Considering that sweeteners are included in animal feed worldwide, determination of taste receptor specificities in different species is essential for the development of scientifically-based dietary formulations.

Sweetness Perception is not Involved in the Regulation of Blood Glucose after Oral Application of Sucrose and Glucose Solutions in Healthy Male Subjects

SCOPE

This study investigates the effect of the sweetness of a sucrose versus an isocaloric glucose solution in dietary concentrations on blood glucose regulation by adjusting the sweetness level using the sweet taste inhibitor lactisole.

METHODS AND RESULTS

A total of 27 healthy males participated in this randomized, crossover study with four treatments: 10% glucose, 10% sucrose, 10% sucrose + 60 ppm lactisole, and 10% glucose + 60 ppm lactisole. Plasma glucose, insulin, glucagon-like peptide 1, and glucagon levels are measured at baseline and 15, 30, 60, 90, and 120 min after beverage consumption. Test subjects rated the sucrose solution to be sweeter than the isocaloric glucose solution, whereas no difference in sweetness is reported after addition of lactisole to the sucrose solution. Administration of the less sweet glucose solution versus sucrose led to higher blood glucose levels after 30 min, as reflected by a lower ΔAUC for sucrose (1072 ± 136) than for glucose (1567 ± 231). Application of lactisole leads to no differences in glucose, insulin, or glucagon responses induced by sucrose or glucose.

CONCLUSION

The results indicate that the structure of the carbohydrate has a stronger impact on the regulation of blood glucose levels than the perceived sweetness.

Bioelectrical signal associated with sweet taste transduction in humans is a hyperpolarizing potential on the lingual epithelium

The lingual surface potential (LSP), which hyperpolarizes in response to salt and bitter stimuli, is thought to be a bioelectrical signal associated with taste transduction in humans. In contrast, a recent study reported sweet and sour stimuli to evoke a depolarization of the LSP. We questioned the origin of such a depolarization because liquid junction potentials (JPs), which arise at the interfaces of recording electrode and taste solutions, are neglected in the report. We recorded the LSPs to sucrose and NaCl solutions on the human tongue using an Ag/AgCl electrode. To estimate JPs generated by each taste solution, we made an agar model to simulate the human tongue. The lingual surface was rinsed with a 10 mM NaCl solution that mimics the sodium content of the lingual fluid. In the human tongue, sucrose dissolved in distilled water evoked a depolarizing LSP that could be attributed to JPs, resulting from the change in electrolyte concentration of the taste solution. Sucrose dissolved in 10 mM NaCl solution evoked a hyperpolarizing LSP which became more negative in a concentration-dependent manner (300-1500 mM). Lactisole (3.75 mM), an inhibitor of sweet taste, significantly reduced the LSPs and decreased perceived intensity of sweetness by human subjects. The negative JPs generated by 100 mM NaCl in the agar model were not different from the LSPs to 100 mM NaCl. When the electrolyte environment on the lingual surface is controlled for JPs, the bioelectrical signal associated with sweet taste transduction is a hyperpolarizing potential.

G Protein-Coupled Receptors in Taste Physiology and Pharmacology

Heterotrimeric G protein-coupled receptors (GPCRs) comprise the largest receptor family in mammals and are responsible for the regulation of most physiological functions. Besides mediating the sensory modalities of olfaction and vision, GPCRs also transduce signals for three basic taste qualities of sweet, umami (savory taste), and bitter, as well as the flavor sensation kokumi. Taste GPCRs reside in specialised taste receptor cells (TRCs) within taste buds. Type I taste GPCRs (TAS1R) form heterodimeric complexes that function as sweet (TAS1R2/TAS1R3) or umami (TAS1R1/TAS1R3) taste receptors, whereas Type II are monomeric bitter taste receptors or kokumi/calcium-sensing receptors. Sweet, umami and kokumi receptors share structural similarities in containing multiple agonist binding sites with pronounced selectivity while most bitter receptors contain a single binding site that is broadly tuned to a diverse array of bitter ligands in a non-selective manner. Tastant binding to the receptor activates downstream secondary messenger pathways leading to depolarization and increased intracellular calcium in TRCs, that in turn innervate the gustatory cortex in the brain. Despite recent advances in our understanding of the relationship between agonist binding and the conformational changes required for receptor activation, several major challenges and questions remain in taste GPCR biology that are discussed in the present review. In recent years, intensive integrative approaches combining heterologous expression, mutagenesis and homology modeling have together provided insight regarding agonist binding site locations and molecular mechanisms of orthosteric and allosteric modulation. In addition, studies based on transgenic mice, utilizing either global or conditional knock out strategies have provided insights to taste receptor signal transduction mechanisms and their roles in physiology. However, the need for more functional studies in a physiological context is apparent and would be enhanced by a crystallized structure of taste receptors for a more complete picture of their pharmacological mechanisms.

Sweet Taste Antagonist Lactisole Administered in Combination with Sucrose, But Not Glucose, Increases Energy Intake and Decreases Peripheral Serotonin in Male Subjects

Knowledge regarding the involvement of sweetness perception on energy intake is scarce. Here, the impact of glucose and sucrose sweetness, beyond their caloric load, on subsequent food intake and biomarkers of satiation was evaluated by co-administration of the sweet taste receptor inhibitor lactisole. A total of 27 healthy, male subjects received solutions of either 10% glucose w/o 60 ppm lactisole or 10% sucrose w/o 60 ppm lactisole. Subsequent food intake from a standardized breakfast was evaluated 2 h after receiving the respective test solution. Changes in postprandial plasma concentrations of cholecystokinin, ghrelin, and serotonin were determined over a period of 120 min, as was the body temperature. Administration of lactisole to the sucrose solution increased the energy intake from the subsequent standardized breakfast by 12.9 ± 5.8% (p = 0.04), led to a decreased Δ AUC of the body core temperature by 46 ± 20% (p = 0.01), and time-dependently reduced Δ serotonin plasma concentrations (−16.9 ± 6.06 ng/mL vs. −0.56 ± 3.7 ng/mL after sucrose administration, p = 0.03). The present study shows that lactisole increases energy intake and decreases plasma serotonin concentrations as well as body core temperature induced by sucrose, but not glucose. This finding may be associated with the different binding affinities of sucrose and glucose to the sweet taste receptor.

Sucralose can improve glucose tolerance and upregulate expression of sweet taste receptors and glucose transporters in an obese rat model

OBJECTIVES

Non-nutritive sweeteners (NNS) are widely used as replacements for table sugar in beverages and dessert. However, the metabolic effects of NNS remain controversial. This study aimed to investigate the effects of various sucralose loads on glucose metabolism and expression of sweet taste receptors (STR) and glucose transporters in a high-fat diet (HFD) rats.

METHODS

Four-week-old male Sprague Dawley rats were fed a HFD for 8 weeks, then randomly divided into eight groups (6 in each group). All were gavaged with either saline, sucralose (0.54 mM or 0.78 mM), or sucrose (324 mM) with/without gurmarin, a sweet taste inhibitor, for 4 weeks, followed by an intragastric glucose tolerance test (IGGTT) with blood glucose, and plasma insulin, GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) measurements. In the following week, the rats were sacrificed and the small intestine was removed for measurement of sweet taste receptor and glucose transporter expression by quantitative Reverse Transcription-Polymerase Chain Reaction.

RESULTS

In HFD rats, blood glucose levels were decreased at 30, 60, and 120 min during the IGGTT after 4 weeks supplementation with 0.78 mM sucralose. TIR3 expression was increased in the duodenum and TIR2 was increased in the ileum after 324 mM sucrose supplementation. T1R3 expression was increased after 0.54 mM and 0.78 mM sucralose in the ileum, but there was no change in the expression of TIRs in the duodenum after sucralose treatments. SGLT-1 expression was increased after both 0.78 mM sucralose and 324 mM sucrose in the ileum, and only increased in the duodenum after 324 mM sucrose supplementation.

CONCLUSIONS

The effects of sucralose on glucose metabolism in HFD rats are dose-dependent and related to enhanced expression of sweet taste receptors and glucose transporters. Further studies are needed to clarify the molecular mechanisms involved.

Current Progress in Understanding the Structure and Function of Sweet Taste Receptor

The sweet taste receptor, which was identified approximately 20 years ago, mediates sweet taste recognition in humans and other vertebrates. With the development of genomics, metabonomics, structural biology, evolutionary biology, physiology, and neuroscience, as well as technical advances in these areas, our understanding of this important protein has resulted in substantial progress. This article reviews the structure, function, genetics, and evolution of the sweet taste receptor and offers meaningful insights into this G protein-coupled receptor, which may be helpful guidances for personalized feeding, diet, and medicine. Prospective directions for research on sweet taste receptors have also been proposed.

Asymmetric Synthesis of Photophore-Containing Lactisole Derivatives to Elucidate Sweet Taste Receptors

Lactisole, which has a 2-phenoxy propionic acid skeleton, is well-known as an inhibitor of sweet taste receptors. We recently revealed some of the structure–activity relationships of the aromatic ring and chiral center of lactisole. Photoaffinity labeling is one of the common chemical biology methods to elucidate the interaction between bioactive compounds and biomolecules. In this paper, the novel asymmetric synthesis of lactisole derivatives with common photophores (benzophenone, azide and trifluoromethyldiazirine) for photoaffinity labeling is described. The synthetic compounds are subjected to cell-based sweet taste receptors, and the substitution with trifluoromethyldiazirinyl photophore shows the highest affinity to the receptor of the synthesized compounds.

Sweet Thermal Taste: Perceptual Characteristics in Water and Dependence on TAS1R2/TAS1R3

The initial objective of this study was to determine if activation of the sweet taste receptor TAS1R2/TAS1R3 is necessary for perception of sweet thermal taste (swTT). Our approach was to inhibit the receptor with the inverse agonist lactisole using a temperature-controlled flow gustometer. Because all prior studies of thermal taste (TT) used metal thermodes to heat the tongue tip, we first investigated whether it could be generated in heated water. Experiment 1 showed that sweetness could be evoked when deionized water was heated from 20 to 35 °C, and testing with static temperatures between 20 and 35 °C demonstrated the importance of heating from a cool temperature. As in previous studies, thermal sweetness was reported by only a subset of participants, and replicate measurements found variability in reports of sweetness across trials and between sessions. Experiment 2 then showed that exposure to 8 mM lactisole blocked perception of swTT. Confirmation of the involvement of TAS1R2/TAS1R3 led to an investigation of possible sensory and cognitive interactions between thermal and chemical sweetness. Using sucrose as a sweet stimulus and quinine as a nonsweet control, we found that dynamic heating capable of producing thermal sweetness did not increase the sweetness of sucrose compared with static heating at 35 °C. However, swTT was disrupted if trials containing sucrose (but not quinine) were interspersed among heating-only trials. These findings provide new information relevant to understanding the perceptual processes and receptor mechanisms of swTT, as well as the heat sensitivity of sweet taste in general.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.