Five canonical tastes, bitter, sweet, umami (amino acid), salty, and sour (acid), are detected by animals as diverse as fruit flies and humans, consistent with a near-universal drive to consume ...fundamental nutrients and to avoid toxins or other harmful compounds. Surprisingly, despite this strong conservation of basic taste qualities between vertebrates and invertebrates, the receptors and signaling mechanisms that mediate taste in each are highly divergent. The identification over the last two decades of receptors and other molecules that mediate taste has led to stunning advances in our understanding of the basic mechanisms of transduction and coding of information by the gustatory systems of vertebrates and invertebrates. In this Review, we discuss recent advances in taste research, mainly from the fly and mammalian systems, and we highlight principles that are common across species, despite stark differences in receptor types.
In this Review, Liman, Zhang, and Montell discuss recent advances in taste research, mainly from the fly and mammalian systems, highlighting principles that are common across species despite stark differences in receptor types.
Detection of NaCl by the gustatory system is fundamental for salt intake and tissue homeostasis. Yet, signal transduction mechanisms for salty taste have remained obscure. In this issue of Neuron, ...Nomura et al. (2020) report that the epithelial sodium channel ENaC, which serves as the salty receptor, is co-expressed with the voltage-activated ATP release channel CALHM1/3 in a subset of taste cells and that these cells mediate amiloride-sensitive salty taste.
Detection of NaCl by the gustatory system is fundamental for salt intake and tissue homeostasis. Yet, signal transduction mechanisms for salty taste have remained obscure. In this issue of Neuron, Nomura et al. (2020) report that the epithelial sodium channel ENaC, which serves as the salty receptor, is co-expressed with the voltage-activated ATP release channel CALHM1/3 in a subset of taste cells and that these cells mediate amiloride-sensitive salty taste.
The ability to visualize endogenous proteins in living neurons provides a powerful means to interrogate neuronal structure and function. Here we generate recombinant antibody-like proteins, termed ...Fibronectin intrabodies generated with mRNA display (FingRs), that bind endogenous neuronal proteins PSD-95 and Gephyrin with high affinity and that, when fused to GFP, allow excitatory and inhibitory synapses to be visualized in living neurons. Design of the FingR incorporates a transcriptional regulation system that ties FingR expression to the level of the target and reduces background fluorescence. In dissociated neurons and brain slices, FingRs generated against PSD-95 and Gephyrin did not affect the expression patterns of their endogenous target proteins or the number or strength of synapses. Together, our data indicate that PSD-95 and Gephyrin FingRs can report the localization and amount of endogenous synaptic proteins in living neurons and thus may be used to study changes in synaptic strength in vivo.
•Recombinant intrabodies (FingRs) generated with mRNA display bind PSD-95 or Gephyrin•PSD-95 and Gephyrin FingRs accurately label endogenous targets in living neurons•Transcriptional control matches expression of FingRs to targets, reducing background•PSD-95 and Gephyrin FingRs do not affect neuronal morphology or function
New methods are needed to monitor synaptic structure in vivo in real time. Gross et al. report the generation of genetically encoded intrabodies (FingRs) that bind Gephryn and PSD-95 and can be used to fluorescently label inhibitory and excitatory synapses.
Ion channels form the basis for cellular electrical signaling. Despite the scores of genetically identified ion channels selective for other monatomic ions, only one type of proton-selective ion ...channel has been found in eukaryotic cells. By comparative transcriptome analysis of mouse taste receptor cells, we identified Otopetrin1 (OTOP1), a protein required for development of gravity-sensing otoconia in the vestibular system, as forming a proton-selective ion channel. We found that murine OTOP1 is enriched in acid-detecting taste receptor cells and is required for their zinc-sensitive proton conductance. Two related murine genes,
and
, and a
ortholog also encode proton channels. Evolutionary conservation of the gene family and its widespread tissue distribution suggest a broad role for proton channels in physiology and pathophysiology.
Receptors for bitter, sugar, and other tastes have been identified in the fruit fly
, while a broadly tuned receptor for the taste of acid has been elusive. Previous work showed that such a receptor ...was unlikely to be encoded by a gene within one of the two major families of taste receptors in
, the "gustatory receptors" and "ionotropic receptors." Here, to identify the acid taste receptor, we tested the contributions of genes encoding proteins distantly related to the mammalian Otopertrin1 (OTOP1) proton channel that functions as a sour receptor in mice. RNA interference (RNAi) knockdown or mutation by CRISPR/Cas9 of one of the genes,
(
), but not of the others (
or
) severely impaired the behavioral rejection to a sweet solution laced with high levels of HCl or carboxylic acids and greatly reduced acid-induced action potentials measured from taste hairs. An isoform of
that we isolated from the proboscis was sufficient to restore behavioral sensitivity and acid-induced action potential firing in
mutant flies. At lower concentrations, HCl was attractive to the flies, and this attraction was abolished in the
mutant. Cell type-specific rescue experiments showed that
functions in distinct subsets of gustatory receptor neurons for repulsion and attraction to high and low levels of protons, respectively. This work highlights a functional conservation of a sensory receptor in flies and mammals and shows that the same receptor can function in both appetitive and repulsive behaviors.
Abstract
Ammonium (NH
4
+
), a breakdown product of amino acids that can be toxic at high levels, is detected by taste systems of organisms ranging from
C. elegans
to humans and has been used for ...decades in vertebrate taste research. Here we report that OTOP1, a proton-selective ion channel expressed in sour (Type III) taste receptor cells (TRCs), functions as sensor for ammonium chloride (NH
4
Cl). Extracellular NH
4
Cl evoked large dose-dependent inward currents in HEK-293 cells expressing murine OTOP1 (mOTOP1), human OTOP1 and other species variants of OTOP1, that correlated with its ability to alkalinize the cell cytosol. Mutation of a conserved intracellular arginine residue (R292) in the mOTOP1 tm 6-tm 7 linker specifically decreased responses to NH
4
Cl relative to acid stimuli. Taste responses to NH
4
Cl measured from isolated Type III TRCs, or gustatory nerves were strongly attenuated or eliminated in an
Otop1
−/−
mouse strain. Behavioral aversion of mice to NH
4
Cl, reduced in
Skn-1a
−/−
mice lacking Type II TRCs, was entirely abolished in a double knockout with
Otop1
. These data together reveal an unexpected role for the proton channel OTOP1 in mediating a major component of the taste of NH
4
Cl and a previously undescribed channel activation mechanism.
The sense of taste allows animals to sample chemicals in the environment prior to ingestion. Of the five basic tastes, sour, the taste of acids, had remained among the most mysterious. Acids are ...detected by type III taste receptor cells (TRCs), located in taste buds across the tongue and palate epithelium. The first step in sour taste transduction is believed to be entry of protons into the cell cytosol, which leads to cytosolic acidification and the generation of action potentials. The proton-selective ion channel Otop1 is expressed in type III TRCs and is a candidate sour receptor. Here, we tested the contribution of Otop1 to taste cell and gustatory nerve responses to acids in mice in which Otop1 was genetically inactivated (Otop1-KO mice). We first show that Otop1 is required for the inward proton current in type III TRCs from different parts of the tongue that are otherwise molecularly heterogeneous. We next show that in type III TRCs from Otop1-KO mice, intracellular pH does not track with extracellular pH and that moderately acidic stimuli do not elicit trains of action potentials, as they do in type III TRCs from wild-type mice. Moreover, gustatory nerve responses in Otop1-KO mice were severely and selectively attenuated for acidic stimuli, including citric acid and HCl. These results establish that the Otop1 proton channel plays a critical role in acid detection in the mouse gustatory system, evidence that it is a bona fide sour taste receptor.
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•Genetic inactivation of Otop1 in mice eliminates proton currents in taste cells•Otop1-KO mice have severely attenuated cellular responses to acids•Otop1-KO mice have severely attenuated gustatory nerve responses to acids•Otop1 is a sour taste receptor
Otop1 forms a proton channel expressed in taste receptor cells that detect sour stimuli. Teng et al. show that mice in which Otop1 is genetically inactivated have severely diminished cellular and gustatory nerve response to acids, indicating that Otop1 functions as a sour taste receptor.
Five tastes have been identified, each of which is transduced by a separate set of taste cells. Of these sour, which is associated with acid stimuli, is the least understood. Genetic ablation ...experiments have established that sour is detected by a subset of taste cells that express the TRP channel PKD2L1 and its partner PKD1L3, however the mechanisms by which this subset of cells detects acids remain unclear. Previous efforts to understand sour taste transduction have been hindered because sour responsive cells represent only a small fraction of cells in a taste bud, and numerous ion channels with no role in sour sensing are sensitive to acidic pH. To identify acid-sensitive conductances unique to sour cells, we created genetically modified mice in which sour cells were marked by expression of YFP under the control of the PKD2L1 promoter. To measure responses to sour stimuli we developed a method in which suction electrode recording is combined with UV photolysis of NPE-caged proton. Using these methods, we report that responses to sour stimuli are not mediated by Na⁺ permeable channels as previously thought, but instead are mediated by a proton conductance specific to PKD2L1-expressing taste cells. This conductance is sufficient to drive action potential firing in response to acid stimuli, is enriched in the apical membrane of PKD2L1-expressing taste cells and is not affected by targeted deletion of the PKD1L3 gene. We conclude that, during sour transduction, protons enter through an apical proton conductance to directly depolarize the taste cell membrane.
Sour taste, the taste of acids, is one of the most enigmatic of the five basic taste qualities; its function is unclear and its receptor was until recently unknown. Sour tastes are transduced in ...taste buds on the tongue and palate epithelium by a subset of taste receptor cells, known as type III cells. Type III cells express a number of unique markers, which allow for their identification and manipulation. These cells respond to acid stimuli with action potentials and release neurotransmitters onto afferent nerve fibers, with cell bodies in geniculate and petrosal ganglia. Here, we review classical studies of sour taste leading up to the identification of the sour receptor as the proton channel OTOP1.
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