Chemical synapses without synaptic vesicles: Purinergic neurotransmission through a CALHM1 channel-mitochondrial signaling complex

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.

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.

Tuft cell-derived acetylcholine promotes epithelial chloride secretion and intestinal helminth clearance

Epithelial cells secrete chloride to regulate water release at mucosal barriers, supporting both homeostatic hydration and the "weep" response that is critical for type 2 immune defense against parasitic worms (helminths). Epithelial tuft cells in the small intestine sense helminths and release cytokines and lipids to activate type 2 immune cells, but whether they regulate epithelial secretion is unknown. Here, we found that tuft cell activation rapidly induced epithelial chloride secretion in the small intestine. This response required tuft cell sensory functions and tuft cell-derived acetylcholine (ACh), which acted directly on neighboring epithelial cells to stimulate chloride secretion, independent of neurons. Maximal tuft cell-induced chloride secretion coincided with immune restriction of helminths, and clearance was delayed in mice lacking tuft cell-derived ACh, despite normal type 2 inflammation. Thus, we have uncovered an epithelium-intrinsic response unit that uses ACh to couple tuft cell sensing to the secretory defenses of neighboring epithelial cells.

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.

Semirational engineering of Cytophaga hutchinsonii polyphosphate kinase for developing a cost-effective, robust, and efficient adenosine 5'-triphosphate regeneration system

IMPORTANCE

The adenosine 5'-triphosphate (ATP) regeneration system can significantly reduce the cost of many biocatalytic processes. Numerous studies have endeavored to utilize the ATP regeneration system based on Cytophaga hutchinsonii PPK (ChPPK). However, the wild-type ChPPK enzyme possesses limitations such as low enzymatic activity, poor stability, and limited substrate tolerance, impeding its application in catalytic reactions. To enhance the performance of ChPPK, we employed a semi-rational design approach to obtain the variant ChPPK/A79G/S106C/I108F/L285P. The enzymatic kinetic parameters and the catalytic performance in the synthesis of nicotinamide mononucleotide demonstrated that the variant ChPPK/A79G/S106C/I108F/L285P exhibited superior enzymatic properties than the wild-type enzyme. All data indicated that our engineered ATP regeneration system holds inherent potential for implementation in biocatalytic processes.

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.

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.

Endocrinology of Taste with Aging

Taste is one of our five primary senses, and taste impairment has been shown to increase with aging. The ability to taste allows us to enjoy the food we eat and to avoid foods that are potentially spoiled or poisonous. Recent advances in our understanding of the molecular mechanisms of taste receptor cells located within taste buds help us decipher how taste works. The discoveries of "classic" endocrine hormones in taste receptor cells point toward taste buds being actual endocrine organs. A better understanding of how taste works may help in reversing taste impairment associated with aging.

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.

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".

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.

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 insights into human taste perception and umami tastants: A review

Understanding taste is key for optimizing the palatability of seaweeds and other non-animal-based foods rich in protein. The lingual papillae in the mouth hold taste buds with taste receptors for the five gustatory taste qualities. Each taste bud contains three distinct cell types, of which Type II cells carry various G protein-coupled receptors that can detect sweet, bitter, or umami tastants, while type III cells detect sour, and likely salty stimuli. Upon ligand binding, receptor-linked intracellular heterotrimeric G proteins initiate a cascade of downstream events which activate the afferent nerve fibers for taste perception in the brain. The taste of amino acids depends on the hydrophobicity, size, charge, isoelectric point, chirality of the alpha carbon, and the functional groups on their side chains. The principal umami ingredient monosodium l-glutamate, broadly known as MSG, loses umami taste upon acetylation, esterification, or methylation, but is able to form flat configurations that bind well to the umami taste receptor. Ribonucleotides such as guanosine monophosphate and inosine monophosphate strongly enhance umami taste when l-glutamate is present. Ribonucleotides bind to the outer section of the venus flytrap domain of the receptor dimer and stabilize the closed conformation. Concentrations of glutamate, aspartate, arginate, and other compounds in food products may enhance saltiness and overall flavor. Umami ingredients may help to reduce the consumption of salts and fats in the general population and increase food consumption in the elderly.

Cryo-EM structure of the heptameric calcium homeostasis modulator 1 channel

Calcium homeostasis modulator 1 (CALHM1) is a voltage- and Ca²⁺-gated ATP channel that plays an important role in neuronal signaling. However, as the previously reported CALHM structures are all in the ATP-conducting state, the gating mechanism of ATP permeation is still elusive. Here, we report cryo-EM reconstructions of two Danio rerio CALHM1 heptamers with ordered or flexible long C-terminal helices at resolutions of 3.2 Å and 2.9 Å, respectively, and one D. rerio CALHM1 octamer with flexible long C-terminal helices at a resolution of 3.5 Å. Structural analysis shows that the heptameric CALHM1s are in an ATP-nonconducting state with a central pore diameter of approximately 6.6 Å. Compared with those inside the octameric CALHM1, the N-helix inside the heptameric CALHM1 is in the “down” position to avoid steric clashing with the adjacent TM1 helix. Molecular dynamics simulations show that as the N-helix moves from the “down” position to the “up” position, the pore size of ATP molecule permeation increases significantly. Our results provide important information for elucidating the mechanism of ATP molecule permeation in the CALHM1 channel.

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.

The presynaptic active zone protein Bassoon as a marker for synapses between Type III cells and afferent nerve fibers in taste buds

Taste buds are receptor organs for gustation. Two types of taste receptor cells have been identified in taste buds: Type II and Type III cells. Type III cells connect with afferent fibers through conventional chemical synapses. In the present study, we used immunocytochemistry to examine the distribution pattern of Bassoon, a scaffolding protein of the cytomatrix at the active zones of conventional synapses in mouse taste buds. Bassoon was predominantly detected as small puncta in Type III cells. Bassoon-immunoreactive puncta were observed in proximity to or partially overlapping with intragemmal nerve fibers. The distribution pattern of Bassoon in taste buds was similar among circumvallate, fungiform, and foliate taste buds. Immunoelectron microscopy showed Bassoon at the active zones of the conventional synapses of Type III cells in circumvallate taste buds. The present results demonstrate that Bassoon is a marker for synapses between Type III cells and afferent fibers in mouse taste buds.

Adaptive immune cells shape obesity-associated type 2 diabetes mellitus and less prominent comorbidities

Obesity and type 2 diabetes mellitus (T2DM) are increasing in prevalence owing to decreases in physical activity levels and a shift to diets that include addictive and/or high-calorie foods. These changes are associated with the adoption of modern lifestyles and the presence of an obesogenic environment, which have resulted in alterations to metabolism, adaptive immunity and endocrine regulation. The size and quality of adipose tissue depots in obesity, including the adipose tissue immune compartment, are critical determinants of overall health. In obesity, chronic low-grade inflammation can occur in adipose tissue that can progress to systemic inflammation; this inflammation contributes to the development of insulin resistance, T2DM and other comorbidities. An improved understanding of adaptive immune cell dysregulation that occurs during obesity and its associated metabolic comorbidities, with an appreciation of sex differences, will be critical for repurposing or developing immunomodulatory therapies to treat obesity and/or T2DM-associated inflammation. This Review critically discusses how activation and metabolic reprogramming of lymphocytes, that is, T cells and B cells, triggers the onset, development and progression of obesity and T2DM. We also consider the role of immunity in under-appreciated comorbidities of obesity and/or T2DM, such as oral cavity inflammation, neuroinflammation in Alzheimer disease and gut microbiome dysbiosis. Finally, we discuss previous clinical trials of anti-inflammatory medications in T2DM and consider the path forward.

"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.

Contribution of non-selective membrane channels and receptors in epilepsy

Overcoming refractory epilepsy's resistance to the combination of antiepileptic drugs (AED), mitigating side effects, and preventing sudden unexpected death in epilepsy are critical goals for therapy of this disorder. Current therapeutic strategies are based primarily on neurocentric mechanisms, overlooking the participation of astrocytes and microglia in the pathophysiology of epilepsy. This review is focused on a set of non-selective membrane channels (permeable to ions and small molecules), including channels and ionotropic receptors of neurons, astrocytes, and microglia, such as: the hemichannels formed by Cx43 and Panx1; the purinergic P2X7 receptors; the transient receptor potential vanilloid (TRPV1 and TRPV4) channels; calcium homeostasis modulators (CALHMs); transient receptor potential canonical (TRPC) channels; transient receptor potential melastatin (TRPM) channels; voltage-dependent anion channels (VDACs) and volume-regulated anion channels (VRACs), which all have in common being activated by epileptic activity and the capacity to exacerbate seizure intensity. Specifically, we highlight evidence for the activation of these channels/receptors during epilepsy including neuroinflammation and oxidative stress, discuss signaling pathways and feedback mechanisms, and propose the functions of each of them in acute and chronic epilepsy. Studying the role of these non-selective membrane channels in epilepsy and identifying appropriate blockers for one or more of them could provide complementary therapies to better alleviate the disease.

On the molecular nature of large-pore channels

Membrane transport is a fundamental means to control basic cellular processes such as apoptosis, inflammation, and neurodegeneration and is mediated by a number of transporters, pumps, and channels. Accumulating evidence over the last half century has shown that a type of so-called "large-pore channel" exists in various tissues and organs in gap-junctional and non-gap-junctional forms in order to flow not only ions but also metabolites such as ATP. They are formed by a number of protein families with little or no evolutionary linkages including connexin, innexin, pannexin, leucine-rich repeat-containing 8 (LRRC8), and calcium homeostasis modulator (CALHM). This review summarizes the history and concept of large-pore channels starting from connexin gap junction channels to the more recent developments in innexin, pannexin, LRRC8, and CALHM. We describe structural and functional features of large-pore channels that are crucial for their diverse functions on the basis of available structures.

Posttranslational regulation of CALHM1/3 channel: N-linked glycosylation and S-palmitoylation

Among calcium homeostasis modulator (CALHM) family members, CALHM1 and 3 together form a voltage-gated large-pore ion channel called CALHM1/3. CALHM1/3 plays an essential role in taste perception by mediating neurotransmitter release at channel synapses of taste bud cells. However, it is poorly understood how CALHM1/3 is regulated. Biochemical analyses of the two subunits following site-directed mutagenesis and pharmacological treatments established that both CALHM1 and 3 were N-glycosylated at single Asn residues in their second extracellular loops. Biochemical and electrophysiological studies revealed that N-glycan acquisition on CALHM1 and 3, respectively, controls the biosynthesis and gating kinetics of the CALHM1/3 channel. Furthermore, failure in subsequent remodeling of N-glycans decelerated the gating kinetics. Thus, the acquisition of N-glycans on both subunits and their remodeling differentially contribute to the functional expression of CALHM1/3. Meanwhile, metabolic labeling and acyl-biotin exchange assays combined with genetic modification demonstrated that CALHM3 was reversibly palmitoylated at three intracellular Cys residues. Screening of the DHHC protein acyltransferases identified DHHC3 and 15 as CALHM3 palmitoylating enzymes. The palmitoylation-deficient mutant CALHM3 showed a normal degradation rate and interaction with CALHM1. However, the same mutation markedly attenuated the channel activity but not surface localization of CALHM1/3, suggesting that CALHM3 palmitoylation is a critical determinant of CALHM1/3 activity but not its formation or forward trafficking. Overall, this study characterized N-glycosylation and S-palmitoylation of CALHM1/3 subunits and clarified their differential contributions to its functional expression, providing insights into the fine control of the CALHM1/3 channel and associated physiological processes.

Overlapping distributions of mammalian types I, II, and III taste cell markers in chicken taste buds

Mammalian taste buds comprise types I, II, and III taste cells, with each type having specific characteristics: glia-like supporting cells (type I), taste receptor cells (type II), and presynaptic cells (type III). In this study, to characterize the peripheral taste-sensing systems in chickens, we analyzed the distributions of the mammalian types I, II, and III taste cell markers in chicken taste buds: glutamate-aspartate transporter (GLAST) for type I; taste receptor type 1 members 1 and 3 (T1R1 and T1R3), taste receptor type 2 member 7 (T2R7), and α-gustducin for type II; and synaptosomal protein 25 (SNAP25) and neural cell adhesion molecule (NCAM) for type III. We found that most GLAST⁺ taste cells expressed α-gustducin and SNAP25 and that high percentages of T1R3⁺ or α-gustducin⁺ taste cells expressed SNAP25 and NCAM. These results demonstrated a unique subset of chicken taste cells expressing multiple taste cell type marker proteins. Taken together, these results provide new insights into the taste-sensing mechanisms in vertebrate taste buds.

Variable Branching Characteristics of Peripheral Taste Neurons Indicates Differential Convergence

Taste neurons are functionally and molecularly diverse, but their morphologic diversity remains completely unexplored. Using sparse cell genetic labeling, we provide the first reconstructions of peripheral taste neurons. The branching characteristics across 96 taste neurons show surprising diversity in their complexities. Individual neurons had 1-17 separate arbors entering between one and seven taste buds, 18 of these neurons also innervated non-taste epithelia. Axon branching characteristics are similar in gustatory neurons from male and female mice. Cluster analysis separated the neurons into four groups according to branch complexity. The primary difference between clusters was the amount of the nerve fiber within the taste bud available to contact taste-transducing cells. Consistently, we found that the maximum number of taste-transducing cells capable of providing convergent input onto individual gustatory neurons varied with a range of 1-22 taste-transducing cells. Differences in branching characteristics across neurons indicate that some neurons likely receive input from a larger number of taste-transducing cells than other neurons (differential convergence). By dividing neurons into two groups based on the type of taste-transducing cell most contacted, we found that neurons contacting primarily sour transducing cells were more heavily branched than those contacting primarily sweet/bitter/umami transducing cells. This suggests that neuron morphologies may differ across functional taste quality. However, the considerable remaining variability within each group also suggests differential convergence within each functional taste quality. Each possibility has functional implications for the system.SIGNIFICANCE STATEMENT Taste neurons are considered relay cells, communicating information from taste-transducing cells to the brain, without variation in morphology. By reconstructing peripheral taste neuron morphologies for the first time, we found that some peripheral gustatory neurons are simply branched, and can receive input from only a few taste-transducing cells. Other taste neurons are heavily branched, contacting many more taste-transducing cells than simply branched neurons. Based on the type of taste-transducing cell contacted, branching characteristics are predicted to differ across (and within) quality types (sweet/bitter/umami vs sour). Therefore, functional differences between neurons likely depends on the number of taste-transducing cells providing input and not just the type of cell providing input.

Optogenetic Activation of Type III Taste Cells Modulates Taste Responses

Studies have suggested that communication between taste cells shapes the gustatory signal before transmission to the brain. To further explore the possibility of intragemmal signal modulation, we adopted an optogenetic approach to stimulate sour-sensitive (Type III) taste cells using mice expressing Cre recombinase under a specific Type III cell promoter, Pkd2l1 (polycystic kidney disease-2-like 1), crossed with mice expressing Cre-dependent channelrhodopsin (ChR2). The application of blue light onto the tongue allowed for the specific stimulation of Type III cells and circumvented the nonspecific effects of chemical stimulation. To understand whether taste modality information is preprocessed in the taste bud before transmission to the sensory nerves, we recorded chorda tympani nerve activity during light and/or chemical tastant application to the tongue. To assess intragemmal modulation, we compared nerve responses to various tastants with or without concurrent light-induced activation of the Type III cells. Our results show that light significantly decreased taste responses to sweet, bitter, salty, and acidic stimuli. On the contrary, the light response was not consistently affected by sweet or bitter stimuli, suggesting that activation of Type II cells does not affect nerve responses to stimuli that activate Type III cells.

Slow recovery from the inactivation of voltage-gated sodium channel Nav1.3 in mouse taste receptor cells

Action potentials play an important role in neurotransmitter release in response to taste. Here, I have investigated voltage-gated Na⁺ channels, a primary component of action potentials, in respective cell types of mouse fungiform taste bud cells (TBCs) with in situ whole-cell clamping and single-cell RT-PCR techniques. The cell types of TBCs electrophysiologically examined were determined immunohistochemically using the type III inositol 1,4,5-triphoshate receptor as a type II cell marker and synaptosomal-associated protein 25 as a type III cell marker. I show that type II cells, type III cells, and TBCs not immunoreactive to these markers (likely type I cells) generate voltage-gated Na⁺ currents. The recovery following inactivation of these currents was well fitted with double exponential curves. The time constants in type III cells (~20 ms and ~ 1 s) were significantly slower than respective time constants in other cell types. RT-PCR analysis indicated the expression of Nav1.3, Nav1.5, Nav1.6, and β1 subunit mRNAs in TBCs. Pharmacological inhibition and single-cell RT-PCR studies demonstrated that type II and type III cells principally express tetrodotoxin (TTX)-sensitive Nav1.3 channels and that ~ 30% of type I cells express TTX-resistant Nav1.5 channels. The auxiliary β1 subunit that modulates gating kinetics was rarely detected in TBCs. As the β1 subunit co-expressed with an α subunit is known to accelerate the recovery from inactivation, it is likely that voltage-gated Na⁺ channels in TBCs may function without β subunits. Slow recovery from inactivation, especially in type III cells, may limit high-frequency firing in response to taste substances.

Cellular Diversity and Regeneration in Taste Buds

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

Lung epithelial endoplasmic reticulum and mitochondrial 3D ultrastructure: a new frontier in lung diseases

It has long been appreciated that the endoplasmic reticulum (ER) and mitochondria, organelles important for regular cell function and survival, also play key roles in pathogenesis of various lung diseases, including asthma, fibrosis, and infections. Alterations in processes regulated within these organelles, including but not limited to protein folding in the ER and oxidative phosphorylation in the mitochondria, are important in disease pathogenesis. In recent years it has also become increasingly apparent that organelle structure dictates function. It is now clear that organelles must maintain precise organization and localization for proper function. Newer microscopy capabilities have allowed the scientific community to reveal, via 3D imaging, that the structure of these organelles and their interactions with each other are a main component of regulating function and, therefore, effects on the disease state. In this review, we will examine how 3D imaging through techniques could allow advancements in knowledge of how the ER and mitochondria function and the roles they may play in lung epithelia in progression of lung disease.

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

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

Is there a role for GABA in peripheral taste processing

In the peripheral neurons and circuits for hearing, balance, touch and pain, GABA plays diverse and important roles. In some cases, GABA is an essential player in the maintenance of sensory receptors and afferent neurons. In other instances, GABA modulates the sensory signal before it reaches CNS neurons. And in yet other instances, tonic GABA-mediated signals set the resting tone and excitability of afferent neurons. GABA_A receptors are present on gustatory afferent neurons that carry taste signals from taste buds to central circuits in the brainstem. Yet, the functional significance of these receptors is unexplored. Here, I outline some of the roles of GABA in other peripheral sensory systems. I then consider whether similar functions may be ascribed to GABA signaling in the taste periphery.

Sour taste: receptors, cells and circuits

Sour taste, which is evoked by low pH, is one of the original four fundamental taste qualities, recognized as a distinct taste sensation for centuries, and universally aversive across diverse species. It is generally assumed to have evolved for detection of acids in unripe fruit and spoiled food. But despite decades of study, only recently have the receptor, the neurotransmitter, and the circuits for sour taste been identified. In this review, we describe studies leading up to the identification of the sour receptor as OTOP1, an ion channel that is selectively permeable to protons. We also describe advances in our understanding of how information is transmitted from the taste receptor cells to gustatory neurons, leading to behavioral aversion to acids.

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 Receptor Signaling

All organisms have the ability to detect chemicals in the environment, which likely evolved out of organisms' needs to detect food sources and avoid potentially harmful compounds. The taste system detects chemicals and is used to determine whether potential food items will be ingested or rejected. The sense of taste detects five known taste qualities: bitter, sweet, salty, sour, and umami, which is the detection of amino acids, specifically glutamate. These different taste qualities encompass a wide variety of chemicals that differ in their structure and as a result, the peripheral taste utilizes numerous and diverse mechanisms to detect these stimuli. In this chapter, we will summarize what is currently known about the signaling mechanisms used by taste cells to transduce stimulus signals.

N-cadherin localization in taste buds of mouse circumvallate papillae

Taste buds, the receptor organs for taste, contain 50-100 taste bud cells. Although these cells undergo continuous turnover, the structural and functional integrity of taste buds is maintained. The molecular mechanisms by which synaptic connectivity between taste buds and afferent fibers is formed and maintained remain ambiguous. In the present study, we examined the localization of N-cadherin in the taste buds of the mouse circumvallate papillae because N-cadherin, one of the classical cadherins, is important for the formation and maintenance of synapses. At the light microscopic level, N-cadherin was predominantly detected in type II cells and nerve fibers in the connective tissues in and around the vallate papillae. At the ultrastructural level, N-cadherin immunoreactivity appears along the cell membrane and in the intracellular vesicles of type II cells. N-cadherin immunoreactivity also is evident in the membranes of afferent terminals at the contact sites to N-cadherin-positive type II cells. At channel type synapses between type II cells and nerve fibers, N-cadherin is present surrounding, but not within, the presumed neurotransmitter release zone, identified by large mitochondria apposed to the taste cells. The present results suggest that N-cadherin is important for the formation or maintenance of type II cell afferent synapses in taste buds.

Age-related electrophysiological changes in mouse taste receptor cells

NEW FINDINGS

What is the central question of this study? Loss of taste or inability to distinguish between different tastes progresses with age. The purpose was to evaluate the age-dependent changes in taste by studying the electrophysiological properties of taste receptor cells. What is the main finding and its importance? Ageing decreased the voltage-gated Na⁺ and K⁺ current densities of type III cells (sour and/or salt receptor cells) but did not affect the current densities in type II cells. At the peripheral levels, the excitability of type III cells was reduced due to ageing, which may affect the signal transduction to taste nerves.

ABSTRACT

The loss of taste due to normal ageing in mammals is assumed to be caused by the ageing of taste receptor cells. We examined the electrophysiological properties of taste receptor cells in the fungiform taste buds of ∼20-month-old mice in situ and subsequently identified their cell types with immunological markers: the inositol 1,4,5-trisphosphate (IP₃ ) receptor (IP₃ R3) for type II cells and a SNARE protein, synaptosomal-associated protein 25 (SNAP-25), for type III cells. Other cells are referred to as non-immunoreactive cells (non-IRCs). Cell types of some cells that could not be identified using cell-type markers were identified based on the electrophysiological feature of the respective cell types. All cell types generated action potentials and a variety of voltage-gated currents. The type II cells mainly expressed tetraethylammonium (TEA)-insensitive and slowly activating outwardly rectifying currents and generated tail currents in repolarization. In contrast, the type III cells expressed TEA-sensitive and faster activating K⁺ currents and did not generate tail currents. These cell type-specific characteristics of voltage-gated currents in ∼20-month-old mice were similar to their respective cell types in ∼2-month-old mice. Also, we showed an age-dependent decrease in Na⁺ and K⁺ current densities in type III cells and an age-dependent increase in outwardly rectifying current density in non-IRCs. Ageing did not affect the voltage-gated current densities in type II cells. The decreased Na⁺ and K⁺ current densities, i.e. the decreased excitability of type III cells, due to ageing may affect the signal transduction to taste nerves.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

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.

Peripheral and central mechanisms of cough hypersensitivity

Chronic cough is a difficult to treat symptom of many respiratory and some non-respiratory diseases, indicating that varied pathologies can underpin the development of chronic cough. However, clinically and experimentally it has been useful to collate these different pathological processes into the single unifying concept of cough hypersensitivity. Cough hypersensitivity syndrome is reflected by troublesome cough often precipitated by levels of stimuli that ordinarily don't cause cough in healthy people, and this appears to be a hallmark feature in many patients with chronic cough. Accordingly, a strong argument has emerged that changes in the excitability and/or normal regulation of the peripheral and central neural circuits responsible for cough are instrumental in establishing cough hypersensitivity and for causing excessive cough in disease. In this review, we explore the current peripheral and central neural mechanisms that are believed to be involved in altered cough sensitivity and present possible links to the mechanism of action of novel therapies that are currently undergoing clinical trials for chronic cough.

Cryo-electron microscopy structure of CLHM1 ion channel from Caenorhabditis elegans

Calcium homeostasis modulators (CALHMs/CLHMs) comprise a family of pore-forming protein complexes assembling into voltage-gated, Ca2+ -sensitive, nonselective channels. These complexes contain an ion-conduction pore sufficiently wide to permit the passing of ATP molecules serving as neurotransmitters. While their function and structure information is accumulating, the precise mechanisms of these channel complexes remain to be full understood. Here, we present the structure of the Caenorhabditis elegans CLHM1 channel in its open state solved through single-particle cryo-electron microscopy at 3.7-Å resolution. The transmembrane region of the channel structure of the dominant class shows an assembly of 10-fold rotational symmetry in one layer, and its cytoplasmic region is involved in additional twofold symmetrical packing in a tail-to-tail manner. Furthermore, we identified a series of amino acid residues critical for the regulation of CeCLHM1 channel using functional assays, electrophysiological analyses as well as structural-based analysis. Our structure and function analyses provide new insights into the mechanisms of CALHM channels.

COVID-19 and the Chemical Senses: Supporting Players Take Center Stage

The main neurological manifestation of COVID-19 is loss of smell or taste. The high incidence of smell loss without significant rhinorrhea or nasal congestion suggests that SARS-CoV-2 targets the chemical senses through mechanisms distinct from those used by endemic coronaviruses or other common cold-causing agents. Here we review recently developed hypotheses about how SARS-CoV-2 might alter the cells and circuits involved in chemosensory processing and thereby change perception. Given our limited understanding of SARS-CoV-2 pathogenesis, we propose future experiments to elucidate disease mechanisms and highlight the relevance of this ongoing work to understanding how the virus might alter brain function more broadly.

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.

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.

Function, Innervation, and Neurotransmitter Signaling in Mice Lacking Type-II Taste Cells

The Skn-1a transcription factor (Pou2f3) is required for Type II taste cell differentiation in taste buds. Taste buds in Skn-1a^-/- mice lack Type II taste cells but have a concomitant expansion of Type III cells, providing an ideal model to determine the relative role of taste cell types in response specificity. We confirmed that chorda tympani responses to sweet, bitter, and umami stimuli were greatly reduced in the knock-outs (KOs) compared with wild-type (WT) littermates. Skn-1a^-/- mice also had reductions to NaCl that were partially amiloride-insensitive, suggesting that both Type II and Type III cells contribute to amiloride-insensitive salt detection in anterior tongue. We also confirmed that responses to sour stimuli are equivalent in the KOs, despite the large increase in the number of Type III taste cells. To examine their innervation, we crossed the Htr3a-GFP (5-HT_3A-GFP) reporter mouse with the Skn-1a^-/- mice and examined geniculate ganglion neurons for GFP expression and responses to 5-HT. We found no change in the number of 5-HT_3A-expressing neurons with KO of Skn-1a. Calcium imaging showed that only 5-HT_3A-expressing neurons respond to exogenous 5-HT, while most neurons respond to ATP, similar to WT mice. Interestingly, despite loss of all Type II cells, the P2X3 antagonist AF353 blocked all chorda tympani responses. These data collectively raise questions pertaining the source of ATP signaling in the absence of Type II taste cells and whether the additional Type III cells are innervated by fibers that would have normally innervated Type II cells.

Function, Innervation, and Neurotransmitter Signaling in Mice Lacking Type-II Taste Cells

The Skn-1a transcription factor (Pou2f3) is required for Type II taste cell differentiation in taste buds. Taste buds in Skn-1a ^-/- mice lack Type II taste cells but have a concomitant expansion of Type III cells, providing an ideal model to determine the relative role of taste cell types in response specificity. We confirmed that chorda tympani responses to sweet, bitter, and umami stimuli were greatly reduced in the knock-outs (KOs) compared with wild-type (WT) littermates. Skn-1a ^-/- mice also had reductions to NaCl that were partially amiloride-insensitive, suggesting that both Type II and Type III cells contribute to amiloride-insensitive salt detection in anterior tongue. We also confirmed that responses to sour stimuli are equivalent in the KOs, despite the large increase in the number of Type III taste cells. To examine their innervation, we crossed the Htr3a-GFP (5-HT_3A-GFP) reporter mouse with the Skn-1a ^-/- mice and examined geniculate ganglion neurons for GFP expression and responses to 5-HT. We found no change in the number of 5-HT_3A-expressing neurons with KO of Skn-1a Calcium imaging showed that only 5-HT_3A-expressing neurons respond to exogenous 5-HT, while most neurons respond to ATP, similar to WT mice. Interestingly, despite loss of all Type II cells, the P2X3 antagonist AF353 blocked all chorda tympani responses. These data collectively raise questions pertaining the source of ATP signaling in the absence of Type II taste cells and whether the additional Type III cells are innervated by fibers that would have normally innervated Type II cells.