Expression of calcium-activated chloride channels Ano1 and Ano2 in mouse taste cells

The complexities of salt taste reception: insights into the role of TMC4 in chloride taste detection

Although salt is an essential substance vital to life, excessive salt intake could cause various health issues. Therefore, new technologies and strategies should be developed to reduce salt intake without compromising taste. However, the underlying physiological mechanisms of salt taste reception is complex and not completely understood. Sodium chloride is a typical salty substance. It is widely believed that only sodium is important for the generation of salty taste. On the other hand, from a psychophysical perspective, the importance of chloride in salty taste has been indicated. Thus, understanding the mechanisms of both sodium- and chloride-tastes generation is necessary to completely comprehended the fundamentals of salt taste reception. However, the mechanism for detecting chloride taste has remained unclear for many years. Recently, we have identified transmembrane channel-like 4 (TMC4) as the first molecule that mediates the reception of chloride taste. TMC4 functions as a voltage-dependent chloride channel and plays an important role in the reception of the chloride taste by detecting chloride ions. In this mini-review, we first introduce the known reception mechanism of salty taste, and then discuss the roles of TMC4 in the salt taste reception. The finding of TMC4 may serve as a basis for developing new technologies and formulating strategies to reduce salt intake without compromising taste.

The physiological roles of anoctamin2/TMEM16B and anoctamin1/TMEM16A in chemical senses

Chemical senses allow animals to detect and discriminate a vast array of molecules. The olfactory system is responsible of the detection of small volatile molecules, while water dissolved molecules are detected by taste buds in the oral cavity. Moreover, many animals respond to signaling molecules such as pheromones and other semiochemicals through the vomeronasal organ. The peripheral organs dedicated to chemical detection convert chemical signals into perceivable information through the employment of diverse receptor types and the activation of multiple ion channels. Two ion channels, TMEM16B, also known as anoctamin2 (ANO2) and TMEM16A, or anoctamin1 (ANO1), encoding for Ca2+-activated Cl¯ channels, have been recently described playing critical roles in various cell types. This review aims to discuss the main properties of TMEM16A and TMEM16B-mediated currents and their physiological roles in chemical senses. In olfactory sensory neurons, TMEM16B contributes to amplify the odorant response, to modulate firing, response kinetics and adaptation. TMEM16A and TMEM16B shape the pattern of action potentials in vomeronasal sensory neurons increasing the interspike interval. In type I taste bud cells, TMEM16A is activated during paracrine signaling mediated by ATP. This review aims to shed light on the regulation of diverse signaling mechanisms and neuronal excitability mediated by Ca-activated Cl¯ channels, hinting at potential new roles for TMEM16A and TMEM16B in the chemical senses.

Anion efflux mediates transduction in the hair cells of the zebrafish lateral line

Hair cells are the principal sensory receptors of the vertebrate auditory system, where they transduce sounds through mechanically gated ion channels that permit cations to flow from the surrounding endolymph into the cells. The lateral line of zebrafish has served as a key model system for understanding hair cell physiology and development, often with the belief that these hair cells employ a similar transduction mechanism. In this study, we demonstrate that these hair cells are exposed to an unregulated external environment with cation concentrations that are too low to support transduction. Our results indicate that hair cell excitation is instead mediated by a substantially different mechanism involving the outward flow of anions. Further investigation of hair cell transduction in a diversity of sensory systems and species will likely yield deep insights into the physiology of these unique cells.

Glial regulators of ions and solutes required for specific chemosensory functions in Caenorhabditis elegans

Glia and accessory cells regulate the microenvironment around neurons and primary sensory cells. However, the impact of specific glial regulators of ions and solutes on functionally diverse primary cells is poorly understood. Here, we systemically investigate the requirement of ion channels and transporters enriched in Caenorhabditis elegans Amsh glia for the function of chemosensory neurons. Although Amsh glia ablated worms show reduced function of ASH, AWC, AWA, and ASE neurons, we show that the loss of glial enriched ion channels and transporters impacts these neurons differently, with nociceptor ASH being the most affected. Furthermore, our analysis underscores the importance of K+, Cl-, and nucleoside homeostasis in the Amphid sensory organ and uncovers the contribution of glial genes implicated in neurological disorders. Our findings build a unique fingerprint of each glial enriched ion channel and transporter and may provide insights into the function of supporting cells of mammalian sensory organs.

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.

"Tripartite Synapses" in Taste Buds: A Role for Type I Glial-like Taste Cells

In mammalian taste buds, Type I cells comprise half of all cells. These are termed "glial-like" based on morphologic and molecular features, but there are limited studies describing their function. We tested whether Type I cells sense chemosensory activation of adjacent chemosensory (i.e., Types II and III) taste bud cells, similar to synaptic glia. Using Gad2;;GCaMP3 mice of both sexes, we confirmed by immunostaining that, within taste buds, GCaMP expression is predominantly in Type I cells (with no Type II and ≈28% Type III cells expressing weakly). In dissociated taste buds, GCaMP+ Type I cells responded to bath-applied ATP (10-100 μm) but not to 5-HT (transmitters released by Type II or III cells, respectively). Type I cells also did not respond to taste stimuli (5 μm cycloheximide, 1 mm denatonium). In lingual slice preparations also, Type I cells responded to bath-applied ATP (10-100 μm). However, when taste buds in the slice were stimulated with bitter tastants (cycloheximide, denatonium, quinine), Type I cells responded robustly. Taste-evoked responses of Type I cells in the slice preparation were significantly reduced by desensitizing purinoceptors or by purinoceptor antagonists (suramin, PPADS), and were essentially eliminated by blocking synaptic ATP release (carbenoxolone) or degrading extracellular ATP (apyrase). Thus, taste-evoked release of afferent ATP from type II chemosensory cells, in addition to exciting gustatory afferent fibers, also activates glial-like Type I taste cells. We speculate that Type I cells sense chemosensory activation and that they participate in synaptic signaling, similarly to glial cells at CNS tripartite synapses.SIGNIFICANCE STATEMENT Most studies of taste buds view the chemosensitive excitable cells that express taste receptors as the sole mediators of taste detection and transmission to the CNS. Type I "glial-like" cells, with their ensheathing morphology, are mostly viewed as responsible for clearing neurotransmitters and as the "glue" holding the taste bud together. In the present study, we demonstrate that, when intact taste buds respond to their natural stimuli, Type I cells sense the activation of the chemosensory cells by detecting the afferent transmitter. Because Type I cells synthesize GABA, a known gliotransmitter, and cognate receptors are present on both presynaptic and postsynaptic elements, Type I cells may participate in GABAergic synaptic transmission in the manner of astrocytes at tripartite synapses.

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.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Calcium- and Voltage-Dependent Dual Gating ANO1 is an Intrinsic Determinant of Repolarization in Rod Bipolar Cells of the Mouse Retina

TMEM16A/anoctamin1 (ANO1), a calcium (Ca²⁺)-activated chloride (Cl^-) channel, has many functions in various excitable cells and modulates excitability in both Ca²⁺- and voltage-gating modes. However, its gating characteristics and role in primary neural cells remain unclear. Here, we characterized its Ca²⁺- and voltage-dependent components in rod bipolar cells using dissociated and slice preparations of the mouse retina. The I-V curves of Ca²⁺-dependent ANO1 tail current and voltage-gated Ca²⁺ channel (VGCC) are similar; as ANO1 is blocked by VGCC inhibitors, ANO1 may be gated by Ca²⁺ influx through VGCC. The voltage-dependent component of ANO1 has outward rectifying and sustained characteristics and is clearly isolated by the inhibitory effect of Cl^- reduction and T16Ainh-A01, a selective ANO1 inhibitor, in high EGTA, a Ca²⁺ chelator. The voltage-dependent component disappears due to VGCC inhibition, suggesting that Ca²⁺ is the essential trigger for ANO1. In perforated current-clamping method, the application of T16Ainh-A01 and reduction of Cl^- extended excitation periods in rod bipolar cells, revealing that ANO1 induces repolarization during excitation. Overall, ANO1 opens by VGCC activation during physiological excitation of the rod bipolar cell and has a voltage-dependent component. These two gating-modes concurrently provide the intrinsic characteristics of the membrane potential in rod bipolar cells.

Contributions of glutamate transporters and Ca2+-activated Cl- currents to feedback from horizontal cells to cone photoreceptors

Lateral inhibitory feedback from horizontal cells (HCs) to cones establishes center-surround receptive fields and color opponency in the retina. When HCs hyperpolarize to light, inhibitory feedback to cones increases activation of cone Ca²⁺ currents (I_Ca) that can in turn activate additional currents. We recorded simultaneously from cones and HCs to analyze cone currents activated by HC feedback in salamander retina. Depolarization-activated inward tail currents in cones were inhibited by CaCCinh-A01 that inhibits both Ano1 and Ano2 Ca²⁺-activated Cl^- currents (I_Cl(Ca)). An Ano1-selective inhibitor Ani9 was less effective suggesting that Ano2 is the predominant I_Cl(Ca) subtype in cones. CaCCinh-A01 inhibited feedback currents more strongly when intracellular Ca²⁺ in cones was buffered with 0.05 mM EGTA compared to stronger buffering with 5 mM EGTA. By contrast, blocking glutamate transporter anion currents (I_Cl(Glu)) with TBOA had stronger inhibitory effects on cone feedback currents when Ca²⁺ buffering was strong. Inward feedback currents ran down at rates intermediate between rundown of glutamate release and I_Cl(Ca), consistent with contributions to feedback from both I_Cl(Ca) and I_Cl(Glu). These results suggest that Cl^- channels coupled to glutamate transporters help to speed inward feedback currents initiated by local changes in intracellular [Ca²⁺] close to synaptic ribbons of cones whereas Ano2 Ca²⁺-activated Cl^- channels contribute to slower components of feedback regulated by spatially extensive changes in intracellular [Ca²⁺].

Calcium-Activated Chloride Channels in Newly Differentiating Mouse Lens Fiber Cells and Their Role in Volume Regulation

Purpose

Chloride channels have been proposed to play an important role in the regulation of lens volume. Unfortunately, little information is available about the molecular identity of these channels or how they are regulated in the lens due to the difficulties in isolating mouse fiber cells. Recently, our laboratory has developed a new technique for isolating these cells by using transgenic mouse lenses that lack both Cx50 and Cx46. The purpose of this study was to test the hypothesis that newly differentiating mouse fiber cells express calcium-activated chloride channels (CaCCs) by using this technique.

Methods

Differentiating fiber cells were isolated from lenses of double knockout mice that lack both Cx50 and Cx46 by using collagenase. Membrane currents were studied using the whole-cell patch clamp technique. The molecular identity and distribution of CaCCs were investigated using RT-PCR and immunofluorescence.

Results

Our electrophysiologic experiments suggest that peripheral fiber cells express a calcium-activated chloride current. The voltage gating properties, calcium sensitivity, and pharmacologic properties of this current resembled those of TMEM16 CaCCs. RT-PCR analysis demonstrated the presence of TMEM16A and TMEM16B transcripts in wild-type and double knockout mouse lenses. Both TMEM16A and TMEM16B proteins were detected in the differentiating epithelial cells and newly elongating fiber cells near the equator of the lens by immunohistochemistry.

Conclusions

Our results demonstrate that membrane conductance of peripheral fiber cells contain CaCCs that can be attributed to TMEM16A and TMEM16B. Given their critical role in volume regulation in other tissues, we speculate that these channels play a similar role in the lens.

Ca2+-activated Cl- currents in the murine vomeronasal organ enhance neuronal spiking but are dispensable for male-male aggression

Ca²⁺-activated Cl^- currents have been observed in many physiological processes, including sensory transduction in mammalian olfaction. The olfactory vomeronasal (or Jacobson's) organ (VNO) detects molecular cues originating from animals of the same species or from predators. It then triggers innate behaviors such as aggression, mating, or flight. In the VNO, Ca²⁺-activated Cl^- channels (CaCCs) are thought to amplify the initial pheromone-evoked receptor potential by mediating a depolarizing Cl^- efflux. Here, we confirmed the co-localization of the Ca²⁺-activated Cl^- channels anoctamin 1 (Ano1, also called TMEM16A) and Ano2 (TMEM16B) in microvilli of apically and basally located vomeronasal sensory neurons (VSNs) and their absence in supporting cells of the VNO. Both channels were expressed as functional isoforms capable of giving rise to Ca²⁺-activated Cl^- currents. Although these currents persisted in the VNOs of mice lacking Ano2, they were undetectable in olfactory neuron-specific Ano1 knockout mice irrespective of the presence of Ano2 The loss of Ca²⁺-activated Cl^- currents resulted in diminished spontaneous and drastically reduced pheromone-evoked spiking of VSNs. Although this indicated an important role of anoctamin channels in VNO signal amplification, the lack of this amplification did not alter VNO-dependent male-male territorial aggression in olfactory Ano1/Ano2 double knockout mice. We conclude that Ano1 mediates the bulk of Ca²⁺-activated Cl^- currents in the VNO and that Ano2 plays only a minor role. Furthermore, vomeronasal signal amplification by CaCCs appears to be dispensable for the detection of male-specific pheromones and for near-normal aggressive behavior in mice.