CALHM3 Is Essential for Rapid Ion Channel-Mediated Purinergic Neurotransmission of GPCR-Mediated Tastes

Channel synapse mediates neurotransmission of airway protective chemoreflexes

Neural reflexes to chemicals in the throat protect the airway from aspiration and infection. Mechanistic understanding of these reflexes remains premature, exemplified by chronic cough-a sensitized cough reflex-being a prevalent unmet clinical need. Here, in mice, a whole-body search for channel synapses-featuring CALHM1/3 channel-mediated neurotransmitter release-and single-cell transcriptomics uncovered subclasses of the Pou2f3+ chemosensory cell family in the throat communicating with vagal neurons via this synapse. They express G protein-coupled receptors (GPCRs) for noxious chemicals, T2Rs, which upon stimulation trigger swallow and cough-like expulsive reflexes in the hypopharynx and larynx, respectively. These reflexes were abolished by Calhm3 and Pou2f3 knockout and could be triggered by targeted optogenetic stimulation. Furthermore, aeroallergen exposure augmented CALHM3-dependent expulsive reflex. This study identifies Pou2f3+ epithelial cells with channel synapses as chemosensory end organs of airway protective reflexes and sites of their hyperresponsiveness, advancing mechanistic understanding of airway defense programs with distinct therapeutic potential.

A deadly taste: linking bitter taste receptors and apoptosis

Humans can perceive five canonical tastes: salty, sour, umami, sweet, and bitter. These tastes are transmitted through the activation of ion channels and receptors. Bitter taste receptors (Taste Family 2 Receptors; T2Rs) are a sub-family of 25 G-protein coupled receptor (GPCR) isoforms that were first identified in type II taste bud cells. T2Rs are activated by a broad array of bitter agonists, which cause an increase in intracellular calcium (Ca2+) and a decrease in cyclic adenosine 3',5'-monophosphate (cAMP). Interestingly, T2Rs are expressed beyond the oral cavity, where they play diverse non-taste roles in cell physiology and disease. Here, we summarize the literature that explores the role of T2Rs in apoptosis. Activation of T2Rs with bitter agonists induces apoptosis in several cancers, the airway epithelia, smooth muscle, and more. In many of these tissues, T2R activation causes mitochondrial Ca2+ overload, a main driver of apoptosis. This response may be a result of T2R cellular localization, nuclear Ca2+ mobilization and/or a remnant of the established immunological roles of T2Rs in other cell types. T2R-induced apoptosis could be pharmacologically leveraged to treat diseases of altered cellular proliferation. Future work must explore additional extra-oral T2R-expressing tissues for apoptotic responses, develop methods for in-vivo studies, and discover high affinity bitter agonists for clinical application.

A mechanism of CALHM1 ion channel gating

The calcium homeostasis modulator (CALHM) proteins comprise a family of six genes, some of which have been demonstrated to function as ion channels. CALHM1, the founding member, is an extracellular Ca2+- and voltage-gated large-pore nonselective ion channel. The mechanisms by which Ca2+ and voltage regulate CALHM1 channel gating are unknown. Cryo-electron microscopic structures of CALHM1 and its paralogs have provided little insight into these features, although they have suggested that the amino-termini, including an amino-terminal helix (NTH) and the first transmembrane helix (TM1), may possess significant flexibility. Here, we investigated the role of the amino-terminus in the gating regulation of human CALHM1 channels expressed in Xenopus oocytes. Deletion of the NTH and the proximal end of TM1 markedly reduced the voltage dependence of channel gating, whereas extracellular Ca2+ retained the ability to close the channel, indicating that the amino-terminus is not the Ca2+-regulated gate. Furthermore, inhibition of channel currents by ruthenium red was independent of the presence of the amino-terminus and was mediated by effects on channel gating rather than pore block. The introduction of a cysteine residue into the proximal end of TM1 enabled complete inhibition of the channel by a cross-linking reagent under conditions in which the channel was in a closed state. Our findings indicate that although the NTH plays a role in voltage-dependent gating, it does not act as the gate itself. Instead, our results suggest that the gate in CALHM1 is formed by proximal regions of the first transmembrane domain.NEW & NOTEWORTHY CALHM1 is a voltage- and extracellular Ca2+-regulated large-pore ion channel that plays an essential role in taste perception. The mechanisms that regulate the opening and the closing of the channel are unknown. Here we explored the role of the amino-terminal region of the channel in gating regulation. Our data define the roles of the amino-terminus in channel gating, establishing components essential for the opening and closing of the CALHM1 channel gate.

Functional characterization of Type IV basal cells in rat fungiform taste buds

Taste buds, the end organs of taste, consist of a diverse population of sensory cells that is constantly renewed. Cell differentiation begins with Type IV basal cells, which are ovoid elements located inside the taste bud near its base. These cells are postmitotic precursors that give rise to all other cell types, including glial-like cells (Type I cells) and chemoreceptors (Type II and Type III cells). Despite their critical role in cell turnover, Type IV basal cells are relatively unknown in terms of functional features. Here, we used Lucifer yellow labeling and patch-clamp technique to investigate their electrophysiological properties in the rat fungiform taste buds. All Type IV basal cells showed voltage-gated sodium currents (INa), albeit at a far lower density (17 pA/pF) than chemoreceptors (444 pA/pF), which fire action potentials during sensory transduction. Furthermore, they lacked calcium homeostasis modulator currents, which are required for neurotransmitter release by some chemoreceptor types. Amiloride-sensitive epithelial sodium channel (ENaC) was found to be only present in a subset of Type IV basal cells. Interestingly, Type IV basal cells shared some membrane features with glial-like cells, such as high cell capacitance and low INa density; however, input resistance was greater in Type IV basal cells than in glial-like cells. Thus, although Type IV basal cells may eventually differentiate into distinct cell lineages, our findings indicate that they are quite homogeneous in terms of the electrophysiological characteristics, with the exception of functional ENaCs, which appear to be only expressed in one subset.

Structural comparisons of human and mouse fungiform taste buds

Taste buds are commonly studied in rodent models, but some differences exist between mice and humans in terms of gustatory mechanisms and sensitivities. Whether these functional differences are reflected in structural differences between species is unclear. Using immunofluorescent image stacks, we compared the morphological and molecular characteristics of mouse and human fungiform taste buds. The results suggest that while the general features of fungiform taste buds are similar between mice and humans, several characteristics differ significantly. Human taste buds are larger and taller than those of mice, yet they contain similar numbers of taste cells. Taste buds in humans are more heavily innervated by gustatory nerve fibers expressing the purinergic receptor P2X3 showing a 40% higher innervation density than in mice. Like type II cells of mice, a subset (about 30%) of cells in human taste buds is immunoreactive for phospholipase C beta (PLCβ2). These PLCβ2-immunoreactive cells display calcium homeostasis modulator 1 (CALHM1)-immunoreactive puncta closely opposed to gustatory nerve fibers suggestive of channel-type synapses in type II cells in mice. These puncta, used as a measure of synaptic contact, are significantly larger in humans compared to mice suggesting a higher efflux of adenosine triphosphate (ATP) neurotransmitter in humans. Altogether these findings suggest that while many similarities exist in the organization of murine and human fungiform taste buds, significant differences do exist in taste bud size, innervation density, and size of synaptic contacts that may impact gustatory signal transmission.

Tracheal tuft cells release ATP and link innate to adaptive immunity in pneumonia

Tracheal tuft cells shape immune responses in the airways. While some of these effects have been attributed to differential release of either acetylcholine, leukotriene C4 and/or interleukin-25 depending on the activating stimuli, tuft cell-dependent mechanisms underlying the recruitment and activation of immune cells are incompletely understood. Here we show that Pseudomonas aeruginosa infection activates mouse tuft cells, which release ATP via pannexin 1 channels. Taste signaling through the Trpm5 channel is essential for bacterial tuft cell activation and ATP release. We demonstrate that activated tuft cells recruit dendritic cells to the trachea and lung. ATP released by tuft cells initiates dendritic cell activation, phagocytosis and migration. Tuft cell stimulation also involves an adaptive immune response through recruitment of IL-17A secreting T helper cells. Collectively, the results provide a molecular framework defining tuft cell dependent regulation of both innate and adaptive immune responses in the airways to combat bacterial infection.

Artificial intelligence and food flavor: How AI models are shaping the future and revolutionary technologies for flavor food development

The food flavor science, traditionally reliant on experimental methods, is now entering a promising era with the help of artificial intelligence (AI). By integrating existing technologies with AI, researchers can explore and develop new flavor substances in a digital environment, saving time and resources. More and more research will use AI and big data to enhance product flavor, improve product quality, meet consumer needs, and drive the industry toward a smarter and more sustainable future. In this review, we elaborate on the mechanisms of flavor recognition and their potential impact on nutritional regulation. With the increase of data accumulation and the development of internet information technology, food flavor databases and food ingredient databases have made great progress. These databases provide detailed information on the nutritional content, flavor molecules, and chemical properties of various food compounds, providing valuable data support for the rapid evaluation of flavor components and the construction of screening technology. With the popularization of AI in various fields, the field of food flavor has also ushered in new development opportunities. This review explores the mechanisms of flavor recognition and the role of AI in enhancing food flavor analysis through high-throughput omics data and screening technologies. AI algorithms offer a pathway to scientifically improve product formulations, thereby enhancing flavor and customized meals. Furthermore, it discusses the safety challenges of integrating AI into the food flavor industry.

International Union of Basic and Clinical Pharmacology. CXVII: Taste 2 receptors-Structures, functions, activators, and blockers

For most vertebrates, bitter perception plays a critical role in the detection of potentially harmful substances in food items. The detection of bitter compounds is facilitated by specialized receptors located in the taste buds of the oral cavity. This work focuses on these receptors, including their sensitivities, structure-function relationships, agonists, and antagonists. The existence of numerous bitter taste receptor variants in the human population and the fact that several of them profoundly affect individual perceptions of bitter tastes are discussed as well. Moreover, the identification of bitter taste receptors in numerous tissues outside the oral cavity and their multiple proposed roles in these tissues are described briefly. Although this work is mainly focused on human bitter taste receptors, it is imperative to compare human bitter taste with bitter taste of other animals to understand which forces might have shaped the evolution of bitter taste receptors and their functions and to distinguish apparently typical human features from rather general ones. For readers who are not very familiar with the gustatory system, short descriptions of taste anatomy, signal transduction, and oral bitter taste receptor expression are included in the beginning of this article. SIGNIFICANCE STATEMENT: Apart from their role as sensors for potentially harmful substances in the oral cavity, the numerous additional roles of bitter taste receptors in tissues outside the gustatory system have recently received much attention. For careful assessment of their functions inside and outside the taste system, a solid knowledge of the specific and general pharmacological features of these receptors and the growing toolbox available for studying them is imperative and provided in this work.

Tuft cells in the intestine, immunity and beyond

Tuft cells have gained substantial attention over the past 10 years due to numerous reports linking them with type 2 immunity and microorganism-sensing capacity in many mucosal tissues. This heightened interest is fuelled by their unique ability to produce an array of biological effector molecules, including IL-25, allergy-related eicosanoids, and the neurotransmitter acetylcholine, enabling downstream responses in diverse cell types. Operating through G protein-coupled receptor-mediated signalling pathways reminiscent of type II taste cells in oral taste buds, tuft cells emerge as chemosensory sentinels that integrate luminal conditions, eliciting appropriate responses in immune, epithelial and neuronal populations. How tuft cells promote tissue alterations and adaptation to the variety of stimuli at mucosal surfaces has been explored in multiple studies in the past few years. Since the initial recognition of the role of tuft cells, the discovery of diverse tuft cell effector functions and associated feedback loops have also revealed the complexity of tuft cell biology. Although earlier work largely focused on extraintestinal tissues, novel genetic tools and recent mechanistic studies on intestinal tuft cells established fundamental concepts of tuft cell activation and functions. This Review is an overview of intestinal tuft cells, providing insights into their development, signalling and interaction modules in immunity and other states.

Sodium-dependent glucose co-transport proteins (SGLTs) are not involved in human glucose taste detection

The sweet taste of saccharides, such as sucrose and glucose, and other sweeteners is known to result from activation of the TAS1R2/R3 receptor expressed in taste receptor cells (TRCs) of the taste bud. Recent reports have suggested the existence of an additional sweet taste signaling pathway for metabolizable saccharides that is dependent on the activity of glucose transporters, especially SGLT1, also expressed in TRCs. We have investigated the potential contribution of SGLT1 to glucose taste signaling in humans. Concentration-response analysis of glucose mediated changes in membrane potential measured in Chinese hamster ovary (CHO) cells transiently expressing the human SGLT1 (hSGLT1) yielded an EC50 value of 452 μM. The SGLT inhibitor phlorizin inhibited the membrane potential response to 10 mM glucose with an IC50 of 3.5 μM. In contrast, EC50 values of 127 and 132 mM were obtained from concentration-response analysis of glucose taste in vehicles of water or 20 mM NaCl, respectively, by rapid throughput taste discrimination with human subjects. Lactisole, an antagonist of TAS1R2/R3, at a concentration of 1 mM completely inhibited taste responses to glucose concentrations of 250 mM and below. Phlorizin (0.2 mM) and the high potency SGLT1-selective inhibitor mizagliflozin (10 μM) failed to inhibit glucose taste detection measured at peri-threshold concentrations in the rapid throughput taste discrimination assay. A Yes/No experiment using the taste discrimination assay revealed that 0.2 mM phlorizin was discriminable from water for some subjects. Taken together the results indicate that agonist activation of TAS1R2/R3 is sufficient to account for all glucose taste without contribution by an alternative SGLT-mediated signaling pathway. Furthermore, the taste of phlorizin could be a confounding variable for studies evaluating a role for SGLTs in taste.

Mechanosensitive release of ATP in the urinary bladder mucosa

The urinary bladder mucosa (urothelium and suburothelium/lamina propria) functions as a barrier between the content of the urine and the underlying bladder tissue. The bladder mucosa is also a mechanosensitive tissue that releases signaling molecules that affect functions of cells in the bladder wall interconnecting the mucosa with the detrusor muscle and the CNS. Adenosine 5'-triphosphate (ATP) is a primary mechanotransduction signal that is released from cells in the bladder mucosa in response to bladder wall distention and activates cell membrane-localized P2X and P2Y purine receptors on urothelial cells, sensory and efferent neurons, interstitial cells, and detrusor smooth muscle cells. The amounts of ATP at active receptor sites depend significantly on the amounts of extracellularly released ATP. Spontaneous and distention-induced release of ATP appear to be under differential control. This review is focused on mechanisms underlying urothelial release of ATP in response to mechanical stimulation. First, we present a brief overview of studies that report mechanosensitive ATP release in bladder cells or tissues. Then, we discuss experimental evidence for mechanosensitive release of urothelial ATP by vesicular and non-vesicular mechanisms and roles of the stretch-activated channels PIEZO channels, transient receptor potential vanilloid type 4, and pannexin 1. This is followed by brief discussion of possible involvement of calcium homeostasis modulator 1, acid-sensing channels, and connexins in the release of urothelial ATP. We conclude with brief discussion of limitations of current research and of needs for further studies to increase our understanding of mechanotransduction in the bladder wall and of purinergic regulation of bladder function.

Biology, function and structure of the calcium homeostasis modulator family

Calcium homeostasis modulators (CALHMs) are the most recently discovered members of the large-pore channel family. They mediate the conductance of ions and larger molecules, such as ATP, and play critical roles in pathways related to Alzheimer's disease, neuroinflammation, neuromodulation, taste perception and innate immune responses. Since the inaugural report on CALHM1 in 2008, significant breakthroughs have revealed their biological roles, ion and ATP channel functions, and structures, positioning the field for further advancements. In this review, we discuss the overall progress and recent developments in understanding the biological roles, functions and molecular structures of CALHM proteins.

Topical application of a P2X2/P2X3 purine receptor inhibitor suppresses the bitter taste of medicines and other taste qualities

BACKGROUND AND PURPOSE

Many medications taste intensely bitter. The innate aversion to bitterness affects medical compliance, especially in children. There is a clear need to develop bitter blockers to suppress the bitterness of vital medications. Bitter taste is mediated by TAS2R receptors. Because different pharmaceutical compounds activate distinct sets of TAS2Rs, targeting specific receptors may only suppress bitterness for certain, but not all, bitter-tasting compounds. Alternative strategies are needed to identify universal bitter blockers that will improve the acceptance of every medication. Taste cells in the mouth transmit signals to afferent gustatory nerve fibres through the release of ATP, which activates the gustatory nerve-expressed purine receptors P2X2/P2X3. We hypothesized that blocking gustatory nerve transmission with P2X2/P2X3 inhibitors (e.g. 5-(5-iodo-4-methoxy-2-propan-2-ylphenoxy)pyrimidine-2,4-diamine [AF-353]) would reduce bitterness for all medications and bitter compounds.

EXPERIMENTAL APPROACH

Human sensory taste testing and mouse behavioural analyses were performed to determine if oral application of AF-353 blocks perception of bitter taste and other taste qualities but not non-gustatory oral sensations (e.g. tingle).

KEY RESULTS

Rinsing the mouth with AF-353 in humans or oral swabbing it in mice suppressed the bitter taste and avoidance behaviours of all compounds tested. We further showed that AF-353 suppressed other taste qualities (i.e. salt, sweet, sour and savoury) but had no effects on other oral or nasal sensations (e.g, astringency and oral tingle).

CONCLUSION AND IMPLICATIONS

This is the first time a universal, reversible taste blocker in humans has been reported. Topical application of P2X2/P2X3 inhibitor to suppress bitterness may improve medical compliance.

One of the major challenges of masking the bitter taste in medications: an overview of quantitative methods for bitterness

Objective

The aim of the present study was to carry out a systematic research on bitterness quantification to provide a reference for scholars and pharmaceutical developers to carry out drug taste masking research. Significance: The bitterness of medications poses a significant concern for clinicians and patients. Scientifically measuring the intensity of drug bitterness is pivotal for enhancing drug palatability and broadening their clinical utility.

Methods

The current study was carried out by conducting a systematic literature review that identified relevant papers from indexed databases. Numerous studies and research are cited and quoted in this article to summarize the features, strengths, and applicability of quantitative bitterness assessment methods.

Results

In our research, we systematically outlined the classification and key advancements in quantitative research methods for assessing drug bitterness, including in vivo quantification techniques such as traditional human taste panel methods, as well as in vitro quantification methods such as electronic tongue analysis. It focused on the quantitative methods and difficulties of bitterness of natural drugs with complex system characteristics and their difficulties in quantification, and proposes possible future research directions.

Conclusion

The quantitative methods of bitterness were summarized, which laid an important foundation for the construction of a comprehensive bitterness quantification standard system and the formulation of accurate, efficient and rich taste masking strategies.

Maintenance of taste receptor cell presynaptic sites requires gustatory nerve fibers

The turnover and re-establishment of peripheral taste synapses is vital to maintain connectivity between the primary taste receptor cells and the gustatory neurons which relay taste information from the tongue to the brain. Despite the importance of neuron-taste cell reconnection, mechanisms governing synapse assembly and the specificity of synaptic connections is largely unknown. Here we use the expression of presynaptic proteins, CALHM1 and Bassoon, to probe whether nerve fiber connectivity is an initiating factor for the recruitment of presynaptic machinery in different populations of taste cells. Under homeostatic conditions, the vast majority (>90%) of presynaptic sites are directly adjacent to nerve fibers. In the days immediately following gustatory nerve transection and complete denervation, Bassoon and CALHM1 puncta are markedly reduced. This suggests that nerve fiber innervation is crucial for the recruitment and maintenance of presynaptic sites. In support of this, we find that expression of Bassoon and Calhm1 mRNA transcripts are significantly reduced after denervation. During nerve fiber regeneration into the taste bud, presynaptic sites begin to replenish, but are not as frequently connected to nerve fibers as intact controls (∼50% compared to >90%). This suggests that gustatory neuron proximity, rather than direct contact, likely drives taste receptor cells to express and aggregate presynaptic proteins at the cell membrane. Together, these data support the idea that trophic factors secreted by gustatory nerve fibers prompt taste receptor cells to produce presynaptic specializations at the cell membrane, which in turn may guide neurons to form mature synapses. These findings provide new insights into the mechanisms driving synaptogenesis and synaptic plasticity within the rapidly changing taste bud environment.

Structural comparisons of human and mouse fungiform taste buds

Taste buds are commonly studied in rodent models, but some differences exist between mice and humans in terms of gustatory mechanisms and sensitivities. Whether these functional differences are reflected in structural differences between species is unclear. Using immunofluorescent image stacks, we compared morphological and molecular characteristics of mouse and human fungiform taste buds. The results suggest that while the general features of fungiform taste buds are similar between mice and humans, several characteristics differ significantly. Human taste buds are larger and taller than those of mice, yet they contain similar numbers of taste cells. Taste buds in humans are more heavily innervated by gustatory nerve fibers expressing the purinergic receptor P2X3 showing a 40% higher innervation density than in mice. Like Type II cells of mice, a subset (about 30%) of cells in human taste buds is immunoreactive for PLCβ2. These PLCβ2-immunoreactive cells display CALHM1-immunoreactive puncta closely apposed to gustatory nerve fibers suggestive of channel-type synapses described in mice. These puncta, used as a measure of synaptic contact, are however significantly larger in humans compared to mice. Altogether these findings suggest that while many similarities exist in the structural organization of murine and human fungiform taste buds, significant differences do exist in taste bud size, innervation density, and size of synaptic contacts that may impact gustatory signal transmission.

Role of taste receptors in salty taste perception of minerals and amino acids and developments in salt reduction strategies: A review

Salt (sodium chloride) plays a key role in maintaining the textural, microbiological, and sensorial aspects of the foods. However high dietary salt intake in the population has led to a series of health problems. Currently manufacturers are under pressure to reduce the sodium levels in foods without compromising the consumer experience. Because of the clean salty taste produced by sodium chloride, it has been challenging for the food industry to develop a suitable salt substitute. Studies have shown that different components within a food matrix can influence the perception of saltiness. This review aims to comprehend the potential synergistic effect of compounds such as minerals and amino acids on the perception of saltiness and covers the mechanism of perception where relevant to taste resulting from sodium ions and other metallic ions (such as K, Mg, Ca), as well as various amino acids and their derivatives. Finally, the review summarizes various salt reduction strategies explored by researchers, government organizations and food industry, including the potential use of plant-based extracts.

Structural features of heteromeric channels composed of CALHM2 and CALHM4 paralogs

The CALHM proteins constitute a family of large pore channels that contains six closely related paralogs in humans. Two family members, CALHM1 and 3, have been associated with the release of ATP during taste sensation. Both proteins form heteromeric channels that activate at positive potential and decreased extracellular Ca2+ concentration. Although the structures of several family members displayed large oligomeric organizations of different size, their function has in most cases remained elusive. Our previous study has identified the paralogs CALHM2, 4 and, 6 to be highly expressed in the placenta and defined their structural properties as membrane proteins exhibiting features of large pore channels with unknown activation properties (Drożdżyk et al., 2020). Here, we investigated whether these placental paralogs would form heteromers and characterized heteromeric complexes consisting of CALHM2 and CALHM4 subunits using specific binders as fiducial markers. Both proteins assemble with different stoichiometries with the largest population containing CALHM2 as the predominant component. In these oligomers, the subunits segregate and reside in their preferred conformation found in homomeric channels. Our study has thus revealed the properties that govern the formation of CALHM heteromers in a process of potential relevance in a cellular context.

Influence of extracellular ATP on mammalian sperm physiology

In addition to its central role in cellular metabolism, adenosine 5'-triphosphate (ATP) is an important extracellular signalling molecule involved in various physiological processes. In reproduction, extracellular ATP participates in both autocrine and paracrine paths regulating gametogenesis, gamete maturation and fertilisation. This review focusses on how extracellular ATP modulates sperm physiology with emphasis on the mammalian acrosome reaction. The presence of extracellular ATP in the reproductive tract is primarily determined by the ion channels and transporters that influence its movement within the cells comprising the tract. The main targets of extracellular ATP in spermatozoa are its own transporters, particularly species-specific sperm purinergic receptors. We also discuss notable phenotypes from knock-out mouse models and human Mendelian inheritance related to ATP release mechanisms, along with immunological, proteomic, and functional observations regarding sperm purinergic receptors and their involvement in sperm signalling.

A hyperpolarizing neuron recruits undocked innexin hemichannels to transmit neural information in Caenorhabditis elegans

While depolarization of the neuronal membrane is known to evoke the neurotransmitter release from synaptic vesicles, hyperpolarization is regarded as a resting state of chemical neurotransmission. Here, we report that hyperpolarizing neurons can actively signal neural information by employing undocked hemichannels. We show that UNC-7, a member of the innexin family in Caenorhabditis elegans, functions as a hemichannel in thermosensory neurons and transmits temperature information from the thermosensory neurons to their postsynaptic interneurons. By monitoring neural activities in freely behaving animals, we find that hyperpolarizing thermosensory neurons inhibit the activity of the interneurons and that UNC-7 hemichannels regulate this process. UNC-7 is required to control thermotaxis behavior and functions independently of synaptic vesicle exocytosis. Our findings suggest that innexin hemichannels mediate neurotransmission from hyperpolarizing neurons in a manner that is distinct from the synaptic transmission, expanding the way of neural circuitry operations.

Calcium homeostasis modulator 2 (Calhm2) as slowly activating membrane current channel in mouse B cells

In mouse B lymphocytes, an unidentified slow-activating voltage-dependent current resembling the characteristics of the Calhm family ion channel (ICalhm-L) was investigated. RT-PCR analysis revealed the presence of Calhm2 and 6 transcripts, with subsequent whole-cell patch-clamp studies indicating that the ICalhm-L is augmented by heat, alkaline pH, and low extracellular [Ca2+]. Overexpression of Calhm2, but not Calhm6, in N2A cells recapitulated ICalhm-L. Moreover, Calhm2 knockdown in Bal-17 cells abolished ICalhm-L. We firstly identify the voltage-dependent ion channel function of the Calhm2 in the mouse immune cells. ATP release assays in primary mouse B cells suggested a significant contribution of Calhm2 for purinergic signaling at physiological temperature.

Chronic social defeat stress broadly inhibits gene expression in the peripheral taste system and alters taste responses in mice

Human studies have linked stress exposure to unhealthy eating behavior. However, the mechanisms that drive stress-associated changes in eating behavior remain incompletely understood. The sense of taste plays important roles in food preference and intake. In this study, we use a chronic social defeat stress (CSDS) model in mice to address whether chronic stress impacts taste sensation and gene expression in taste buds and the gut. Our results showed that CSDS significantly elevated circulating levels of corticosterone and acylated ghrelin while lowering levels of leptin, suggesting a change in metabolic hormones that promotes food consumption. Stressed mice substantially increased their intake of food and water 3-5 days after the stress onset and gradually gained more body weight than that of controls. Moreover, CSDS significantly decreased the expression of multiple taste receptors and signaling molecules in taste buds and reduced mRNA levels of several taste progenitor/stem cell markers and regulators. Stressed mice showed significantly reduced sensitivity and response to umami and sweet taste compounds in behavioral tests. In the small intestine, the mRNA levels of Gnat3 and Tas1r2 were elevated in CSDS mice. The increased Gnat3 was mostly localized in a type of Gnat3+ and CD45+ immune cells, suggesting changes of immune cell distribution in the gut of stressed mice. Together, our study revealed broad effects of CSDS on the peripheral taste system and the gut, which may contribute to stress-associated changes in eating behavior.

Structural basis for sugar perception by Drosophila gustatory receptors

Insects rely on a family of seven transmembrane proteins called gustatory receptors (GRs) to encode different taste modalities, such as sweet and bitter. We report structures of Drosophila sweet taste receptors GR43a and GR64a in the apo and sugar-bound states. Both GRs form tetrameric sugar-gated cation channels composed of one central pore domain (PD) and four peripheral ligand-binding domains (LBDs). Whereas GR43a is specifically activated by the monosaccharide fructose that binds to a narrow pocket in LBDs, disaccharides sucrose and maltose selectively activate GR64a by binding to a larger and flatter pocket in LBDs. Sugar binding to LBDs induces local conformational changes, which are subsequently transferred to the PD to cause channel opening. Our studies reveal a structural basis for sugar recognition and activation of GRs.

Expression of the umami taste receptor T1R1/T1R3 in porcine testis of: Function in regulating testosterone synthesis and autophagy in Leydig cells

Testosterone is a vital male hormone responsible for male sexual characteristics. The taste receptor family 1 subunit 3 (T1R3) regulates testosterone synthesis and autophagy in non-taste cells, and the links with the taste receptor family 1 subunit 1 (T1R1) for umami perception. However, little is known about these mechanisms. Thus, we aimed to determine the relationship between the umami taste receptor (T1R1/T1R3) and testosterone synthesis or autophagy in testicular Leydig cells of the Xiang pig. There was a certain proportion of spermatogenic tubular dysplasia in the Xiang pig at puberty, in which autophagy was enhanced, and the testosterone level was increased with a weak expression of T1R3. Silenced T1R3 decreased testosterone level and intracellular cyclic adenosine monophosphate (cAMP) content and inhibited the messenger RNA (mRNA) expression levels of testosterone synthesis enzyme genes [steroidogenic acute regulatory protein (StAR), hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 1 (3β-HSD1), cytochrome P450 family 17 subfamily A member 1 (CYP17A1) and hydroxysteroid 17-beta dehydrogenase 3 (17β-HSD3)]. In addition, T1R3 increased the number of acidic autophagy bubbles and upregulated the expression levels of autophagy markers [Microtubule-associated protein 1 A/1B-light chain 3 (LC3) and Beclin-1] in testicular Leydig cells of the Xiang pig. Using an umami tasting agonist (10 mM L-glutamate for 6 h), the activation of T1R1/T1R3 enhanced the testosterone synthesis ability by increasing the intracellular cAMP level and upregulated the expression levels of StAR, 3β-HSD1, CYP17A1 and 17β-HSD3 in Leydig cells. Furthermore, the number of acidic autophagy bubbles decreased in the T1R1/T1R3-activated group with the downregulation of the expression levels of the autophagy markers, including LC3 and Beclin-1. These data suggest that the function of T1R1/T1R3 expressed in testicular Leydig cells of the Xiang pig is related to testosterone synthesis and autophagy.

Bitter taste receptors in the reproductive system: Function and therapeutic implications

Type 2 taste receptors (TAS2Rs), traditionally known for their role in bitter taste perception, are present in diverse reproductive tissues of both sexes. This review explores our current understanding of TAS2R functions with a particular focus on reproductive health. In males, TAS2Rs are believed to play potential roles in processes such as sperm chemotaxis and male fertility. Genetic insights from mouse models and human polymorphism studies provide some evidence for their contribution to male infertility. In female reproduction, it is speculated that TAS2Rs influence the ovarian milieu, shaping the functions of granulosa and cumulus cells and their interactions with oocytes. In the uterus, TAS2Rs contribute to uterine relaxation and hold potential as therapeutic targets for preventing preterm birth. In the placenta, they are proposed to function as vigilant sentinels, responding to infection and potentially modulating mechanisms of fetal protection. In the cervix and vagina, their analogous functions to those in other extraoral tissues suggest a potential role in infection defense. In addition, TAS2Rs exhibit altered expression patterns that profoundly affect cancer cell proliferation and apoptosis in reproductive cancers. Notably, TAS2R agonists show promise in inducing apoptosis and overcoming chemoresistance in these malignancies. Despite these advances, challenges remain, including a lack of genetic and functional studies. The application of techniques such as single-cell RNA sequencing and clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated endonuclease 9 gene editing could provide deeper insights into TAS2Rs in reproduction, paving the way for novel therapeutic strategies for reproductive disorders.

Evolution of protein domain repertoires of CALHM6

Calcium (Ca2 +) homeostasis is essential in conducting various cellular processes including nerve transmission, muscular movement, and immune response. Changes in Ca2 + concentration in the cytoplasm are significant in bringing about various immune responses such as pathogen clearance and apoptosis. Various key players are involved in calcium homeostasis such as calcium binders, pumps, and channels. Sequence-based evolutionary information has recently been exploited to predict the biophysical behaviors of proteins, giving critical clues about their functionality. Ion channels are reportedly the first channels developed during evolution. Calcium homeostasis modulator protein 6 (CALHM6) is one such channel. Comprised of a single domain called Ca_hom_mod, CALHM6 is a stable protein interacting with various other proteins in calcium regulation. No previous attempt has been made to trace the exact evolutionary events in the domain of CALHM6, leaving plenty of room for exploring its evolution across a wide range of organisms. The current study aims to answer the questions by employing a computational-based strategy that used profile Hidden Markov Models (HMMs) to scan for the CALHM6 domain, integrated the data with a time-calibrated phylogenetic tree using BEAST and Mesquite, and visualized through iTOL. Around 4,000 domains were identified, and 14,000 domain gain, loss, and duplication events were observed at the end which also included various protein domains other than CALHM6. The data were analyzed concerning CALHM6 evolution as well as the domain gain, loss, and duplication of its interacting partners: Calpain, Vinculin, protein S100-A7, Thioredoxin, Peroxiredoxin, and Calmodulin-like protein 5. Duplication events of CALHM6 near higher eukaryotes showed its increasing complexity in structure and function. This in-silico phylogenetic approach applied to trace the evolution of CALHM6 was an effective approach to get a better understanding of the protein CALHM6.

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.

Give-and-take of gustation: the interplay between gustatory neurons and taste buds

Mammalian taste buds are highly regenerative and can restore themselves after normal wear and tear of the lingual epithelium or following physical and chemical insults, including burns, chemotherapy, and nerve injury. This is due to the continual proliferation, differentiation, and maturation of taste progenitor cells, which then must reconnect with peripheral gustatory neurons to relay taste signals to the brain. The turnover and re-establishment of peripheral taste synapses are vital to maintain this complex sensory system. Over the past several decades, the signal transduction and neurotransmitter release mechanisms within taste cells have been well delineated. However, the complex dynamics between synaptic partners in the tongue (taste cell and gustatory neuron) are only partially understood. In this review, we highlight recent findings that have improved our understanding of the mechanisms governing connectivity and signaling within the taste bud and the still-unresolved questions regarding the complex interactions between taste cells and gustatory neurons.

Physiological roles of chloride ions in bodily and cellular functions

Physiological roles of Cl-, a major anion in the body, are not well known compared with those of cations. This review article introduces: (1) roles of Cl- in bodily and cellular functions; (2) the range of cytosolic Cl- concentration ([Cl-]c); (3) whether [Cl-]c could change with cell volume change under an isosmotic condition; (4) whether [Cl-]c could change under conditions where multiple Cl- transporters and channels contribute to Cl- influx and efflux in an isosmotic state; (5) whether the change in [Cl-]c could be large enough to act as signals; (6) effects of Cl- on cytoskeletal tubulin polymerization through inhibition of GTPase activity and tubulin polymerization-dependent biological activity; (7) roles of cytosolic Cl- in cell proliferation; (8) Cl--regulatory mechanisms of ciliary motility; (9) roles of Cl- in sweet/umami taste receptors; (10) Cl--regulatory mechanisms of with-no-lysine kinase (WNK); (11) roles of Cl- in regulation of epithelial Na+ transport; (12) relationship between roles of Cl- and H+ in body functions.

Recent advances in the structure and activation mechanisms of metabolite-releasing Pannexin 1 channels

Pannexin 1 (PANX1) is a widely expressed large-pore ion channel located in the plasma membrane of almost all vertebrate cells. It possesses a unique ability to act as a conduit for both inorganic ions (e.g. potassium or chloride) and bioactive metabolites (e.g. ATP or glutamate), thereby activating varying signaling pathways in an autocrine or paracrine manner. Given its crucial role in cell-cell interactions, the activity of PANX1 has been implicated in maintaining homeostasis of cardiovascular, immune, and nervous systems. Dysregulation of PANX1 has also been linked to numerous diseases, such as ischemic stroke, seizure, and inflammatory disorders. Therefore, the mechanisms underlying different modes of PANX1 activation and its context-specific channel properties have gathered significant attention. In this review, we summarize the roles of PANX1 in various physiological processes and diseases, and analyze the accumulated lines of evidence supporting diverse molecular mechanisms associated with different PANX1 activation modalities. We focus on examining recent discoveries regarding PANX1 regulations by reversible post-translational modifications, elevated intracellular calcium concentration, and protein-protein interactions, as well as by irreversible cleavage of its C-terminal tail. Additionally, we delve into the caveats in the proposed PANX1 gating mechanisms and channel open-closed configurations by critically analyzing the structural insights derived from cryo-EM studies and the unitary properties of PANX1 channels. By doing so, we aim to identify potential research directions for a better understanding of the functions and regulations of PANX1 channels.

Rapid structural remodeling of peripheral taste neurons is independent of taste cell turnover

Taste bud cells are constantly replaced in taste buds as old cells die and new cells migrate into the bud. The perception of taste relies on new taste bud cells integrating with existing neural circuitry, yet how these new cells connect with a taste ganglion neuron is unknown. Do taste ganglion neurons remodel to accommodate taste bud cell renewal? If so, how much of the structure of taste axons is fixed and how much remodels? Here, we measured the motility and branching of individual taste arbors (the portion of the axon innervating taste buds) in mice over time with two-photon in vivo microscopy. Terminal branches of taste arbors continuously and rapidly remodel within the taste bud. This remodeling is faster than predicted by taste bud cell renewal, with terminal branches added and lost concurrently. Surprisingly, blocking entry of new taste bud cells with chemotherapeutic agents revealed that remodeling of the terminal branches on taste arbors does not rely on the renewal of taste bud cells. Although terminal branch remodeling was fast and intrinsically controlled, no new arbors were added to taste buds, and few were lost over 100 days. Taste ganglion neurons maintain a stable number of arbors that are each capable of high-speed remodeling. We propose that terminal branch plasticity permits arbors to locate new taste bud cells, while stability of arbor number supports constancy in the degree of connectivity and function for each neuron over time.

Effects of temperature on action potentials and ion conductances in type II taste-bud cells

Temperature strongly influences the intensity of taste, but it remains understudied despite its physiological, hedonic, and commercial implications. The relative roles of the peripheral gustatory and somatosensory systems innervating the oral cavity in mediating thermal effects on taste sensation and perception are poorly understood. Type II taste-bud cells, responsible for sensing sweet, bitter umami, and appetitive NaCl, release neurotransmitters to gustatory neurons by the generation of action potentials, but the effects of temperature on action potentials and the underlying voltage-gated conductances are unknown. Here, we used patch-clamp electrophysiology to explore the effects of temperature on acutely isolated type II taste-bud cell electrical excitability and whole cell conductances. Our data reveal that temperature strongly affects action potential generation, properties, and frequency and suggest that thermal sensitivities of underlying voltage-gated Na+ and K+ channel conductances provide a mechanism for how and whether voltage-gated Na+ and K+ channels in the peripheral gustatory system contribute to the influence of temperature on taste sensitivity and perception.NEW & NOTEWORTHY The temperature of food affects how it tastes. Nevertheless, the mechanisms involved are not well understood, particularly whether the physiology of taste-bud cells in the mouth is involved. Here we show that the electrical activity of type II taste-bud cells that sense sweet, bitter, and umami substances is strongly influenced by temperature. These results suggest a mechanism for the influence of temperature on the intensity of taste perception that resides in taste buds themselves.

Structure of human CALHM1 reveals key locations for channel regulation and blockade by ruthenium red

Calcium homeostasis modulator 1 (CALHM1) is a voltage-dependent channel involved in neuromodulation and gustatory signaling. Despite recent progress in the structural biology of CALHM1, insights into functional regulation, pore architecture, and channel blockade remain limited. Here we present the cryo-EM structure of human CALHM1, revealing an octameric assembly pattern similar to the non-mammalian CALHM1s and the lipid-binding pocket conserved across species. We demonstrate by MD simulations that this pocket preferentially binds a phospholipid over cholesterol to stabilize its structure and regulate the channel activities. Finally, we show that residues in the amino-terminal helix form the channel pore that ruthenium red binds and blocks.

Contribution of TRPC3-mediated Ca2+ entry to taste transduction

The current concept of taste transduction implicates the TASR/PLCβ2/IP₃R3/TRPM5 axis in mediating chemo-electrical coupling in taste cells of the type II. While generation of IP₃ has been verified as an obligatory step, DAG appears to be a byproduct of PIP₂ cleavage by PLCβ2. Here, we provide evidence that DAG-signaling could play a significant and not yet recognized role in taste transduction. In particular, we found that DAG-gated channels are functional in type II cells but not in type I and type III cells. The DAG-gated current presumably constitutes a fraction of the generator current triggered by taste stimulation in type II cells. Bitter stimuli and DAG analogs produced Ca²⁺ transients in type II cells, which were greatly decreased at low bath Ca²⁺, indicating their dependence on Ca²⁺ influx. Among DAG-gated channels, transcripts solely for TRPC3 were detected in the taste tissue, thus implicating this channel in mediating DAG-regulated Ca²⁺ entry. Release of the afferent neurotransmitter ATP from CV papillae was monitored online by using the luciferin/luciferase method and Ussing-like chamber. It was shown that ATP secretion initiated by bitter stimuli and DAG analogs strongly depended on mucosal Ca²⁺. Based on the overall findings, we speculate that in taste transduction, IP₃-driven Ca²⁺ release is transient and mainly responsible for rapid activation of Ca²⁺-gated TRPM5 channels, thus forming the initial phase of receptor potential. DAG-regulated Ca²⁺ entry through apically situated TRPC3 channels extends the primary Ca²⁺ signal and preserves TRPM5 activity, providing a needful prolongation of the receptor potential.

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

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

Taste arbor structural variability analyzed across taste regions

Taste ganglion neurons are functionally and molecularly diverse, but until recently morphological diversity was completely unexplored. Specifically, taste arbors (the portion of the neuron within the taste bud) vary in structure, but the reason for this variability is unclear. Here, we analyzed structural variability in taste arbors to determine which factors determine their morphological diversity. To characterize arbor morphology and its relationship to taste bud cells capable of transducing taste stimuli (taste-transducing cell) number and type, we utilized sparse cell genetic labeling of taste ganglion neurons in combination with whole-mount immunohistochemistry. Reconstruction of 151 taste arbors revealed variation in arbor size, complexity, and symmetry. Overall, taste arbors exist on a continuum of complexity, cannot be categorized into discrete morphological groups, and do not have stereotyped endings. Arbor size/complexity was not related to the size of the taste bud in which it was located or the type of taste-transducing cell contacted (membranes within 180 nm). Instead, arbors could be broadly categorized into three groups: large asymmetrical arbors contacting many taste-transducing cells, small symmetrical arbors contacting one or two taste-transducing cells, and unbranched arbors. Neurons with multiple arbors had arbors in more than one of these categories, indicating that this variability is not an intrinsic feature of neuron type. Instead, we speculate that arbor structure is determined primarily by nerve fiber remodeling in response to cell turnover and that large asymmetrical arbors represent a particularly plastic state.

The ion channel CALHM6 controls bacterial infection-induced cellular cross-talk at the immunological synapse

Membrane ion channels of the calcium homeostasis modulator (CALHM) family promote cell-cell crosstalk at neuronal synapses via ATP release, where ATP acts as a neurotransmitter. CALHM6, the only CALHM highly expressed in immune cells, has been linked to the induction of natural killer (NK) cell anti-tumour activity. However, its mechanism of action and broader functions in the immune system remain unclear. Here, we generated Calhm6-/- mice and report that CALHM6 is important for the regulation of the early innate control of Listeria monocytogenes infection in vivo. We find that CALHM6 is upregulated in macrophages by pathogen-derived signals and that it relocates from the intracellular compartment to the macrophage-NK cell synapse, facilitating ATP release and controlling the kinetics of NK cell activation. Anti-inflammatory cytokines terminate CALHM6 expression. CALHM6 forms an ion channel when expressed in the plasma membrane of Xenopus oocytes, where channel opening is controlled by a conserved acidic residue, E119. In mammalian cells, CALHM6 is localised to intracellular compartments. Our results contribute to the understanding of neurotransmitter-like signal exchange between immune cells that fine-tunes the timing of innate immune responses.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Channel-mediated ATP release in the nervous system

ATP is well established as a transmitter and modulator in the peripheral and central nervous system. While conventional exocytotic release of ATP at synapses occurs, this transmitter is unusual in also being released into the extracellular space via large-pored plasma membrane channels. This review considers the channels that are known to be permeable to ATP and some of the functions of channel-mediated ATP release. While the possibility of ATP release via channels mediating volume transmission has been known for some time, localised ATP release via channels at specialised synapses made by taste cells to the afferent nerve has recently been documented in taste buds. This raises the prospect that "channel synapses" may occur in other contexts. However, volume transmission and channel synapses are not necessarily mutually exclusive. We suggest that certain glial cells in the brain stem and hypothalamus, which possess long processes and are known to release ATP, may be candidates for both modes of ATP release -channel-mediated volume transmission in the region of their somata and more localised transmission possibly via either conventional or channel synapses from their processes at distal targets. Finally, we consider the different characteristics of vesicular and channel synapses and suggest that channel synapses may be advantageous in requiring less energy than their conventional vesicular counterparts. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Physiology of the tongue with emphasis on taste transduction

The tongue is a complex multifunctional organ that interacts and senses both interoceptively and exteroceptively. Although it is easily visible to almost all of us, it is relatively understudied and what is in the literature is often contradictory or is not comprehensively reported. The tongue is both a motor and a sensory organ: motor in that it is required for speech and mastication, and sensory in that it receives information to be relayed to the central nervous system pertaining to the safety and quality of the contents of the oral cavity. Additionally, the tongue and its taste apparatus form part of an innate immune surveillance system. For example, loss or alteration in taste perception can be an early indication of infection as became evident during the present global SARS-CoV-2 pandemic. Here, we particularly emphasize the latest updates in the mechanisms of taste perception, taste bud formation and adult taste bud renewal, and the presence and effects of hormones on taste perception, review the understudied lingual immune system with specific reference to SARS-CoV-2, discuss nascent work on tongue microbiome, as well as address the effect of systemic disease on tongue structure and function, especially in relation to taste.

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.

Defining the role of TRPM4 in broadly responsive taste receptor cells

Peripheral taste receptor cells use multiple signaling pathways to transduce taste stimuli into output signals that are sent to the brain. We have previously identified a subpopulation of Type III taste cells that are broadly responsive (BR) and respond to multiple taste stimuli including bitter, sweet, umami, and sour. These BR cells use a PLCβ3/IP₃R1 signaling pathway to detect bitter, sweet, and umami stimuli and use a separate pathway to detect sour. Currently, the downstream targets of the PLCβ3 signaling pathway are unknown. Here we identify TRPM4, a monovalent selective TRP channel, as an important downstream component in this signaling pathway. Using live cell imaging on isolated taste receptor cells from mice, we show that inhibition of TRPM4 abolished the taste-evoked sodium responses and significantly reduced the taste-evoked calcium responses in BR cells. Since BR cells are a subpopulation of Type III taste cells, they have conventional chemical synapses that require the activation of voltage-gated calcium channels (VGCCs) to cause neurotransmitter release. We found that TRPM4-dependent membrane depolarization selectively activates L-type VGCCs in these cells. The calcium influx through L-type VGCCs also generates a calcium-induced calcium release (CICR) via ryanodine receptors that enhances TRPM4 activity. Together these signaling events amplify the initial taste response to generate an appropriate output signal.

Bidirectional sensitivity of CALHM1 channel to protons from both sides of plasma membrane

Calcium homeostasis modulator 1 (CALHM1), a newly discovered voltage-dependent nonselective ion channel, has drawn attention for its role in neuronal activity and taste sensation. Its sluggish voltage-dependent activation is facilitated by lowering extracellular Ca²⁺ concentration ([Ca²⁺]_e). Here, we investigated the effects of extracellular and intracellular pH (pH_e and pH_i) on human CALHM1. When normalized to the amplitude of the CALHM1 current (I_CALHM1) under whole cell patch clamp at symmetrical pH 7.4, I_CALHM1 decreased at acidic pH_e or pH_i, whereas it sharply increased at alkaline pH_e or pH_i. The effects of pH were preserved in the inside-out configuration. The voltage dependence of I_CALHM1 showed leftward and rightward shifts at alkaline and acidic pH_e and pH_i, respectively. Site-directed mutagenesis of the water-accessible charged residues of the pore and nearby domains revealed that E17, K229, E233, D257, and E259 are nonadditively responsible for facilitation at alkaline pH_i. Identification of the pH_e-sensing residue was not possible because mutation of putative residues impaired membrane expression, resulting in undetectable I_CALHM1. Alkaline pH_e-dependent facilitation appeared gradually with depolarization, suggesting that the sensitivity to pH_e might be due to H⁺ diffusion through the open-state CALHM1. At pH_e 6.2, decreased [Ca²⁺]_e could not recover the inhibited I_CALHM1 but further augmented the increased I_CALHM1 at pH_e 8.6, suggesting that unidentified common residues might contribute to the [Ca²⁺]_e and acidic pH_e. This study is the first, to our knowledge, to demonstrate the remarkable pH sensitivity of CALHM1, which might contribute to the pH-dependent modulation of neuronal excitability or taste sensation.

E-cigarette Flavors, Sensory Perception, and Evoked Responses

The chemosensory experiences evoked by flavors encompass a number of unique sensations that include olfactory stimuli (smell), gustatory stimuli (taste, i.e., salty, sweet, sour, bitter, and umami (also known as "savoriness")), and chemesthesis (touch). As such, the responses evoked by flavors are complex and, as briefly stated above, involve multiple perceptive mechanisms. The practice of adding flavorings to tobacco products dates back to the 17th century but is likely much older. More recently, the electronic cigarette or "e-cigarette" and its accompanying flavored e-liquids emerged on to the global market. These new products contain no combustible tobacco but often contain large concentrations (reported from 0 to more than 50 mg/mL) of nicotine as well as numerous flavorings and/or flavor chemicals. At present, there are more than 400 e-cigarette brands available along with potentially >15,000 different/unique flavored products. However, surprisingly little is known about the flavors/flavor chemicals added to these products, which can account for >1% by weight of some e-liquids, and their resultant chemosensory experiences, and the US FDA has done relatively little, until recently, to regulate these products. This article will discuss e-cigarette flavors and flavor chemicals, their elicited responses, and their sensory effects in some detail.

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

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

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.

Kinesin-2 motors differentially impact biogenesis of extracellular vesicle subpopulations shed from sensory cilia

Extracellular vesicles (EVs) are bioactive lipid-bilayer enclosed particles released from nearly all cells. One specialized site for EV shedding is the primary cilium. Here, we discover the conserved ion channel CLHM-1 as a ciliary EV cargo. Imaging of EVs released from sensory neuron cilia of Caenorhabditis elegans expressing fluorescently tagged CLHM-1 and TRP polycystin-2 channel PKD-2 shows enrichment of these cargoes in distinct EV subpopulations that are differentially shed in response to mating partner availability. PKD-2 alone is present in EVs shed from the cilium distal tip, whereas CLHM-1 EVs bud from a secondary site(s), including the ciliary base. Heterotrimeric and homodimeric kinesin-2 motors have discrete impacts on PKD-2 and CLHM-1 colocalization in both cilia and EVs. Total loss of kinesin-2 activity decreases shedding of PKD-2 but not CLHM-1 EVs. Our data demonstrate that anterograde intraflagellar transport is required for selective enrichment of protein cargoes into heterogeneous EVs with different signaling potentials.

The neural basis of sugar preference

When it comes to food, one tempting substance is sugar. Although sweetness is detected by the tongue, the desire to consume sugar arises from the gut. Even when sweet taste is impaired, animals can distinguish sugars from non-nutritive sweeteners guided by sensory cues arising from the gut epithelium. Here, we review the molecular receptors, cells, circuits and behavioural consequences associated with sugar sensing in the gut. Recent work demonstrates that some duodenal cells, termed neuropod cells, can detect glucose using sodium-glucose co-transporter 1 and release glutamate onto vagal afferent neurons. Based on these and other data, we propose a model in which specific populations of vagal neurons relay these sensory cues to distinct sets of neurons in the brain, including neurons in the caudal nucleus of the solitary tract, dopaminergic reward circuits in the basal ganglia and homeostatic feeding circuits in the hypothalamus, that alter current and future sugar consumption. This emerging model highlights the critical role of the gut in sensing the chemical properties of ingested nutrients to guide appetitive decisions.

Taste Receptors beyond Taste Buds

Taste receptors are responsible for detecting their ligands not only in taste receptor cells (TRCs) but also in non-gustatory organs. For several decades, many research groups have accumulated evidence for such “ectopic” expression of taste receptors. More recently, some of the physiologic functions (apart from taste) of these ectopic taste receptors have been identified. Here, we summarize our current understanding of these ectopic taste receptors across multiple organs. With a particular focus on the specialized epithelial cells called tuft cells, which are now considered siblings of type II TRCs, we divide the ectopic expression of taste receptors into two categories: taste receptors in TRC-like cells outside taste buds and taste receptors with surprising ectopic expression in completely different cell types.