Transgenic labeling of taste receptor cells in model fish under the control of the 5'-upstream region of medaka phospholipase C-beta 2 gene

Structure and function of the larval teleost fish gill

The fish gill is a multifunctional organ that is important in multiple physiological processes such as gas transfer, ionoregulation, and chemoreception. This characteristic organ of fishes has received much attention, yet an often-overlooked point is that larval fishes in most cases do not have a fully developed gill, and thus larval gills do not function identically as adult gills. In addition, large changes associated with gas exchange and ionoregulation happen in gills during the larval phase, leading to the oxygen and ionoregulatory hypotheses examining the environmental constraint that resulted in the evolution of gills. This review thus focuses exclusively on the larval fish gill of teleosts, summarizing the development of teleost larval fish gills and its function in gas transfer, ionoregulation, and chemoreception, and comparing and contrasting it to adult gills where applicable, while providing some insight into the oxygen vs ionoregulatory hypotheses debate.

The control of breathing in fishes - historical perspectives and the path ahead

The study of breathing in fishes has featured prominently in Journal of Experimental Biology (JEB), particularly during the latter half of the past century. Indeed, many of the seminal discoveries in this important sub-field of comparative respiratory physiology were reported first in JEB. The period spanning 1960-1990 (the 'golden age of comparative respiratory physiology') witnessed intense innovation in the development of methods to study the control of breathing. Many of the guiding principles of piscine ventilatory control originated during this period, including our understanding of the dominance of O2 as the driver of ventilation in fish. However, a critical issue - the identity of the peripheral O2 chemoreceptors - remained unanswered until methods for cell isolation, culture and patch-clamp recording established that gill neuroepithelial cells (NECs) respond to hypoxia in vitro. Yet, the role of the NECs and other putative peripheral or central chemoreceptors in the control of ventilation in vivo remains poorly understood. Further progress will be driven by the implementation of genetic tools, most of which can be used in zebrafish (Danio rerio). These tools include CRISPR/Cas9 for selective gene knockout, and Tol2 systems for transgenesis, the latter of which enables optogenetic stimulation of cellular pathways, cellular ablation and in vivo cell-specific biosensing. Using these methods, the next period of discovery will see the identification of the peripheral sensory pathways that initiate ventilatory responses, and will elucidate the nature of their integration within the central nervous system and their link to the efferent motor neurons that control breathing.

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.

Distribution and Origin of VIP-, SP-, and Phospholipase Cβ2 -Immunoreactive Nerves in the Tongue of the Bullfrog, Rana catesbeiana

Previous studies have found a few intralingual ganglionic cells that were immunoreactive to vasoactive intestinal polypeptide (VIP) in the frog. A recent study reported a large number of such cells, and the possibility of the release of substance P (SP) from these. The aim of the present study was to investigate the distribution, origin, and colocalization of VIP- and SP- immunoreactive nerves in the tongue of the bullfrog, R. catesbeiana. In addition, the study also examined the colocalization of SP and phospholipase Cβ2 (PLCβ2 ) in the tongue and jugular ganglion. VIP immunoreactivity was seen in unipolar cells that were sparse in nerve bundles in the submucosal and muscle layers. The density of VIP-immunoreactive cells was approximately 4.8 cells/mm(3) . Their fibers terminated in the vicinity of the epithelial basal layer of the fungiform papillae. SP immunoreactivity was not seen in the VIP-immunoreactive cells, but was observed in pseudounipolar cells in the jugular ganglion. The SP fibers terminated close to the free surface, showing spindle- and button-like profiles. Transection of glossopharyngeal nerve resulted in the persistence of VIP-immunoreactive cells and the disappearance of SP-immunoreactive fibers in the tongue. SP immunoreactivity was co-expressed with PLCβ2 in both the tongue and jugular ganglia. No PLCβ2 immunoreactivity was seen in cells comprising the epithelial taste disk. These findings indicate that the origin of VIP nerve fibers are unipolar cells in the tongue, and SP and PLCβ2 fibers originate from pseudounipolar cells that may be able to release SP primarily in the jugular ganglion. Anat Rec, 299:929-942, 2016. © 2016 Wiley Periodicals, Inc.

Diversity in cell motility reveals the dynamic nature of the formation of zebrafish taste sensory organs

Taste buds are sensory organs in jawed vertebrates, composed of distinct cell types that detect and transduce specific taste qualities. Taste bud cells differentiate from oropharyngeal epithelial progenitors localized mainly in proximity of the forming organs. Despite recent progress in elucidating the molecular interactions required for taste bud cell development and function, the cell behaviour underlying the organ assembly is poorly defined. Here, we used time-lapse imaging to observe the formation of taste buds in live zebrafish larvae. We found that tg(fgf8a.dr17) expressing cells form taste buds and get rearranged within the forming organs. In addition, differentiating cells move from the epithelium to the forming organs and can be displaced between developing organs. During organ formation, taste bud tg(fgf8a.dr17) and Type-II cells are displaced in random, directed or confined mode relative to the taste bud they join or are maintained. Finally, ascl1a activity in the 5-HT/Type-III cell is required to direct and maintain tg(fgf8a.dr17) expressing cells into the taste bud. We propose diversity in displacement modes of differentiating cells as a key mechanism for the highly dynamic process of taste bud assembly.

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 microfluidic device to study neuronal and motor responses to acute chemical stimuli in zebrafish

Zebrafish larva is a unique model for whole-brain functional imaging and to study sensory-motor integration in the vertebrate brain. To take full advantage of this system, one needs to design sensory environments that can mimic the complex spatiotemporal stimulus patterns experienced by the animal in natural conditions. We report on a novel open-ended microfluidic device that delivers pulses of chemical stimuli to agarose-restrained larvae with near-millisecond switching rate and unprecedented spatial and concentration accuracy and reproducibility. In combination with two-photon calcium imaging and recordings of tail movements, we found that stimuli of opposite hedonic values induced different circuit activity patterns. Moreover, by precisely controlling the duration of the stimulus (50-500 ms), we found that the probability of generating a gustatory-induced behavior is encoded by the number of neurons activated. This device may open new ways to dissect the neural-circuit principles underlying chemosensory perception.

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.

Fgf signaling controls pharyngeal taste bud formation through miR-200 and Delta-Notch activity

Taste buds, the taste sensory organs, are conserved in vertebrates and composed of distinct cell types, including taste receptor, basal/presynaptic and support cells. Here, we characterize zebrafish taste bud development and show that compromised Fgf signaling in the larva results in taste bud reduction and disorganization. We determine that Fgf activity is required within pharyngeal endoderm for formation of Calb2b(+) cells and reveal miR-200 and Delta-Notch signaling as key factors in this process. miR-200 knock down shows that miR-200 activity is required for taste bud formation and in particular for Calb2b(+) cell formation. Compromised delta activity in mib(-/-) dramatically reduces the number of Calb2b(+) cells and increases the number of 5HT(+) cells. Conversely, larvae with increased Notch activity and ascl1a(-/-) mutants are devoid of 5HT(+) cells, but have maintained and increased Calb2b(+) cells, respectively. These results show that Delta-Notch signaling is required for intact taste bud organ formation. Consistent with this, Notch activity restores Calb2b(+) cell formation in pharyngeal endoderm with compromised Fgf signaling, but fails to restore the formation of these cells after miR-200 knock down. Altogether, this study provides genetic evidence that supports a novel model where Fgf regulates Delta-Notch signaling, and subsequently miR-200 activity, in order to promote taste bud cell type differentiation.

Gustation in fish: search for prototype of taste perception

Fish perceive water-soluble chemicals at the taste buds that are distributed on oropharyngeal and trunk epithelia. Recent progress in molecular analyses has revealed that teleosts and mammals share pivotal signaling components to transduce taste stimuli. The fish orthologs of taste receptors, fT1R and fT2R, show mutually exclusive expression in taste buds, and both are coexpressed with phospholipase C-beta2 and the transient receptor potential M5 channel as common downstream components of taste receptor signals. Interestingly, fT1R heteromers are activated by various L-amino acids but not by sugars. This may reflects that in fish the energy metabolism depends primarily on gluconeogenesis from amino acids. fT2Rs are activated by denatonium benzoate, which is a bitter substance for mammals. It is thus likely that the preferable and aversive tastes for vertebrates, though their taste modalities somewhat vary, are transduced by the sensory conserved pathways. The comparative molecular biology of the fish taste system would lead to understanding a general logic of encoding taste modalities in vertebrates.

Construction of a taste-blind medaka fish and quantitative assay of its preference-aversion behavior

In vertebrates, the taste system provides information used in the regulation of food ingestion. In mammals, each cell group within the taste buds expresses either the T1R or the T2R taste receptor for preference–aversion discrimination. However, no such information is available regarding fish. We developed a novel system for quantitatively assaying taste preference–aversion in medaka fish. In this study, we prepared fluorescently labeled foods with fine cavities designed to retain tastants until they were bitten by the fish. The subjects were fed food containing a mixture of amino acids and inosine monophosphate (AN food), denatonium benzoate (DN food) or no tastant (NT food), and the amounts of ingested food were measured by fluorescence microscopy. Statistical analysis of the fluorescence intensities yielded quantitative measurements of AN food preference and DN food aversion. We then generated a transgenic fish expressing dominant-negative Gα_i2 both in T1R-expressing and in T2R-expressing cells. The feeding assay revealed that the transgenic fish was unable to show a preference for AN food and an aversion to DN food. The assay system was useful for evaluating taste-blind behaviors, and the results indicate that the two taste signaling pathways conveying preferable and aversive taste information are conserved in fish as well as in mammals.

Transient receptor potential channel M5 and phospholipaseC-β2 colocalizing in zebrafish taste receptor cells

In mammals, transient receptor potential (TRP) channel M5 (TRPM5) is coexpressed with phospholipaseC-beta2 (PLC-beta2) in the taste receptor cells, and both PLC-beta2 and TRPM5 are essential elements in the signal transduction of sweet, bitter and umami stimuli. In this study, we identified the zebrafish homologue of TRPM5 (zfTRPM5) and examined its expression in the gustatory system by in-situ hybridization. Using a transgenic zebrafish line that expressed green fluorescent protein under the control of the PLC-beta2 promoter, we showed that zfTRPM5 is expressed in green fluorescent protein-labeled cells of the taste buds. These results demonstrate that zfTRPM5 and PLC-beta2 colocalize in zebrafish taste receptor cells, suggesting their crucial roles in taste signaling via the fish taste receptors.

Characterization of ligands for fish taste receptors

Recent progress in the molecular biology of taste reception has revealed that in mammals, the heteromeric receptors T1R1/3 and T1R2/3 respond to amino acids and sweeteners, respectively, whereas T2Rs are receptors for bitter tastants. Similar taste receptors have also been characterized in fish, but their ligands have not been identified yet. In the present study, we conducted a series of experiments to identify the fish taste receptor ligands. Facial nerve recordings in zebrafish (Danio rerio) demonstrated that the fish perceived amino acids and even denatonium, which is a representative of aversive bitter compounds for mammals and Drosophila. Calcium imaging analysis of T1Rs in zebrafish and medaka fish (Oryzias latipes) using an HEK293T heterologous expression system revealed that both T1R1/3 and a series of T1R2/3 responded to amino acids but not to sugars. A triple-labeling, in situ hybridization analysis demonstrated that cells expressing T1R1/3 and T1R2/3s exist in PLCbeta2-expressing taste bud cells of medaka fish. Functional analysis using T2Rs showed that zfT2R5 and mfT2R1 responded to denatonium. Behavior observations confirmed that zebrafish prefer amino acids and avoid denatonium. These results suggest that, although there may be some fish-specific way of discriminating ligands, vertebrates could have a conserved gustatory mechanism by which T1Rs and T2Rs respond to attractive and aversive tastants, respectively.