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

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.

Cryo-EM structures of calcium homeostasis modulator channels in diverse oligomeric assemblies

Calcium homeostasis modulator (CALHM) family proteins are Ca²⁺-regulated adenosine triphosphate (ATP)-release channels involved in neural functions including neurotransmission in gustation. Here, we present the cryo-electron microscopy (EM) structures of killifish CALHM1, human CALHM2, and Caenorhabditis elegans CLHM-1 at resolutions of 2.66, 3.4, and 3.6 Å, respectively. The CALHM1 octamer structure reveals that the N-terminal helix forms the constriction site at the channel pore in the open state and modulates the ATP conductance. The CALHM2 undecamer and CLHM-1 nonamer structures show the different oligomeric stoichiometries among CALHM homologs. We further report the cryo-EM structures of the chimeric construct, revealing that the intersubunit interactions at the transmembrane domain (TMD) and the TMD-intracellular domain linker define the oligomeric stoichiometry. These findings advance our understanding of the ATP conduction and oligomerization mechanisms of CALHM channels.

Cryo-EM structure of the calcium homeostasis modulator 1 channel

Calcium homeostasis modulator 1 (CALHM1) is a voltage-gated ATP release channel that plays an important role in neural gustatory signaling and the pathogenesis of Alzheimer's disease. Here, we present a cryo-electron microscopy structure of full-length Ca²⁺-free CALHM1 from Danio rerio at an overall resolution of 3.1 Å. Our structure reveals an octameric architecture with a wide pore diameter of ~20 Å, presumably representing the active conformation. The overall structure is substantially different from that of the isoform CALHM2, which forms both undecameric hemichannels and gap junctions. The N-terminal small helix folds back to the pore and forms an antiparallel interaction with transmembrane helix 1. Structural analysis revealed that the extracellular loop 1 region within the dimer interface may contribute to oligomeric assembly. A positive potential belt inside the pore was identified that may modulate ion permeation. Our structure offers insights into the assembly and gating mechanism of the CALHM1 channel.

A missense variant in specificity protein 6 (SP6) is associated with amelogenesis imperfecta

Amelogenesis is the process of enamel formation. For amelogenesis to proceed, the cells of the inner enamel epithelium (IEE) must first proliferate and then differentiate into the enamel-producing ameloblasts. Amelogenesis imperfecta (AI) is a heterogeneous group of genetic conditions that result in defective or absent tooth enamel. We identified a 2 bp variant c.817_818GC>AA in SP6, the gene encoding the SP6 transcription factor, in a Caucasian family with autosomal dominant hypoplastic AI. The resulting missense protein change, p.(Ala273Lys), is predicted to alter a DNA-binding residue in the first of three zinc fingers. SP6 has been shown to be crucial to both proliferation of the IEE and to its differentiation into ameloblasts. SP6 has also been implicated as an AI candidate gene through its study in rodent models. We investigated the effect of the missense variant in SP6 (p.(Ala273Lys)) using surface plasmon resonance protein-DNA binding studies. We identified a potential SP6 binding motif in the AMBN proximal promoter sequence and showed that wild-type (WT) SP6 binds more strongly to it than the mutant protein. We hypothesize that SP6 variants may be a very rare cause of AI due to the critical roles of SP6 in development and that the relatively mild effect of the missense variant identified in this study is sufficient to affect amelogenesis causing AI, but not so severe as to be incompatible with life. We suggest that current AI cohorts, both with autosomal recessive and dominant disease, be screened for SP6 variants.

Cell-type-independent expression of inwardly rectifying potassium currents in mouse fungiform taste bud cells

Inwardly rectifying potassium (Kir) channels play key roles in functions, including maintaining the resting membrane potential and regulating the action potential duration in excitable cells. Using in situ whole-cell recordings, we investigated Kir currents in mouse fungiform taste bud cells (TBCs) and immunologically identified the cell types (type I-III) expressing these currents. We demonstrated that Kir currents occur in a cell-type-independent manner. The activation potentials we measured were -80 to -90 mV, and the magnitude of the currents increased as the membrane potentials decreased, irrespective of the cell types. The maximum current densities at -120 mV showed no significant differences among cell types (p>0.05, one-way ANOVA). The density of Kir currents was not correlated with the density of either transient inward currents or outwardly rectifying currents, although there was significant correlation between transient inward and outwardly rectifying current densities (p<0.05, test for no correlation). RT-PCR studies employing total RNA extracted from peeled lingual epithelia detected mRNAs for Kir1, Kir2, Kir4, Kir6, and Kir7 families. These findings indicate that TBCs express several types of Kir channels functionally, which may contribute to regulation of the resting membrane potential and signal transduction of taste.

Cryo-EM structures and functional properties of CALHM channels of the human placenta

The transport of substances across the placenta is essential for the development of the fetus. Here, we were interested in the role of channels of the calcium homeostasis modulator (CALHM) family in the human placenta. By transcript analysis, we found the paralogs CALHM2, 4, and 6 to be highly expressed in this organ and upregulated during trophoblast differentiation. Based on electrophysiology, we observed that activation of these paralogs differs from the voltage- and calcium-gated channel CALHM1. Cryo-EM structures of CALHM4 display decameric and undecameric assemblies with large cylindrical pore, while in CALHM6 a conformational change has converted the pore shape into a conus that narrows at the intracellular side, thus describing distinct functional states of the channel. The pore geometry alters the distribution of lipids, which occupy the cylindrical pore of CALHM4 in a bilayer-like arrangement whereas they have redistributed in the conical pore of CALHM6 with potential functional consequences.

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.

Bitter taste receptors stimulate phagocytosis in human macrophages through calcium, nitric oxide, and cyclic-GMP signaling

Bitter taste receptors (T2Rs) are GPCRs involved in detection of bitter compounds by type 2 taste cells of the tongue, but are also expressed in other tissues throughout the body, including the airways, gastrointestinal tract, and brain. These T2Rs can be activated by several bacterial products and regulate innate immune responses in several cell types. Expression of T2Rs has been demonstrated in immune cells like neutrophils; however, the molecular details of their signaling are unknown. We examined mechanisms of T2R signaling in primary human monocyte-derived unprimed (M0) macrophages (M[Formula: see text]s) using live cell imaging techniques. Known bitter compounds and bacterial T2R agonists activated low-level calcium signals through a pertussis toxin (PTX)-sensitive, phospholipase C-dependent, and inositol trisphosphate receptor-dependent calcium release pathway. These calcium signals activated low-level nitric oxide (NO) production via endothelial and neuronal NO synthase (NOS) isoforms. NO production increased cellular cGMP and enhanced acute phagocytosis ~ threefold over 30-60 min via protein kinase G. In parallel with calcium elevation, T2R activation lowered cAMP, also through a PTX-sensitive pathway. The cAMP decrease also contributed to enhanced phagocytosis. Moreover, a co-culture model with airway epithelial cells demonstrated that NO produced by epithelial cells can also acutely enhance M[Formula: see text] phagocytosis. Together, these data define M[Formula: see text] T2R signal transduction and support an immune recognition role for T2Rs in M[Formula: see text] cell physiology.

Regulation of immune responses by tuft cells

Tuft cells are rare, secretory epithelial cells that generated scant immunological interest until contemporaneous reports in 2016 linked tuft cells with type 2 immunity in the small intestine. Tuft cells have the capacity to produce an unusual spectrum of biological effector molecules, including IL-25, eicosanoids implicated in allergy (such as cysteinyl leukotrienes and prostaglandin D₂) and the neurotransmitter acetylcholine. In most cases, the extracellular signals controlling tuft cell effector function are unknown, but signal transduction is thought to proceed via canonical, G protein-coupled receptor-dependent pathways involving components of the signalling pathway used by type II taste bud cells to sense sweet, bitter and umami compounds. Tuft cells are ideally positioned as chemosensory sentinels that can detect and relay information from diverse luminal substances via what appear to be stereotyped outputs to initiate both positive and aversive responses through populations of immune and neuronal cells. Despite recent insights, numerous questions remain regarding tuft cell lineage, diversity and effector mechanisms and how tuft cells interface with the immunological niche in the tissues where they reside.

Structure and assembly of calcium homeostasis modulator proteins

The biological membranes of many cell types contain large-pore channels through which a wide variety of ions and metabolites permeate. Examples include connexin, innexin and pannexin, which form gap junctions and/or bona fide cell surface channels. The most recently identified large-pore channels are the calcium homeostasis modulators (CALHMs), through which ions and ATP permeate in a voltage-dependent manner to control neuronal excitability, taste signaling and pathologies of depression and Alzheimer's disease. Despite such critical biological roles, the structures and patterns of their oligomeric assembly remain unclear. Here, we reveal the structures of two CALHMs, chicken CALHM1 and human CALHM2, by single-particle cryo-electron microscopy (cryo-EM), which show novel assembly of the four transmembrane helices into channels of octamers and undecamers, respectively. Furthermore, molecular dynamics simulations suggest that lipids can favorably assemble into a bilayer within the larger CALHM2 pore, but not within CALHM1, demonstrating the potential correlation between pore size, lipid accommodation and channel activity.

A subset of taste receptor cells express biocytin-permeable channels activated by reducing extracellular Ca2+ concentration

Taste receptor cells (type II cells) transmit taste information to taste nerve fibres via ATP-permeable channels, including calcium homeostasis modulator (CALHM), connexin and/or pannexin1 channels, via the paracrine release of adenosine triphosphate (ATP) as a predominant transmitter. In the present study, we demonstrate that extracellular Ca²⁺ -dependent biocytin-permeable channels are present in a subset of type II cells in mouse fungiform taste buds using biocytin uptake, immunohistochemistry and in situ whole-cell recordings. Type II cells were labelled with biocytin in an extracellular Ca²⁺ concentration ([Ca²⁺ ]_out )-sensitive manner. We found that the ratio of biocytin-labelled type II cells to type II cells per taste bud was approximately 20% in 2 mM Ca²⁺ saline, and this ratio increased to approximately 50% in nominally Ca²⁺ -free saline. The addition of 300 µM GdCl₃ , which inhibits various channels including CALHM1 channels, significantly inhibited biocytin labelling in nominally Ca²⁺ -free saline, whereas the addition of 20 µM ruthenium red did not. Moreover, Cs⁺ -insensitive currents increased in nominally Ca²⁺ -free saline in approximately 40% of type II cells. These increased currents appeared at a potential of above -35 mV, reversed at approximately +10 mV and increased with depolarization. These results suggest that biocytin labels type II cells via ion channels activated by [Ca²⁺ ]_out reduction, probably "CALHM-like" channels, on the basolateral membrane and that taste receptor cells can be categorized into two groups based on differences in the expression levels of [Ca²⁺ ]_out -dependent biocytin-permeable channels. These data indicate electrophysiological and pharmacologically relevant properties of biocytin-permeable channels and suggest their contributions to taste signal transduction.

New frontiers in probing the dynamics of purinergic transmitters in vivo

Purinergic transmitters such as adenosine, ADP, ATP, UTP, and UDP-glucose play important roles in a wide range of physiological processes, including the sleep-wake cycle, learning and memory, cardiovascular function, and the immune response. Moreover, impaired purinergic signaling has been implicated in various pathological conditions such as pain, migraine, epilepsy, and drug addiction. Examining the function of purinergic transmission in both health and disease requires direct, sensitive, non-invasive tools for monitoring structurally similar purinergic transmitters; ideally, these tools should have high spatial and temporal resolution in in vivo applications. Here, we review the recent progress with respect to the development and application of new methods for detecting purinergic transmitters, focusing on optical tools; in addition, we provide discussion regarding future perspectives.

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.

The structures and gating mechanism of human calcium homeostasis modulator 2

Calcium homeostasis modulators (CALHMs) are voltage-gated, Ca²⁺-inhibited nonselective ion channels that act as major ATP release channels, and have important roles in gustatory signalling and neuronal toxicity^1-3. Dysfunction of CALHMs has previously been linked to neurological disorders¹. Here we present cryo-electron microscopy structures of the human CALHM2 channel in the Ca²⁺-free active or open state and in the ruthenium red (RUR)-bound inhibited state, at resolutions up to 2.7 Å. Our work shows that purified CALHM2 channels form both gap junctions and undecameric hemichannels. The protomer shows a mirrored arrangement of the transmembrane domains (helices S1-S4) relative to other channels with a similar topology, such as connexins, innexins and volume-regulated anion channels^4-8. Upon binding to RUR, we observed a contracted pore with notable conformational changes of the pore-lining helix S1, which swings nearly 60° towards the pore axis from a vertical to a lifted position. We propose a two-section gating mechanism in which the S1 helix coarsely adjusts, and the N-terminal helix fine-tunes, the pore size. We identified a RUR-binding site near helix S1 that may stabilize this helix in the lifted conformation, giving rise to channel inhibition. Our work elaborates on the principles of CALHM2 channel architecture and symmetry, and the mechanism that underlies channel inhibition.

Therapeutic potential of ectopic olfactory and taste receptors

Olfactory and taste receptors are expressed primarily in the nasal olfactory epithelium and gustatory taste bud cells, where they transmit real-time sensory signals to the brain. However, they are also expressed in multiple extra-nasal and extra-oral tissues, being implicated in diverse biological processes including sperm chemotaxis, muscle regeneration, bronchoconstriction and bronchodilatation, inflammation, appetite regulation and energy metabolism. Elucidation of the physiological roles of these ectopic receptors is revealing potential therapeutic and diagnostic applications in conditions including wounds, hair loss, asthma, obesity and cancers. This Review outlines current understanding of the diverse functions of ectopic olfactory and taste receptors and assesses their potential to be therapeutically exploited.

Three-dimensional reconstructions of mouse circumvallate taste buds using serial blockface scanning electron microscopy: I. Cell types and the apical region of the taste bud

Taste buds comprise four types of taste cells: three mature, elongate types, Types I-III; and basally situated, immature postmitotic type, Type IV cells. We employed serial blockface scanning electron microscopy to delineate the characteristics and interrelationships of the taste cells in the circumvallate papillae of adult mice. Type I cells have an indented, elongate nucleus with invaginations, folded plasma membrane, and multiple apical microvilli in the taste pore. Type I microvilli may be either restricted to the bottom of the pore or extend outward reaching midway up into the taste pore. Type II cells (aka receptor cells) possess a large round or oval nucleus, a single apical microvillus extending through the taste pore, and specialized "atypical" mitochondria at functional points of contact with nerve fibers. Type III cells (aka "synaptic cells") are elongate with an indented nucleus, possess a single, apical microvillus extending through the taste pore, and are characterized by a small accumulation of synaptic vesicles at points of contact with nerve fibers. About one-quarter of Type III cells also exhibit an atypical mitochondrion near the presynaptic vesicle clusters at the synapse. Type IV cells (nonproliferative "basal cells") have a nucleus in the lower quarter of the taste bud and a foot process extending to the basement membrane often contacting nerve processes along the way. In murine circumvallate taste buds, Type I cells represent just over 50% of the population, whereas Types II, III, and IV (basal cells) represent 19, 15, and 14%, respectively.

Taste bud formation depends on taste nerves

It has been known for more than a century that, in adult vertebrates, the maintenance of taste buds depends on their afferent nerves. However, the initial formation of taste buds is proposed to be nerve-independent in amphibians, and evidence to the contrary in mammals has been endlessly debated, mostly due to indirect and incomplete means to impede innervation during the protracted perinatal period of taste bud differentiation. Here, by genetically ablating, in mice, all somatic (i.e. touch) or visceral (i.e. taste) neurons for the oral cavity, we show that the latter but not the former are absolutely required for the proper formation of their target organs, the taste buds.

Opening a "Wide" Window onto Taste Signal Transmission

Taste bud cells for sweet, umami, and bitter transmit sensory signals without a synapse. A study by Ma et al. (2018) finds a key ATP-permeable pore-forming subunit required for rapid neurotransmission from the tongue to secondary taste neurons.

Bitter tastants and artificial sweeteners activate a subset of epithelial cells in acute tissue slices of the rat trachea

Bitter and sweet receptors (T2Rs and T1Rs) are expressed in many extra-oral tissues including upper and lower airways. To investigate if bitter tastants and artificial sweeteners could activate physiological responses in tracheal epithelial cells we performed confocal Ca²⁺ imaging recordings on acute tracheal slices. We stimulated the cells with denatonium benzoate, a T2R agonist, and with the artificial sweeteners sucralose, saccharin and acesulfame-K. To test cell viability we measured responses to ATP. We found that 39% of the epithelial cells responding to ATP also responded to bitter stimulation with denatonium benzoate. Moreover, artificial sweeteners activated different percentages of the cells, ranging from 5% for sucralose to 26% for saccharin, and 27% for acesulfame-K. By using carbenoxolone, a gap junction blocker, we excluded that responses were mainly mediated by Ca²⁺ waves through cell-to-cell junctions. Pharmacological experiments showed that both denatonium and artificial sweeteners induced a PLC-mediated release of Ca²⁺ from internal stores. In addition, bitter tastants and artificial sweeteners activated a partially overlapping subpopulation of tracheal epithelial cells. Our results provide new evidence that a subset of ATP-responsive tracheal epithelial cells from rat are activated by both bitter tastants and artificial sweeteners.

CALHM1/CALHM3 channel is intrinsically sorted to the basolateral membrane of epithelial cells including taste cells

The CALHM1/CALHM3 channel in the basolateral membrane of polarized taste cells mediates neurotransmitter release. However, mechanisms regulating its localization remain unexplored. Here, we identified CALHM1/CALHM3 in the basolateral membrane of type II taste cells in discrete puncta localized close to afferent nerve fibers. As in taste cells, CALHM1/CALHM3 was present in the basolateral membrane of model epithelia, although it was distributed throughout the membrane and did not show accumulation in puncta. We identified canonical basolateral sorting signals in CALHM1 and CALHM3: tyrosine-based and dileucine motifs. However, basolateral sorting remained intact in mutated channels lacking those signals, suggesting that non-canonical signals reside elsewhere. Our study demonstrates intrinsic basolateral sorting of CALHM channels in polarized cells, and provides mechanistic insights.

Taste Receptors: New Players in Sperm Biology

Taste receptors were first described as sensory receptors located on the tongue, where they are expressed in small clusters of specialized epithelial cells. However, more studies were published in recent years pointing to an expression of these proteins not only in the oral cavity but throughout the body and thus to a physiological role beyond the tongue. The recent observation that taste receptors and components of the coupled taste transduction cascade are also expressed during the different phases of spermatogenesis as well as in mature spermatozoa from mouse to humans and the overlap between the ligand spectrum of taste receptors with compounds in the male and female reproductive organs makes it reasonable to assume that sperm "taste" these different cues in their natural microenvironments. This assumption is assisted by the recent observations of a reproductive phenotype of different mouse lines carrying a targeted deletion of a taste receptor gene as well as the finding of a significant correlation between human male infertility and some polymorphisms in taste receptors genes. In this review, we depict recent findings on the role of taste receptors in male fertility, especially focusing on their possible involvement in mechanisms underlying spermatogenesis and post testicular sperm maturation. We also highlight the impact of genetic deletions of taste receptors, as well as their polymorphisms on male reproduction.

Spilanthol Enhances Sensitivity to Sodium in Mouse Taste Bud Cells

Overconsumption of NaCl has been linked to increased hypertension-related morbidity. Compounds that can enhance NaCl responses in taste cells could help reduce human NaCl consumption without sacrificing perceived saltiness. Spilanthol is an unsaturated alkylamide isolated from the Jambu plant (Acmella oleracea) that can induce tingling, pungency, and numbing in the mouth. Structurally similar fatty acid amides, such as sanshool, elicit numbing and tingling sensations by inhibiting 2-pore-domain potassium leak channels on trigeminal sensory neurons. Even when insufficient to induce action potential firing, leak current inhibition causes depolarization and increased membrane resistance, which combine to make cells more sensitive to subsequent depolarizing stimuli, such as NaCl. Using calcium imaging, we tested whether spilanthol alters sensitivity to NaCl in isolated circumvallate taste bud cells and trigeminal sensory neurons of mice (Mus musculus). Micromolar spilanthol elicited little to no response in taste bud cells or trigeminal neurons. These same perithreshold concentrations of spilanthol significantly enhanced responses to NaCl (140 and 200 mM) in taste bud cells. Trigeminal neurons, however, exhibited response enhancement only at the highest concentrations of NaCl and spilanthol tested. Using a combination of potassium depolarization, immunohistochemistry, and Trpm5-GFP and Tas1r3-GFP mice to characterize taste bud cells by type, we found spilanthol enhancement of NaCl responses most prevalent in NaCl-responsive type III cells, and commonly observed in NaCl-responsive type II cells. Our results indicate that spilanthol enhances NaCl responses in taste bud cells and point to a family of compounds that may have utility as salty taste enhancers.

A Pharmacological Perspective on the Study of Taste

The study of taste has been guided throughout much of its history by the conceptual framework of psychophysics, where the focus was on quantification of the subjective experience of the taste sensations. By the mid-20th century, data from physiologic studies had accumulated sufficiently to assemble a model for the function of receptors that must mediate the initial stimulus of tastant molecules in contact with the tongue. But the study of taste as a receptor-mediated event did not gain momentum until decades later when the actual receptor proteins and attendant signaling mechanisms were identified and localized to the highly specialized taste-responsive cells of the tongue. With those discoveries a new opportunity to examine taste as a function of receptor activity has come into focus. Pharmacology is the science designed specifically for the experimental interrogation and quantitative characterization of receptor function at all levels of inquiry from molecules to behavior. This review covers the history of some of the major concepts that have shaped thinking and experimental approaches to taste, the seminal discoveries that have led to elucidation of receptors for taste, and how applying principles of receptor pharmacology can enhance understanding of the mechanisms of taste physiology and perception.

Innate and acquired tolerance to bitter stimuli in mice

Tolerance to bitter foods and its potentiation by repetitive exposure are commonly experienced and potentially underlie the consumption of bitter foods, but it remains unknown whether permissive and adaptive responses are general phenomena for bitter-tasting substances or specific to certain substances, and they have not been rigorously studied in mice. Here, we investigated the effects of prolonged exposure to a bitter compound on both recognition and rejection behaviors to the same compound in mice. Paired measurements of rejection (RjT) and apparent recognition (aRcT) thresholds were conducted using brief-access two-bottle choice tests before and after taste aversion conditioning, respectively. First, RjT was much higher than aRcT for the bitter amino acids L-tryptophan and L-isoleucine, which mice taste daily in their food, indicating strong acceptance of those familiar stimuli within the concentration range between RjT and aRcT. Next, we tested five other structurally dissimilar bitter compounds, to which mice were naive at the beginning of experiments: denatonium benzoate, quinine-HCl, caffeine, salicin, and epigallocatechin gallate. RjT was moderately higher than aRcT for all the compounds tested, indicating the presence of innate acceptance to these various, unfamiliar bitter stimuli in mice. Lastly, a 3-week forced exposure increased RjT for all the bitter compounds except salicin, demonstrating that mice acquire tolerance to a broad array of bitter compounds after long-term exposure to them. Although the underlying mechanisms remain to be determined, our studies provide behavioral evidence of innate and acquired tolerance to various bitter stimuli in mice, suggesting its generality among bitterants.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

Extracellular nucleotides and nucleosides as signalling molecules

Extracellular nucleotides, mainly ATP, but also ADP, UTP, UDP and UDP-sugars, adenosine, and adenine base participate in the "purinergic signalling" pathway, an ubiquitous system of cell-to-cell communication. Fundamental pathophysiological processes such as tissue homeostasis, wound healing, neurodegeneration, immunity, inflammation and cancer are modulated by purinergic signalling. Nucleotides can be released from cells via unspecific or specific mechanisms. A non-regulated nucleotide release can occur from damaged or dying cells, whereas exocytotic granules, plasma membrane-derived microvesicles, membrane channels (connexins, pannexins, calcium homeostasis modulator (CALHM) channels and P2X7 receptor) or specific ATP binding cassette (ABC) transporters are involved in the controlled release. Four families of specific receptors, i.e. nucleotide P2X and P2Y receptors, adenosine P1 receptors, and the adenine-selective P0 receptor, and several ecto- nucleotidases are essential components of the "purinergic signalling" pathway. Thanks to the activity of ecto-nucleotidases, ATP (and possibly other nucleotides) are degraded into additional messenger molecules with specific action. The final biological effects depend on the type and amount of released nucleotides, their modification by ecto-nucleotidases, and their possible cellular re-uptake. Overall, these processes confer a remarkable level of selectivity and plasticity to purinergic signalling that makes this network one of the most relevant extracellular messenger systems in higher organisms.

The Proposal of Molecular Mechanisms of Weak Organic Acids Intake-Induced Improvement of Insulin Resistance in Diabetes Mellitus via Elevation of Interstitial Fluid pH

Blood contains powerful pH-buffering molecules such as hemoglobin (Hb) and albumin, while interstitial fluids have little pH-buffering molecules. Thus, even under metabolic disorder conditions except severe cases, arterial blood pH is kept constant within the normal range (7.35~7.45), but the interstitial fluid pH under metabolic disorder conditions becomes lower than the normal level. Insulin resistance is one of the most important key factors in pathogenesis of diabetes mellitus, nevertheless the molecular mechanism of insulin resistance occurrence is still unclear. Our studies indicate that lowered interstitial fluid pH occurs in diabetes mellitus, causing insulin resistance via reduction of the binding affinity of insulin to its receptor. Therefore, the key point for improvement of insulin resistance occurring in diabetes mellitus is development of methods or techniques elevating the lowered interstitial fluid pH. Intake of weak organic acids is found to improve the insulin resistance by elevating the lowered interstitial fluid pH in diabetes mellitus. One of the molecular mechanisms of the pH elevation is that: (1) the carboxyl group (R-COO-) but not H⁺ composing weak organic acids in foods is absorbed into the body, and (2) the absorbed the carboxyl group (R-COO-) behaves as a pH buffer material, elevating the interstitial fluid pH. On the other hand, high salt intake has been suggested to cause diabetes mellitus; however, the molecular mechanism is unclear. A possible mechanism of high salt intake-caused diabetes mellitus is proposed from a viewpoint of regulation of the interstitial fluid pH: high salt intake lowers the interstitial fluid pH via high production of H⁺ associated with ATP synthesis required for the Na⁺,K⁺-ATPase to extrude the high leveled intracellular Na⁺ caused by high salt intake. This review article introduces the molecular mechanism causing the lowered interstitial fluid pH and insulin resistance in diabetes mellitus, the improvement of insulin resistance via intake of weak organic acid-containing foods, and a proposal mechanism of high salt intake-caused diabetes mellitus.