The cell biology of taste

A novel bionic in vitro bioelectronic tongue based on cardiomyocytes and microelectrode array for bitter and umami detection

Electronic tongues (ETs) have been developed and widely used in food, beverage and pharmaceutical fields, but limited in sensitivity and specificity. In recent years, bioelectronic tongues (BioETs) integrating biological materials and various types of transducers are proposed to bridge the gap between ET system and biological taste. In this work, a bionic in vitro cell-based BioET is developed for bitter and umami detection, utilizing rat cardiomyocytes as a primary taste sensing element and microelectrode arrays (MEAs) as a secondary transducer for the first time. The primary cardiomyocytes of Sprague Dawley (SD) rats, which endogenously express bitter and umami taste receptors, were cultured on MEAs. Cells attached and grew well on the sensor surface, and syncytium was formed for potential conduction and mechanical beating, indicating the good biocompatibility of surface coating. The specificity of this BioET was verified by testing different tastants and bitter compounds. The results show that the BioET responds to bitter and umami compounds specifically among five basic tastants. For bitter recognition, only those can activate receptors in cardiomyocytes can be recognized by the BioET, and different bitter substances could be discriminated by principal component analysis (PCA). Moreover, the specific detections of two bitters (Denatonium Benzoate, Diphenidol) and an umami compound (Monosodium Glutamate) were realized with a detection limit of 10^-6 M. The cardiomyocytes-based BioET proposed in this work provides a new approach for the construction of BioETs and has promising applications in taste detection and pharmaceutical study.

Low intake of digestible carbohydrates ameliorates duodenal absorption of carbohydrates in mice with glucose metabolism disorders induced by artificial sweeteners

BACKGROUND

Long-term artificial sweetener consumption has been reported to induce glucose intolerance, and the intestinal microbiota seems as an important target. While the impacts of artificial sweeteners on energy balance remain controversial, this work aimed to evaluate the protective effects in mice of a low digestible carbohydrate (LDC) diet on plasma glucose, plasma fasting insulin, sweet taste receptors, glucose transporters, and absorption of carbohydrates, together with consumption of acesulfame potassium (AK) or saccharin (SAC).

RESULTS

Artificial sweetener was administered to mice for 12 weeks to induce glucose metabolism disorders; mice were treated with an LDC diet for the final 6 weeks. The experimental groups were treated with an LDC diet that had the same energy as the normal-diet group. Prolonged administration of artificial sweeteners led to metabolic dysfunction, characterized by significantly increased plasma glucose, insulin resistance, sweet taste receptors, glucose transporters, and absorption of carbohydrates. Treatment with an LDC diet positively modulated these altered parameters, suggesting overall beneficial effects of an LDC diet on detrimental changes associated with artificial sweeteners.

CONCLUSIONS

Reducing digestible carbohydrates in the diet can significantly reduce the absorption of carbohydrates and improve glucose metabolism disorders caused by dietary factors. These effects may be due to the fact that reducing the amount of digestible carbohydrates in the feed can reduce the number of intestinal sweet receptors induced by exposure to artificial sweeteners. © 2019 Society of Chemical Industry.

Changes in taste function and ingestive behavior following bariatric surgery

Bariatric surgery is the most effective treatment for severe obesity and its related comorbidities. Roux-en-Y Gastric Bypass (RYGB) and Sleeve Gastrectomy (SG) are currently the most popular weight-loss surgeries used worldwide. Following these surgeries, many patients self-report changes in taste perception and decreased preference for unhealthy foods. These reported changes might account for increased adherence to healthier diets and successful weight loss after surgeries. However, researchers have used a variety of methodologies to assess patients' reported changes andresults are discrepant. The goal of this review is to summarize the literature regarding changes to taste function and ingestive behavior following RYGB and SG to examine differences in findings by methodology (indirect vs. direct measurements). We focused our review around changes in sweets, fats, and alcohol because most of the documented changes in ingestive behavior post-surgery are related to changes in these dietary items. We found that studies using surveys and questionnaires generally find that subjects self-report changes in taste and decrease their preference and cravings for energy-dense foods (particularly, sweets and high-fats). However, studies using validated sensory techniques that include oral sampling or by using direct food intake measurements find little to no change in subjects' ability to perceive taste or their preference for energy-dense foods. Therefore, reported changes in taste and food preferences are unlikely to be explained by alterations in taste intensity and diet selection, and are rather related to changes in the rewarding value of food. Further, that RYGB, and likely SG, is associated with increased alcohol consumption and arisk to develop an alcohol use disorder) supports the notion that these surgeries alter central circuits of reward that are critical in the regulation of ingestive behavior.

Impairment of Bitter Taste Sensor Transient Receptor Potential Channel M5-Mediated Aversion Aggravates High-Salt Intake and Hypertension

Excessive salt consumption leads to cardiovascular diseases. Despite various measures designed to reduce salt intake, daily salt intake remains at a high level. Appropriate salt intake is balanced by salt taste preference triggered by epithelium sodium channel and salt taste aversion evoked by bitter taste sensor, transient receptor potential channel M5 (TRPM5). However, the behavioral mechanism of excessive salt intake remains largely elusive. In this study, wild type and TRPM5^-/- mice were applied to study the influence of high-salt administration on epithelium sodium channel/TRPM5 and the associated behavior to salt consumption. We found that long-term high-salt intake impaired the aversive behavior to high-salt stimulation but did not alter the preference to low salt in mice. The mechanistic evidence demonstrated that high-salt intake blunted the TRPM5-mediated aversive behavior to noxious salt stimulation through inhibiting PKC (protein kinase C) activity and PKC-dependent threonine phosphorylation in the tongue epithelium but did not affect the epithelium sodium channel-dependent salt taste preference. Inhibition of TRPM5 also resulted in an impaired aversive response to high salt, with reduced taste perception in bitter cortical field of mice. TRPM5^-/- mice showed a lowered aversion to high-salt diet and developed salt-induced hypertension. The impaired perception to bitter taste evoked by high-salt intake also existed in hypertensive patients with high-salt consumption. We demonstrate that long-term high-salt consumption impairs aversive response to concentrated salt by downregulating bitter taste sensor TRPM5. It suggests that enhancing TRPM5 function might antagonize excessive salt intake and high salt-induced hypertension.

The Role of the Anion in Salt (NaCl) Detection by Mouse Taste Buds

How taste buds detect NaCl remains poorly understood. Among other problems, applying taste-relevant concentrations of NaCl (50-500 mm) onto isolated taste buds or cells exposes them to unphysiological (hypo/hypertonic) conditions. To overcome these limitations, we used the anterior tongue of male and female mice to implement a slice preparation in which fungiform taste buds are in a relatively intact tissue environment and stimuli are limited to the taste pore. Taste-evoked responses were monitored using confocal Ca²⁺ imaging via GCaMP3 expressed in Type 2 and Type 3 taste bud cells. NaCl evoked intracellular mobilization of Ca²⁺ in the apical tips of a subset of taste cells. The concentration dependence and rapid adaptation of NaCl-evoked cellular responses closely resembled behavioral and afferent nerve responses to NaCl. Importantly, taste cell responses were not inhibited by the diuretic, amiloride. Post hoc immunostaining revealed that >80% of NaCl-responsive taste bud cells were of Type 2. Many NaCl-responsive cells were also sensitive to stimuli that activate Type 2 cells but never to stimuli for Type 3 cells. Ion substitutions revealed that amiloride-insensitive NaCl responses depended on Cl^- rather than Na⁺ Moreover, choline chloride, an established salt taste enhancer, was equally effective a stimulus as sodium chloride. Although the apical transducer for Cl^- remains unknown, blocking known chloride channels and cotransporters had little effect on NaCl responses. Together, our data suggest that chloride, an essential nutrient, is a key determinant of taste transduction for amiloride-insensitive salt taste.SIGNIFICANCE STATEMENT Sodium and chloride are essential nutrients and must be regularly consumed to replace excreted NaCl. Thus, understanding salt taste, which informs salt appetite, is important from a fundamental sensory perspective and forms the basis for interventions to replace/reduce excess Na⁺ consumption. This study examines responses to NaCl in a semi-intact preparation of mouse taste buds. We identify taste cells that respond to NaCl in the presence of amiloride, which is significant because much of human salt taste also is amiloride-insensitive. Further, we demonstrate that Cl^-, not Na⁺, generates these amiloride-insensitive salt taste responses. Intriguingly, choline chloride, a commercial salt taste enhancer, is also a highly effective stimulus for these cells.

The Role of the Anion in Salt (NaCl) Detection by Mouse Taste Buds

How taste buds detect NaCl remains poorly understood. Among other problems, applying taste-relevant concentrations of NaCl (50-500 mm) onto isolated taste buds or cells exposes them to unphysiological (hypo/hypertonic) conditions. To overcome these limitations, we used the anterior tongue of male and female mice to implement a slice preparation in which fungiform taste buds are in a relatively intact tissue environment and stimuli are limited to the taste pore. Taste-evoked responses were monitored using confocal Ca2+ imaging via GCaMP3 expressed in Type 2 and Type 3 taste bud cells. NaCl evoked intracellular mobilization of Ca2+ in the apical tips of a subset of taste cells. The concentration dependence and rapid adaptation of NaCl-evoked cellular responses closely resembled behavioral and afferent nerve responses to NaCl. Importantly, taste cell responses were not inhibited by the diuretic, amiloride. Post hoc immunostaining revealed that >80% of NaCl-responsive taste bud cells were of Type 2. Many NaCl-responsive cells were also sensitive to stimuli that activate Type 2 cells but never to stimuli for Type 3 cells. Ion substitutions revealed that amiloride-insensitive NaCl responses depended on Cl- rather than Na+ Moreover, choline chloride, an established salt taste enhancer, was equally effective a stimulus as sodium chloride. Although the apical transducer for Cl- remains unknown, blocking known chloride channels and cotransporters had little effect on NaCl responses. Together, our data suggest that chloride, an essential nutrient, is a key determinant of taste transduction for amiloride-insensitive salt taste.SIGNIFICANCE STATEMENT Sodium and chloride are essential nutrients and must be regularly consumed to replace excreted NaCl. Thus, understanding salt taste, which informs salt appetite, is important from a fundamental sensory perspective and forms the basis for interventions to replace/reduce excess Na+ consumption. This study examines responses to NaCl in a semi-intact preparation of mouse taste buds. We identify taste cells that respond to NaCl in the presence of amiloride, which is significant because much of human salt taste also is amiloride-insensitive. Further, we demonstrate that Cl-, not Na+, generates these amiloride-insensitive salt taste responses. Intriguingly, choline chloride, a commercial salt taste enhancer, is also a highly effective stimulus for these cells.

Taste loss with obesity in mice and men

BACKGROUND

Our sense of taste is critical in defining our food choices and habits. Located primarily in our tongue, taste buds are small assemblies of constantly renewing sensory cells, tasked with evaluating oral stimuli before the food we eat is consumed.

METHODS

Using both mice and a free-living human population, we tracked taste papilla abundancy with weight gain, to test for deficiencies in the taste system of obese mice and humans with increased adiposity.

RESULTS

Mice fed a high-fat diet for 8 weeks expressed markers for all subtypes of taste cells at a lower level than chow-fed counterparts. This came alongside the loss of markers for taste cell proliferation (Ki-67) and development (β-catenin), as well as lower fungiform papillae density, consistent with earlier results showing lower circumvallate taste bud abundance in obese mice. Likewise, in a population of college students tracked through 4 years of college attendance, the change in density of fungiform papillae, which house taste buds in the anterior tongue, was negatively correlated with change in neck circumference, a marker of adiposity.

CONCLUSIONS

These results highlight changes in taste during weight gain as a potentially important consideration in the study of obesity.

Uncovering the Cells and Circuits of Touch in Normal and Pathological Settings

The sense of touch is fundamental as it provides vital, moment-to-moment information about the nature of our physical environment. Primary sensory neurons provide the basis for this sensation in the periphery; however, recent work demonstrates that touch transduction mechanisms also occur upstream of the sensory neurons via non-neuronal cells such as Merkel cells and keratinocytes. Within the spinal cord, deep dorsal horn circuits transmit innocuous touch centrally and also transform touch into pain in the setting of injury. Here non-neuronal cells play a key role in the induction and maintenance of persistent mechanical pain. This review highlights recent advances in our understanding of mechanosensation, including a growing appreciation for the role of non-neuronal cells in both touch and pain.

Bitter Taste Receptors for Asthma Therapeutics

Clinical management of asthma and chronic obstructive pulmonary disease (COPD) has primarily relied on the use of beta 2 adrenergic receptor agonists (bronchodilators) and corticosteroids, and more recently, monoclonal antibody therapies (biologics) targeting specific cytokines and their functions. Although these approaches provide relief from exacerbations, questions remain on their long-term efficacy and safety. Furthermore, current therapeutics do not address progressive airway remodeling (AR), a key pathological feature of severe obstructive lung disease. Strikingly, agonists of the bitter taste receptors (TAS2Rs) deliver robust bronchodilation, curtail allergen-induced inflammatory responses in the airways and regulate airway smooth muscle (ASM) cell proliferation and mitigate features of AR in vitro and in animal models. The scope of this review is to provide a comprehensive and systematic insight into our current understanding of TAS2Rs with an emphasis on the molecular events that ensue TAS2R activation in distinct airway cell types and expand on the pleiotropic effects of TAS2R targeting in mitigating various pathological features of obstructive lung diseases. Finally, we will discuss specific opportunities that could help the development of selective agonists for specific TAS2R subtypes in the treatment of asthma.

Insights on modulators in perception of taste modalities: a review

A major challenge in taste research is to overcome the flavour imperfections in food products and to build nutritious strategies to combat against obesity as well as other related metabolic syndromes. The field of molecular taste research and chemical senses has contributed to an enormous development in understanding the taste receptors and mechanisms of taste perception. Accordingly, the development of taste-modifying compounds or taste modulators that alter the perception of basic taste modalities has gained significant prominence in the recent past. The beneficial aspects of these substances are overwhelming while considering their potential taste-modifying properties. The objective of the present review is to provide an impression about the taste-modulating compounds and their distinctive taste-modifying properties with reference to their targets and proposed mechanisms of action. The present review also makes an effort to discuss the basic mechanism involved in oro-gustatory taste perception as well as on the effector molecules involved in signal transduction downstream to the activation of taste receptors.

Cyclophosphamide-Induced Disruptions to Appetitive Qualities and Detection Thresholds of NaCl: Comparison of Single-Dose and Dose Fractionation Effects

Chemotherapy is one of the most common treatments for cancer; however, a side effect is often altered taste. This study examined how cyclophosphamide, a chemotherapy drug, affects salt taste in mice. On the basis of previous findings, it was predicted that cyclophosphamide-induced disruptions in salt taste would be observed near days 2-4, 8-12, and 22-24 posttreatment, and that multiple, smaller doses would cause more severe disruptions to taste. To test these predictions, two experiments were performed, one using brief access testing to measure appetitive qualities, and another using operant conditioning to measure detection thresholds. After a single 100 mg/kg cyclophosphamide injection, peak alterations in brief access lick rates were seen near days 5-8 and 15 posttreatment, whereas peak alterations in detection thresholds were seen days 6, 14, and 20 posttreatment. After five 20 mg/kg injections of cyclophosphamide, brief access lick rates revealed disruptions only on postinjection day 8 whereas thresholds appeared to cycle, gradually increased to and decreased from peak elevations on posttreatment days 4, 10, 15, 20, and 23. Although salt taste functions were disrupted by cyclophosphamide, the patterns of these disruptions were less severe and shorter than expected from cell morphology studies, suggesting a functional adjustment to maintain behavioral accuracy. Fractionation of cyclophosphamide dosing had minimum effect on brief access responses but caused longer, cyclic-like disruptions of detection thresholds compared to single-dose administration.

FGF- and SHH-based molecular signals regulate barbel and craniofacial development in catfish

BACKGROUND

Catfish (Siluriformes) are characterized by unique morphologies, including enlarged jaws with movable barbels and taste buds covering the entire body surface. Evolution of these characteristics was a crucial step in their adaptive radiation to freshwater environments. However, the developmental processes of the catfish craniofacial region and taste buds remain to be elucidated; moreover, little is known about the molecular mechanisms underlying the morphogenesis of these structures.

RESULTS

In Amur catfish (Silurus asotus), three pairs of barbel primordia are formed by 2 days post-fertilization (dpf). Innervation of the peripheral nerves and formation of muscle precursors are also established during early development. Taste buds from the oral region to the body trunk are formed by 4 dpf. We then isolated catfish cognates Shh (SaShh) and Fgf8 (SaFgf8), which are expressed in maxillary barbel primordium at 1-2 dpf. Further, SHH signal inhibition induces reduction of mandibular barbels with abnormal morphology of skeletal elements, whereas it causes no apparent abnormality in the trigeminal and facial nerve morphology. We also found that mandibular barbel lengths and number of taste buds are reduced by FGF inhibition, as seen in SHH signal inhibition. However, unlike with SHH inhibition, the abnormal morphology of the trigeminal and facial nerves was observed in FGF signal-inhibited embryos.

CONCLUSION

The developmental processes of Amur catfish are consistent with those reported for other catfish species. Thus, developmental aspects of craniofacial structures and taste buds may be conserved in Siluriformes. Our findings also suggest that SHH signaling plays a crucial role in the formation of barbels and taste buds, without affecting nerve projection, while FGF signaling is required for the development of barbels, taste buds, and branchial nerves. Thus, SHH and FGF signaling plays key roles in the ontogenesis and evolution of some catfish-specific characteristics.

Association between taste perception and adiposity in overweight or obese older subjects with metabolic syndrome and identification of novel taste-related genes

BACKGROUND

The relation between taste perception, diet, and adiposity remains controversial. Additionally, there is a lack of knowledge on the polymorphisms influencing taste given the scarcity of genome-wide association studies (GWASs) published.

OBJECTIVES

We studied the relation between perception of the basic tastes, i.e., sweet, salty, bitter, sour, and umami (separately and jointly in a "taste score"), and anthropometric measurements in older subjects with metabolic syndrome (MetS). GWASs were undertaken to identify genes associated with basic tastes and their score.

METHODS

Taste perception was cross-sectionally determined by challenging subjects (381 older individuals with MetS) with solutions (5 concentrations) of the basic tastes with the use of standard prototypical tastants (phenylthiocarbamide and 6-n-propylthiouracil, NaCl, sucrose, monopotassium glutamate, and citric acid, for bitter, salt, sweet, umami, and sour, respectively). Taste perception intensities were expressed on a scale. A total taste score was derived.

RESULTS

The total taste score was inversely associated with body weight, body mass index, and waist circumference (P < 0.05). Subjects having a total taste score higher than or equal to the median (11 points for concentration V) were less likely to be classified as obese than subjects below the median (OR: 0.36; 95% CI: 0.22, 0.59; P < 0.001). Associations were similar, albeit less strong, for some taste qualities. In the GWASs, the highest associations were for bitter taste (rs1726866-TAS2R38, with P = 7.74 × 10-18 for phenylthiocarbamide and P = 3.96 × 10-19 for 6-n-propylthiouracil). For other tastes, several single-nucleotide polymorphisms (SNPs) exceeded the P threshold of 1 × 10-5. However, the top-ranked SNPs independently explained a low percentage of taste variability, hence their use as single proxies for the association between taste perception and adiposity is limited.

CONCLUSIONS

We found a strong inverse association between greater taste perception and body weight, body mass index, and waist circumference in older subjects with MetS and identified some taste-related SNPs. It would be advantageous to identify additional genetic proxies for taste and to develop polygenic scores. Data used in this study were derived from the clinical trial PREDIMED PLUS at baseline, registered at http://www.isrctn.com as ISRCTN89898870.

Exploring Associations between Interindividual Differences in Taste Perception, Oral Microbiota Composition, and Reported Food Intake

The role of taste perception, its relationship with oral microbiota composition, and their putative link with eating habits and food intake were the focus of the present study. A sample of 59 reportedly healthy adults (27 male, 32 female; age: 23.3 ± 2.6 years) were recruited for the study and taste thresholds for basic tastes, food intake, and oral microbiota composition were evaluated. Differences in taste perception were associated with different habitual food consumption (i.e., frequency) and actual intake. Subjects who were orally hyposensitive to salty taste reported consuming more bakery and salty baked products, saturated-fat-rich products, and soft drinks than hypersensitive subjects. Subjects hyposensitive to sweet taste reported consuming more frequently sweets and desserts than the hypersensitive group. Moreover, subjects hypersensitive to bitter taste showed higher total energy and carbohydrate intakes compared to those who perceived the solution as less bitter. Some bacterial taxa on tongue dorsum were associated with gustatory functions and with vegetable-rich (e.g., Prevotella) or protein/fat-rich diets (e.g., Clostridia). Future studies will be pivotal to confirm the hypothesis and the potential exploitation of oral microbiome as biomarker of long-term consumption of healthy or unhealthy diets.

Changes in Oral Health Behaviors Associated With a Nursing Intervention in Primary Care

Objective. To describe changes in oral health behaviors following implementation of a nursing intervention targeting children at risk for early childhood caries at an urban 2-site primary care practice. Methods. Nurses used a proprietary Nursing Caries Assessment Tool (N-CAT) to identify behaviors associated with early childhood caries risk, then provided brief focused dental education, fluoride varnish applications, and dental referrals to children without a dental home. We used generalized estimating equation logistic regression models, adjusted for age at visit, to analyze changes in oral health behaviors over time including the following: (1) tooth brushing frequency, (2) use of fluoride toothpaste, and (3) adult help with brushing among children younger than 5 years of age who had at least 2 N-CATs documented during well care visits between April 2013 and June 2015. We also evaluated dietary habits including going to bed with a bottle or sippy cup and sugar-sweetened beverage consumption, as secondary study outcomes during the same time frame. Results. A total of 2097 children with a mean age of 15.8 (SD 7.6) months at the initial visit were included in the analysis; 51% were boys; 28% were black, 36% Hispanic/Latino, 5% white, 2% Asian, and 19% other; 75% were publicly insured. During the study period, significant (P < .05) improvements were noted across the 3 oral health behaviors studied among children younger than 18 months. Conclusion. Nursing interventions show promise for promoting preventive dental care in primary care settings and deserve further study.

G-Protein-Coupled Receptors Are Dynamic Regulators of Digestion and Targets for Digestive Diseases

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane signaling proteins. In the gastrointestinal tract, GPCRs expressed by epithelial cells sense contents of the lumen, and GPCRs expressed by epithelial cells, myocytes, neurons, and immune cells participate in communication among cells. GPCRs control digestion, mediate digestive diseases, and coordinate repair and growth. GPCRs are the target of more than one third of therapeutic drugs, including many drugs used to treat digestive diseases. Recent advances in structural, chemical, and cell biology research have shown that GPCRs are not static binary switches that operate from the plasma membrane to control a defined set of intracellular signals. Rather, GPCRs are dynamic signaling proteins that adopt distinct conformations and subcellular distributions when associated with different ligands and intracellular effectors. An understanding of the dynamic nature of GPCRs has provided insights into the mechanism of activation and signaling of GPCRs and has shown opportunities for drug discovery. We review the allosteric modulation, biased agonism, oligomerization, and compartmentalized signaling of GPCRs that control digestion and digestive diseases. We highlight the implications of these concepts for the development of selective and effective drugs to treat diseases of the gastrointestinal tract.

Transient receptor potential vanilloid 4 mediates sour taste sensing via type III taste cell differentiation

Taste buds are comprised of taste cells, which are classified into types I to IV. Transient receptor potential (TRP) channels play a significant role in taste perception. TRP vanilloid 4 (TRPV4) is a non-selective cation channel that responds to mechanical, thermal, and chemical stimuli. The present study aimed to define the function and expression of TRPV4 in taste buds using Trpv4-deficient mice. In circumvallate papillae, TRPV4 colocalized with a type IV cell and epithelial cell marker but not type I, II, or III markers. Behavioural studies showed that Trpv4 deficiency reduced sensitivity to sourness but not to sweet, umami, salty, and bitter tastes. Trpv4 deficiency significantly reduced the expression of type III cells compared with that in wild type (WT) mice in vivo and in taste bud organoid experiments. Trpv4 deficiency also significantly reduced Ki67-positive cells and β-catenin expression compared with those in WT circumvallate papillae. Together, the present results suggest that TRPV4 contributes to sour taste sensing by regulating type III taste cell differentiation in mice.

Coping with noxious effects of quinine by praying mantids (Mantodea) and spiders (Araneae)

Because of its bitter taste, quinine elicits strong antifeedant and toxic effects in animals including arthropods. In the present study, two mantis, Sphodromantis viridis, Hierodula membranacea, and two spider species, Nephila edulis, Selenocosmia javanensis, were offered a quinine solution or prey (crickets) contaminated or injected with quinine, which they ingested and survived without apparent toxic symptoms. Analysis of their faeces and, in the case of spiders, of silk from their web revealed that quinine was excreted over a period of 8-12 (mantids) or 7 days (spiders). Interestingly, the silk glands of the spiders served as an additional excretory organ of quinine. Both, mantids and spiders were shown to tolerate high amounts of quinine in their prey. Obviously, the bitter taste of this compound is not perceived by their gustatory receptors and consequently triggers no aversive reactions.

Non-Nutritive Sweeteners and Their Implications on the Development of Metabolic Syndrome

Individuals widely use non-nutritive sweeteners (NNS) in attempts to lower their overall daily caloric intake, lose weight, and sustain a healthy diet. There are insufficient scientific data that support the safety of consuming NNS. However, recent studies have suggested that NNS consumption can induce gut microbiota dysbiosis and promote glucose intolerance in healthy individuals that may result in the development of type 2 diabetes mellitus (T2DM). This sequence of events may result in changes in the gut microbiota composition through microRNA (miRNA)-mediated changes. The mechanism(s) by which miRNAs alter gene expression of different bacterial species provides a link between the consumption of NNS and the development of metabolic changes. Another potential mechanism that connects NNS to metabolic changes is the molecular crosstalk between the insulin receptor (IR) and G protein-coupled receptors (GPCRs). Here, we aim to highlight the role of NNS in obesity and discuss IR-GPCR crosstalk and miRNA-mediated changes, in the manipulation of the gut microbiota composition and T2DM pathogenesis.

Detection of Succinate by Intestinal Tuft Cells Triggers a Type 2 Innate Immune Circuit

In the small intestine, type 2 responses are regulated by a signaling circuit that involves tuft cells and group 2 innate lymphoid cells (ILC2s). Here, we identified the microbial metabolite succinate as an activating ligand for small intestinal (SI) tuft cells. Sequencing analyses of tuft cells isolated from the small intestine, gall bladder, colon, thymus, and trachea revealed that expression of tuft cell chemosensory receptors is tissue specific. SI tuft cells expressed the succinate receptor (SUCNR1), and providing succinate in drinking water was sufficient to induce a multifaceted type 2 immune response via the tuft-ILC2 circuit. The helminth Nippostrongylus brasiliensis and a tritrichomonad protist both secreted succinate as a metabolite. In vivo sensing of the tritrichomonad required SUCNR1, whereas N. brasiliensis was SUCNR1 independent. These findings define a paradigm wherein tuft cells monitor microbial metabolites to initiate type 2 immunity and suggest the existence of other sensing pathways triggering the response to helminths.

Ion Chromatographic Fingerprinting of STC-1 Cellular Response for Taste Sensing

Taste sensing is of great importance in both the pharmaceutical and foodstuff industries, and is currently mainly based on human sensory evaluation. Many approaches based on chemical sensors have been proposed, leading to the development of various electronic tongue systems. However, this approach is limited by the applied recognition methods, which do not consider natural receptors. Biorecognition elements such as taste receptor proteins or whole cells can be involved in the development of taste sensing biosensors usually equipped with various electrochemical transducers. Here, we propose a new approach: intestinal secretin tumor cell line (STC-1) chemosensory cells were applied for taste recognition, and their taste-specific cellular response was decoded from ion chromatographic fingerprints with the use of multivariate data processing by partial least squares discriminant analysis (PLS-DA). This approach could be useful for the development of various non-invasive taste sensing assays, as well as for studying taste transduction mechanisms in vitro.

A role for taste receptors in (neuro)endocrinology?

The sense of taste is positioned at the forefront when it comes to the interaction of our body with foodborne chemicals. However, the role of our taste system, and in particular its associated taste receptors, is not limited to driving food preferences leading to ingestion or rejection before other organs take over responsibility for nutrient digestion, absorption and metabolic regulation. Taste sensory elements do much more. On the one hand, extra-oral taste receptors from the brain to the gut continue to sense nutrients and noxious substances after ingestion and, on the other hand, the nutritional state feeds back on the taste system. This intricate regulatory network is orchestrated by endocrine factors that are secreted in response to taste receptor signalling and, in turn regulate the taste receptor cells themselves. The present review summarises current knowledge on the endocrine regulation of the taste perceptual system and the release of hunger/satiety regulating factors by gastrointestinal taste receptors. Furthermore, the regulation of blood glucose levels via the activation of pancreatic sweet taste receptors and subsequent insulin secretion, as well as the influence of bitter compounds on thyroid hormone release, is addressed. Finally, the central effects of tastants are discussed briefly.

Sweet and bitter taste stimuli activate VTA projection neurons in the parabrachial nucleus

This study investigated neural projections from the parabrachial nucleus (PBN), a gustatory and visceral processing area in the brainstem, to the ventral tegmental area (VTA) in the midbrain. The VTA contains a large population of dopaminergic neurons that have been shown to play a role in reward processing. Anterograde neural tracing methods were first used to confirm that a robust projection from the caudal PBN terminates in the dorsal VTA; this projection was larger on the contralateral side. In the next experiment, we combined dual retrograde tracing from the VTA and the gustatory ventral posteromedial thalamus (VPMpc) with taste-evoked Fos protein expression, which labels activated neurons. Mice were stimulated through an intraoral cannula with sucrose, quinine, or water, and PBN sections were processed for immunofluorescent detection of Fos and retrograde tracers. The distribution of tracer-labeled PBN neurons demonstrated that the populations of cells projecting to the VTA or VPMpc are largely independent. Quantification of cells double labeled for Fos and either tracer demonstrated that sucrose and quinine were effective in activating both pathways. These results indicate that information about both appetitive and aversive tastes is delivered to a key midbrain reward interface via direct projections from the PBN.

Taste Receptors: New Players in Sperm Biology

Taste receptors were first described as sensory receptors located on the tongue, where they are expressed in small clusters of specialized epithelial cells. However, more studies were published in recent years pointing to an expression of these proteins not only in the oral cavity but throughout the body and thus to a physiological role beyond the tongue. The recent observation that taste receptors and components of the coupled taste transduction cascade are also expressed during the different phases of spermatogenesis as well as in mature spermatozoa from mouse to humans and the overlap between the ligand spectrum of taste receptors with compounds in the male and female reproductive organs makes it reasonable to assume that sperm "taste" these different cues in their natural microenvironments. This assumption is assisted by the recent observations of a reproductive phenotype of different mouse lines carrying a targeted deletion of a taste receptor gene as well as the finding of a significant correlation between human male infertility and some polymorphisms in taste receptors genes. In this review, we depict recent findings on the role of taste receptors in male fertility, especially focusing on their possible involvement in mechanisms underlying spermatogenesis and post testicular sperm maturation. We also highlight the impact of genetic deletions of taste receptors, as well as their polymorphisms on male reproduction.

DNA-mediated self-assembly of taste cells and neurons for taste signal transmission

Cells can communicate with one another through physical connections and chemical signaling, activating various signaling pathways that can affect cellular functions and behaviors. In taste buds, taste cells transmit taste information to neurons via paracrine signaling. However, no previous studies have reported the in vitro co-culture of taste and neuronal cells, which allows us to monitor intercellular communications and better understand the mechanism of taste perception. Here, we introduce the first investigation on the proximate assembly and co-culture of taste cells and neurons to monitor the intercellular transmission of taste signals. Taste cells and neurons are placed closely using a pair of single-stranded oligonucleotides conjugated with polyethylene glycol and a phospholipid. Complementary oligonucleotide conjugates are anchored into the cellular membrane of neonatal taste cells and embryonic hippocampal neuronal cells, respectively, and then the cells are self-assembled into a functional multicellular unit for taste perception. Treatment of the assembled cells with a bitter tastant generates the sequential influx of calcium ions into the cytoplasm in taste cells and then in neuronal cells. Our work demonstrates that the cellular self-assembly is critical for efficient taste signal transduction, which can be used as a promising platform to construct cell-based biosensors for taste sensing.

A bioinspired in vitro bioelectronic tongue with human T2R38 receptor for high-specificity detection of N-C=S-containing compounds

Detection and identification of bitter compounds draw great attention in pharmaceutical and food industry. Several well-known agonists of specific bitter taste receptors have been found to exhibit anti-cancer effects. For example, N-C=S-containing compounds, such as allyl-isothiocyanates, have shown cancer chemo-preventive effects. It is worth noting that human T2R38 receptor is specific for compounds containing N-C=S moiety. Here, a bioinspired cell-based bioelctronic tongue (BioET) is developed for the high-specificity isothiocyanate-induced bitter detection, utilizing human Caco-2 cells as a primary sensing element and interdigitated impedance sensor as a secondary transducer. As an intestinal carcinoma cell line, Caco-2 endogenously expresses human bitter receptor T2R38, and the activation of T2R38 induces the changes of cellular morphology which can be detected by electric cell-substrate impedance sensing (ECIS). After configuration and optimization of parameters including timing of compound administration and cell density, quantitative bitter evaluation models were built for two well-known bitter compounds, phenylthiocarbamide (PTC) and propylthiouracil (PROP). The bitter specific detection of this BioET is inhibited by probenecid and U-73122, and is not elicited by other taste modalities or bitter ligands that do not activate T2R38. Moreover, by combining different computational tools, we designed a ligand-based virtual screening (LBVS) protocol to select ligands that are likely to activate T2R38 receptor. Three computationally predicted agonists of T2R38 were selected using the LBVS protocol, and the BioET presented response to the predicted agonists, validating the capability of the LBVS protocol. This study suggests this unique cell-based BioET paves a general and promising way to specifically detect N-C=S-containing compounds that can be used for pharmaceutical study and drug development.

Unilateral nasal obstruction alters sweet taste preference and sweet taste receptors in rat circumvallate papillae

Nasal obstruction causes mouth breathing, and affects the growth and development of craniofacial structures, muscle function in the stomatognathic system, and the taste perceptive system. However, the detailed mechanism underlying the effects of nasal obstruction on taste perception has not been fully elucidated. In this study, we investigated this mechanism using the two-bottle taste preference test, immunohistological analysis, and quantification of the mRNA expression of taste-related molecules in the circumvallate papillae. Neonatal male Wistar rats were divided randomly into control and experimental groups. Rats in the experimental group underwent unilateral nasal obstruction by cauterization of the external nostril at the age of 8 days. Arterial oxygen saturation (SpO₂) was recorded in awake rats using collar clip sensors. Taste preference for five basic taste solutions was evaluated. Immunohistochemical analysis and quantitative real-time polymerase chain reaction (RT-PCR) were conducted to evaluate the expressions of taste-related molecules in the taste cells of the circumvallate papillae. Body weights were similar between the two groups throughout the experimental period. The SpO₂ in the 7- to 12-week-old rats in the experimental group was significantly lower than that in the age-matched rats in the control group. In the two-bottle taste preference test, the sensitivities to sweet taste decreased in the experimental group. The mRNA expression of T1R2, T1R3, α-gustducin, and PLCβ2 was significantly lower in the experimental group than in the control group as determined by quantitative RT-PCR, and the immunohistochemical staining for α-gustducin and PLCβ2 was less prominent. These findings suggest that nasal obstruction may affect sweet taste perception via the reduced expression of taste-related molecules in the taste cells in rat circumvallate papillae.

Is there adaptation in the human genome for taste perception and phase I biotransformation?

Background

During the modern human expansion, new environmental pressures may have driven adaptation, especially in genes related to the perception of ingested substances and their detoxification. Consequently, positive (adaptive) selection may have occurred in genes related to taste, and in those related to the CYP450 system due to its role in biotransformation of potentially toxic compounds. A total of 91 genes (taste receptors and CYP450 superfamily) have been studied using Hierarchical Boosting, a powerful combination of different selection tests, to detect signatures of recent positive selection in three continental human populations: Northern Europeans (CEU), East Asians (CHB) and Africans (YRI). Analyses have been refined with selection analyses of the 26 populations of 1000 Genomes Project Phase 3.

Results

Genes related to taste perception have not been positively selected in the three continental human populations. This finding suggests that, contrary to results of previous studies, different allele frequencies among populations in genes such as TAS2R38 and TAS2R16 are not due to positive selection but to genetic drift. CYP1 and CYP2 genes, also previously considered to be under positive selection, did not show signatures of selective sweeps. However, three genes belonging to the CYP450 system have been identified by the Hierarchical Boosting as positively selected: CYP3A4 and CYP3A43 in CEU, and CYP27A1 in CHB.

Conclusions

No main adaptive differences are found in known taste receptor genes among the three continental human populations studied. However, there are important genetic adaptations in the cytochrome P450 system related to the Out of Africa expansion of modern humans. We confirmed that CYP3A4 and CYP3A43 are under selection in CEU, and we report for the first time CYP27A1 to be under positive selection in CHB.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1366-7) contains supplementary material, which is available to authorized users.

Spilanthol Enhances Sensitivity to Sodium in Mouse Taste Bud Cells

Overconsumption of NaCl has been linked to increased hypertension-related morbidity. Compounds that can enhance NaCl responses in taste cells could help reduce human NaCl consumption without sacrificing perceived saltiness. Spilanthol is an unsaturated alkylamide isolated from the Jambu plant (Acmella oleracea) that can induce tingling, pungency, and numbing in the mouth. Structurally similar fatty acid amides, such as sanshool, elicit numbing and tingling sensations by inhibiting 2-pore-domain potassium leak channels on trigeminal sensory neurons. Even when insufficient to induce action potential firing, leak current inhibition causes depolarization and increased membrane resistance, which combine to make cells more sensitive to subsequent depolarizing stimuli, such as NaCl. Using calcium imaging, we tested whether spilanthol alters sensitivity to NaCl in isolated circumvallate taste bud cells and trigeminal sensory neurons of mice (Mus musculus). Micromolar spilanthol elicited little to no response in taste bud cells or trigeminal neurons. These same perithreshold concentrations of spilanthol significantly enhanced responses to NaCl (140 and 200 mM) in taste bud cells. Trigeminal neurons, however, exhibited response enhancement only at the highest concentrations of NaCl and spilanthol tested. Using a combination of potassium depolarization, immunohistochemistry, and Trpm5-GFP and Tas1r3-GFP mice to characterize taste bud cells by type, we found spilanthol enhancement of NaCl responses most prevalent in NaCl-responsive type III cells, and commonly observed in NaCl-responsive type II cells. Our results indicate that spilanthol enhances NaCl responses in taste bud cells and point to a family of compounds that may have utility as salty taste enhancers.

Obesity is associated with altered gene expression in human tastebuds

BACKGROUND

The role of taste perception in the development and persistence of obesity is currently unclear due to conflicting results from psychophysical and other studies. No study to date has assessed whether there is an underlying fundamental difference in the physiology of taste tissue between lean and obese individuals.

METHOD/SUBJECTS

We analysed the transcriptomic profile (RNA-seq) of human fungiform taste papillae biopsied from lean (n = 23) and obese (n = 13) Caucasian females (age range 18-55) to identify differences in gene expression.

RESULTS

Obesity status was the major contributor to variance in global gene expression between individuals. A total of 62 genes had significantly different gene expression levels between lean and obese (P < 0.0002), with the specific taste associated genes phospholipase C beta 2 (PLCβ2) and sonic hedge-hog (SHH) having significantly reduced expression in obese group. Genes associated with inflammation and immune response were the top enriched biological pathways differing between the lean and the obese groups. Analysis of a broader gene set having a twofold change in expression (2619 genes) identified three enriched theme groups (sensory perception, cell and synaptic signalling, and immune response). Further, analysis of taste associated genes identified a consistent reduction in the expression of taste-related genes (in particular reduced type II taste cell genes) in the obese compared to the lean group.

CONCLUSION

The findings show obesity is associated with altered gene expression in tastebuds. Furthermore, the results suggest the tastebud microenvironment is distinctly different between lean and obese persons and, that changes in sensory gene expression contribute to this altered microenvironment. This research provides new evidence of a link between obesity and altered taste and in the future may help design strategies to combat obesity.

Taste disorders are partly genetically determined: Role of the TAS2R38 gene, a pilot study

OBJECTIVES/HYPOTHESIS

Taste sensitivity varies greatly among individuals influencing eating behavior and health, consequently the disorders of this sense can affect the quality of life. The ability to perceive the bitter of thiourea compounds, such as phenylthiocarbamide (PTC), has been largely reported as a marker of the general taste sensitivity, food preferences, and health. PTC sensitivity is mediated by the TAS2R38 receptor and its genetic common variants. We study the role of the TAS2R38 receptor in taste disorders with the aim of understanding if these can be genetically determined.

STUDY DESIGN

Prospective cohort study.

METHODS

Differences in the PTC responsiveness between the patients cohort and healthy controls were assessed. All subjects received standardized tests for smell and taste function and were genotyped for the TAS2R38 gene.

RESULTS

PAV/PAV homozygous patients gave high PTC ratings, whereas PAV/AVI genotypes reported lower values, which are similar to those determined in AVI/AVI or rare genotypes. In addition, the patients cohort did not meet the Hardy-Weinberg equilibrium at the TAS2R38 locus, showing a very low frequency of subjects carrying the PAV/AVI diplotype. Independently, in healthy controls who were in equilibrium at the locus, PAV/PAV homozygous and heterozygous rated PTC bitterness higher compared to AVI/AVI or rare genotypes.

CONCLUSIONS

Our findings, by showing that an only taster haplotype (PAV) is not sufficient to evoke high responses of TAS2R38 receptor in patients with taste disorders, suggest that the genetic constitution may represent a risk factor for the development of taste disorders.

LEVEL OF EVIDENCE

2c Laryngoscope, 129:E307-E312, 2019.

Hen protein-derived peptides as the blockers of human bitter taste receptors T2R4, T2R7 and T2R14

Bitter sensation is mediated by various bitter taste receptors (T2Rs), thus T2R antagonists are actively explored. Our objective was to look for novel T2R blockers in hen protein hydrolysate (HPH). We screened the least bitter HPH fractions using electronic tongue, and analyzed their peptide sequences and calcium mobilization in HEK293T cells expressing T2Rs. The results showed that the HPH fractions with higher bitterness intensity had higher hydrophobicity, more hydrophobic amino acids, and more positively charged peptides, but fewer known umami peptides. The peptide fractions from the least bitter HPH fraction significantly inhibited quinine bitterness (P < 0.05), and also significantly inhibited quinine- or diphenhydramine-dependent calcium mobilization of HEK293T cells expressing human T2R4, T2R7, or T2R14 (P < 0.05). Among them, the first eluted (least bitter) peptide fraction showed the strongest bitter-inhibitory effect. In conclusion, HPH peptides are the blockers of T2R4, T2R7, and T2R14.

Umami as an 'Alimentary' Taste. A New Perspective on Taste Classification

Applied taste research is increasingly focusing on the relationship with diet and health, and understanding the role the sense of taste plays in encouraging or discouraging consumption. The concept of basic tastes dates as far back 3000 years, where perception dominated classification with sweet, sour, salty, and bitter consistently featuring on basic taste lists throughout history. Advances in molecular biology and the recent discovery of taste receptors and ligands has increased the basic taste list to include umami and fat taste. There is potential for a plethora of other new basic tastes pending the discovery of taste receptors and ligands. Due to the possibility for an ever-growing list of basic tastes it is pertinent to critically evaluate whether new tastes, including umami, are suitably positioned with the four classic basic tastes (sweet, sour, salty, and bitter). The review critically examines the evidence that umami, and by inference other new tastes, fulfils the criteria for a basic taste, and proposes a subclass named ‘alimentary’ for tastes not meeting basic criteria.

Mammalian Taste Bud Cells Utilize Extragemmal 5-Hydroxy-L-Tryptophan to Biosynthesize the Neurotransmitter Serotonin

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter that is found in mammalian taste buds and can regulate the output of intragemmal signaling networks onto afferent nerve fibers. However, it is unclear how 5-HT is produced, synthesized locally inside taste buds or absorbed from outside sources. In this study, we attempt to address this question by delineating the process of possible 5-HT biosynthesis within taste buds. First, we verified that the rate-limiting enzyme tryptophan hydroxylase (TPH2) responsible for converting L-tryptophan into the intermediate 5-hydroxy-L-tryptophan (5-HTP) is expressed in a subset of type II taste bud cells (TBCs) whereas the enzyme aromatic L-aromatic amino acid decarboxylase (AADC) capable of converting 5-HTP into 5-HT is found in type III TBCs. And abolishment of TPH2 did not affect the production of intragemmal 5-HT or alter TBCs; the mutant mice did not show any changes in behavioral responses to all five primary taste qualities: sweet, umami, bitter, salty, and sour. Then we identified that 5-HTP as well as AADC are abundant in type III TBCs; and application of an AADC inhibitor significantly blocked the production of 5-HT in taste buds. In contrast, administration of an inhibitor on serotonin-reuptake transporters had minimal impact on the 5-HT amount in taste buds, indicating that exogenous 5-HT is not a major source for the intragemmal transmitter. Taken together, our data indicate that intragemmal serotonin is not biosynthesized de novo from tryptophan; instead, it is produced by AADC-mediated conversion of 5-HTP absorbed from the plasma and/or nerve fibers into 5-HT. Thus, our results suggest that the overall bodily 5-HTP level in the plasma and nervous system can regulate taste buds' physiological function, and provide an important molecular mechanism connecting these peripheral taste organs with the circulatory and nervous systems.

Morphology of GNAT3-immunoreactive chemosensory cells in the rat larynx

The upper airways play important roles in respiratory defensive reflexes. Although solitary chemosensory cells and chemosensory cell clusters have been reported in the laryngeal mucosa of mammalian species, the distribution and cellular morphology of chemosensory cells remain unclear. In the present study, the distribution and morphology of solitary chemosensory cells and chemosensory cell clusters were examined by immunofluorescence for GNAT3 on whole-mount preparations of the rat laryngeal mucosa. Electrophysiological experiments were performed to analyze the respiratory reflexes evoked by bitter stimuli to the laryngeal cavity. In the whole area of the laryngeal mucosa, the numbers of GNAT3-immunoreactive solitary chemosensory cells and chemosensory clusters were 421.0 ± 20.3 and 62.7 ± 6.9, respectively. GNAT3-immunoreactive solitary chemosensory cells were mainly distributed in the mucosa overlying epiglottic and arytenoid cartilage, and chemosensory clusters were mainly distributed on the edge of the epiglottis and aryepiglottic fold. GNAT3-immunoreactive solitary chemosensory cells were slender with elongated processes or had a flask-like/columnar shape. The number of GNAT3-immunoreactive cells in chemosensory clusters was 6.1 ± 0.4, ranging between 2 and 14 cells. GNAT3-immunoreactive cells in the cluster were variform and the tips of apical processes gathered at one point at the surface of the epithelium. The tips of apical cytoplasmic processes in solitary chemosensory cells and cells in the cluster were immunoreactive for espin, and faced the laryngeal cavity. Physiological experiments showed that the application of 10 mm quinine hydrochloride to the laryngeal cavity decreased respiratory frequency. The present results revealed the chemosensory field of the larynx and the morphological characteristics of the laryngeal chemosensory system for respiratory depression.

ENaC in Cholinergic Brush Cells

Cholinergic polymodal chemosensory cells in the mammalian urethra (urethral brush cells = UBC) functionally express the canonical bitter and umami taste transduction signaling cascade. Here, we aimed to determine whether UBC are functionally equipped for the perception of salt through ENaC (epithelial sodium channel). Cholinergic UBC were isolated from ChAT-eGFP reporter mice (ChAT = choline acetyltransferase). RT-PCR showed mRNA expression of ENaC subunits Scnn1a, Scnn1b, and Scnn1g in urethral epithelium and isolated UBC. Scnn1a could also be detected by next generation sequencing in 4/6 (66%) single UBC, two of them also expressed the bitter receptor Tas2R108. Strong expression of Scnn1a was seen in some urothelial umbrella cells and in 65% of UBC (30/46 cells) in a Scnn1a reporter mouse strain. Intracellular [Ca²⁺] was recorded in isolated UBC stimulated with the bitter substance denatonium benzoate (25 mM), ATP (0.5 mM) and NaCl (50 mM, on top of 145 mM Na⁺ and 153 mM Cl⁻ baseline in buffer); mannitol (150 mM) served as osmolarity control. NaCl, but not mannitol, evoked an increase in intracellular [Ca²⁺] in 70% of the tested UBC. The NaCl-induced effect was blocked by the ENaC inhibitor amiloride (IC₅₀ = 0.47 μM). When responses to both NaCl and denatonium were tested, all three possible positive response patterns occurred in a balanced distribution: 42% NaCl only, 33% denatonium only, 25% to both stimuli. A similar reaction pattern was observed with ATP and NaCl as test stimuli. About 22% of the UBC reacted to all three stimuli. Thus, NaCl evokes calcium responses in several UBC, likely involving an amiloride-sensitive channel containing α-ENaC. This feature does not define a new subpopulation of UBC, but rather emphasizes their polymodal character. The actual function of α-ENaC in cholinergic UBC—salt perception, homeostatic ion transport, mechanoreception—remains to be determined.

SOX2 regulation by hedgehog signaling controls adult lingual epithelium homeostasis

Adult tongue epithelium is continuously renewed from epithelial progenitor cells, a process that requires hedgehog (HH) signaling. In mice, pharmacological inhibition of the HH pathway causes taste bud loss within a few weeks. Previously, we demonstrated that sonic hedgehog (SHH) overexpression in lingual progenitors induces ectopic taste buds with locally increased SOX2 expression, suggesting that taste bud differentiation depends on SOX2 downstream of HH. To test this, we inhibited HH signaling in mice and observed a rapid decline in Sox2 and SOX2-GFP expression in taste epithelium. Upon conditional deletion of Sox2, differentiation of both taste and non-taste epithelial cells was blocked, and progenitor cell number increased. In contrast to basally restricted proliferation in controls, dividing cells were overabundant and spread to suprabasal epithelial layers in mutants. SOX2 loss in progenitors also led non-cell-autonomously to taste cell apoptosis, dramatically shortening taste cell lifespans. Finally, in tongues with conditional Sox2 deletion and SHH overexpression, ectopic and endogenous taste buds were not detectable; instead, progenitor hyperproliferation expanded throughout the lingual epithelium. In summary, we show that SOX2 functions downstream of HH signaling to regulate lingual epithelium homeostasis.

Hydrogen Bond Length as a Key To Understanding Sweetness

Neutron diffraction experiments have been performed to investigate and compare the structure of the hydration shell of three monosaccharides, namely, fructose, glucose, and mannose. It is found that despite their differences with respect to many thermodynamical quantities, bioprotective properties against environmental stresses, and taste, the influence of these monosaccharides on the bulk water solvent structure is virtually identical. Conversely, these sugars interact with the neighboring water molecules by forming H bonds of different length and strength. Interestingly, the sweetness of these monosaccharides, along with that of the disaccharide trehalose, is correlated with the length of these H bonds. This suggests that the small differences in stereochemistry between the different sugars determine a relevant change in polarity, which has a fundamental impact on the behavior of these molecules in vivo.

Biogastronomy: Factors that determine the biological response to meal ingestion

BACKGROUND

The biological response to a meal includes physiological changes, primarily related to the digestive process, and a sensory experience, involving sensations related to the homeostatic control of food consumption, eg, satiety and fullness, with a hedonic dimension, ie associated with changes in digestive well-being and mood. The responses to a meal include a series of events before, during and after ingestion. While much attention has been paid to the events before and during ingestion, relatively little is known about the postprandial sensations, which are key to the gastronomical experience.

PURPOSE

The aim of this narrative review is to provide a comprehensive overview and to define the framework to investigate the factors that determine the postprandial experience. Based on a series of proof-of-concept studies and related information, we propose that the biological responses to a meal depend on the characteristics of the meal, primarily its palatability and composition, and the responsiveness of the guest, which may be influenced by multiple previous and concurrent conditioning factors. This information provides the scientific backbone to the development of personalized gastronomy.

Expression of the candidate fat taste receptors in human fungiform papillae and the association with fat taste function

Significant experimental evidence supports fat as a taste modality; however, the associated peripheral mechanisms are not well established. Several candidate taste receptors have been identified, but their expression pattern and potential functions in human fungiform papillae remain unknown. The aim of this study is to identify the fat taste candidate receptors and ion channels that were expressed in human fungiform taste buds and their association with oral sensory of fatty acids. For the expression analysis, quantitative RT-PCR (qRT-PCR) from RNA extracted from human fungiform papillae samples was used to determine the expression of candidate fatty acid receptors and ion channels. Western blotting analysis was used to confirm the presence of the proteins in fungiform papillae. Immunohistochemistry analysis was used to localise the expressed receptors or ion channels in the taste buds of fungiform papillae. The correlation study was analysed between the expression level of the expressed fat taste receptors or ion channels indicated by qRT-PCR and fat taste threshold, liking of fatty food and fat intake. As a result, qRT-PCR and western blotting indicated that mRNA and protein of CD36, FFAR4, FFAR2, GPR84 and delayed rectifying K+ channels are expressed in human fungiform taste buds. The expression level of CD36 was associated with the liking difference score (R −0·567, β=−0·04, P=0·04) between high-fat and low-fat food and FFAR2 was associated with total fat intake (ρ=−0·535, β=−0·01, P=0·003) and saturated fat intake (ρ=−0·641, β=−0·02, P=0·008).