Sodium-Taste Cells Require Skn-1a for Generation and Share Molecular Features with Sweet, Umami, and Bitter Taste Cells

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, PLCβ2, 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. PLCβ2 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.

The effect of bariatric surgery on the expression of gastrointestinal taste receptors: A systematic review

Gastrointestinal nutrient sensing via taste receptors may contribute to weight loss, metabolic improvements, and a reduced preference for sweet and fatty foods following bariatric surgery. This review aimed to investigate the effect of bariatric surgery on the expression of oral and post-oral gastrointestinal taste receptors and associations between taste receptor alterations and clinical outcomes of bariatric surgery. A systematic review was conducted to capture data from both human and animal studies on changes in the expression of taste receptors in oral or post-oral gastrointestinal tissue following any type of bariatric surgery. Databases searched included Medline, Embase, Emcare, APA PsychInfo, Cochrane Library, and CINAHL. Two human and 21 animal studies were included. Bariatric surgery alters the quantity of many sweet, umami, and fatty acid taste receptors in the gastrointestinal tract. Changes to the expression of sweet and amino acid receptors occur most often in intestinal segments surgically repositioned more proximally, such as the alimentary limb after gastric bypass. Conversely, changes to fatty acid receptors were observed more frequently in the colon than in the small intestine. Significant heterogeneity in the methodology of included studies limited conclusions regarding the direction of change in taste receptor expression induced by bariatric surgeries. Few studies have investigated associations between taste receptor expression and clinical outcomes of bariatric surgery. As such, future studies should look to investigate the relationship between bariatric surgery-induced changes to gut taste receptor expression and function and the impact of surgery on taste preferences, food palatability, and eating behaviour.Registration code in PROSPERO: CRD42022313992.

Ultrastructural localization of calcium homeostasis modulator 1 in mouse taste buds

Taste receptor cells are morphologically classified as types II and III. Type II cells form a unique type of synapses referred to as channel synapses where calcium homeostasis modulator 1 (CALHM1) together with CALHM3 forms voltage-gated channels that release the neurotransmitter, adenosine triphosphate (ATP). To validate the proposed structural model of channel synapses, the ultrastructural localization of CALHM1 in type II cells of both fungiform and circumvallate taste buds was examined. A monoclonal antibody against CALHM1 was developed and its localization was evaluated via immunofluorescence and immunoelectron microscopy using the immunogold-silver labeling technique. CALHM1 was detected as puncta using immunofluorescence and along the presynaptic membrane of channel synapses facing atypical mitochondria, which provide ATP, by immunoelectron microscopy. In addition, it was detected along the plasma membrane lined by subsurface cisternae at sites apposed to afferent nerve fibers. Our results support the validity of a previously proposed structural model for channel synapses and provide insights into the function of subsurface cisternae whose function in taste receptor cells is unknown. We also examined the localization of CALHM1 in hybrid synapses of type III cells, which are conventional chemical synapses accompanied by mitochondria similar to atypical mitochondria of channel synapses. CALHM1 was not detected in the six hybrid synapses examined using immunoelectron microscopy. We further performed double immunolabeling for CALHM1 and Bassoon, which is detected as puncta corresponding to conventional vesicular synapses in type III cells. Our observations suggest that at least some, and probably most, hybrid synapses are not accompanied by CALHM1.

Regeneration of taste through sonic hedgehog upregulation by photobiomodulation

This study investigated photobiomodulation (PBM) effects in Sonic hedgehog (Shh) signaling as a potential approach to taste preservation and regeneration. Primary taste cell (TC) cultures were treated with Shh antagonist vismodegib and irradiated using a continuous wave type 630 nm light-emitting diode (10 mW/cm2 ) array, with single or multiple doses of 30 J/cm2 to determine dose inducing significant upregulation effect. Shh, Ptch, Smo, and Gli1 were significantly upregulated at 120 J/cm2 , used as the minimum dose in vivo. Vismodegib was administered via daily oral gavage for 21 days (30 mg/kg) to induce Shh inhibition in the tongue of rat animal models resulting in taste bud damage and taste dysfunction. PBM treatment using a 630 nm laser (3 W/cm2 ) at a radiant exposure of 120 J/cm2 (24 J/cm2  × 5) successfully upregulated the Shh protein expression, regenerated taste buds, and recovered taste function.

The proton channel OTOP1 is a sensor for the taste of ammonium chloride

Ammonium (NH4+), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from C. elegans to humans and has been used for decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH4Cl). Extracellular NH4Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH4Cl relative to acid stimuli. Taste responses to NH4Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an Otop1-/- mouse strain. Behavioral aversion of mice to NH4Cl, reduced in Skn-1a-/- mice lacking Type II TRCs, was entirely abolished in a double knockout with Otop1. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH4Cl and a previously undescribed channel activation mechanism.

A Transcription Factor Etv1/Er81 Is Involved in the Differentiation of Sweet, Umami, and Sodium Taste Cells

Taste cells are maintained by continuous turnover throughout a lifetime, yet the mechanisms of taste cell differentiation, and how taste sensations remain constant despite this continuous turnover, remain poorly understood. Here, we report that a transcription factor Etv1 (also known as Er81) is involved in the differentiation of taste cells responsible for the preference for sweet, umami, and salty tastes. Molecular analyses revealed that Etv1 is expressed by a subset of taste cells that depend on Skn-1a (also known as Pou2f3) for their generation and express T1R genes (responsible for sweet and umami tastes) or Scnn1a (responsible for amiloride-sensitive salty taste). Etv1CreERT2/CreERT2 mice express Etv1 isoform(s) but not Etv1 in putative proprioceptive neurons as comparable to wild-type mice, yet lack expression of Etv1 or an isoform in taste cells. These Etv1CreERT2/CreERT2 mice have the same population of Skn-1a-dependent cells in taste buds as wild-type mice but have altered gene expression in taste cells, with regional differences. They have markedly decreased electrophysiological responses of chorda tympani nerves to sweet and umami tastes and to amiloride-sensitive salty taste evoked by sodium cation, but they have unchanged responses to bitter or sour tastes. Our data thus show that Etv1 is involved in the differentiation of the taste cells responsible for sweet, umami, and salty taste preferences.

Recent Advances in Bitterness-Sensing Systems

Bitterness is one of the basic tastes, and sensing bitterness plays a significant role in mammals recognizing toxic substances. The bitter taste of food and oral medicines may decrease consumer compliance. As a result, many efforts have been made to mask or decrease the bitterness in food and oral pharmaceutical products. The detection of bitterness is critical to evaluate how successful the taste-masking technology is, and many novel taste-sensing systems have been developed on the basis of various interaction mechanisms. In this review, we summarize the progress of bitterness response mechanisms and the development of novel sensors in detecting bitterness ranging from commercial electronic devices based on modified electrodes to micro-type sensors functionalized with taste cells, polymeric membranes, and other materials in the last two decades. The challenges and potential solutions to improve the taste sensor quality are also discussed.

Molecular and Cellular Mechanisms of Salt Taste

Salt taste, the taste of sodium chloride (NaCl), is mechanistically one of the most complex and puzzling among basic tastes. Sodium has essential functions in the body but causes harm in excess. Thus, animals use salt taste to ingest the right amount of salt, which fluctuates by physiological needs: typically, attraction to low salt concentrations and rejection of high salt. This concentration-valence relationship is universally observed in terrestrial animals, and research has revealed complex peripheral codes for NaCl involving multiple taste pathways of opposing valence. Sodium-dependent and -independent pathways mediate attraction and aversion to NaCl, respectively. Gustatory sensors and cells that transduce NaCl have been uncovered, along with downstream signal transduction and neurotransmission mechanisms. However, much remains unknown. This article reviews classical and recent advances in our understanding of the molecular and cellular mechanisms underlying salt taste in mammals and insects and discusses perspectives on human salt taste.

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.

Changes of the biophysical properties of voltage-gated Na+ currents during maturation of the sodium-taste cells in rat fungiform papillae

Taste cells are a heterogeneous population of sensory receptors that undergo continuous turnover. Different chemo-sensitive cell lines rely on action potentials to release the neurotransmitter onto nerve endings. The electrical excitability is due to the presence of a tetrodotoxin-sensitive, voltage-gated sodium current (I_Na ) similar to that found in neurons. Since the biophysical properties of neuronal I_Na change during development, we wondered whether the same also occurred in taste cells. Here, we used the patch-clamp recording technique to study I_Na in salt-sensing cells (sodium cells) of rat fungiform papillae. We identified these cells by exploiting the known blocking effect of amiloride on ENaC, the sodium (salt) receptor. Based on the amplitude of I_Na , which is known to increase during development, we subdivided sodium cells into two groups: cells with small sodium current (SSC cells; I_Na  < 1 nA) and cells with large sodium current (LSC cells; I_Na  > 1 nA). We found that: the voltage dependence of activation and inactivation significantly differed between these subsets; a slowly inactivating sodium current was more prominent in LSC cells; membrane capacitance in SSC cells was larger than in LSC cells. mRNA expression analysis of the α-subunits of voltage-gated sodium channels in fungiform taste buds supported the functional data. Lucifer Yellow labelling of recorded cells revealed that our electrophysiological criterion for distinguishing two broad groups of taste cells was in good agreement with morphological observations for cell maturity. Thus, all these findings are consistent with developmental changes in the voltage-dependent properties of sodium-taste cells. KEY POINTS: Taste cells are sensory receptors that undergo continuous turnover while they detect food chemicals and communicate with afferent nerve fibres. The voltage-gated sodium current (I_Na ) is a key ion current for generating action potentials in fully differentiated and chemo-sensitive taste cells, which use electrical signalling to release neurotransmitters. Here we show that, during the maturation of rat taste cells involved in salt detection (sodium cells), the biophysical properties of I_Na , such as voltage dependence of activation and inactivation, change significantly. Our results help reveal how taste cells gain electrical excitability during turnover, a property critical to their operation as chemical detectors that relay sensory information to nerve fibres.

Relationship between ENaC Regulators and SARS-CoV-2 Virus Receptor (ACE2) Expression in Cultured Adult Human Fungiform (HBO) Taste Cells

In addition to the α, β, and γ subunits of ENaC, human salt-sensing taste receptor cells (TRCs) also express the δ-subunit. At present, it is not clear if the expression and function of the ENaC δ-subunit in human salt-sensing TRCs is also modulated by the ENaC regulatory hormones and intracellular signaling effectors known to modulate salt responses in rodent TRCs. Here, we used molecular techniques to demonstrate that the G-protein-coupled estrogen receptor (GPER1), the transient receptor potential cation channel subfamily V member 1 (TRPV1), and components of the renin-angiotensin-aldosterone system (RAAS) are expressed in δ-ENaC-positive cultured adult human fungiform (HBO) taste cells. Our results suggest that RAAS components function in a complex with ENaC and TRPV1 to modulate salt sensing and thus salt intake in humans. Early, but often prolonged, symptoms of COVID-19 infection are the loss of taste, smell, and chemesthesis. The SARS-CoV-2 spike protein contains two subunits, S1 and S2. S1 contains a receptor-binding domain, which is responsible for recognizing and binding to the ACE2 receptor, a component of RAAS. Our results show that the binding of a mutated S1 protein to ACE2 decreases ACE2 expression in HBO cells. We hypothesize that changes in ACE2 receptor expression can alter the balance between the two major RAAS pathways, ACE1/Ang II/AT1R and ACE2/Ang-(1-7)/MASR1, leading to changes in ENaC expression and responses to NaCl in salt-sensing human fungiform taste cells.

Taste receptor gene expression is associated with decreased eGFR in patients with diabetes

Dysgeusia is not only associated with zinc deficiency but also with certain drugs or diseases, including diabetes and renal failure. It often lowers the patient's quality of life and hinders access to proper nutrition. The underlying mechanism is unclear and there is a lack of awareness among patients. Here, we focused on lingual taste receptor gene expression in diabetes and elucidated the relationship between taste receptor gene expression and renal function. Forty-seven patients with diabetes and 10 healthy subjects (control group) were enrolled. Lingual foliate papillae were scraped and the derived cDNA was quantified by real-time polymerase chain reaction. Dysgeusia was assessed using SALSAVE?. All statistical analyses were performed using JMP? software 13. The expression of T1R1 and T1R2 was significantly upregulated in type 2 diabetes patients as compared with that in healthy subjects (P<0.01) but did not change in type 1 diabetes patients. T1R3 expression positively correlated and Scnn1 expression negatively correlated with estimated glomerular filtration rate, suggesting that altered taste receptor gene expression could reflect impaired renal function. Thus, alterations in T1R3 and Scnn1 expression in diabetes correlated with renal function. Taste receptor gene expression dysregulation could indicate dysgeusia associated with impaired renal function in patients with diabetes. J. Med. Invest. 69 : 120-126, February, 2022.

Intranasal calcitonin gene-related peptide administration impairs fear memory retention in mice through the PKD/p-HDAC5/Npas4 pathway

The calcitonin gene-related peptide (CGRP) suppresses fear memory retention in mice. Although intracerebroventricular administration of CGRP alters the fear memory processes, making it a promising therapeutic strategy for post-traumatic stress disorder (PTSD), direct brain injection into patients is not practical. Therefore, we propose that intranasal application may be an effective way to deliver CGRP to the brain. This study tested whether CGRP nasal administration exerts the same effect as intracerebroventricular administration using C57BL6J mice. The amount of CGRP in the cerebrospinal fluid and hippocampus 30 min after nasal administration of CGRP was significantly higher when compared with saline. Intranasal CGRP also elicited photophobic behaviors similar to intracerebroventricular injection. Moreover, intranasal CGRP decreased fear memory retention but did not affect reactivation and extinction of fear memory. We found intranasal CGRP significantly increased the expression of protein kinase D (PKD), phosphorylated histone deacetylase 5 (p-HDAC5) and neuronal PAS domain protein 4 (Npas4) in the hippocampus. CGRP-mediated impairment of fear memory and Npas4 expression increases were attenuated significantly by the CGRP receptor antagonist BIBN4096. Together, our data demonstrate that intranasal CGRP delivery activates the PKD/p-HDAC5/Npas4 pathway, decreases fear memory retention.

Longitudinal trajectories and determinants of human fungiform papillae density

Tongue fungiform papillae contain taste buds crucial for taste and hormone-producing taste receptor cells; therefore, they may be considered as endocrine organs and have important age-associated physiological implications. We examine the cross-sectional and longitudinal trajectories of fungiform papillae density in 1084 participants from the Baltimore Longitudinal Study of Aging using linear regression models and mixed effects models. At baseline, the mean age was 67.86 ± 14.20 years, with a mean follow-up time among those with repeat visits of 4.24 ± 1.70 years. Women (53%) were younger (66.85 ± 13.78 vs. 69.04 ± 14.61 years, p < 0.001) and had a higher fungiform papillae density than men (16.14 ± 9.54 vs. 13.77 ± 8.61 papillae/cm², p < 0.001). Whites (67%) had a lower fungiform papillae density than non-Whites after adjusting for age and sex. Factors cross-sectionally associated with a lower fungiform papillae density included a higher waist-hip ratio (β = −8.525, p = 0.029), current smoking status (β = −5.133, p = 0.014), and alcohol use within the past 12 months (β = −1.571, p = 0.025). Longitudinally, fungiform papillae density decreased linearly with follow-up time (β = −0.646, p < 0.001). The rate of decline was not affected by sex, race, BMI, waist-hip ratio, smoking, or alcohol use. The longitudinal decline of fungiform papillae density over time needs to be explored further in order to identify other possible age-associated physiological determinants.

Dysgeusia: A review in the context of COVID-19

Background

Taste disorders in general, and dysgeusia in particular, are relatively common disorders that may be a sign of a more complex acute or chronic medical condition. During the COVID-19 pandemic, taste disorders have found their way into the realm of general as well as specialty dentistry, with significance in screening for patients who potentially may have the virus.

Types of Studies Reviewed

The authors searched electronic databases (PubMed, Embase, Web of Science, Google Scholar) for studies focused on dysgeusia, ageusia, and other taste disorders and their relationship to local and systemic causes.

Results

The authors found pertinent literature explaining the normal physiology of taste sensation, proposals for suggested new tastes, presence of gustatory receptors in remote tissues of the body, and etiology and pathophysiology of taste disorders, in addition to the valuable knowledge gained about gustatory disorders in the context of COVID-19. Along with olfactory disorders, taste disorders are one of the earliest suggestive symptoms of COVID-19 infection.

Conclusions

Gustatory disorders are the result of local or systemic etiology or both. Newer taste sensations, such as calcium and fat tastes, have been discovered, as well as taste receptors that are remote from the oropharyngeal area. Literature published during the COVID-19 pandemic to date reinforces the significance of early detection of potential patients with COVID-19 by means of screening for recent-onset taste disorders.

Practical Implications

Timely screening and identification of potential gustatory disorders are paramount for the dental care practitioner to aid in the early diagnosis of COVID-19 and other serious systemic disorders.

Cellular Diversity and Regeneration in Taste Buds

Taste buds are the sensory end organs for gustation, mediating sensations of salty, sour, bitter, sweet and umami as well as other possible modalities, e.g. fat and kokumi. Understanding of the structure and function of these sensory organs has increased greatly in the last decades with advances in ultrastructural methods, molecular genetics, and in vitro models. This review will focus on the cellular constituents of taste buds, and molecular regulation of taste bud cell renewal and differentiation.

Interactions between Chemesthesis and Taste: Role of TRPA1 and TRPV1

In addition to the sense of taste and olfaction, chemesthesis, the sensation of irritation, pungency, cooling, warmth, or burning elicited by spices and herbs, plays a central role in food consumption. Many plant-derived molecules demonstrate their chemesthetic properties via the opening of transient receptor potential ankyrin 1 (TRPA1) and transient receptor potential vanilloid 1 (TRPV1) channels. TRPA1 and TRPV1 are structurally related thermosensitive cation channels and are often co-expressed in sensory nerve endings. TRPA1 and TRPV1 can also indirectly influence some, but not all, primary taste qualities via the release of substance P and calcitonin gene-related peptide (CGRP) from trigeminal neurons and their subsequent effects on CGRP receptor expressed in Type III taste receptor cells. Here, we will review the effect of some chemesthetic agonists of TRPA1 and TRPV1 and their influence on bitter, sour, and salt taste qualities.

Salt Sensation and Regulation

Taste sensation and regulation are highly conserved in insects and mammals. Research conducted over recent decades has yielded major advances in our understanding of the molecular mechanisms underlying the taste sensors for a variety of taste sensations and the processes underlying regulation of ingestion depending on our internal state. Salt (NaCl) is an essential ingested nutrient. The regulation of internal sodium concentrations for physiological processes, including neuronal activity, fluid volume, acid–base balance, and muscle contraction, are extremely important issues in animal health. Both mammals and flies detect low and high NaCl concentrations as attractive and aversive tastants, respectively. These attractive or aversive behaviors can be modulated by the internal nutrient state. However, the differential encoding of the tastes underlying low and high salt concentrations in the brain remain unclear. In this review, we discuss the current view of taste sensation and modulation in the brain with an emphasis on recent advances in this field. This work presents new questions that include but are not limited to, “How do the fly’s neuronal circuits process this complex salt code?” and “Why do high concentrations of salt induce a negative valence only when the need for salt is low?” A better understanding of regulation of salt homeostasis could improve our understanding of why our brains enjoy salty food so much.

Whole-Mount Staining, Visualization, and Analysis of Fungiform, Circumvallate, and Palate Taste Buds

Taste buds are collections of taste-transducing cells specialized to detect subsets of chemical stimuli in the oral cavity. These transducing cells communicate with nerve fibers that carry this information to the brain. Because taste-transducing cells continuously die and are replaced throughout adulthood, the taste-bud environment is both complex and dynamic, requiring detailed analyses of its cell types, their locations, and any physical relationships between them. Detailed analyses have been limited by tongue-tissue heterogeneity and density that have significantly reduced antibody permeability. These obstacles require sectioning protocols that result in splitting taste buds across sections so that measurements are only approximated, and cell relationships are lost. To overcome these challenges, the methods described herein involve collecting, imaging, and analyzing whole taste buds and individual terminal arbors from three taste regions: fungiform papillae, circumvallate papillae, and the palate. Collecting whole taste buds reduces bias and technical variability and can be used to report absolute numbers for features including taste-bud volume, total taste-bud innervation, transducing-cell counts, and the morphology of individual terminal arbors. To demonstrate the advantages of this method, this paper provides comparisons of taste bud and innervation volumes between fungiform and circumvallate taste buds using a general taste-bud marker and a label for all taste fibers. A workflow for the use of sparse-cell genetic labeling of taste neurons (with labeled subsets of taste-transducing cells) is also provided. This workflow analyzes the structures of individual taste-nerve arbors, cell type numbers, and the physical relationships between cells using image analysis software. Together, these workflows provide a novel approach for tissue preparation and analysis of both whole taste buds and the complete morphology of their innervating arbors.

GAD65Cre Drives Reporter Expression in Multiple Taste Cell Types

In taste buds, Type I cells represent the majority of cells (50-60%) and primarily have a glial-like function in taste buds. However, recent studies suggest that they have additional sensory and signaling functions including amiloride-sensitive salt transduction, oxytocin modulation of taste, and substance P mediated GABA release. Nonetheless, the overall function of Type I cells in transduction and signaling remains unclear, primarily because of the lack of a reliable reporter for this cell type. GAD65 expression is specific to Type I taste cells and GAD65 has been used as a Cre driver to study Type I cells in salt taste transduction. To test the specificity of transgene-driven expression, we crossed GAD65Cre mice with floxed tdTomato and Channelrhodopsin (ChR2) lines and examined the progeny with immunochemistry, chorda tympani recording, and calcium imaging. We report that while many tdTomato+ taste cells express NTPDase2, a specific marker of Type I cells, we see some expression of tdTomato in both Gustducin and SNAP25-positive taste cells. We also see ChR2 in cells just outside the fungiform taste buds. Chorda tympani recordings in the GAD65Cre/ChR2 mice show large responses to blue light. Furthermore, several isolated tdTomato-positive taste cells responded to KCl depolarization with increases in intracellular calcium, indicating the presence of voltage-gated calcium channels. Taken together, these data suggest that GAD65Cre mice drive expression in multiple taste cell types and thus cannot be considered a reliable reporter of Type I cell function.