Capacitance measurements of regulated exocytosis in mouse taste cells

Expression profile of the zinc transporter ZnT3 in taste cells of rat circumvallate papillae and its role in zinc release, a potential mechanism for taste stimulation

Zinc is an essential trace element, and its deficiency causes taste dysfunction. Zinc accumulates in zinc transporter (ZnT)3-expressing presynaptic vesicles in hippocampal neurons and acts as a neurotransmitter in the central nervous system. However, the distribution of zinc and its role as a signal transmitter in taste buds remain unknown. Therefore, we examined the distribution of zinc and expression profiles of ZnT3 in taste cells and evaluated zinc release from isolated taste cells upon taste stimuli. Taste cells with a spindle or pyriform morphology were revealed by staining with the fluorescent zinc dye ZnAF-2DA and autometallography in the taste buds of rat circumvallate papillae. Znt3 mRNA levels were detected in isolated taste buds. ZnT3-immunoreactivity was found in phospholipase-β2-immunopositive type II taste cells and aromatic amino acid decarboxylase-immunopositive type III cells but not in nucleoside triphosphate diphosphohydrolase 2-immunopositive type I cells. Moreover, we examined zinc release from taste cells using human transient receptor potential A1-overexpressing HEK293 as zinc-sensor cells. These cells exhibited a clear response to isolated taste cells exposed to taste stimuli. However, pretreatment with magnesium-ethylenediaminetetraacetic acid, an extracellular zinc chelator - but not with zinc-ethylenediaminetetraacetic acid, used as a negative control - significantly decreased the response ratio of zinc-sensor cells. These findings suggest that taste cells release zinc to the intercellular area in response to taste stimuli and that zinc may affect signaling within taste buds.

Recent Progress in the Study of Taste Characteristics and the Nutrition and Health Properties of Organic Acids in Foods

Organic acids could improve the food flavor, maintain the nutritional value, and extend the shelf life of food. This review summarizes the detection methods and concentrations of organic acids in different foods, as well as their taste characteristics and nutritional properties. The composition of organic acids varies in different food. Fruits and vegetables often contain citric acid, creatine is a unique organic acid found in meat, fermented foods have a high content of acetic acid, and seasonings have a wide range of organic acids. Determination of the organic acid contents among different food matrices allows us to monitor the sensory properties, origin identification, and quality control of foods, and further provides a basis for food formulation design. The taste characteristics and the acid taste perception mechanisms of organic acids have made some progress, and binary taste interaction is the key method to decode multiple taste perception. Real food and solution models elucidated that the organic acid has an asymmetric interaction effect on the other four basic taste attributes. In addition, in terms of nutrition and health, organic acids can provide energy and metabolism regulation to protect the human immune and myocardial systems. Moreover, it also exhibited bacterial inhibition by disrupting the internal balance of bacteria and inhibiting enzyme activity. It is of great significance to clarify the synergistic dose-effect relationship between organic acids and other taste sensations and further promote the application of organic acids in food salt reduction.

The Role of ATP and Purinergic Receptors in Taste Signaling

This review summarizes our understanding of ATP signaling in taste and describes new directions for research. ATP meets all requisite criteria to be considered a neurotransmitter: (1) presence in taste cells, as in all cells; (2) release upon appropriate taste stimulation; (3) binding to cognate purinergic receptors P2X2 and P2X3 on gustatory afferent neurons, and (4) after release, enzymatic degradation to adenosine and other nucleotides by the ectonucleotidase, NTPDase2, expressed on the Type I, glial-like cells in the taste bud. Importantly, double knockout of P2X2 and P2X3 or pharmacological inhibition of P2X3 abolishes transmission of all taste qualities. In Type II taste cells (those that respond to sweet, bitter, or umami stimuli), ATP is released non-vesicularly by a large conductance ion channel composed of CALHM1 and CALHM3, which form a so-called channel synapse at areas of contact with afferent taste nerve fibers. Although ATP release has been detected only from Type II cells, it is also required for the transmission of salty and sour stimuli, which are mediated primarily by the Type III taste cells. The source of the ATP required for Type III cell signaling to afferent fibers is still unclear and is a focus for future experiments. The ionotropic purinergic receptor, P2X3, is widely expressed on many sensory afferents and has been a therapeutic target for treating chronic cough and pain. However, its requirement for taste signaling has complicated efforts at treatment since patients given P2X3 antagonists report substantial disturbances of taste and become non-compliant.

Functional expression of TMEM16A in taste bud cells

Key points

Taste transduction occurs in taste buds in the tongue epithelium.

The Ca²⁺‐activated Cl^– channels TMEM16A and TMEM16B play relevant physiological roles in several sensory systems.

Here, we report that TMEM16A, but not TMEM16B, is expressed in the apical part of taste buds.

Large Ca²⁺‐activated Cl⁻ currents blocked by Ani‐9, a selective inhibitor of TMEM16A, are measured in type I taste cells but not in type II or III taste cells.

ATP indirectly activates Ca²⁺‐activated Cl^– currents in type I cells through TMEM16A channels.

These results indicate that TMEM16A is functional in type I taste cells and contribute to understanding the largely unknown physiological roles of these cells.

Abstract

The Ca²⁺‐activated Cl^– channels TMEM16A and TMEM16B have relevant roles in many physiological processes including neuronal excitability and regulation of Cl^– homeostasis. Here, we examined the presence of Ca²⁺‐activated Cl^– channels in taste cells of mouse vallate papillae by using immunohistochemistry and electrophysiological recordings. By using immunohistochemistry we showed that only TMEM16A, and not TMEM16B, was expressed in taste bud cells where it largely co‐localized with the inwardly rectifying K⁺ channel KNCJ1 in the apical part of type I cells. By using whole‐cell patch‐clamp recordings in isolated cells from taste buds, we measured an average current of −1083 pA at −100 mV in 1.5 μm Ca²⁺ and symmetrical Cl^– in type I cells. Ion substitution experiments and blockage by Ani‐9, a specific TMEM16A channel blocker, indicated that Ca²⁺ activated anionic currents through TMEM16A channels. We did not detect any Ca²⁺‐activated Cl^– currents in type II or III taste cells. ATP is released by type II cells in response to various tastants and reaches type I cells where it is hydrolysed by ecto‐ATPases. Type I cells also express P2Y purinergic receptors and stimulation of type I cells with extracellular ATP produced large Ca²⁺‐activated Cl⁻ currents blocked by Ani‐9, indicating a possible role of TMEM16A in ATP‐mediated signalling. These results provide a definitive demonstration that TMEM16A‐mediated currents are functional in type I taste cells and provide a foundation for future studies investigating physiological roles for these often‐neglected taste cells.

Taste transduction occurs in taste buds in the tongue epithelium.

The Ca²⁺‐activated Cl^– channels TMEM16A and TMEM16B play relevant physiological roles in several sensory systems.

Here, we report that TMEM16A, but not TMEM16B, is expressed in the apical part of taste buds.

Large Ca²⁺‐activated Cl⁻ currents blocked by Ani‐9, a selective inhibitor of TMEM16A, are measured in type I taste cells but not in type II or III taste cells.

ATP indirectly activates Ca²⁺‐activated Cl^– currents in type I cells through TMEM16A channels.

These results indicate that TMEM16A is functional in type I taste cells and contribute to understanding the largely unknown physiological roles of these cells.

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.

Taste transduction and channel synapses in taste buds

The variety of taste sensations, including sweet, umami, bitter, sour, and salty, arises from diverse taste cells, each of which expresses specific taste sensor molecules and associated components for downstream signal transduction cascades. Recent years have witnessed major advances in our understanding of the molecular mechanisms underlying transduction of basic tastes in taste buds, including the identification of the bona fide sour sensor H⁺ channel OTOP1, and elucidation of transduction of the amiloride-sensitive component of salty taste (the taste of sodium) and the TAS1R-independent component of sweet taste (the taste of sugar). Studies have also discovered an unconventional chemical synapse termed "channel synapse" which employs an action potential-activated CALHM1/3 ion channel instead of exocytosis of synaptic vesicles as the conduit for neurotransmitter release that links taste cells to afferent neurons. New images of the channel synapse and determinations of the structures of CALHM channels have provided structural and functional insights into this unique synapse. In this review, we discuss the current view of taste transduction and neurotransmission with emphasis on recent advances in the field.

All-Electrical Ca2+-Independent Signal Transduction Mediates Attractive Sodium Taste in Taste Buds

Sodium taste regulates salt intake. The amiloride-sensitive epithelial sodium channel (ENaC) is the Na⁺ sensor in taste cells mediating attraction to sodium salts. However, cells and intracellular signaling underlying sodium taste in taste buds remain long-standing enigmas. Here, we show that a subset of taste cells with ENaC activity fire action potentials in response to ENaC-mediated Na⁺ influx without changing the intracellular Ca²⁺ concentration and form a channel synapse with afferent neurons involving the voltage-gated neurotransmitter-release channel composed of calcium homeostasis modulator 1 (CALHM1) and CALHM3 (CALHM1/3). Genetic elimination of ENaC in CALHM1-expressing cells as well as global CALHM3 deletion abolished amiloride-sensitive neural responses and attenuated behavioral attraction to NaCl. Together, sodium taste is mediated by cells expressing ENaC and CALHM1/3, where oral Na⁺ entry elicits suprathreshold depolarization for action potentials driving voltage-dependent neurotransmission via the channel synapse. Thus, all steps in sodium taste signaling are voltage driven and independent of Ca²⁺ signals. This work also reveals ENaC-independent salt attraction.

Recent advances in taste transduction and signaling

In the last few years, single-cell profiling of taste cells and ganglion cells has advanced our understanding of transduction, encoding, and transmission of information from taste buds as relayed to the central nervous system. This review focuses on new knowledge from these molecular approaches and attempts to place this in the context of previous questions and findings in the field. The individual taste cells within a taste bud are molecularly specialized for detection of one of the primary taste qualities: salt, sour, sweet, umami, and bitter. Transduction and transmitter release mechanisms differ substantially for taste cells transducing sour (Type  III cells) compared with those transducing the qualities of sweet, umami, or bitter (Type II cells), although ultimately all transmission of taste relies on activation of purinergic P2X receptors on the afferent nerves. The ganglion cells providing innervation to the taste buds also appear divisible into functional and molecular subtypes, and each ganglion cell is primarily but not exclusively responsive to one taste quality.

ATP Release Channels

Adenosine triphosphate (ATP) has been well established as an important extracellular ligand of autocrine signaling, intercellular communication, and neurotransmission with numerous physiological and pathophysiological roles. In addition to the classical exocytosis, non-vesicular mechanisms of cellular ATP release have been demonstrated in many cell types. Although large and negatively charged ATP molecules cannot diffuse across the lipid bilayer of the plasma membrane, conductive ATP release from the cytosol into the extracellular space is possible through ATP-permeable channels. Such channels must possess two minimum qualifications for ATP permeation: anion permeability and a large ion-conducting pore. Currently, five groups of channels are acknowledged as ATP-release channels: connexin hemichannels, pannexin 1, calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs, also known as volume-sensitive outwardly rectifying (VSOR) anion channels), and maxi-anion channels (MACs). Recently, major breakthroughs have been made in the field by molecular identification of CALHM1 as the action potential-dependent ATP-release channel in taste bud cells, LRRC8s as components of VRACs, and SLCO2A1 as a core subunit of MACs. Here, the function and physiological roles of these five groups of ATP-release channels are summarized, along with a discussion on the future implications of understanding these channels.

Type III Cells in Anterior Taste Fields Are More Immunohistochemically Diverse Than Those of Posterior Taste Fields in Mice

Activation of Type III cells in mammalian taste buds is implicated in the transduction of acids (sour) and salty stimuli. Several lines of evidence suggest that function of Type III cells in the anterior taste fields may differ from that of Type III cells in posterior taste fields. Underlying anatomy to support this observation is, however, scant. Most existing immunohistochemical data characterizing this cell type focus on circumvallate taste buds in the posterior tongue. Equivalent data from anterior taste fields-fungiform papillae and soft palate-are lacking. Here, we compare Type III cells in four taste fields: fungiform, soft palate, circumvallate, and foliate in terms of reactivity to four canonical markers of Type III cells: polycystic kidney disease 2-like 1 (PKD2L1), synaptosomal associated protein 25 (SNAP25), serotonin (5-HT), and glutamate decarboxylase 67 (GAD67). Our findings indicate that while PKD2L1, 5-HT, and SNAP25 are highly coincident in posterior taste fields, they diverge in anterior taste fields. In particular, a subset of taste cells expresses PKD2L1 without the synaptic markers, and a subset of SNAP25 cells lacks expression of PKD2L1. In posterior taste fields, GAD67-positive cells are a subset of PKD2L1 expressing taste cells, but anterior taste fields also contain a significant population of GAD67-only expressing cells. These differences in expression patterns may underlie the observed functional differences between anterior and posterior taste fields.

Taste buds: cells, signals and synapses

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Action potentials and ion conductances in wild-type and CALHM1-knockout type II taste cells

Taste bud type II cells fire action potentials in response to tastants, triggering nonvesicular ATP release to gustatory neurons via voltage-gated CALHM1-associated ion channels. Whereas CALHM1 regulates mouse cortical neuron excitability, its roles in regulating type II cell excitability are unknown. In this study, we compared membrane conductances and action potentials in single identified TRPM5-GFP-expressing circumvallate papillae type II cells acutely isolated from wild-type (WT) and Calhm1 knockout (KO) mice. The activation kinetics of large voltage-gated outward currents were accelerated in cells from Calhm1 KO mice, and their associated nonselective tail currents, previously shown to be highly correlated with ATP release, were completely absent in Calhm1 KO cells, suggesting that CALHM1 contributes to all of these currents. Calhm1 deletion did not significantly alter resting membrane potential or input resistance, the amplitudes and kinetics of Na⁺ currents either estimated from action potentials or recorded from steady-state voltage pulses, or action potential threshold, overshoot peak, afterhyperpolarization, and firing frequency. However, Calhm1 deletion reduced the half-widths of action potentials and accelerated the deactivation kinetics of transient outward currents, suggesting that the CALHM1-associated conductance becomes activated during the repolarization phase of action potentials.NEW & NOTEWORTHY CALHM1 is an essential ion channel component of the ATP neurotransmitter release mechanism in type II taste bud cells. Its contribution to type II cell resting membrane properties and excitability is unknown. Nonselective voltage-gated currents, previously associated with ATP release, were absent in cells lacking CALHM1. Calhm1 deletion was without effects on resting membrane properties or voltage-gated Na⁺ and K⁺ channels but contributed modestly to the kinetics of action potentials.

Glutamate: Tastant and Neuromodulator in Taste Buds

In taste buds, glutamate plays a double role as a gustatory stimulus and neuromodulator. The detection of glutamate as a tastant involves several G protein-coupled receptors, including the heterodimer taste receptor type 1, member 1 and 3 as well as metabotropic glutamate receptors (mGluR1 and mGluR4). Both receptor types participate in the detection of glutamate as shown with knockout animals and selective antagonists. At the basal part of taste buds, ionotropic glutamate receptors [N-methyl-d-aspartate (NMDA) and non-NMDA] are expressed and participate in the modulation of the taste signal before its transmission to the brain. Evidence suggests that glutamate has an efferent function on taste cells and modulates the release of other neurotransmitters such as serotonin and ATP. This short article reviews the recent developments in the field with regard to glutamate receptors involved in both functions as well as the influence of glutamate on the taste signal.

Mice Lacking Pannexin 1 Release ATP and Respond Normally to All Taste Qualities

Adenosine triphosphate (ATP) is required for the transmission of all taste qualities from taste cells to afferent nerve fibers. ATP is released from Type II taste cells by a nonvesicular mechanism and activates purinergic receptors containing P2X2 and P2X3 on nerve fibers. Several ATP release channels are expressed in taste cells including CALHM1, Pannexin 1, Connexin 30, and Connexin 43, but whether all are involved in ATP release is not clear. We have used a global Pannexin 1 knock out (Panx1 KO) mouse in a series of in vitro and in vivo experiments. Our results confirm that Panx1 channels are absent in taste buds of the knockout mice and that other known ATP release channels are not upregulated. Using a luciferin/luciferase assay, we show that circumvallate taste buds from Panx1 KO mice normally release ATP upon taste stimulation compared with wild type (WT) mice. Gustatory nerve recordings in response to various tastants applied to the tongue and brief-access behavioral testing with SC45647 also show no difference between Panx1 KO and WT. These results confirm that Panx1 is not required for the taste evoked release of ATP or for neural and behavioral responses to taste stimuli.

Glucagon-like peptide-1 is specifically involved in sweet taste transmission

Five fundamental taste qualities (sweet, bitter, salty, sour, umami) are sensed by dedicated taste cells (TCs) that relay quality information to gustatory nerve fibers. In peripheral taste signaling pathways, ATP has been identified as a functional neurotransmitter, but it remains to be determined how specificity of different taste qualities is maintained across synapses. Recent studies demonstrated that some gut peptides are released from taste buds by prolonged application of particular taste stimuli, suggesting their potential involvement in taste information coding. In this study, we focused on the function of glucagon-like peptide-1 (GLP-1) in initial responses to taste stimulation. GLP-1 receptor (GLP-1R) null mice had reduced neural and behavioral responses specifically to sweet compounds compared to wild-type (WT) mice. Some sweet responsive TCs expressed GLP-1 and its receptors were expressed in gustatory neurons. GLP-1 was released immediately from taste bud cells in response to sweet compounds but not to other taste stimuli. Intravenous administration of GLP-1 elicited transient responses in a subset of sweet-sensitive gustatory nerve fibers but did not affect other types of fibers, and this response was suppressed by pre-administration of the GLP-1R antagonist Exendin-4(3-39). Thus GLP-1 may be involved in normal sweet taste signal transmission in mice.

Postsynaptic P2X3-containing receptors in gustatory nerve fibres mediate responses to all taste qualities in mice

Taste buds release ATP to activate ionotropic purinoceptors composed of P2X2 and P2X3 subunits, present on the taste nerves. Mice with genetic deletion of P2X2 and P2X3 receptors (double knockout mice) lack responses to all taste stimuli presumably due to the absence of ATP-gated receptors on the afferent nerves. Recent experiments on the double knockout mice showed, however, that their taste buds fail to release ATP, suggesting the possibility of pleiotropic deficits in these global knockouts. To test further the role of postsynaptic P2X receptors in afferent signalling, we used AF-353, a selective antagonist of P2X3-containing receptors to inhibit the receptors acutely during taste nerve recording and behaviour. The specificity of AF-353 for P2X3-containing receptors was tested by recording Ca(2+) transients to exogenously applied ATP in fura-2 loaded isolated geniculate ganglion neurons from wild-type and P2X3 knockout mice. ATP responses were completely inhibited by 10 μm or 100 μm AF-353, but neither concentration blocked responses in P2X3 single knockout mice wherein the ganglion cells express only P2X2-containing receptors. Furthermore, AF-353 had no effect on taste-evoked ATP release from taste buds. In wild-type mice, i.p. injection of AF-353 or simple application of the drug directly to the tongue, inhibited taste nerve responses to all taste qualities in a dose-dependent fashion. A brief access behavioural assay confirmed the electrophysiological results and showed that preference for a synthetic sweetener, SC-45647, was abolished following i.p. injection of AF-353. These data indicate that activation of P2X3-containing receptors is required for transmission of all taste qualities.

Synaptic communication and signal processing among sensory cells in taste buds

Taste buds (sensory structures embedded in oral epithelium) show a remarkable diversity of transmitters synthesized and secreted locally. The known transmitters accumulate in a cell type selective manner, with 5-HT and noradrenaline being limited to presynaptic cells, GABA being synthesized in both presynaptic and glial-like cells, and acetylcholine and ATP used for signalling by receptor cells. Each of these transmitters participates in local negative or positive feedback circuits that target particular cell types. Overall, the role of ATP is the best elucidated. ATP serves as a principal afferent transmitter, and also is the key trigger for autocrine positive feedback and paracrine circuits that result in potentiation (via adenosine) or inhibition (via GABA or 5-HT). While many of the cellular receptors and mechanisms for these circuits are known, their impact on sensory detection and perception remains to be elaborated in most instances. This brief review examines what is known, and some of the open questions and controversies surrounding the transmitters and circuits of the taste periphery.

A taste for ATP: neurotransmission in taste buds

Not only is ATP a ubiquitous source of energy but it is also used widely as an intercellular signal. For example, keratinocytes release ATP in response to numerous external stimuli including pressure, heat, and chemical insult. The released ATP activates purinergic receptors on nerve fibers to generate nociceptive signals. The importance of an ATP signal in epithelial-to-neuronal signaling is nowhere more evident than in the taste system. The receptor cells of taste buds release ATP in response to appropriate stimulation by tastants and the released ATP then activates P2X2 and P2X3 receptors on the taste nerves. Genetic ablation of the relevant P2X receptors leaves an animal without the ability to taste any primary taste quality. Of interest is that release of ATP by taste receptor cells occurs in a non-vesicular fashion, apparently via gated membrane channels. Further, in keeping with the crucial role of ATP as a neurotransmitter in this system, a subset of taste cells expresses a specific ectoATPase, NTPDase2, necessary to clear extracellular ATP which otherwise will desensitize the P2X receptors on the taste nerves. The unique utilization of ATP as a key neurotransmitter in the taste system may reflect the epithelial rather than neuronal origins of the receptor cells.

Cell-type-dependent action potentials and voltage-gated currents in mouse fungiform taste buds

Taste receptor cells fire action potentials in response to taste substances to trigger non-exocytotic neurotransmitter release in type II cells and exocytotic release in type III cells. We investigated possible differences between these action potentials fired by mouse taste receptor cells using in situ whole-cell recordings, and subsequently we identified their cell types immunologically with cell-type markers, an IP3 receptor (IP3 R3) for type II cells and a SNARE protein (SNAP-25) for type III cells. Cells not immunoreactive to these antibodies were examined as non-IRCs. Here, we show that type II cells and type III cells fire action potentials using different ionic mechanisms, and that non-IRCs also fire action potentials with either of the ionic mechanisms. The width of action potentials was significantly narrower and their afterhyperpolarization was deeper in type III cells than in type II cells. Na(+) current density was similar in type II cells and type III cells, but it was significantly smaller in non-IRCs than in the others. Although outwardly rectifying current density was similar between type II cells and type III cells, tetraethylammonium (TEA) preferentially suppressed the density in type III cells and the majority of non-IRCs. Our mathematical model revealed that the shape of action potentials depended on the ratio of TEA-sensitive current density and TEA-insensitive current one. The action potentials of type II cells and type III cells under physiological conditions are discussed.

How do taste cells lacking synapses mediate neurotransmission? CALHM1, a voltage-gated ATP channel

CALHM1 was recently demonstrated to be a voltage-gated ATP-permeable ion channel and to serve as a bona fide conduit for ATP release from sweet-, umami-, and bitter-sensing type II taste cells. Calhm1 is expressed in taste buds exclusively in type II cells and its product has structural and functional similarities with connexins and pannexins, two families of channel protein candidates for ATP release by type II cells. Calhm1 knockout in mice leads to loss of perception of sweet, umami, and bitter compounds and to impaired gustatory nerve responses to these tastants. These new studies validate the concept of ATP as the primary neurotransmitter from type II cells to gustatory neurons. Furthermore, they identify voltage-gated ATP release through CALHM1 as an essential molecular mechanism of ATP release in taste buds. We discuss these new findings, as well as unresolved issues in peripheral taste signaling that we hope will stimulate future research.

Ryanodine Receptors Selectively Interact with L Type Calcium Channels in Mouse Taste Cells

Introduction

We reported that ryanodine receptors are expressed in two different types of mammalian peripheral taste receptor cells: Type II and Type III cells. Type II cells lack voltage-gated calcium channels (VGCCs) and chemical synapses. In these cells, ryanodine receptors contribute to the taste-evoked calcium signals that are initiated by opening inositol trisphosphate receptors located on internal calcium stores. In Type III cells that do have VGCCs and chemical synapses, ryanodine receptors contribute to the depolarization-dependent calcium influx.

Methodology/Principal Findings

The goal of this study was to establish if there was selectivity in the type of VGCC that is associated with the ryanodine receptor in the Type III taste cells or if the ryanodine receptor opens irrespective of the calcium channels involved. We also wished to determine if the ryanodine receptors and VGCCs require a physical linkage to interact or are simply functionally associated with each other. Using calcium imaging and pharmacological inhibitors, we found that ryanodine receptors are selectively associated with L type VGCCs but likely not through a physical linkage.

Conclusions/Significance

Taste cells are able to undergo calcium induced calcium release through ryanodine receptors to increase the initial calcium influx signal and provide a larger calcium response than would otherwise occur when L type channels are activated in Type III taste cells.

High-resolution membrane capacitance measurements for the study of exocytosis and endocytosis

In order to understand exocytosis and endocytosis, it is necessary to study these processes directly. An elegant way to do this is by measuring plasma membrane capacitance (C(m)), a parameter proportional to cell surface area, the fluctuations of which are due to fusion and fission of secretory and other vesicles. Here we describe protocols that enable high-resolution C(m) measurements in macroscopic and microscopic modes. Macroscopic mode, performed in whole-cell configuration, is used for measuring bulk C(m) changes in the entire membrane area, and it enables the introduction of exocytosis stimulators or inhibitors into the cytosol through the patch pipette. Microscopic mode, performed in cell-attached configuration, enables measurements of C(m) with attofarad resolution and allows characterization of fusion pore properties. Although we usually apply these protocols to primary pituitary cells and astrocytes, they can be adapted and used for other cell types. After initial hardware setup and culture preparation, several C(m) measurements can be performed daily.

Cellular mechanisms of developmental and sex differences in the rapid hormonal modulation of a social communication signal

Some gymnotiform electric fish species rapidly modify their electric signal waveforms by altering the action potential (AP) waveforms of their electrocytes, the excitable cells that produce the electric organ discharge (EOD). The fish Brachyhypopomus gauderio alters EOD amplitude and pulse duration as a social signal in accordance with the prevailing social conditions, under the dual regulation of melanocortins and androgens. We show here that B. gauderio uses two distinct cellular mechanisms to change signal amplitude, and its use of these two mechanisms varies with age and sex of the signaler. EOD amplitude and waveform are regulated by the coordinated timing and shaping of two APs generated from two opposing excitable membranes in each electrocyte. The two membranes fire in sequence within 100 μs of each other with the second AP being broader than the first. We have shown previously that mature males increase EOD amplitude and duration when melanocortin peptide hormones act directly on electrocytes to selectively broaden the second AP and increase the delay between the two APs by approximately 25 μs. Here we show that females selectively broaden only the second AP as males do, but increase amplitude of both APs with no change in delay between them, a previously unreported second mechanism of EOD amplitude change in B. gauderio. Juvenile fish broaden both APs and increase the delay between the APs. Cellular mechanisms of EOD plasticity are therefore shaped during development, presumably by sex steroids, becoming sexually dimorphic at maturity.

Evolutionary origins of taste buds: phylogenetic analysis of purinergic neurotransmission in epithelial chemosensors

Taste buds are gustatory endorgans which use an uncommon purinergic signalling system to transmit information to afferent gustatory nerve fibres. In mammals, ATP is a crucial neurotransmitter released by the taste cells to activate the afferent nerve fibres. Taste buds in mammals display a characteristic, highly specific ecto-ATPase (NTPDase2) activity, suggesting a role in inactivation of the neurotransmitter. The purpose of this study was to test whether the presence of markers of purinergic signalling characterize taste buds in anamniote vertebrates and to test whether similar purinergic systems are employed by other exteroceptive chemosensory systems. The species examined include several teleosts, elasmobranchs, lampreys and hagfish, the last of which lacks vertebrate-type taste buds. For comparison, Schreiner organs of hagfish and solitary chemosensory cells (SCCs) of teleosts, both of which are epidermal chemosensory end organs, were also examined because they might be evolutionarily related to taste buds. Ecto-ATPase activity was evident in elongate cells in all fish taste buds, including teleosts, elasmobranchs and lampreys. Neither SCCs nor Schreiner organs show specific ecto-ATPase activity, suggesting that purinergic signalling is not crucial in those systems as it is for taste buds. These findings suggest that the taste system did not originate from SCCs but arose independently in early vertebrates.

CALHM1 ion channel mediates purinergic neurotransmission of sweet, bitter and umami tastes

Recognition of sweet, bitter and umami tastes requires the non-vesicular release from taste bud cells of ATP, which acts as a neurotransmitter to activate afferent neural gustatory pathways. However, how ATP is released to fulfil this function is not fully understood. Here we show that calcium homeostasis modulator 1 (CALHM1), a voltage-gated ion channel, is indispensable for taste-stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells. Calhm1 knockout mice have severely impaired perceptions of sweet, bitter and umami compounds, whereas their recognition of sour and salty tastes remains mostly normal. Calhm1 deficiency affects taste perception without interfering with taste cell development or integrity. CALHM1 is expressed specifically in sweet/bitter/umami-sensing type II taste bud cells. Its heterologous expression induces a novel ATP permeability that releases ATP from cells in response to manipulations that activate the CALHM1 ion channel. Knockout of Calhm1 strongly reduces voltage-gated currents in type II cells and taste-evoked ATP release from taste buds without affecting the excitability of taste cells by taste stimuli. Thus, CALHM1 is a voltage-gated ATP-release channel required for sweet, bitter and umami taste perception.

Taste buds as peripheral chemosensory processors

Taste buds are peripheral chemosensory organs situated in the oral cavity. Each taste bud consists of a community of 50-100 cells that interact synaptically during gustatory stimulation. At least three distinct cell types are found in mammalian taste buds - Type I cells, Receptor (Type II) cells, and Presynaptic (Type III) cells. Type I cells appear to be glial-like cells. Receptor cells express G protein-coupled taste receptors for sweet, bitter, or umami compounds. Presynaptic cells transduce acid stimuli (sour taste). Cells that sense salt (NaCl) taste have not yet been confidently identified in terms of these cell types. During gustatory stimulation, taste bud cells secrete synaptic, autocrine, and paracrine transmitters. These transmitters include ATP, acetylcholine (ACh), serotonin (5-HT), norepinephrine (NE), and GABA. Glutamate is an efferent transmitter that stimulates Presynaptic cells to release 5-HT. This chapter discusses these transmitters, which cells release them, the postsynaptic targets for the transmitters, and how cell-cell communication shapes taste bud signaling via these transmitters.

Virtual population generator for human cardiomyocytes parameters: in silico drug cardiotoxicity assessment

BACKGROUND

The anatomical and histological parameters of the human ventricle depend on many factors including age and sex. Myocyte volume and electric capacitance are significant physiological parameters of left ventricle cardiomyocyte mathematical models. They allow the assessment of inter-individual variability during in vitro-in vivo extrapolation of the drug cardiotoxic effect.

OBJECTIVE

The current research was carried out to analyze the relationship between age, human left ventricle cardiomyocyte volume, and electric capacitance in a healthy population.

METHODS

In order to collect data describing cardiomyocyte volume and membrane area, literature searches were performed. It was assumed that the cardiomyocyte volume (VOL) and area (AREA) distribution have non-negative support and are skewed to the right. A log-linear model with constant variance was used. A simulation study was run to assess the influence of physiological parameters on action potential duration.

RESULTS

The coefficient of determination for the proposed model R(2) = 0.95, that is, 95% of the variability observed in log cardiomyocyte volume can be explained by the estimated regression equation. To allow simple calculation and model performance validation, a simple Excel file was developed (Supplementary material).

CONCLUSIONS

To the best of our knowledge, there is no other model available, combining age, cardiomyocyte volume, and area. The main limitations of the proposed models result from the assumptions made at the data analysis stage. The limited amount of information available in the literature and the lack of differentiation between sexes results in one common equation. The developed model is a part of the computational system for drug cardiotoxicity assessment.

Acid stimulation (sour taste) elicits GABA and serotonin release from mouse taste cells

Several transmitter candidates including serotonin (5-HT), ATP, and norepinephrine (NE) have been identified in taste buds. Recently, γ-aminobutyric acid (GABA) as well as the associated synthetic enzymes and receptors have also been identified in taste cells. GABA reduces taste-evoked ATP secretion from Receptor cells and is considered to be an inhibitory transmitter in taste buds. However, to date, the identity of GABAergic taste cells and the specific stimulus for GABA release are not well understood. In the present study, we used genetically-engineered Chinese hamster ovary (CHO) cells stably co-expressing GABA(B) receptors and Gαqo5 proteins to measure GABA release from isolated taste buds. We recorded robust responses from GABA biosensors when they were positioned against taste buds isolated from mouse circumvallate papillae and the buds were depolarized with KCl or a stimulated with an acid (sour) taste. In contrast, a mixture of sweet and bitter taste stimuli did not trigger GABA release. KCl- or acid-evoked GABA secretion from taste buds was Ca(2+)-dependent; removing Ca(2+) from the bathing medium eliminated GABA secretion. Finally, we isolated individual taste cells to identify the origin of GABA secretion. GABA was released only from Presynaptic (Type III) cells and not from Receptor (Type II) cells. Previously, we reported that 5-HT released from Presynaptic cells inhibits taste-evoked ATP secretion. Combined with the recent findings that GABA depresses taste-evoked ATP secretion, the present results indicate that GABA and 5-HT are inhibitory transmitters in mouse taste buds and both likely play an important role in modulating taste responses.