Gustducin is a taste-cell-specific G protein closely related to the transducins

Determination of the depth of penetration of the alpha subunit of retinal G protein in membranes: a spectroscopic study

This paper reports the fluorescence quenching of the alpha subunit of retinal rod outer segment G protein (Gtalpha) by vesicles of brominated phospholipids. Two different brominated phospholipids with the bromine quencher groups attached at the 6-7 and 9-10 positions in one of the fatty acyl chains have been used to estimate the depth of penetration of the Gtalpha protein in the lipid vesicles using steady-state fluorescence quenching techniques. Our studies provide evidence of the interaction between Gtalpha protein, in its active conformation, with the lipid vesicles mimicking natural membranes. This study demonstrates that in vitro the distance between fluorescent tryptophan site of Gtalpha and the membrane surface is approximately 6.5 A.

Localization of a G protein Gi2 in the cilia of rat ependyma, oviduct and trachea

In previous studies, the localization of a pertussis toxin-sensitive G protein was demonstrated in ependymal cilia, but the identification of the subtype of G protein was inconsistent. To clarify this issue, we studied the localization of Goalpha, Gi1alpha, Gi3alpha and Gi2alpha in the ciliated ependymal cells and in the cilia of some other tissues of rats using specific antibodies. The cilia of the ependymal cells that line the ventricular cavity of the brain were intensely immunoreactive for Gi2alpha, but not for Goalpha, Gi1alpha or Gi3alpha. Immunoblot analysis demonstrated higher levels of Gi2alpha in the ependymal cilia-rich pellet than in the motor area of the parietal cortex. At the ultrastructural level, the immunoreactivity specific for Gi2alpha was found predominantly in the cilia, but rarely in the microvilli or the basal bodies of ependymal cells. In cross-sections, the immunoreactivity specific for Gi2alpha was observed only in cell membranes, in particular, in the inner electron-dense leaflet of the trilaminar structure. In addition to that in the ependymal cilia, such specific localization of Gi2alpha was observed in the motile cilia in other tissues, including the oviduct and trachea. By contrast, the stereocilia in the ductus deferens were not immunopositive for Gi2alpha. These findings suggest that Gi2 might play an important role in the signal transduction in ciliary membrane-associated function(s) of the ependymal cells, oviduct and trachea.

Is signal transduction modulated by an interaction between heterotrimeric G-proteins and tubulin?

Although it is generally accepted that tubulin plays an important role in G-protein-mediated signal transduction in a variety of systems, the mechanism of this phenomenon is not completely understood. G-protein-tubulin interaction at the cell membrane and the cytosol, and the influence of such an interaction on cellular signaling are discussed in this review article. Because the diameter of a microtubule is 25 nm and the plasma membrane is 9-11 nm thick, it is not possible for membrane-associated tubulin to assemble into a complete microtubule in the membrane environment. However, tubulin heterodimers may be able to function in the membrane environment as individual heterodimers or as polymers arranged into short protofilaments. At the cell membrane, membrane-associated tubulin may influence hormone-receptor interaction, receptor-G-protein coupling, and G-protein-effector coupling. Structural proteins, such as tubulin, can participate in cellular signaling by communicating through physical forces. By virtue of its interaction with the submembranous network of cytoskeletal proteins, tubulin, when perturbed in one locus, can transmit large changes in conformations to other points. Thus, GTP binding to membrane-associated tubulin might lead to a conformational change in either receptors or G proteins. This may, in turn, influence the binding of an agonist to its receptor. On the other hand, in the cell cytosol, subsequent to agonist-induced translocation of G-proteins from the membrane compartment to the cytosol, G-proteins may affect microtubule formation. In GH3 and AtT-20 cells (stably expressing TRH receptor), transiently transfected with Gq alpha cDNA, soluble tubulin levels decreased in Gq alpha-transfected GH3 and AtT-20 cells, by 33% and 52%, respectively. These results suggest that G-proteins may have a direct effect on the microtubule function in vivo. Because tubulin and G-protein families are ubiquitous and highly conserved, an interaction between these two protein families may occur in vivo, and this, in turn, can have an impact on signal transduction. However, the physiological significance of this interaction remains to be demonstrated.

Expression of the putative membrane fatty acid transporter (FAT) in taste buds of the circumvallate papillae in rats

The putative membrane fatty acid transporter (FAT) protein and its mRNA, originally expressed in adipose tissue, were found in the tongue of rats. Northern blot analysis showed a significant expression of FAT mRNA in the epithelial layer of circumvallate papillae. Immunohistochemical staining revealed that immunoreactivity for FAT is specifically localized in the apical part of taste bud cells, possibly gustatory cells, in the circumvallate papillae.

Taste buds have a cyclic nucleotide-activated channel, CNGgust

Cyclic nucleotide-gated (CNG) channels have been characterized as important factors involved in physiological processes including sensory reception for vision and olfaction. The possibility thus exists that a certain CNG channel functions in gustation as well. In the present study, we carried out reverse transcription-polymerase chain reaction and genomic DNA cloning and characterized a CNG channel (CNGgust) as a cyclic nucleotide-activated species expressed in rat tongue epithelial tissues where taste reception takes place. Several types of 5'-rapid amplification of cDNA ends clones of CNGgust cDNA were obtained with various 5'-terminal sequences. As the CNGgust gene was a single copy, the formation of such CNGgust variants should result from alternative splicing. The encoded protein was homologous to known vertebrate CNG channels with 50-80% similarities in amino acid sequence, and particularly homologous to bovine testis CNG channel and human cone CNG channel with 82% similarities. CNGgust was functional when expressed in human embryonic kidney cells, where it opened upon the addition of cGMP or cAMP. Immunohistochemical analysis using an antibody raised against a CNGgust peptide demonstrated the channel to be localized on the pore side of each taste bud in the circumvallate papillae, with no signal observed for degenerated taste buds after denervation of the glossopharyngeal nerve. All these results, together with the indication that cyclic nucleotides play a role gustatory signaling pathway(s), strongly suggest the involvement of CNGgust in taste signal transduction.

Role of subunit diversity in signaling by heterotrimeric G proteins

The heterotrimeric G proteins are extensively involved in the regulation of cells by extracellular signals. The receptors that control them are often the targets of drugs. There are many isoforms of each of the three subunits that make up these proteins. Thus far, genes for at least sixteen alpha subunits, five beta subunits, and eleven gamma subunits have been identified. In addition, some of these proteins have splice variants or are differentially modified. Based upon what is already known, there are well over a thousand possible G protein heterotrimer combinations. The role of subunit diversity in heterotrimer formation and its effect on signaling by G proteins are still not well understood. However, many current lines of research are leading toward an understanding of these roles. The functional significance of subunit heterogeneity is related to the mechanisms used by G proteins to transmit and integrate the many signals coming into cells through this system. Described here are the basic mechanisms by which G proteins integrate cellular responses, the possible role of subunit heterogeneity in these mechanisms, and the evidence for and against their physiological significance. Recent studies suggest the likely possibility that subunit heterogeneity plays an important role in signaling by G proteins. This role has the potential to extend substantially the flexibility of G proteins in mediating cellular responses to extracellular signals. However, the details of this are yet to be worked out, and they are the subject of many different avenues of research.

Differential expression of alpha-gustducin in taste bud populations of the rat and hamster

The G-protein subunit alpha-gustducin, which is similar to rod transducin, has been implicated in the transduction of both sweet- and bitter-tasting substances. In rodents, there are differences in sensitivity to sweet and bitter stimuli in different populations of taste buds. Rat fungiform taste buds are more responsive to salts than to sweet stimuli, whereas those on the palate respond predominantly to sweet substances. In contrast, hamster fungiform taste buds are more sensitive to sweet-tasting stimuli. Taste buds in the vallate and foliate papillae of both species are sensitive to bitter compounds. These differences in sensitivity should be reflected in the numbers of gustducin-containing cells in different taste bud populations. We examined taste buds in the rat and hamster for immunoreactivity to an antibody against alpha-gustducin. Immunofluorescence of labeled taste cells was examined by confocal microscopy, and the cells were counted. Gustducin-positive cells were seen in all taste bud regions; they were spindle-shaped, with circular cross-sections and apical processes that extended to the taste pore. Cells with this characteristic shape in rat vallate taste buds are Type II (light) cells. In the rat, taste buds of the fungiform papillae had fewer gustducin-positive cells (3.1/taste bud) than those of other regions, including the posterior tongue and palate (>8.9/taste bud). Hamster fungiform taste buds contained twice as many gustducin-expressing cells (6.8/taste bud) as those of the rat. These data support the hypothesis that alpha-gustducin is involved in the transduction of both sweet- and bitter-tasting stimuli by mammalian taste receptor cells.

Analysis and comparison of partial sequences of clones from a taste-bud-enriched cDNA library

Differential patterns of cellular development and function are determined, at least in part, by the specific gene expression of particular cells. Thus, determination of differential patterns of gene expression between tissues is likely to help elucidate molecular details of tissue-specific processes. Our hypothesis was that cells of the circumvallate papilla involved in taste perception would express genes that are not expressed in the surrounding epithelium and that determination of the nature of these genes could be helpful in our understanding of the molecular details of taste. Using partial sequencing of clones derived from rat circumvallate papillae, we have begun to characterize genes that could be important in taste. We prepared a cDNA library of whole circumvallate papillae and, by means of a novel subtraction procedure, enriched taste-specific clones. Characterization of the libraries showed that subtraction resulted in good enrichment of taste-specific clones. Here we report the partial sequencing and analysis of 410 cDNA clones from the taste-bud-enriched cDNA library. Approximately 25% of the genes were identified on the basis of their high homology to known transcripts. These included the developmentally important molecules Pax-1, esp1, Notch 1, and Notch 3 that may play roles in the continuous turnover of taste receptor cells. A further 20% of the genes had no significant homology to known DNA sequences and were identified as taste-specific by Southern blot analysis.

The Effects of G Protein Modulators on the Labellar Taste Receptor Cells of the Fleshfly (Boettcherisca peregrina)

A nonhydrolyzable G protein activator (guanosine 5'-O-(3-thiotriphosphate); GTPgammaS) and a G protein inhibitor (guanosine 5'-O-(2-thiodiphosphate); GDPbetaS) were introduced into the labellar taste receptor cells of the fleshfly by treatment of their receptive membranes beneath the tip opening of the chemosensory hair with each reagent in 0.03% deoxycholate solution for 4 min. After treatment with GTPgammaS, the responses of the sugar receptor cell to D-glucose, D-fructose, L-phenylalanine and L-valine and that of the salt receptor cell to cyclic AMP were markedly enhanced, compared with those after treatment with deoxycholate alone. Treatment with GDPbetaS depressed these responses. These results strongly suggest that the responses are mediated by G protein. However, the response of the salt receptor cell to NaCl was not affected by treatment with either GTPgammaS or GDPbetaS, and thus the response to NaCl clearly is not elicited through a G protein-regulated mechanism. Copyright 1997 Elsevier Science Ltd. All rights reserved

Light and dark cells of rat vallate taste buds are morphologically distinct cell types

Cells of mammalian taste buds have been classified into morphological types based on ultrastructural criteria, but investigators have disagreed as to whether these are distinct cell types or the extremes of a continuum. To address this issue, we examined taste buds from rat vallate papillae that had been sectioned transversely, rather than longitudinally, to their longest axis. In these transverse sections, dark (Type I) and light (Type II) cells were easily distinguished by their relative electron density, shape and topological relationships. Cells with electron-lucent cytoplasm (light cells) were circular or oval in outline, while those with electron-dense cytoplasm (dark cells) had an irregular outline with sheetlike cytoplasmic projections that separated adjacent light cells. A hierarchical cluster analysis of 314 cells across five morphological parameters (cell shape and area, and nuclear ellipticity, electron density and invagination) revealed two distinct groups of cells, which largely corresponded to the dark and light cells identified visually. These cells were not continuously distributed within a principal components factor solution. Differences in the means for dark and light cells were highly significant for each morphological parameter, but within either cell type, changes in one parameter correlated little with changes in any other. These analyses all failed to reveal cells with a consistent set of intermediate characteristics, suggesting that dark and light cells of rat vallate taste buds are distinct cell types rather than extremes of a continuum. Sections of taste buds were stained with antibodies to several carbohydrates, then observed by indirect immunofluorescence. Optical sections taken with a confocal laser-scanning microscope showed that the Lewis antigen was present only on spindle-shaped cells with circular or oval outlines and lacking transverse projections; these characteristic shapes matched those of light cells seen by electron microscopy. The H blood group antigen and the 2B8 epitope appeared at most cell-cell interfaces in the bud and are present on dark cells and possibly on some light cells. These findings relate molecular markers to morphological phenotypes and should facilitate future studies of taste cell turnover, development and regeneration.

Signalling functions and biochemical properties of pertussis toxin-resistant G-proteins

Pertussis toxin (PTX) has been widely used as a reagent to characterize the involvement of heterotrimeric G-proteins in signalling. This toxin catalyses the ADP-ribosylation of specific G-protein alpha subunits of the Gi family, and this modification prevents the occurrence of the receptor-G-protein interaction. This review focuses on the biochemical properties and signalling of those G-proteins historically classified as 'PTX-resistant' due to the inability of the toxin to influence signalling through them. These G-proteins include members of the Gq and G12 families and one Gi family member, i.e. Gz. Signalling pathways controlled by these G-proteins are well characterized only for Gq family members, which activate specific isoforms of phospholipase C, resulting in increases in intracellular calcium and activation of protein kinase C (PKC), among other responses. While members of the G12 family have been implicated in processes that regulate cell growth, and Gz has been shown to inhibit adenylate cyclase, the specific downstream targets to these G-proteins in vivo have not been clearly established. Since two of these proteins, G12 alpha and Gz alpha, are excellent substrates for PKC, there is the potential for cross-talk between their signalling and Gq-dependent processes leading to activation of PKC. In tissues that express these G-proteins, a number of guanine-nucleotide-dependent, PTX-resistant, signalling pathways have been defined for which the G-protein involved has not been identified. This review summarizes these pathways and discusses the evidence both for the participation of specific PTX-resistant G-proteins in them and for the regulation of these processes by PKC.

New perspectives in a gustatory physiology: transduction, development, and plasticity

Major advances in the understanding of mammalian gustatory transduction mechanisms have occurred in the past decade. Recent research has revealed that a remarkable diversity of cellular mechanisms are involved in taste stimulus reception. These mechanisms range from G protein-and second messenger-linked receptor systems to stimulus-gated and stimulus-admitting ion channels. Contrary to widely held ideas, new data show that some taste stimuli interact with receptive sites that are localized on both the apical and basolateral membranes of taste cells. Studies of taste system development in several species indicate that the transduction pathways for some stimuli are modulated significantly during the early postnatal period. In addition, recent investigations of adult peripheral gustatory system plasticity strongly suggest that the function of the Na+ sensing system can be modulated by circulating hormones, growth factors, or cytokines.

G protein alpha subunit multigene family in the Japanese puffer fish Fugu rubripes: PCR from a compact vertebrate genome

We compare the complexity and organization of the G protein alpha subunit multigene family in the vertebrate genomes of mammals and the Japanese puffer fish Fugu rubripes. Fourteen Fugu G alpha genes were identified of the 16 genes characterized previously in mammals, including Fugu genes from the four classes of alpha subunits Gs, Gi, Gq, and G12. Fugu and mammalian G alpha coding sequences are highly homologous, and the intron/exon structure of the fish and mammalian orthologs is identical throughout the coding regions. A novel G alpha gene, G alpha p1, was also identified in Fugu rubripes and two other species of puffer fish. The complete sequence of Gnaz and the tandemly duplicated genes Gnai2 and Gnat1 were obtained from a Fugu genomic cosmid library. Introns in the puffer fish G alpha genes lacked repeat DNA sequences, other than simple sequence length repeats, and most introns were significantly shorter in Fugu than in mammalian orthologs. The compact genome of puffer fish provides a unique vertebrate model for characterizing multigene families and identifying novel genes directly from genomic DNA by PCR amplification with degenerate primers. The fact that Fugu encodes most, if not all, of the G protein alpha subunits identified in mammals strongly supports Fugu as a model organism for vertebrate genome research.

Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation and chemosensation in C. elegans

The C. elegans tax-4 mutants are abnormal in multiple sensory behaviors: they fail to respond to temperature or to water-soluble or volatile chemical attractants. We show that the predicted tax-4 gene product is highly homologous to vertebrate cyclic nucleotide-gated channels. Tax-4 protein expressed in cultured cells functions as a cyclic nucleotide-gated channel. The green fluorescent protein (GFP)-tagged functional Tax-4 protein is expressed in thermosensory, gustatory, and olfactory neurons mediating all the sensory behaviors affected by the tax-4 mutations. The Tax-4::GFP fusion is partly localized at the sensory endings of these neurons. The results suggest that a cyclic nucleotide-gated channel is required for thermosensation and chemosensation and that cGMP is an important intracellular messenger in C. elegans sensory transduction.

Chemoreception: tasting the sweet and the bitter

Our understanding of the molecular basis of taste transduction has lagged behind that of other senses, but now a signalling protein-the G protein alpha subunit gustducin-has been shown to be taste-tissue specific and essential for both bitter and sweet tastes.

Detection of cone alpha transducin mRNA in human fetal cochlea: negative mutation analysis in Usher syndrome

Cone alpha transducin (GNAT2), known to be expressed in photoreceptors, was found to be transcribed in human fetal cochlea. Due to the unexpected finding of expression of this gene in the inner ear and the success of the candidate gene approach in identifying mutations for a variety of heritable disorders, we investigated the possible role of this gene in Usher syndrome type I and type II. Single-strand conformation polymorphism (SSCP) was used to screen the GNAT2 coding region, as well as splice donor and acceptor sites, for mutations in a total of 140 unrelated patients. Two nucleotide changes leading to two silent amino acid changes and one rare polymorphism were found. In view of these results and those of a previously published Southern blot analysis, it is unlikely that mutations in GNAT2 are a common gene abnormality in Usher syndrome type I or type II.

Mechanisms of taste transduction

Taste cells use a wide variety of mechanisms for transduction. Ionic stimuli, such as salts and acids, interact directly with ion channels to depolarize taste cells. More complex stimuli, such as sugars and amino acids, utilize apically located receptors for transduction. Recent molecular biological results suggest that the metabotropic glutamate receptor mGluR4 may function in glutamate taste transduction. New biochemical studies have identified a bitter-responsive receptor that activates gustducin. Unexpected results with knockout mice suggest that gustducin may be directly involved in both bitter and sweet transduction. Electrophysiological experiments indicate that both inositol trisphosphate and cyclic nucleotides function in both bitter and sweet transduction events.

The taste of monosodium glutamate: membrane receptors in taste buds

Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (RT)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.

Responses of the hamster chorda tympani nerve to binary component taste stimuli: evidence for peripheral gustatory mixture interactions

Studies of taste mixtures suggest that stimuli which elicit different perceptual taste qualities physiologically interact in the gustatory system and thus, are not independently processed. The present study addressed the role of the peripheral gustatory system in these physiological interactions by measuring the effects of three heterogeneous taste mixtures on responses of the chorda tympani (CT) nerve in the hamster (Mesocricetus auratus). Binary taste stimuli were presented to the anterior tongue and multi-fiber neural responses were recorded from the whole CT. Stimuli consisted of a concentration series of quinine.HCl (QHCl: 1-30 mM), sodium chloride (NaCl: 10-250 mM), sucrose (50-500 mM) and binary combinations of the three different chemicals. Each mixture produced a unique pattern of results on CT response magnitudes measured 10 s into the response. Sucrose responses were inhibited by quinine in QHCl-sucrose mixtures. Neural activity did not increase when quinine was added to 50-250 mM NaCl in QHCl-NaCl mixtures. However, the neural activity elicited by sucrose-NaCl mixtures was greater than the activity elicited by either component stimulus presented alone. The results demonstrate that gustatory mixture interactions are initiated at the level of the taste bud or peripheral nerve. Mechanisms for these interactions are unknown. The results are consistent with one component stimulus modifying the interaction of the other component stimulus with its respective transduction mechanism. Alternatively, peripheral inhibitory mechanisms may come into play when appetitive and aversive stimuli are simultaneously presented to the taste receptors.

Transduction of bitter and sweet taste by gustducin

Several lines of evidence suggest that both sweet and bitter tastes are transduced via receptors coupled to heterotrimeric guanine-nucleotide-binding proteins (G proteins). Gustducin is a taste receptor cell (TRC)-specific G protein that is closely related to the transducins. Gustducin and rod transducin, which is also expressed in TRCs, have been proposed to couple bitter-responsive receptors to TRC-specific phosphodiesterases to regulate intracellular cyclic nucleotides. Here we investigate gustducin's role in taste transduction by generating and characterizing mice deficient in the gustducin alpha-subunit (alpha-gustducin). As predicted, the mutant mice showed reduced behavioural and electrophysiological responses to bitter compounds, whereas they were indistinguishable from wild-type controls in their responses to salty and sour stimuli. Unexpectedly, mutant mice also exhibited reduced behavioural and electrophysiological responses to sweet compounds. Our results suggest that gustducin is a principal mediator of both bitter and sweet signal transduction.

Taste receptor-like cells in the rat gut identified by expression of alpha-gustducin

The alpha-subunit of the trimeric G-protein complex specific for taste receptor cells of the tongue, alpha-gustducin, is described here to be also expressed in the stomach and intestine. The alpha-gustducin-containing cells were identified as brush cells that are scattered throughout the surface epithelium of the gut and share structural features of taste receptor cells of the tongue. These findings provide clues to the long-sought molecular and cellular basis for chemoreception in the gut.

Making sense of chemicals

The sensation of taste is initiated when chemicals interact with peripheral receptors in the oral cavity, activating a cascade of cellular events that lead to changes in neurotransmitter release onto afferent nerve fibers. The mechanisms of taste transduction are diverse and involve a rich array of signaling components.

Rapid kinetics of second messenger production in bitter taste

The tasting of bitter compounds may have evolved as a protective mechanism against ingestion of potentially harmful substances. We have identified second messengers involved in bitter taste and show here for the first time that they are rapid and transient. Using a quench-flow system, we have studied bitter taste signal transduction in a pair of mouse strains that differ in their ability to taste the bitter stimulus sucrose octaacetate (SOA); however, both strains taste the bitter agent denatonium. In both strains of mice, denatonium (10 mM) induced a transient and rapid increase in levels of the second messenger inositol 1,4,5-trisphosphate (IP3) with a maximal production near 75-100 ms after stimulation. In contrast, SOA (100 microM) brought about a similar increase in IP3 only in SOA-taster mice. The response to SOA was potentiated in the presence of GTP (1 microM). The GTP-enhanced SOA-response supports a G protein-mediated response for this bitter compound. The rapid kinetics, transient nature, and specificity of the bitter taste stimulus-induced IP3 formation are consistent with the role of IP3 as a second messenger in the chemoelectrical transduction of bitter taste.

SC1: a marker for astrocytes in the adult rodent brain is upregulated during reactive astrocytosis

Astrocytes are the most abundant cell type in the mammalian central nervous system (CNS), and are involved in many processes critical for normal CNS maintenance and function. We have used double-label immunocytochemistry and in situ analysis to show that the SPARC (secreted protein acidic and rich in cysteine)-related protein SC1, co-localizes with the astrocyte marker glial fibrillary acidic protein (GFAP) in the adult rodent brain. Thus, SC1 is an astrocyte marker that may be used to investigate astrocyte heterogeneity and analyze glial cell lineages during neural development. Consistent with the presence of SC1 and GFAP in astrocytes, both proteins were markedly upregulated following reactive astrocytosis induced by focal mechanical trauma. Therefore, SC1 may play an important role in reactive astrocytosis subsequent to a wide variety of neural trauma, including neurodegenerative diseases and acute neural damage.

Effects of fluoride and cholera and pertussis toxins on sensory transduction in the carotid body

The regulation of the chemoreceptor cell function by G proteins has been studied by measuring the release of 3H-labeled catecholamines ([3H]CA) in carotid bodies (CBs) treated with fluoride, cholera toxin (CTX), and pertussis toxin (PTX). Fluoride augmented the basal release of [3H]CA in a dose- (5-20 mM) and Ca(2+)-dependent manner. Nisoldipine (1 microM) and ethylisopropyl amiloride (EIPA; 10 microM) inhibited this effect by approximately 60%, and both drugs combined inhibited it in full. BAY K 8644 (1 microM) doubled the effect of fluoride. The effects of fluoride on the stimulus-evoked release of [3H]CA varied with the type of stimulus and the duration of the treatment. Simultaneous application of fluoride with the stimulus increased by five times the release evoked by hypoxia and by two times that by K+ and dinitrophenol (DNP). Preincubation with fluoride for 1 h caused an inhibition (approximately 70%) of the release evoked by high K+ and veratridine, whereas that evoked by DNP and low PO2 was still augmented (approximately 2 times). Preincubation (4 h) of the CBs with CTX (3 micrograms/ml) reduced by 54% the release of [3H]CA evoked by 35 mM K+ but did not affect that evoked by low PO2 or DNP. A similar treatment with PTX (1 microgram/ml) affected only the release of [3H]CA evoked by DNP, reducing it by 65%. The data show that fluoride, CTX, and PTX have different effects on the release of [3H]CA evoked by high external K+, DNP, and low PO2, indicating that the stimulus-secretion coupling process for each stimulus is differently regulated by G proteins.

GNAZ in human fetal cochlea: expression, localization, and potential role in inner ear function

Dissociation of an activated alpha-subunit from the beta-gamma complex directly regulates secondary messenger proteins. To address the potential role of G proteins expressed in human fetal cochlea, degenerate oligonucleotide primers corresponding to the 3'-end of the conserved region of alpha-subunits were used for polymerase chain reaction amplification of reverse-transcribed total human fetal cochlear mRNAs; GNAZ and GNAQ were isolated. These two G proteins are unique among the G-protein family because they lack a typical pertussis modification site. GNAZ is expressed in high levels in neural tissue while GNAQ is ubiquitously expressed. We characterized GNAZ expression using Northern blots, tissue in-situ hybridization and immunohistochemistry techniques to elucidate the potential role of this protein in inner ear function. Our data suggest that GNAZ may play a role in maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.

Effects of chlorhexidine on human taste perception

Chlorhexidine gluconate at a dose used to control bacteria in the mouth has a reversible effect on taste perception. Taste-intensity ratings and taste-quality identification for concentration series of sucrose, sodium chloride, citric acid and quinine hydrochloride were obtained from 15 healthy humans. The participants rinsed with 0.12% chlorhexidine for 3 min twice a day. Each individual was tested 3 times: before the 4-day rinse period, 30 min after the final rinse, and 4 days after the rinse period. Chlorhexidine rinses reduced the perceptual intensity of sodium chloride and quinine hydrochloride, not sucrose or citric acid. No effects on taste perception were detected 4 days after the rinse period. The identification of sodium chloride as salty was seriously impaired by chlorhexidine but the identification of quinine hydrochloride as bitter was not affected. Specific sites of action of chlorhexidine on the taste epithelium are not known but its effects on salty taste may be related to its strong positive charge and its effect on bitter taste may be related to its amphiphilicity. Chlorhexidine has promise as a probe of taste transduction, as well as for the management of salty/bitter dysgeusias in humans.

Effect of lipid-derived second messengers on electrophysiological taste responses in the gerbil

Integrated chorda tympani (CT) recordings were made to salty, sour, sweet, bitter, and glutamate tastants before and after a 4-min application of modulators of lipid-derived second messenger systems. The modulators included two membrane-permeable analogues of DAG, 1-oleoyl-2-acetyl glycerol (OAG) and dioctanoyl glycerol (DiC8); thapsigargin, which releases Ca++ from intracellular stores; ionomycin, a calcium ionophore; lanthanum chloride, an inorganic calcium channel blocker; nifedipine, a dihydropyridine calcium channel blocker; quinacrine diHCl, a phospholipase A2 antagonist; melittin, a phospholipase A2 agonist; and indomethacin, which decreases the release of prostaglandins by inhibiting the enzyme cyclo-oxygenase. The main findings were: OAG (125 microM) and DiC8 (100 microM) blocked the responses of several bitter compounds while enhancing the taste response to several sweeteners. Lanthanum chloride blocked all responses, which may be due to the fact that it blocks tight junctions. Quinacrine (1 mM) suppressed several bitter responses while enhancing the response to several sweeteners. The enhancement of sweet taste responses by DAG analogues suggests that there is cross-talk between the adenylate cyclase system and one (or more) pathways involving lipid-derived second messengers in taste cells.

Functional expression of the taste specific G-protein, alpha-gustducin

The taste-specific G-protein alpha-subunit, alpha-gustducin, was expressed using a baculovirus based system. alpha-Gustducin was demonstrated to be myristoylated and was also palmitoylated in insect larval cells. Recombinant alpha-gustducin was purified to homogeneity. Neither receptors nor effectors that interact with gustducin in taste are known. However, alpha-gustducin has a close structural similarity to the visual G-protein, alpha-transducin. Therefore alpha-gustducin was reconstituted with components of the visual system to determine the degree of its functional similarity with alpha-transducin. Despite the fact that the sequences of alpha-gustducin and alpha-transducin share only 80% identity with each other, the interactions and functions of these two proteins were quantitatively identical. These included the interaction with receptor, bovine rhodopsin, with effector, bovine retinal cyclic GMP-phosphodiesterase, and with bovine brain and retinal G-protein beta gamma-heterodimers; receptor-catalysed GDP-GTP exchange and the intrinsic GTPase activity of alpha-gustducin and alpha-transducin were also identical. Gi alpha which is 70% identical with alpha-transducin interacts with different receptor and effector proteins and has very different guanine-nucleotide binding properties. Therefore, the functional equivalence of alpha-gustducin and alpha-transducin suggest that taste buds are likely to contain receptor and effector proteins that share many properties with their retinal equivalents.

A cyclic-nucleotide-suppressible conductance activated by transducin in taste cells

Taste can be divided into four primary sensations: salty, sour, sweet and bitter. Salty and sour are directly transduced by apical channels, whereas sweet and bitter utilize cyclic nucleotide second messengers. We have shown that rod transducin is present in mammalian taste receptor cells, where it is activated by a bitter receptor and in turn activates a phosphodiesterase. Here we introduce into frog taste cells peptides derived from transducin's phosphodiesterase-interaction region, which cause an inward whole-cell current in a subset of cells. We find that the peptides' effects are reversibly suppressed by IBMX and forskolin, indicative of a transducin-activated phosphodiesterase. Cyclic nucleotides suppress the whole-cell current, indicating that cyclic nucleotides may regulate taste-cell conductance. IBMX modifies taste-cell responses to two taste stimuli, implicating phosphodiesterase in taste transduction. Submicromolar cyclic nucleotides directly suppress the conductance of inside-out patches derived from the taste-cell plasma membrane, independently of protein phosphorylation. The channels are unusual in that they are suppressed, rather than activated by cyclic nucleotides. We propose that transducin, via phosphodiesterase, decreases cyclic nucleotide levels to activate the cyclic-nucleotide-suppressible conductance, leading to Ca2+ influx and taste-cell depolarization.

Coupling of bitter receptor to phosphodiesterase through transducin in taste receptor cells

The rod and cone transducins are specific G proteins originally thought to be present only in photoreceptor cells of the vertebrate retina. Transducins convert light stimulation of photoreceptor opsins into activation of cyclic GMP phosphodiesterase (reviewed in refs. 5-7). A transducin-like G protein, gustducin, has been identified and cloned from rat taste cells. We report here that rod transducin is also present in vertebrate taste cells, where it specifically activates a phosphodiesterase isolated from taste tissue. Furthermore, the bitter compound denatonium in the presence of taste-cell membranes activates transducin but not Gi. A peptide that competitively inhibits rhodopsin activation of transducin also blocks taste-cell membrane activation of transducin, arguing for the involvement of a seven-transmembrane-helix G-protein-coupled receptor. These results suggest that rod transducin transduces bitter taste by coupling taste receptor(s) to taste-cell phosphodiesterase. Phosphodieterase-mediated degradation of cyclic nucleotides may lead to taste-cell depolarization through the recently identified cyclic-nucleotide-suppressible conductance.

Nobel Lecture. G proteins and regulation of adenylyl cyclase

The function and structures of G proteins and their role in the regulation of adenylyl cyclase is reviewed.

Pineal opsin: a nonvisual opsin expressed in chick pineal

Pineal opsin (P-opsin), an opsin from chick that is highly expressed in pineal but is not detectable in retina, was cloned by the polymerase chain reaction. It is likely that the P-opsin lineage diverged from the retinal opsins early in opsin evolution. The amino acid sequence of P-opsin is 42 to 46 percent identical to that of the retinal opsins. P-opsin is a seven-membrane spanning, G protein-linked receptor with a Schiff-base lysine in the seventh membrane span and a Schiff-base counterion in the third membrane span. The primary sequence of P-opsin suggests that it will be maximally sensitive to approximately 500-nanometer light and produce a slow and prolonged phototransduction response consistent with the nonvisual function of pineal photoreception.

Expression of GTP-binding protein alpha subunits in human thymocytes

In this report, we investigate G protein alpha subunit diversity in human thymocytes, utilizing common properties shared by these genes and reverse transcription-polymerase chain reaction (RT-PCR). Sequence analysis of PCR amplified gene portions, indicate the presence of members from all four G-protein families that have been described thus far. The alpha subunit genes identified are: G alpha i1-3 and G alpha z but not G alpha o from the Gi family, G alpha s from the Gs family, G alpha 11, G alpha q, and G alpha 16 from the Gq family, and G alpha 12 and G alpha 13 from the G12 family. Also in this report we present the nucleotide and predicted amino acid sequences of the human G alpha 13 cloned from a thymocyte cDNA library. The sequence of the human G alpha 13 has not been previously reported. Comparison of this sequence with the reported murine G alpha 13 shows > 90% identity at the deduced amino acid sequence level. We conclude that thymocytes represent a useful experimental system for the study of G protein involvement in immune responses and lymphocyte development.

Receptors and G proteins as primary components of transmembrane signal transduction. Part 2. G proteins: structure and function

Seven-transmembrane receptors signal through nucleotide-binding proteins (G proteins) into the cell. G proteins are membrane-associated proteins composed of three subunits termed alpha, beta and gamma, of which the G alpha subunit classifies the heterotrimer. So far, 23 different mammalian G alpha subunits are known, which are grouped in four subfamilies (Gs, Gi, Gq, G12) on the basis of their amino acid similarity. They carry an endogenous GTPase activity allowing reversible functional coupling between ligand-bound receptors and effectors such as enzymes and ion channels. In addition, five G beta and seven G gamma subunits have been identified which form tightly associated beta gamma heterodimers. Upon activation by a ligand-bound receptor the G protein dissociates into G alpha and G beta gamma, which both transmit signal by interacting with effectors. On the G protein level, specificity and selectivity of the incoming signal is accomplished by G protein trimers composed of distinct subunits. On the other hand, many receptors have been shown to activate different G proteins, thereby regulating diverse signal transduction pathways.

Generation of inositol phosphates in bitter taste transduction

It is probable that there is a diversity of mechanisms involved in the transduction of bitter taste. One of these mechanisms uses the second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Partial membrane preparations from circumvallate and foliate taste regions of mice tongues responded to the addition of known bitter taste stimuli by increasing the amount of inositol phosphates produced after 30 s incubation. Addition of both the bitter stimulus, sucrose octaacetate and the G-protein stimulant, GTP gamma S, led to an enhanced production of inositol phosphates compared with either alone. Pretreatment of the tissue samples with pertussis toxin eliminated all response to sucrose octaacetate plus GTP gamma S, whereas pretreatment with cholera toxin was without effect. Western blots of solubilized tissue from circumvallate and foliate regions probed with antibodies to the alpha-subunit of several types of G-proteins revealed bands reactive to antibodies against G alpha i1-2 and G alpha o, with no apparent activity to antibodies against G alpha i3. Given the results from the immunoblots and those of the toxin experiments, it is proposed that the transduction of the bitter taste of sucrose octaacetate in mice involves a receptor-mediated activation of a Gi-type protein which activates a phospholipase C to produce the two second messengers, IP3 and DAG.

Molecular cloning of G proteins and phosphodiesterases from rat taste cells

To identify and characterize those proteins involved in taste transduction, we cloned G proteins and phosphodiesterases from rat taste tissue. Using degenerate primers corresponding to conserved regions of G protein alpha subunits, the polymerase chain reaction was used to amplify and clone eight distinct cDNAs: alpha i-2, alpha i-3, alpha 12, alpha 14, a(s), alpha t-rod, alpha t-cone and alpha gustducin. alpha i-3, alpha 14, alpha s, and alpha t-rod are more highly expressed in taste tissue than in the surrounding nonsensory tissue. alpha gustducin is only expressed in taste cells. Rod transducin had previously been found only in the rod cells of the retina, where it converts light stimulation of rhodopsin into activation of cGMP phosphodiesterase. The primary sequence of alpha gustducin shows striking similarities to rod transducin in the receptor interaction domain and the phosphodiesterase activation site. We propose that gustducin and transducin regulate phosphodiesterase activity in taste cells and that this may promote bitter transduction and inhibit sweet transduction. Consistent with this proposal, we cloned two types of cAMP PDE from taste tissue: dnc-1 and PDE-3.