Mutually exclusive expression of Gαia and Gα14 reveals diversification of taste receptor cells in zebrafish

Transcriptome analyses reveal differentially expressed genes associated with development of the palatal organ in bighead carp (Hypophthalmichthys nobilis)

The palatal organ is a filter-feeding related organ and occupies a considerable proportion of the head of bighead carp (Hypophthalmichthys nobilis), a large cyprinid fish intensive aquaculture in Asia. In this study, we performed RNA-seq of the palatal organ during growth periods of two (M2), six (M6) and 15 (M15) months of age after hatching. The numbers of differentially expressed genes (DEGs) were 1384, 481 and 1837 for M2 VS M6, M6 VS M15 and M2 VS M15 respectively. The following signaling pathways of energy metabolism and cytoskeleton function were enriched, including ECM-receptor interaction, Cardiac muscle contraction, Steroid biosynthesis and PPAR signaling pathway. Several members of collagen family (col1a1, col2a1, col6a2, col6a3, col9a2), Laminin gamma 1 (lamc1), integrin alpha 1 (itga1), Fatty acid binding protein 2 (fads2) and lipoprotein lipase (lpl), and Protein tyrosine kinase 7 (Ptk7) are candidate genes for growth and development of basic tissues of the palatal organ. Furthermore, taste-related genes such as fgfrl1, fgf8a, fsta and notch1a were also identified, which may be involved in the development of taste buds of the palatal organ. The transcriptome data obtained in this study provide insights into the understanding functions and development mechanisms of palatal organ, and potential candidate genes that may be related to the genetic modulation of head size of bighead carp.

Zebrafish and medaka T1R (taste receptor type 1) proteins mediate highly sensitive recognition of l-proline

In vertebrates, nutritional tastants, such as amino acids and sugars, are recognized by G-protein-coupled receptors of the taste receptor type 1 (T1R) family. Previous studies have shown that fish T1Rs are functionally distinct from mammalian T1Rs in certain regards. Here, we report the existence of oral receptors with high sensitivity to amino acids in zebrafish and medaka fish. We describe the construction of multiple cell lines stably expressing functional T1Rs (from medaka fish or zebrafish) with a chimeric G-protein (G16gust44) using the Flp-In system. Through functional assays with these cell lines, medaka fish and zebrafish were confirmed to possess particular T1Rs highly sensitive to l-proline, possibly reflecting the physiological importance of l-proline in teleosts, in line with previous studies.

Fatty Acid Sensing in the Gastrointestinal Tract of Rainbow Trout: Different to Mammalian Model?

It is well established in mammals that the gastrointestinal tract (GIT) senses the luminal presence of nutrients and responds to such information by releasing signaling molecules that ultimately regulate feeding. However, gut nutrient sensing mechanisms are poorly known in fish. This research characterized fatty acid (FA) sensing mechanisms in the GIT of a fish species with great interest in aquaculture: the rainbow trout (Oncorhynchus mykiss). Main results showed that: (i) the trout GIT has mRNAs encoding numerous key FA transporters characterized in mammals (FA transporter CD36 -FAT/CD36-, FA transport protein 4 -FATP4-, and monocarboxylate transporter isoform-1 -MCT-1-) and receptors (several free FA receptor -Ffar- isoforms, and G protein-coupled receptors 84 and 119 -Gpr84 and Gpr119-), and (ii) intragastrically-administered FAs differing in their length and degree of unsaturation (i.e., medium-chain (octanoate), long-chain (oleate), long-chain polyunsaturated (α-linolenate), and short-chain (butyrate) FAs) exert a differential modulation of the gastrointestinal abundance of mRNAs encoding the identified transporters and receptors and intracellular signaling elements, as well as gastrointestinal appetite-regulatory hormone mRNAs and proteins. Together, results from this study offer the first set of evidence supporting the existence of FA sensing mechanisms n the fish GIT. Additionally, we detected several differences in FA sensing mechanisms of rainbow trout vs. mammals, which may suggest evolutionary divergence between fish and mammals.

Expression of taste sentinels, T1R, T2R, and PLCβ2, on the passageway for olfactory signals in zebrafish

The senses of taste and smell detect overlapping sets of chemical compounds in fish, e.g. amino acids are detected by both senses. However, so far taste and smell organs appeared morphologically to be very distinct, with a specialized olfactory epithelium for detection of odors and taste buds located in the oral cavity and lip for detection of tastants. Here, we report dense clusters of cells expressing T1R and T2R receptors as well as their signal transduction molecule PLCβ2 in nostrils of zebrafish, i.e. on the entrance funnel through which odor molecules must pass to be detected by olfactory sensory neurons. Quantitative evaluation shows the density of these chemosensory cells in the nostrils to be as high or higher than that in the established taste organs oral cavity and lower lip. Hydrodynamic flow is maximal at the nostril rim enabling high throughput chemosensation in this organ. Taken together, our results suggest a sentinel function for these chemosensory cells in the nostril.

Maintenance and turnover of Sox2+ adult stem cells in the gustatory epithelium

Continuous turnover of taste bud cells in the oral cavity underlies the homeostasis of taste tissues. Previous studies have demonstrated that Sox2⁺ stem cells give rise to all types of epithelial cells including taste bud cells and non-gustatory epithelial cells in the oral epithelium, and Sox2 is required for generating taste bud cells. Here, we show the dynamism of single stem cells through multicolor lineage tracing analyses in Sox2-CreERT2; Rosa26-Confetti mice. In the non-gustatory epithelium, unicolored areas populated by a cluster of cells expressing the same fluorescent protein grew over time, while epithelial cells were randomly labeled with multiple fluorescent proteins by short-term tracing. Similar phenomena were observed in gustatory epithelia. These results suggest that the Sox2⁺ stem cell population is maintained by balancing the increase of certain stem cells with the reduction of the others. In the gustatory epithelia, many single taste buds contained cells labeled with different fluorescent proteins, indicating that a single taste bud is composed of cells derived from multiple Sox2⁺ stem cells. Our results reveal the characteristics of Sox2⁺ stem cells underlying the turnover of taste bud cells and the homeostasis of taste tissues.

Exploring the potential for an evolutionarily conserved role of the taste 1 receptor gene family in gut sensing mechanisms of fish

In this study, we investigated the transcriptional spatio-temporal dynamics of the taste 1 receptor (T1R) gene family repertoire in seabream (Sparus aurata [sa]), during larval ontogeny and in adult tissues. In early larval development, saT1R expression arises heterochronously, i.e. the extraoral taste-related perception in the gastrointestinal tract (GIT) anticipates first exogenous feeding (at 9 days post hatching [dph]), followed by the buccal/intraoral perception from 14 dph onwards, supporting the hypothesis that the early onset of the molecular machinery underlying saT1R expression in the GIT is not induced by food but rather genetically hardwired. During adulthood, we characterized the expression patterns of saT1R within specific tissues (n = 4) distributed in oropharingeal, GIT and brain regions substantiating their functional versatility as chemosensory signaling players to a variety of biological functions beyond oral taste sensation. Further, we provided for the first time direct evidences in fish for mRNA co-expression of a subset of saT1R genes (mostly saT1R3, i.e. the common subunit of the heterodimeric T1R complexes for the detection of “sweet” and “umami” substances), with the selected gut peptides ghrelin (ghr), cholecystokinin (cck), hormone peptide yy (pyy) and proglucagon (pg). Each peptide defines the enteroendocrine cells (ECCs) identity, and establishes on morphological basis, a direct link for T1R chemosensing in the regulation of fish digestive processes. Finally, we analyzed the spatial gene expression patterns of 2 taste signaling components functionally homologous to the mammalian G(i)α subunit gustducin, namely saG(i)α1 and saG(i)α2, and demonstrated their co-localization with the saT1R3 in EECs, thus validating their direct involvement in taste-like transduction mechanisms of the fish GIT. In conclusion, data provide new insights in the evolutionary conservation of gut sensing in fish suggesting a conserved role for nutrient sensors modulating entero-endocrine secretion.

Expression profiles and functional characterization of common carp (Cyprinus carpio) T2Rs

Bitter taste perception is mediated by a family of G protein-coupled receptors (T2Rs) in vertebrates. Common carp (Cyprinus carpio), which has experienced an additional round of whole genome duplication during the course of evolution, has a small number of T2R genes similar to zebrafish, a closely related cyprinid fish species, and their expression pattern at the cellular level or their cognate ligands have not been elucidated yet. Here, we showed through in situ hybridization experiments, that three common carp T2R (ccT2R) genes encoding ccT2R200-1, ccT2R202-1, and ccT2R202-2, were specifically expressed in the subsets of taste receptor cells in the lips and gill rakers. ccT2R200-1 was co-expressed with genes encoding downstream signal transduction molecules, such as PLC-β2 and Gαia. Heterologous expression system revealed that each ccT2R showed narrowly, intermediately, or broadly tuned ligand specificity, as in the case of zebrafish T2Rs. However, ccT2Rs showed different ligand profiles from their orthologous zebrafish T2Rs previously reported. Finally, we identified three ccT2Rs, namely ccT2R200-1, ccT2R200-2, and ccT2R203-1, to be activated by natural bitter compounds, andrographolide and/or picrotoxinin, which elicited no response to zebrafish T2Rs, in a dose-dependent manner. These results suggest that some ccT2Rs may have evolved to function in the oral cavity as taste receptors for natural bitter compounds found in the habitats in a species-specific manner.

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Common carp T2R (ccT2R) gene was co-expressed with genes encoding downstream signal transduction molecules in subsets of taste receptor cells, similar to zebrafish.

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Each ccT2R showed narrowly, intermediately, or broadly tuned ligand specificity, as in the case of zebrafish T2Rs; however, ccT2Rs showed different ligand profiles from their orthologous zebrafish T2Rs previously reported.

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Some ccT2Rs may have evolved to function in the oral cavity as taste receptors for natural bitter compounds found in the habitats in a species-specific manner.

First evidence for the presence of amino acid sensing mechanisms in the fish gastrointestinal tract

This study aimed to characterize amino acid sensing systems in the gastrointestinal tract (GIT) of the carnivorous fish model species rainbow trout. We observed that the trout GIT expresses mRNAs encoding some amino acid receptors described in mammals [calcium-sensing receptor (CaSR), G protein-coupled receptor family C group 6 member A (GPRC6A), and taste receptors type 1 members 1 and 2 (T1r1, T1r2)], while others [taste receptor type 1 member 3 (T1r3) and metabotropic glutamate receptors 1 and 4 (mGlur1, mGlur4)] could not be found. Then, we characterized the response of such receptors, as well as that of intracellular signaling mechanisms, to the intragastric administration of l-leucine, l-valine, l-proline or l-glutamate. Results demonstrated that casr, gprc6a, tas1r1 and tas1r2 mRNAs are modulated by amino acids in the stomach and proximal intestine, with important differences with respect to mammals. Likewise, gut amino acid receptors triggered signaling pathways likely mediated, at least partly, by phospholipase C β3 and β4. Finally, the luminal presence of amino acids led to important changes in ghrelin, cholecystokinin, peptide YY and proglucagon mRNAs and/or protein levels. Present results offer the first set of evidence in favor of the existence of amino acid sensing mechanisms within the fish GIT.

Sodium-Taste Cells Require Skn-1a for Generation and Share Molecular Features with Sweet, Umami, and Bitter Taste Cells

Taste buds are maintained via continuous turnover of taste bud cells derived from local epithelial stem cells. A transcription factor Skn-1a (also known as Pou2f3) is required for the generation of sweet, umami (savory), and bitter taste cells that commonly express TRPM5 and CALHM ion channels. Here, we demonstrate that sodium-taste cells distributed only in the anterior oral epithelia and involved in evoking salty taste also require Skn-1a for their generation. We discovered taste cells in fungiform papillae and soft palate that show similar but not identical molecular feature with sweet, umami, and bitter taste-mediated Type II cells. This novel cell population expresses Plcb2, Itpr3, Calhm3, Skn-1a, and ENaCα (also known as Scnn1a) encoding the putative amiloride-sensitive (AS) salty taste receptor but lacks Trpm5 and Gnat3 Skn-1a-deficient taste buds are predominantly composed of putative non-sensory Type I cells and sour-sensing Type III cells, whereas wild-type taste buds include Type II (i.e., sweet, umami, and bitter taste) cells and sodium-taste cells. Both Skn-1a and Calhm3-deficient mice have markedly decreased chorda tympani nerve responses to sodium chloride, and those decreased responses are attributed to the loss of the AS salty taste response. Thus, AS salty taste is mediated by Skn-1a-dependent taste cells, whereas amiloride-insensitive salty taste is mediated largely by Type III sour taste cells and partly by bitter taste cells. Our results demonstrate that Skn-1a regulates differentiation toward all types of taste cells except sour taste cells.

Expression of Eya1 in mouse taste buds

Taste substances are detected by taste receptor cells in the taste buds in the oral epithelium. Individual taste receptor cells contribute to evoking one of the five taste qualities: sweet, umami, bitter, sour, and salty (sodium). They are continuously replaced every few weeks by new ones generated from local epithelial stem cells. A POU transcription factor, Pou2f3 (also known as Skn-1a), regulates the generation and differentiation of sweet, umami, and bitter cells. However, the molecular mechanisms underlying terminal differentiation into these Pou2f3-dependent taste receptor cells remain unknown. To identify the candidate molecules that regulate the differentiation of these taste receptor cells, we searched for taste receptor type-specific transcription factors using RNA-sequence data of sweet and bitter cells. No transcription factor gene showing higher expression in sweet cells than in bitter cells was found. Eyes absent 1 (Eya1) was identified as the only transcription factor gene showing higher expression in bitter cells than in sweet cells. In situ hybridization revealed that Eya1 was predominantly expressed in bitter cells and also in the putative immature/differentiating taste bud cells in circumvallate and fungiform papillae and soft palate. Eya1 is a candidate molecule that regulates the generation and differentiation of bitter cells.

Insights into the Function and Evolution of Taste 1 Receptor Gene Family in the Carnivore Fish Gilthead Seabream (Sparus aurata)

A plethora of molecular and functional studies in tetrapods has led to the discovery of multiple taste 1 receptor (T1R) genes encoding G-protein coupled receptors (GPCRs) responsible for sweet (T1R2 + T1R3) and umami (T1R1 + T1R3) taste. In fish, the T1R gene family repertoires greatly expanded because of several T1R2 gene duplications, and recent studies have shown T1R2 functional divergence from canonical mammalian sweet taste perceptions, putatively as an adaptive mechanism to develop distinct feeding strategies in highly diverse aquatic habitats. We addressed this question in the carnivore fish gilthead seabream (Sparus aurata), a model species of aquaculture interest, and found that the saT1R gene repertoire consists of eight members including saT1R1, saT1R3 and six saT1R2a-f gene duplicates, adding further evidence to the evolutionary complexity of fishT1Rs families. To analyze saT1R taste functions, we first developed a stable gene reporter system based on Ca²⁺-dependent calcineurin/NFAT signaling to examine specifically in vitro the responses of a subset of saT1R heterodimers to L-amino acids (L-AAs) and sweet ligands. We show that although differentially tuned in sensitivity and magnitude of responses, saT1R1/R3, saT1R2a/R3 and saT1R2b/R3 may equally serve to transduce amino acid taste sensations. Furthermore, we present preliminary information on the potential involvement of the Gi protein alpha subunits saGαi1 and saGαi2 in taste signal transduction.

Oral and pre-absorptive sensing of amino acids relates to hypothalamic control of food intake in rainbow trout

To assess the putative role of taste and pre-absorptive sensing of amino acids in food intake control in fish, we carried out an oral administration with l-leucine, l-valine, l-proline or l-glutamic acid in rainbow trout (Oncorhynchus mykiss). Treatment with proline significantly reduced voluntary food intake at 2 h and 3 h after oral administration, while glutamic acid showed a less pronounced satiating effect at 3 h. The mRNA expression of taste receptor subunits tas1r1, tas1r2a, tas1r2b and tas1r3 was measured in the epithelium overlying the bony basihyal of the fish (analogous to the tetrapod tongue) at 10, 20 or 30 min following treatment. No significant changes were observed, except for a tas1r down-regulation by valine at 30 min. Of the downstream taste signalling genes that were analysed in parallel, plcb2 and possibly trpm5 (non-significant trend) were down-regulated 20 min after proline and glutamic acid treatment. The signal originated in the oropharyngeal and/or gastric cavity presumably relays to the brain as changes in genes involved in the regulation of food intake occurred in hypothalamus 10-30 min after oral treatment with amino acids. In particular, proline induced changes consistent with an increased anorexigenic potential in the hypothalamus. We have therefore demonstrated, for the first time in fish, that the peripheral (pre-absorptive) detection of an amino acid (l-proline), presumably by taste-related mechanisms, elicits a satiety signal that in hypothalamus is translated into changes in cellular signalling and neuropeptides regulating food intake, ultimately resulting in decreased food intake.

Sensory cutaneous papillae in the sea lamprey (Petromyzon marinus L.): I. Neuroanatomy and physiology

Molecules present in an animal's environment can indicate the presence of predators, food, or sexual partners and consequently, induce migratory, reproductive, foraging, or escape behaviors. Three sensory systems, the olfactory, gustatory, and solitary chemosensory cell (SCC) systems detect chemical stimuli in vertebrates. While a great deal of research has focused on the olfactory and gustatory system over the years, it is only recently that significant attention has been devoted to the SCC system. The SCCs are microvillous cells that were first discovered on the skin of fish, and later in amphibians, reptiles, and mammals. Lampreys also possess SCCs that are particularly numerous on cutaneous papillae. However, little is known regarding their precise distribution, innervation, and function. Here, we show that sea lampreys (Petromyzon marinus L.) have cutaneous papillae located around the oral disk, nostril, gill pores, and on the dorsal fins and that SCCs are particularly numerous on these papillae. Tract-tracing experiments demonstrated that the oral and nasal papillae are innervated by the trigeminal nerve, the gill pore papillae are innervated by branchial nerves, and the dorsal fin papillae are innervated by spinal nerves. We also characterized the response profile of gill pore papillae to some chemicals and showed that trout-derived chemicals, amino acids, and a bile acid produced potent responses. Together with a companion study (Suntres et al., Journal of Comparative Neurology, this issue), our results provide new insights on the function and evolution of the SCC system in vertebrates.

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

Binding of sweet, umami, and bitter tastants to G protein-coupled receptors (GPCRs) in apical membranes of type II taste bud cells (TBCs) triggers action potentials that activate a voltage-gated nonselective ion channel to release ATP to gustatory nerves mediating taste perception. Although calcium homeostasis modulator 1 (CALHM1) is necessary for ATP release, the molecular identification of the channel complex that provides the conductive ATP-release mechanism suitable for action potential-dependent neurotransmission remains to be determined. Here we show that CALHM3 interacts with CALHM1 as a pore-forming subunit in a CALHM1/CALHM3 hexameric channel, endowing it with fast voltage-activated gating identical to that of the ATP-release channel in vivo. Calhm3 is co-expressed with Calhm1 exclusively in type II TBCs, and its genetic deletion abolishes taste-evoked ATP release from taste buds and GPCR-mediated taste perception. Thus, CALHM3, together with CALHM1, is essential to form the fast voltage-gated ATP-release channel in type II TBCs required for GPCR-mediated tastes.

Tracing the evolution of the heterotrimeric G protein α subunit in Metazoa

BACKGROUND

Heterotrimeric G proteins are fundamental signaling proteins composed of three subunits, Gα and a Gβγ dimer. The role of Gα as a molecular switch is critical for transmitting and amplifying intracellular signaling cascades initiated by an activated G protein Coupled Receptor (GPCR). Despite their biochemical and therapeutic importance, the study of G protein evolution has been limited to the scope of a few model organisms. Furthermore, of the five primary Gα subfamilies, the underlying gene structure of only two families has been thoroughly investigated outside of Mammalia evolution. Therefore our understanding of Gα emergence and evolution across phylogeny remains incomplete.

RESULTS

We have computationally identified the presence and absence of every Gα gene (GNA-) across all major branches of Deuterostomia and evaluated the conservation of the underlying exon-intron structures across these phylogenetic groups. We provide evidence of mutually exclusive exon inclusion through alternative splicing in specific lineages. Variations of splice site conservation and isoforms were found for several paralogs which coincide with conserved, putative motifs of DNA-/RNA-binding proteins. In addition to our curated gene annotations, within Primates, we identified 15 retrotranspositions, many of which have undergone pseudogenization. Most importantly, we find numerous deviations from previous findings regarding the presence and absence of individual GNA- genes, nuanced differences in phyla-specific gene copy numbers, novel paralog duplications and subsequent intron gain and loss events.

CONCLUSIONS

Our curated annotations allow us to draw more accurate inferences regarding the emergence of all Gα family members across Metazoa and to present a new, updated theory of Gα evolution. Leveraging this, our results are critical for gaining new insights into the co-evolution of the Gα subunit and its many protein binding partners, especially therapeutically relevant G protein - GPCR signaling pathways which radiated in Vertebrata evolution.

Genetic Lineage Tracing in Taste Tissues Using Sox2-CreERT2 Strain

Taste cells in taste buds are epithelial sensory cells. Old taste bud cells die and are replaced by new ones generated from taste stem cells. Identifying and characterizing adult taste stem cells is therefore important to understand how peripheral taste tissues are maintained. SOX2 is expressed in oral epithelium including gustatory papillae and has been proposed to be a marker of adult taste stem/progenitor cells. Nevertheless, this hypothesis has never been directly tested. Here, by single-color genetic lineage tracing using Sox2-CreERT2 strain, we reveal that all types of taste bud cells distributed throughout the oral epithelium are derived from stem cells that express SOX2. Short-term tracing shows that SOX2-positive taste stem cells actively supply taste bud cells. At the base of epithelium outside taste buds are distributed proliferation marker- and SOX2-positive cells. Consistently, taste stem cells identified by Lgr5 expression in the circumvallate papillae also express SOX2. Together, taste stem cells distributed in oral epithelia express SOX2.

Acid-sensing ion channels and transient-receptor potential ion channels in zebrafish taste buds

Sensory information from the environment is required for life and survival, and it is detected by specialized cells which together make up the sensory system. The fish sensory system includes specialized organs that are able to detect mechanical and chemical stimuli. In particular, taste buds are small organs located on the tongue in terrestrial vertebrates that function in the perception of taste. In fish, taste buds occur on the lips, the flanks, and the caudal (tail) fins of some species and on the barbels of others. In fish taste receptor cells, different classes of ion channels have been detected which, like in mammals, presumably participate in the detection and/or transduction of chemical gustatory signals. However, since some of these ion channels are involved in the detection of additional sensory modalities, it can be hypothesized that taste cells sense stimuli other than those specific for taste. This mini-review summarizes current knowledge on the presence of transient-receptor potential (TRP) and acid-sensing (ASIC) ion channels in the taste buds of teleosts, especially adult zebrafish. Up to now ASIC4, TRPC2, TRPA1, TRPV1 and TRPV4 ion channels have been found in the sensory cells, while ASIC2 was detected in the nerves supplying the taste buds.

Gustatory and general visceral centers and their connections in the brain of adult zebrafish: a carbocyanine dye tract-tracing study

The central connections of the gustatory/general visceral system of the adult zebrafish (Danio rerio) were examined by means of carbocyanine dye tracing. Main primary gustatory centers (facial and vagal lobes) received sensory projections from the facial and vagal nerves, respectively. The vagal nerve also projects to the commissural nucleus of Cajal, a general visceral sensory center. These primary centers mainly project on a prominent secondary gustatory and general visceral nucleus (SGN/V) located in the isthmic region. Secondary projections on the SGN/V were topographically organized, those of the facial lobe mainly ending medially to those of the vagal lobe, and those from the commissural nucleus ventrolaterally. Descending facial lobe projections to the medial funicular nucleus were also noted. Ascending fibers originating from the SGN/V mainly projected to the posterior thalamic nucleus and the lateral hypothalamus (lateral torus, lateral recess nucleus, hypothalamic inferior lobe diffuse nucleus) and an intermediate cell- and fiber-rich region termed here the tertiary gustatory nucleus proper, but not to a nucleus formerly considered as the zebrafish tertiary gustatory nucleus. The posterior thalamic nucleus, tertiary gustatory nucleus proper, and nucleus of the lateral recess gave rise to descending projections to the SGN/V and the vagal lobe. The connectivity between diencephalic gustatory centers and the telencephalon was also investigated. The present results showed that the gustatory connections of the adult zebrafish are rather similar to those reported in other cyprinids, excepting the tertiary gustatory nucleus. Similarities between the gustatory systems of zebrafish and other fishes are also discussed. J. Comp. Neurol. 525:333-362, 2017. © 2016 Wiley Periodicals, Inc.

Evolutionary conserved brainstem circuits encode category, concentration and mixtures of taste

Evolutionary conserved brainstem circuits are the first relay for gustatory information in the vertebrate brain. While the brainstem circuits act as our life support system and they mediate vital taste related behaviors, the principles of gustatory computations in these circuits are poorly understood. By a combination of two-photon calcium imaging and quantitative animal behavior in juvenile zebrafish, we showed that taste categories are represented by dissimilar brainstem responses and generate different behaviors. We also showed that the concentration of sour and bitter tastes are encoded by different principles and with different levels of sensitivity. Moreover, we observed that the taste mixtures lead to synergistic and suppressive interactions. Our results suggest that these interactions in early brainstem circuits can result in non-linear computations, such as dynamic gain modulation and discrete representation of taste mixtures, which can be utilized for detecting food items at broad range of concentrations of tastes and rejecting inedible substances.

A physiologic role for serotonergic transmission in adult rat taste buds

Of the multiple neurotransmitters and neuropeptides expressed in the mammalian taste bud, serotonin remains both the most studied and least understood. Serotonin is expressed in a subset of taste receptor cells that form synapses with afferent nerve fibers (type III cells) and was once thought to be essential to neurotransmission (now understood as purinergic). However, the discovery of the 5-HT1A serotonin receptor in a subset of taste receptor cells paracrine to type III cell suggested a role in cell-to-cell communication during the processing of taste information. Functional data describing this role are lacking. Using anatomical and neurophysiological techniques, this study proposes a modulatory role for serotonin during the processing of taste information. Double labeling immunocytochemical and single cell RT-PCR technique experiments documented that 5-HT1A-expressing cells co-expressed markers for type II cells, cells which express T1R or T2R receptors and release ATP. These cells did not co-express type III cells markers. Neurophysiological recordings from the chorda tympani nerve, which innervates anterior taste buds, were performed prior to and during intravenous injection of a 5-HT1A receptor antagonist. These experiments revealed that serotonin facilitates processing of taste information for tastants representing sweet, sour, salty, and bitter taste qualities. On the other hand, injection of ondansetron, a 5-HT3 receptor antagonist, was without effect. Collectively, these data support the hypothesis that serotonin is a crucial element in a finely-tuned feedback loop involving the 5-HT1A receptor, ATP, and purinoceptors. It is hypothesized that serotonin facilitates gustatory signals by regulating the release of ATP through ATP-release channels possibly through phosphatidylinositol 4,5-bisphosphate resynthesis. By doing so, 5-HT1A activation prevents desensitization of post-synaptic purinergic receptors expressed on afferent nerve fibers and enhances the afferent signal. Serotonin may thus play a major modulatory role within peripheral taste in shaping the afferent taste signals prior to their transmission across gustatory nerves.

Genetics of taste receptors

Taste receptors function as one of the interfaces between internal and external milieus. Taste receptors for sweet and umami (T1R [taste receptor, type 1]), bitter (T2R [taste receptor, type 2]), and salty (ENaC [epithelial sodium channel]) have been discovered in the recent years, but transduction mechanisms of sour taste and ENaC-independent salt taste are still poorly understood. In addition to these five main taste qualities, the taste system detects such noncanonical "tastes" as water, fat, and complex carbohydrates, but their reception mechanisms require further research. Variations in taste receptor genes between and within vertebrate species contribute to individual and species differences in taste-related behaviors. These variations are shaped by evolutionary forces and reflect species adaptations to their chemical environments and feeding ecology. Principles of drug discovery can be applied to taste receptors as targets in order to develop novel taste compounds to satisfy demand in better artificial sweeteners, enhancers of sugar and sodium taste, and blockers of bitterness of food ingredients and oral medications.

Enteroendocrine profile of α-transducin immunoreactive cells in the gastrointestinal tract of the European sea bass (Dicentrarchus labrax)

In vertebrates, chemosensitivity of nutrients occurs through the activation of taste receptors coupled with G-protein subunits, including α-transducin (G(αtran)) and α-gustducin (G(αgust)). This study was aimed at characterising the cells expressing G(αtran) immunoreactivity throughout the mucosa of the sea bass gastrointestinal tract. G(αtran) immunoreactive cells were mainly found in the stomach, and a lower number of immunopositive cells were detected in the intestine. Some G(αtran) immunoreactive cells in the stomach contained G(αgust) immunoreactivity. Gastric G(αtran) immunoreactive cells co-expressed ghrelin, obestatin and 5-hydroxytryptamine immunoreactivity. In contrast, G(αtran) immunopositive cells did not contain somatostatin, gastrin/cholecystokinin, glucagon-like peptide-1, substance P or calcitonin gene-related peptide immunoreactivity in any investigated segments of the sea bass gastrointestinal tract. Specificity of G(αtran) and G(αgust) antisera was determined by Western blot analysis, which identified two bands at the theoretical molecular weight of ~45 and ~40 kDa, respectively, in sea bass gut tissue as well as in positive tissue, and by immunoblocking with the respective peptide, which prevented immunostaining. The results of the present study provide a molecular and morphological basis for a role of taste-related molecules in chemosensing in the sea bass gastrointestinal tract.

The vertebrate ancestral repertoire of visual opsins, transducin alpha subunits and oxytocin/vasopressin receptors was established by duplication of their shared genomic region in the two rounds of early vertebrate genome duplications

BACKGROUND

Vertebrate color vision is dependent on four major color opsin subtypes: RH2 (green opsin), SWS1 (ultraviolet opsin), SWS2 (blue opsin), and LWS (red opsin). Together with the dim-light receptor rhodopsin (RH1), these form the family of vertebrate visual opsins. Vertebrate genomes contain many multi-membered gene families that can largely be explained by the two rounds of whole genome duplication (WGD) in the vertebrate ancestor (2R) followed by a third round in the teleost ancestor (3R). Related chromosome regions resulting from WGD or block duplications are said to form a paralogon. We describe here a paralogon containing the genes for visual opsins, the G-protein alpha subunit families for transducin (GNAT) and adenylyl cyclase inhibition (GNAI), the oxytocin and vasopressin receptors (OT/VP-R), and the L-type voltage-gated calcium channels (CACNA1-L).

RESULTS

Sequence-based phylogenies and analyses of conserved synteny show that the above-mentioned gene families, and many neighboring gene families, expanded in the early vertebrate WGDs. This allows us to deduce the following evolutionary scenario: The vertebrate ancestor had a chromosome containing the genes for two visual opsins, one GNAT, one GNAI, two OT/VP-Rs and one CACNA1-L gene. This chromosome was quadrupled in 2R. Subsequent gene losses resulted in a set of five visual opsin genes, three GNAT and GNAI genes, six OT/VP-R genes and four CACNA1-L genes. These regions were duplicated again in 3R resulting in additional teleost genes for some of the families. Major chromosomal rearrangements have taken place in the teleost genomes. By comparison with the corresponding chromosomal regions in the spotted gar, which diverged prior to 3R, we could time these rearrangements to post-3R.

CONCLUSIONS

We present an extensive analysis of the paralogon housing the visual opsin, GNAT and GNAI, OT/VP-R, and CACNA1-L gene families. The combined data imply that the early vertebrate WGD events contributed to the evolution of vision and the other neuronal and neuroendocrine functions exerted by the proteins encoded by these gene families. In pouched lamprey all five visual opsin genes have previously been identified, suggesting that lampreys diverged from the jawed vertebrates after 2R.

Pou2f3/Skn-1a is necessary for the generation or differentiation of solitary chemosensory cells in the anterior nasal cavity

Solitary chemosensory cells in the non-neuronal epithelium of the anterior nasal cavity have bitter taste cell-like molecular characteristics and are involved in the detection of noxious substances. Here, we demonstrate that Pou2f3/Skn-1a, which is necessary for generation of sweet, umami, and bitter taste cells, is also necessary for the generation or differentiation of solitary chemosensory cells.

Taste and odorant receptors of the coelacanth--a gene repertoire in transition

G-protein coupled chemosensory receptors (GPCR-CRs) aid in the perception of odors and tastes in vertebrates. So far, six GPCR-CR families have been identified that are conserved in most vertebrate species. Phylogenetic analyses indicate differing evolutionary dynamics between teleost fish and tetrapods. The coelacanth Latimeria chalumnae belongs to the lobe-finned fishes, which represent a phylogenetic link between these two groups. We searched the genome of L. chalumnae for GPCR-CRs and found that coelacanth taste receptors are more similar to those in tetrapods than in teleost fish: two coelacanth T1R2s co-segregate with the tetrapod T1R2s that recognize sweet substances, and our phylogenetic analyses indicate that the teleost T1R2s are closer related to T1R1s (umami taste receptors) than to tetrapod T1R2s. Furthermore, coelacanths are the first fish with a large repertoire of bitter taste receptors (58 T2Rs). Considering current knowledge on feeding habits of coelacanths the question arises if perception of bitter taste is the only function of these receptors. Similar to teleost fish, coelacanths have a variety of olfactory receptors (ORs) necessary for perception of water-soluble substances. However, they also have seven genes in the two tetrapod OR subfamilies predicted to recognize airborne molecules. The two coelacanth vomeronasal receptor families are larger than those in teleost fish, and similar to tetrapods and form V1R and V2R monophyletic clades. This may point to an advanced development of the vomeronasal organ as reported for lungfish. Our results show that the intermediate position of Latimeria in the phylogeny is reflected in its GPCR-CR repertoire.

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

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

Developing a sense of taste

Taste buds are found in a distributed array on the tongue surface, and are innervated by cranial nerves that convey taste information to the brain. For nearly a century, taste buds were thought to be induced by nerves late in embryonic development. However, this view has shifted dramatically. A host of studies now indicate that taste bud development is initiated and proceeds via processes that are nerve-independent, occur long before birth, and governed by cellular and molecular mechanisms intrinsic to the developing tongue. Here we review the state of our understanding of the molecular and cellular regulation of taste bud development, incorporating important new data obtained through the use of two powerful genetic systems, mouse and zebrafish.

Genetic labeling of Tas1r1 and Tas2r131 taste receptor cells in mice

Characterization of the peripheral taste system relies on the identification and visualization of the different taste bud cell types. So far, genetic strategies to label taste receptor cells are limited to sweet, sour, and salty detecting cells. To visualize Tas1r1 umami and Tas2r131 bitter sensing cells, we generated animals in which the Tas1r1 and Tas2r131 open reading frames are replaced by expression cassettes containing the fluorescent proteins mCherry or hrGFP, respectively. These animals enabled us to visualize and quantify the entire oral Tas1r1 and Tas2r131 cell populations. Tas1r1-mCherry cells were predominantly detected in fungiform papillae, whereas Tas2r131-hrGFP cells, which are ~4-fold more abundant, were mainly present in foliate and vallate papillae. In the palate, both cell types were similarly distributed. Mice carrying both recombinant alleles demonstrated completely segregated Tas1r1 and Tas2r131 cell populations. Only ~50% of the entire bitter cell population expressed hrGFP, indicating that bitter taste receptor cells express a subset of the bitter receptor repertoire. In extragustatory tissues, mCherry fluorescence was observed in testis and hrGFP fluorescence in testis, thymus, vomeronasal organ, and respiratory epithelium, suggesting that only few extraoral sites express Tas2r131 and Tas1r1 receptors at levels comparable to taste tissue.

Functional diversification of taste cells in vertebrates

Tastes are senses resulting from the activation of taste cells distributed in oral epithelia. Sweet, umami, bitter, sour, and salty tastes are called the five "basic" tastes, but why five, and why these five? In this review, we dissect the peripheral gustatory system in vertebrates from molecular and cellular perspectives. Recent behavioral and molecular genetic studies have revealed the nature of functional taste receptors and cells and show that different taste qualities are accounted for by the activation of different subsets of taste cells. Based on this concept, the diversity of basic tastes should be defined by the diversity of taste cells in taste buds, which varies among species.