Aging does not affect the proportion of taste cell types in mice.

Generally, taste sensitivity is known to change with age. However, the molecular mechanisms underlying this phenomenon remain unclear. Mammalian taste buds are classified into type I, II, III, and IV cells; among them, type II and III cells have an important role in the taste detection process. We hypothesized that age-related changes in the proportion of taste cell types would be a factor in changes in taste sensitivity. To test this hypothesis, we compared the expression patterns of type II and III cell markers in taste buds obtained from the circumvallate papillae of young and old mice. Gustducin, SEMA3A, PLCß2, and CAR4 were used as type II and III cell markers, respectively. When we performed double-fluorescence staining using antibodies for these molecules, Gustducin and SEMA3A immune-positive cells were 22.7 ± 1.2% and 27.6 ± 0.9% in young mice and 22.0 ± 0.7% and 25.9 ± 1.1% in old mice, respectively. PLCß2 and CAR4 immune-positive cells were 30.3 ± 1.5% and 20.7 ± 1.3% in young mice and 29.1 ± 0.8% and 21.1 ± 1.2% in old mice, respectively. There were no significant differences in the percentage of immunopositive cells for all antibodies tested between young and old mice. These results suggest that the proportion of type II and III cells does not change with aging.

Mouse TMC4 is involved in the detection of chloride taste of salts.

Licking behavior with various salts in transmembrane channel-like 4 (Tmc4) knockout (KO) mice was observed. In Tmc4 KO mice, a significant decrease in sensitivity to chloride salts, such as NaCl, KCl, and NH4Cl, was observed, while no significant decrease in sensitivity to Na-gluconate was observed. This finding suggests that TMC4 may be involved in the detection of chloride taste.

Effect of salivary gland removal on taste preference in mice.

To evaluate the effect of decreased salivary secretion on taste preference, we investigated taste preference for five basic tastes by a 48 h two-bottle preference test using a mouse model (desalivated mice) that underwent surgical removal of three major salivary glands: the parotid, submandibular, and sublingual glands. In the desalivated mice, the avoidance behaviors for bitter and salty tastes and the attractive behaviors for sweet and umami tastes were significantly decreased. We confirmed that saliva is necessary to maintain normal taste preference. To estimate the cause of the preference changes, we investigated the effects of salivary gland removal on the expression of taste-related molecules in the taste buds. No apparent changes were observed in the expression levels or patterns of taste-related molecules after salivary gland removal. When the protein concentration and composition in the saliva were compared between the control and desalivated mice, the protein concentration decreased and its composition changed after major salivary gland removal. These results suggest that changes in protein concentration and composition in the saliva may be one of the factors responsible for the changes in taste preferences observed in the desalivated mice.

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.

Effect of Addition of Umami and Sour Components on NaCl Taste in Humans.

We investigated the effects of adding umami and sour components to the taste characteristics of NaCl (75 mM and 100 mM) using sensory evaluation. Here, we employed a mixture of 1 mM monosodium glutamate (MSG) and 0.5 mM inosine 5'-monophosphate (IMP) as the umami component and 3 mM citric acid as the sour component. We confirmed that the addition of MSG+IMP significantly enhanced the salty taste intensity of the NaCl solutions. In addition, the addition of MSG+IMP increased the total taste intensity and percentage of participants who sensed umami. The addition of citric acid increased the salty taste intensity of 100 mM NaCl. The addition of citric acid also increased the total taste intensity, a greater percentage of participants discerned a sour taste. Furthermore, we observed the taste characteristics when adding lemon flavor, which evokes sourness, to the citric acid and NaCl mixture. Although the addition of lemon flavor did not affect the salty taste intensity, the number of participants who perceived a sour taste increased compared to those given only citric acid with the NaCl, and improvement in palatability was also observed. These results suggest that the addition of MSG+IMP and citric acid complicates the taste of NaCl solutions, and that they may enhance the salty taste.

Two-generation exposure to a high-fat diet induces the change of salty taste preference in rats.

High-fat diet (HFD) leads to multiple complications, including taste alteration. This study observed the effect of a two-generation exposure to an HFD on the peripheral taste system in offspring. Ten pregnant Wistar rats were assigned a standard diet (SD) (n = 5) or HFD (n = 5) from day 7 of pregnancy through the lactation. Thirty-six male and female 3-week-old offspring were measured for body weight and blood glucose level, and the circumvallate papillae were collected. The other twenty-four 3-week-old offspring were weaned on the same diet as their mothers and raised individually. The taste preference behaviors were studied using the two-bottle taste preference test and analyzed five basic tastes (sweet, bitter, umami, sour, and salty). The expressions of epithelial sodium channel alpha subunit (ENaCα) and angiotensin II receptor type 1 (AT1) in the circumvallate papilla were analyzed by immunohistochemical staining and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). We found increased body weight and salty taste preference of offspring from the HFD group in both sexes. Correspondingly, the AT1 level of the taste bud cells significantly increased in 3-week-old female offspring from the HFD group. An increase in AT1 levels may be a risk factor for changes in salty taste preference.

Molecular logic of salt taste reception in special reference to transmembrane channel-like 4 (TMC4).

The taste is biologically of intrinsic importance. It almost momentarily perceives environmental stimuli for better survival. In the early 2000s, research into taste reception was greatly developed with discovery of the receptors. However, the mechanism of salt taste reception is not fully elucidated yet and many questions still remain. At present, next-generation sequencing and genome-editing technologies are available which would become pivotal tools to elucidate the remaining issues. Here we review current mechanisms of salt taste reception in particular and characterize the properties of transmembrane channel-like 4 as a novel salt taste-related molecule that we found using these sophisticated tools.

Taste Characteristics of Various Amino Acid Derivatives.

Amino acids contribute to the taste of foods. Previous studies on the taste of amino acids focused mainly on α-amino acids, and therefore, the taste characteristics of amino acid derivatives remain unclear. In the present study, we targeted 6 different amino acid derivatives, ß-alanine, citrulline, creatine, γ-aminobutyric acid, taurine, and ornithine, and evaluated their taste characteristics in a human sensory study. All tested amino acid derivatives showed multiple taste qualities; no derivatives had only a single taste quality. However, their taste intensities were relatively weak even at high concentrations. Given that the interactions between amino acid derivatives and nucleotide result in taste enhancements, we investigated the effect of inosine 5'-monophosphate (IMP) on the taste characteristics and found that the taste intensity of ornithine increased in the presence of IMP. This finding will be useful for understanding the role of amino acid derivatives as taste substances in daily foods.

Identification of mouse bitter taste receptors that respond to resveratrol: a bitter-tasting polyphenolic compound.

The mouse bitter taste receptors (Tas2rs) that respond to resveratrol, a bitter-tasting polyphenolic compound, were identified. Among 35 members of the Tas2r family, Tas2r108, 109, 131, and 137 responded to resveratrol treatment. mRNA expression levels of Tas2r108 and Tas2r137 were higher than those of Tas2r109 and Tas2r131 in mouse circumvallate papillae, indicating that Tas2r108 and Tas2r137 may play important roles in detecting the bitterness of resveratrol in the oral cavity. The mRNA expression levels of Tas2r137 and Tas2r108 were also observed in several tissues, suggesting that Tas2r108 and Tas2r137 may also be involved in the physiological action of resveratrol.

Expression of Olfactory-Related Genes in the Olfactory Epithelium of an Alzheimer's Disease Mouse Model.

Using an amyloid precursor protein (App) gene knock-in (KI) mouse of Alzheimer's disease (AD), we investigated the expression of olfactory-related genes in olfactory impairment caused by AD. We observed the change in olfactory behavior in the App-KI mice. There was no significant difference, however, in the mRNA expression levels of olfactory-related genes between the olfactory epithelia of wild-type (WT) and App-KI mice. Amyloid-ß deposition was confirmed throughout the olfactory pathway in App-KI mice, but not in WT mice. These show that the change in olfactory behavior in the App-KI mice might cause by the impairment of the olfactory pathway.

Transmembrane channel-like 4 is involved in pH and temperature-dependent modulation of salty taste.

Human susceptibility to NaCl varies depending on temperature and pH, the molecular mechanisms of which remain unclear. The voltage-dependent chloride channel, transmembrane channel-like 4 (TMC4), is activated at approximately 40 °C and is suppressed at pH 5.5. As these are similar in character to human sensory evaluations, human TMC4 may be involved in human salt taste reception.

TMC4 is a novel chloride channel involved in high-concentration salt taste sensation.

"Salty taste" sensation is evoked when sodium and chloride ions are present together in the oral cavity. The presence of an epithelial cation channel that receives Na+ has previously been reported. However, no molecular entity involving Cl- receptors has been elucidated. We report the strong expression of transmembrane channel-like 4 (TMC4) in the circumvallate and foliate papillae projected to the glossopharyngeal nerve, mediating a high-concentration of NaCl. Electrophysiological analysis using HEK293T cells revealed that TMC4 was a voltage-dependent Cl- channel and the consequent currents were completely inhibited by NPPB, an anion channel blocker. TMC4 allowed permeation of organic anions including gluconate, but their current amplitudes at positive potentials were less than that of Cl-. Tmc4-deficient mice showed significantly weaker glossopharyngeal nerve response to high-concentration of NaCl than the wild-type littermates. These results indicated that TMC4 is a novel chloride channel that responds to high-concentration of NaCl.

Ibuprofen inhibits oral NaCl response through transmembrane channel-like 4.

Nonsteroidal anti-inflammatory drugs, such as ibuprofen, are known to modify salty taste perception in humans. However, the underlying molecular mechanisms remain unknown. We investigated the inhibitory effect of ibuprofen on the NaCl stimulation of epithelium sodium channel (ENaC) and transmembrane channel-like 4 (TMC4), which are involved in salty taste detection. Although ibuprofen only minimally inhibited the response of the ENaC to NaCl, it significantly inhibited the TMC4 response to NaCl with an IC50 at 1.45 mM. These results suggest that ibuprofen interferes with detection of salty taste via inhibition of TMC4.

Change in Taste Preference to Capsaicin and Catechin Due to Aging in Mice.

Taste is a chemical sensation that primarily detects nutrients present in food, and maintenance of taste sensations is important for ensuring that older people have a balanced nutritional diet. While several reports have suggested that taste sensitivity changes with age, the molecular mechanisms underlying this phenomenon are still unclear. Previous studies on the matter have focused mainly on the relationship between aging and taste detection of specific basic taste-inducing substances, and other than for these basic substances, understanding of how aging affects the detection of taste is limited. Therefore, to understand the effect that aging has on the taste detection of some familiar substances found in our daily meals, namely capsaicin and catechin, we investigated age-related changes in taste preferences to capsaicin and catechin in young and old C57BL/6J mice using a 48-h two-bottle preference test. For the capsaicin stimuli, the mice showed avoidance behavior in a concentration-dependent manner. However, we observed that there was no significant difference in the preference ratio for capsaicin between young and old mice. For the catechin stimuli, although both age groups showed avoidance behavior in a concentration-dependent manner, the preference ratio in old mice showed significantly higher values than those in young mice. This suggests that catechin sensitivity is declined due to aging. Thus, we observed that catechin sensitivity decreases with age, but capsaicin sensitivity does not.

Comparison between the timing of the occurrence of taste sensitivity changes and short-term memory decline due to aging in SAMP1 mice.

Several studies have suggested that cognitive impairment affects taste sensitivity. However, the mechanism behind this is still unclear. In this study, we focused on short-term memory. Using senescence-accelerated mouse prone 1 (SAMP1) mice, we compared whether the effects of aging are observed earlier in taste sensitivity or short-term memory. We used 8-week-old mice as the young group, and 70- and 80-week-old mice as aged groups. Taste sensitivity was evaluated using a 48-hour two-bottle preference test, and short-term memory was evaluated using the Y-maze test. SAMP1 mice showed apparently changes in taste sensitivity at 70-weeks-old. However, the influence of aging on spontaneous alternation behavior, which is indicative of short-term memory alterations, was not observed in 70-week-old mice. At 80-weeks-old, the influence of aging was observed, and spontaneous alternation behavior was significantly decreased. This suggests that age-dependent changes in taste sensitivity occur prior to short-term memory function decline. In addition, there was no significant influence of aging on the mRNA expression of long-term potentiation-related genes in the hippocampus of 80-week-old mice. Therefore, the age-related decline of short-term memory may not affect taste sensitivity.

Bitter taste receptor activation by hop-derived bitter components induces gastrointestinal hormone production in enteroendocrine cells.

Matured hop bitter acids (MHBA) are bitter acid oxides derived from hops, widely consumed as food ingredients to add bitterness and flavor in beers. Previous studies have suggested a potential gut-brain mechanism in which MHBA simulates enteroendocrine cells to produce cholecystokinin (CCK), a gastrointestinal hormone which activates autonomic nerves, resulting in body fat reduction and cognitive improvement; however, the MHBA recognition site on enteroendocrine cells has not been fully elucidated. In this study, we report that MHBA is recognized by specific human and mouse bitter taste receptors (human TAS2R1, 8, 10 and mouse Tas2r119, 130, 105) using a heterologous receptor expression system in human embryonic kidney 293T cells. In addition, knockdown of each of these receptors using siRNA transfection partially but significantly suppressed an MHBA-induced calcium response and CCK production in enteroendocrine cells. Furthermore, blocking one of the essential taste signaling components, transient receptor potential cation channel subfamily M member 5, remarkably inhibited the MHBA-induced calcium response and CCK production in enteroendocrine cells. Our results demonstrate that specific bitter taste receptor activation by MHBA drives downstream calcium response and CCK production in enteroendocrine cells. These findings reveal a mechanism by which food ingredients derived from hops in beer activate the gut-brain axis for the first time.

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.

Analysis of Taste Sensitivities in App Knock-In Mouse Model of Alzheimer's Disease.

BACKGROUND: Some studies have reported a decline in taste sensitivities in patients with Alzheimer's disease. However, the detail remains unknown. OBJECTIVE: We investigated the effect of cognitive impairment on taste sensitivity using an App knock-in mouse model of Alzheimer's disease. METHODS: Behavioral assays, a brief access test, and a 48 h two-bottle preference test, to assess taste sensitivities were started from 12 months of age in mice that were confirmed to have impaired cognition. RESULTS: In the assays, there was no significant difference in taste sensitivities between wild type and App knock-in mice. Additionally, no apparent difference was observed in the expression of taste markers in their taste bud cells. CONCLUSION: We concluded that cognitive impairment might not greatly affect taste sensitivity.

Efficacy of Long-Term Feeding of α-Glycerophosphocholine for Aging-Related Phenomena in Old Mice.

α-Glycerophosphocholine (GPC) is a natural source of choline. It reportedly prevents aging-related decline in cognitive function, but the underlying mechanism remains unclear. Although it is understood that aging influences taste sensitivity and energy regulation, whether GPC exerts antiaging effects on such phenomena requires further elucidation. Here, we used old C57BL/6J mice that were fed a GPC-containing diet, to investigate the molecular mechanisms underlying the prevention of a decline in cognitive function associated with aging and examine the beneficial effects of GPC intake on aging-related phenomena, such as taste sensitivity and energy regulation. We confirmed that GPC intake reduces the aging-related decline in the expression levels of genes related to long-term potentiation. Although we did not observe an improvement in aging-related decline in taste sensitivity, there was a notable improvement in the expression levels of ß-oxidation-associated genes in old mice. Our results suggest that the prevention of aging-related decline in cognitive function by GPC intake may be associated with the improvement of gene expression levels of long-term potentiation. Furthermore, GPC intake may positively influence lipid metabolism.