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

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Expression of Taste Receptor 2 Subtypes in Human Testis and Sperm

Taste receptors (TASRs) are expressed not only in the oral cavity but also throughout the body, thus suggesting that they may play different roles in organ systems beyond the tongue. Recent studies showed the expression of several TASRs in mammalian testis and sperm, indicating an involvement of these receptors in male gametogenesis and fertility. This notion is supported by an impaired reproductive phenotype of mouse carrying targeted deletion of taste receptor genes, as well as by a significant correlation between human semen parameters and specific polymorphisms of taste receptor genes. To better understand the biological and thus clinical significance of these receptors for human reproduction, we analyzed the expression of several members of the TAS2Rs family of bitter receptors in human testis and in ejaculated sperm before and after in vitro selection and capacitation. Our results provide evidence for the expression of TAS2R genes, with TAS2R14 being the most expressed bitter receptor subtype in both testis tissue and sperm cells, respectively. In addition, it was observed that in vitro capacitation significantly affects both the expression and the subcellular localization of these receptors in isolated spermatozoa. Interestingly, α-gustducin and α-transducin, two Gα subunits expressed in taste buds on the tongue, are also expressed in human spermatozoa; moreover, a subcellular redistribution of both G protein α-subunits to different sub-compartments of sperm was registered upon in vitro capacitation. Finally, we shed light on the possible downstream transduction pathway initiated upon taste receptor activation in the male reproductive system. Performing ultrasensitive droplets digital PCR assays to quantify RNA copy numbers of a distinct gene, we found a significant correlation between the expression of TAS2Rs and TRPM5 (r = 0.87), the cation channel involved in bitter but also sweet and umami taste transduction in taste buds on the tongue. Even if further studies are needed to clarify the precise functional role of taste receptors for successful reproduction, the presented findings significantly extend our knowledge of the biological role of TAS2Rs for human male fertility.

A Pharmacological Perspective on the Study of Taste

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

Development of Full Sweet, Umami, and Bitter Taste Responsiveness Requires Regulator of G protein Signaling-21 (RGS21)

The mammalian tastes of sweet, umami, and bitter are initiated by activation of G protein-coupled receptors (GPCRs) of the T1R and T2R families on taste receptor cells. GPCRs signal via nucleotide exchange and hydrolysis, the latter hastened by GTPase-accelerating proteins (GAPs) that include the Regulators of G protein Signaling (RGS) protein family. We previously reported that RGS21, uniquely expressed in Type II taste receptor cells, decreases the potency of bitter-stimulated T2R signaling in cultured cells, consistent with its in vitro GAP activity. However, the role of RGS21 in organismal responses to GPCR-mediated tastants was not established. Here, we characterized mice lacking the Rgs21 fifth exon. Eliminating Rgs21 expression had no effect on body mass accumulation (a measure of alimentation), fungiform papillae number and morphology, circumvallate papillae morphology, and taste bud number. Two-bottle preference tests, however, revealed that Rgs21-null mice have blunted aversion to quinine and denatonium, and blunted preference for monosodium glutamate, the sweeteners sucrose and SC45647, and (surprisingly) NaCl. Observed reductions in GPCR-mediated tastant responses upon Rgs21 loss are opposite to original expectations, given that loss of RGS21-a GPCR signaling negative regulator-should lead to increased responsiveness to tastant-mediated GPCR signaling (all else being equal). Yet, reduced organismal tastant responses are consistent with observations of reduced chorda tympani nerve recordings in Rgs21-null mice. Reduced tastant-mediated responses and behaviors exhibited by adult mice lacking Rgs21 expression since birth have thus revealed an underappreciated requirement for a GPCR GAP to establish the full character of tastant signaling.

Activation of airway epithelial bitter taste receptors by Pseudomonas aeruginosa quinolones modulates calcium, cyclic-AMP, and nitric oxide signaling

Bitter taste receptors (taste family 2 bitter receptor proteins; T2Rs), discovered in many tissues outside the tongue, have recently become potential therapeutic targets. We have shown previously that airway epithelial cells express several T2Rs that activate innate immune responses that may be important for treatment of airway diseases such as chronic rhinosinusitis. It is imperative to more clearly understand what compounds activate airway T2Rs as well as their full range of functions. T2R isoforms in airway motile cilia (T2R4, -14, -16, and -38) produce bactericidal levels of nitric oxide (NO) that also increase ciliary beating, promoting clearance of mucus and trapped pathogens. Bacterial quorum-sensing acyl-homoserine lactones activate T2Rs and stimulate these responses in primary airway cells. Quinolones are another type of quorum-sensing molecule used by Pseudomonas aeruginosa To elucidate whether bacterial quinolones activate airway T2Rs, we analyzed calcium, cAMP, and NO dynamics using a combination of fluorescent indicator dyes and FRET-based protein biosensors. T2R-transfected HEK293T cells, several lung epithelial cell lines, and primary sinonasal cells grown and differentiated at the air-liquid interface were tested with 2-heptyl-3-hydroxy-4-quinolone (known as Pseudomonas quinolone signal; PQS), 2,4-dihydroxyquinolone, and 4-hydroxy-2-heptylquinolone (HHQ). In HEK293T cells, PQS activated T2R4, -16, and -38, whereas HHQ activated T2R14. 2,4-Dihydroxyquinolone had no effect. PQS and HHQ increased calcium and decreased both baseline and stimulated cAMP levels in cultured and primary airway cells. In primary cells, PQS and HHQ activated levels of NO synthesis previously shown to be bactericidal. This study suggests that airway T2R-mediated immune responses are activated by bacterial quinolones as well as acyl-homoserine lactones.

Whole transcriptome profiling of taste bud cells

Analysis of single-cell RNA-Seq data can provide insights into the specific functions of individual cell types that compose complex tissues. Here, we examined gene expression in two distinct subpopulations of mouse taste cells: Tas1r3-expressing type II cells and physiologically identified type III cells. Our RNA-Seq libraries met high quality control standards and accurately captured differential expression of marker genes for type II (e.g. the Tas1r genes, Plcb2, Trpm5) and type III (e.g. Pkd2l1, Ncam, Snap25) taste cells. Bioinformatics analysis showed that genes regulating responses to stimuli were up-regulated in type II cells, while pathways related to neuronal function were up-regulated in type III cells. We also identified highly expressed genes and pathways associated with chemotaxis and axon guidance, providing new insights into the mechanisms underlying integration of new taste cells into the taste bud. We validated our results by immunohistochemically confirming expression of selected genes encoding synaptic (Cplx2 and Pclo) and semaphorin signalling pathway (Crmp2, PlexinB1, Fes and Sema4a) components. The approach described here could provide a comprehensive map of gene expression for all taste cell subpopulations and will be particularly relevant for cell types in taste buds and other tissues that can be identified only by physiological methods.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

Regulation of Adipogenesis by Quinine through the ERK/S6 Pathway

Quinine is a bitter tasting compound that is involved in the regulation of body weight as demonstrated in in vivo animal models and in vitro models of the adipogenic system. Arguments exist over the positive or negative roles of quinine in both in vivo animal models and in vitro cell models, which motivates us to further investigate the functions of quinine in the in vitro adipogenic system. To clarify the regulatory functions of quinine in adipogenesis, mouse primary preadipocytes were induced for differentiation with quinine supplementation. The results showed that quinine enhanced adipogenesis in a dose dependent manner without affecting lipolysis. The pro-adipogenic effect of quinine was specific, as other bitter tasting agonists had no effect on adipogenesis. Moreover, the pro-adipogenic effect of quinine was mediated by activation of ERK/S6 (extracellular-signal-regulated kinase/Ribosomal protein S6) signaling. Knockdown of bitter taste receptor T2R106 (taste receptor, type 2, member 106) impaired the pro-adipogenic effect of quinine and suppressed the activation of ERK/S6 signaling. Taken together, quinine stimulates adipogenesis through ERK/S6 signaling, which at least partly functions via T2R106.

Taste receptors in innate immunity

Taste receptors were first identified on the tongue, where they initiate a signaling pathway that communicates information to the brain about the nutrient content or potential toxicity of ingested foods. However, recent research has shown that taste receptors are also expressed in a myriad of other tissues, from the airway and gastrointestinal epithelia to the pancreas and brain. The functions of many of these extraoral taste receptors remain unknown, but emerging evidence suggests that bitter and sweet taste receptors in the airway are important sentinels of innate immunity. This review discusses taste receptor signaling, focusing on the G-protein-coupled receptors that detect bitter, sweet, and savory tastes, followed by an overview of extraoral taste receptors and in-depth discussion of studies demonstrating the roles of taste receptors in airway innate immunity. Future research on extraoral taste receptors has significant potential for identification of novel immune mechanisms and insights into host-pathogen interactions.

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.

Receptors for short-chain fatty acids in brush cells at the "gastric groove"

In the stomach of rodents clusters of brush cells are arranged at the "gastric groove," immediately at the transition zone from the non-glandular reservoir compartment to the glandular digestive compartment. Based on their taste cell-like molecular phenotype it has been speculated that the cells may be capable to sense constituents of the ingested food, however, searches for nutrient receptors have not been successful. In this study, it was hypothesized that the cells may express receptors for short-chain fatty acids, metabolites generated by microorganisms during the storage of ingested food in the murine forestomach, which lacks the acidic milieu of more posterior regions of the stomach and is colonized with numerous microbiota. Experimental approaches, including RT-PCR analysis and immunohistochemical studies, revealed that the majority of these brush cells express the G-protein coupled receptor types GPR41 (FFAR3) and GPR43 (FFAR2), which are activated by short-chain fatty acids. Both, the GPR41 receptor proteins as well as an appropriate G-protein, α-gustducin, were found to be segregated at the apical brush border of the cells, indicating a direct contact with the luminal content of this gastric region. The exposure of microvillar processes with appropriate receptors and signaling elements to the gastric lumen suggests that the brush cells may in fact be capable to sense the short-chain fatty acids which originate from fermentation processes during the retention of ingested food in the anterior part of the stomach.

Genetic loss or pharmacological blockade of testes-expressed taste genes causes male sterility

TAS1R taste receptors and their associated heterotrimeric G protein gustducin are involved in sugar and amino acid sensing in taste cells and in the gastrointestinal tract. They are also strongly expressed in testis and sperm, but their functions in these tissues were previously unknown. Using mouse models, we show that the genetic absence of both TAS1R3, a component of sweet and amino acid taste receptors, and the gustducin α-subunit GNAT3 leads to male-specific sterility. To gain further insight into this effect, we generated a mouse model that expressed a humanized form of TAS1R3 susceptible to inhibition by the antilipid medication clofibrate. Sperm formation in animals without functional TAS1R3 and GNAT3 is compromised, with malformed and immotile sperm. Furthermore, clofibrate inhibition of humanized TAS1R3 in the genetic background of Tas1r3(-/-), Gnat3(-/-) doubly null mice led to inducible male sterility. These results indicate a crucial role for these extraoral "taste" molecules in sperm development and maturation. We previously reported that blocking of human TAS1R3, but not mouse TAS1R3, can be achieved by common medications or chemicals in the environment. We hypothesize that even low levels of these compounds can lower sperm count and negatively affect human male fertility, which common mouse toxicology assays would not reveal. Conversely, we speculate that TAS1R3 and GNAT3 activators may help infertile men, particularly those that are affected by some of the mentioned inhibitors and/or are diagnosed with idiopathic infertility involving signaling pathway of these receptors.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

ROS-GC subfamily membrane guanylate cyclase-linked transduction systems: taste, pineal gland and hippocampus

In the continuous efforts to test the validity of the theme that the Ca(2+)-modulated ROS-GC subfamily system is a universal transduction component of the sensory and sensory-linked network of neurons, this article focuses on the presence and variant biochemical forms of this transduction system in the gustatory epithelium, the site of gustatory transduction; in the pineal, a light-sensitive gland; and in the hippocampus neurons, linked with the perception of all SENSES.

Phototransduction motifs and variations

Seeing begins in the photoreceptors, where light is absorbed and signaled to the nervous system. Throughout the animal kingdom, photoreceptors are diverse in design and purpose. Nonetheless, phototransduction-the mechanism by which absorbed photons are converted into an electrical response-is highly conserved and based almost exclusively on a single class of photoproteins, the opsins. In this Review, we survey the G protein-coupled signaling cascades downstream from opsins in photoreceptors across vertebrate and invertebrate species, noting their similarities as well as differences.

SREBP-2 regulates gut peptide secretion through intestinal bitter taste receptor signaling in mice

Bitter taste-sensing G protein-coupled receptors (type 2 taste receptors [T2Rs]) are expressed in taste receptor cells of the tongue, where they play an important role in limiting ingestion of bitter-tasting, potentially toxic compounds. T2Rs are also expressed in gut-derived enteroendocrine cells, where they have also been hypothesized to play a role in limiting toxin absorption. In this study, we have shown that T2R gene expression in both cultured mouse enteroendocrine cells and mouse intestine is regulated by the cholesterol-sensitive SREBP-2. In addition, T2R stimulation of cholecystokinin (CCK) secretion was enhanced directly by SREBP-2 in cultured cells and in mice fed chow supplemented with lovastatin and ezetimibe (L/E) to decrease dietary sterol absorption and increase nuclear activity of SREBP-2. Low-cholesterol diets are naturally composed of high amounts of plant matter that is likely to contain dietary toxins, and CCK is known to improve dietary absorption of fats, slow gastric emptying, and decrease food intake. Thus, these studies suggest that SREBP-2 activation of bitter signaling receptors in the intestine may sensitize the gut to a low-fat diet and to potential accompanying food-borne toxins that make it past the initial aversive response in the mouth.

The G-protein coupling properties of the human sweet and amino acid taste receptors

The human T1R taste receptors are family C G-protein-coupled receptors (GPCRs) that act as heterodimers to mediate sweet (hT1R2 + hT1R3) and umami (hT1R1 + hT1R3) taste modalities. Each T1R has a large extracellular ligand-binding domain linked to a seven transmembrane-spanning core domain (7TMD). We demonstrate that the 7TMDs of hT1R1 and hT1R2 display robust ligand-independent constitutive activity, efficiently catalyzing the exchange of GDP for GTP on Galpha subunits. In contrast, relative to the 7TMDs of hT1R1 and hT1R2, the 7TMD of hT1R3 couples poorly to G-proteins, suggesting that in vivo signaling may proceed primarily through hT1R1 and hT1R2. In addition, we provide direct evidence that the hT1Rs selectively signal through Galpha(i/o) pathways, coupling to multiple Galpha(i/o) subunits as well as the taste cell specific Gbeta(1)gamma(13) dimer.

Functional characterization of human bitter taste receptors

The T2Rs belong to a multi-gene family of G-protein-coupled receptors responsible for the detection of ingested bitter-tasting compounds. The T2Rs are conserved among mammals with the human and mouse gene families consisting of about 25 members. In the present study we address the signalling properties of human and mouse T2Rs using an in vitro reconstitution system in which both the ligands and G-proteins being assayed can be manipulated independently and quantitatively assessed. We confirm that the mT2R5, hT2R43 and hT2R47 receptors respond selectively to micromolar concentrations of cycloheximide, aristolochic acid and denatonium respectively. We also demonstrate that hT2R14 is a receptor for aristolochic acid and report the first characterization of the ligand specificities of hT2R7, which is a broadly tuned receptor responding to strychnine, quinacrine, chloroquine and papaverine. Using these defined ligand-receptor interactions, we assayed the ability of the ligand-activated T2Rs to catalyse GTP binding on divergent members of the G(alpha) family including three members of the G(alphai) subfamily (transducin, G(alphai1) and G(alphao)) as well as G(alphas) and G(alphaq). The T2Rs coupled with each of the three G(alphai) members tested. However, none of the T2Rs coupled to either G(alphas) or G(alphaq), suggesting the T2Rs signal primarily through G(alphai)-mediated signal transduction pathways. Furthermore, we observed different G-protein selectivities among the T2Rs with respect to both G(alphai) subunits and G(betagamma) dimers, suggesting that bitter taste is transduced by multiple G-proteins that may differ among the T2Rs.

Parietal-eye phototransduction components and their potential evolutionary implications

The parietal-eye photoreceptor is unique because it has two antagonistic light signaling pathways in the same cell-a hyperpolarizing pathway maximally sensitive to blue light and a depolarizing pathway maximally sensitive to green light. Here, we report the molecular components of these two pathways. We found two opsins in the same cell: the blue-sensitive pinopsin and a previously unidentified green-sensitive opsin, which we name parietopsin. Signaling components included gustducin-alpha and Galphao, but not rod or cone transducin-alpha. Single-cell recordings demonstrated that Go mediates the depolarizing response. Gustducin-alpha resembles transducin-alpha functionally and likely mediates the hyperpolarizing response. The parietopsin-Go signaling pair provides clues about how rod and cone phototransduction might have evolved.

RGS21 is a novel regulator of G protein signalling selectively expressed in subpopulations of taste bud cells

Abstract G-protein-mediated signalling processes are involved in sweet and bitter taste transduction. In particular, the G protein alpha-subunit gustducin has been implicated in these processes. One of the limiting factors for the time-course of cellular responses induced by tastants is therefore the intrinsic GTPase activity of alpha-gustducin, which determines the lifetime of the active G protein complex. In several signalling systems specific 'regulator of G protein signalling' (RGS) proteins accelerate the GTPase activity of G protein alpha-subunits. Using differential screening approaches, we have identified a novel RGS protein termed RGS21, which represents the smallest known member of this protein family. Reverse transcription polymerase chain reaction and in situ hybridization experiments demonstrated that RGS21 is expressed selectively in taste tissue where it is found in a subpopulation of sensory cells. Furthermore, it is coexpressed in individual taste cells with bitter and sweet transduction components including alpha-gustducin, phospholipase Cbeta2, T1R2/T1R3 sweet taste receptors and T2R bitter taste receptors. In vitro binding assays demonstrate that RGS21 binds alpha-gustducin in a conformation-dependent manner and has the potential to interact with the same Galpha subtypes as T1R receptors. These results suggest that RGS21 could play a regulatory role in bitter as well as sweet taste transduction processes.

Functional interaction between T2R taste receptors and G-protein alpha subunits expressed in taste receptor cells

Bitter taste perception is a conserved chemical sense against the ingestion of poisonous substances in mammals. A multigene family of G-protein-coupled receptors, T2R (so-called TAS2R or TRB) receptors and a G-protein alpha subunit (Galpha), gustducin, are believed to be key molecules for its perception, but little is known about the molecular basis for its interaction. Here, we use a heterologous expression system to determine a specific domain of gustducin necessary for T2R coupling. Two chimeric Galpha16 proteins harboring 37 and 44 gustducin-specific sequences at their C termini (G16/gust37 and G16/gust44) responded to different T2R receptors with known ligands, but G16/gust 23, G16/gust11, and G16/gust5 did not. The former two chimeras contained a predicted beta6 sheet, an alpha5 helix, and an extreme C terminus of gustducin, and all the domains were indispensable to the expression of T2R activity. We also expressed G16 protein chimeras with the corresponding domain from other Galpha(i) proteins, cone-transducin (Galpha(t2)), Galpha(i2), and Galpha(z) (G16/t2, G16/i2, and G16/z). As a result, G16/t2 and G16/i2 produced specific responses of T2Rs, but G16/z did not. Because Galpha(t2) and Galpha(i2) are expressed in the taste receptor cells, these G-protein alpha(i) subunits may also be involved in bitter taste perception via T2R receptors. The present Galpha16-based chimeras could be useful tools to analyze the functions of many orphan G-protein-coupled taste receptors.

Functional interaction between T2R taste receptors and G-protein alpha subunits expressed in taste receptor cells

Bitter taste perception is a conserved chemical sense against the ingestion of poisonous substances in mammals. A multigene family of G-protein-coupled receptors, T2R (so-called TAS2R or TRB) receptors and a G-protein alpha subunit (Galpha), gustducin, are believed to be key molecules for its perception, but little is known about the molecular basis for its interaction. Here, we use a heterologous expression system to determine a specific domain of gustducin necessary for T2R coupling. Two chimeric Galpha16 proteins harboring 37 and 44 gustducin-specific sequences at their C termini (G16/gust37 and G16/gust44) responded to different T2R receptors with known ligands, but G16/gust 23, G16/gust11, and G16/gust5 did not. The former two chimeras contained a predicted beta6 sheet, an alpha5 helix, and an extreme C terminus of gustducin, and all the domains were indispensable to the expression of T2R activity. We also expressed G16 protein chimeras with the corresponding domain from other Galpha(i) proteins, cone-transducin (Galpha(t2)), Galpha(i2), and Galpha(z) (G16/t2, G16/i2, and G16/z). As a result, G16/t2 and G16/i2 produced specific responses of T2Rs, but G16/z did not. Because Galpha(t2) and Galpha(i2) are expressed in the taste receptor cells, these G-protein alpha(i) subunits may also be involved in bitter taste perception via T2R receptors. The present Galpha16-based chimeras could be useful tools to analyze the functions of many orphan G-protein-coupled taste receptors.

Three sweet receptor genes are clustered in human chromosome 1

A search of the human genome database led us to identify three human candidate taste receptors, hT1R1, hT1R2, and hT1R3, which contain seven transmembrane domains. All three genes map to a small region of Chromosome (Chr) 1. This region is syntenous to the distal end of Chr 4 in mouse, which contains the Sac (saccharin preference) locus that is involved in detecting sweet tastants. A genetic marker, DVL1, which is linked to the Sac locus, is within 1700 bp of human T1R3. Recently, the murine T1Rs and its human ortholog have been independently identified in combination as sweet and umami receptors near the Sac locus. All three hT1Rs genes are expressed selectively in human taste receptor cells in the fungiform papillae, consistent with their role in taste perception.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Biochemical characteristics of guanine nucleotide binding protein alpha-subunit recombinant protein and three mutants: investigation of a domain motion involved in GDP-GTP exchange

Our previous studies using molecular dynamics have shown a hinge bending motion between the helical and the GTPase domains of GalphaT (Mello et al., 1998). The hypothesis that this motion is allowed by residues Gly56 and Gly179 and that this motion may affect the ligand exchange was tested in this work. Mutations of Gly 56 were carried out and the mutant proteins were expressed in Sf9 cells using the Baculovirus expression system. The recombinant proteins were purified using Ni-NTA affinity chromatography. The results for the (GDP/GTP) exchange assays showed that G56S and double mutants (D55G/G56S) proteins differ significantly from the wild type and D55G mutant forms. The Kd values for GTPgammaS binding of those mutants have decreased by approximately 10-fold. No difference in the GTPase activity was detected for the mutants. Thus, the biochemical results obtained support the conclusions of the computational studies.

Dominant loss of responsiveness to sweet and bitter compounds caused by a single mutation in alpha -gustducin

Biochemical and genetic studies have implicated alpha-gustducin as a key component in the transduction of both bitter or sweet taste. Yet, alpha-gustducin-null mice are not completely unresponsive to bitter or sweet compounds. To gain insights into how gustducin mediates responses to bitter and sweet compounds, and to elicit the nature of the gustducin-independent pathways, we generated a dominant-negative form of alpha-gustducin and expressed it as a transgene from the alpha-gustducin promoter in both wild-type and alpha-gustducin-null mice. A single mutation, G352P, introduced into the C-terminal region of alpha-gustducin critical for receptor interaction rendered the mutant protein unresponsive to activation by taste receptor, but left its other functions intact. In control experiments, expression of wild-type alpha-gustducin as a transgene in alpha-gustducin-null mice fully restored responsiveness to bitter and sweet compounds, formally proving that the targeted deletion of the alpha-gustducin gene caused the taste deficits of the null mice. In contrast, transgenic expression of the G352P mutant did not restore responsiveness of the null mice to either bitter or sweet compounds. Furthermore, in the wild-type background, the mutant transgene inhibited endogenous alpha-gustducin's interactions with taste receptors, i.e., it acted as a dominant-negative. That the mutant transgene further diminished the residual bitter and sweet taste responsiveness of the alpha-gustducin-null mice suggests that other guanine nucleotide-binding regulatory proteins expressed in the alpha-gustducin lineage of taste cells mediate these responses.

Reconstitution of the vertebrate visual cascade using recombinant heterotrimeric transducin purified from Sf9 cells

For reconstitution studies with rhodopsin and cGMP phosphodiesterase (PDE), all three subunits of heterotrimeric transducin (T alpha beta gamma) were simultaneously expressed in Sf9 cells at high levels using a baculovirus expression system and purified to homogeneity. Light-activated rhodopsin catalyzed the loading of purified recombinant T alpha with GTP gamma S. In vitro reconstitution of rhodopsin, recombinant transducin, and PDE in detergent solution resulted in cGMP hydrolysis upon illumination, demonstrating that recombinant transducin was able to activate PDE. The rate of cGMP hydrolysis by PDE as a function of GTP gamma S-loaded recombinant transducin (T(*)) concentration gave a Hill coefficient of approximately 2, suggesting that the activation of PDE by T(*) was cooperatively regulated. Furthermore, the kinetic rate constants for the activation of PDE by T(*) suggested that only the complex of PDE with two T(*) molecules, PDE. T(2)(*), was significantly catalytically active under the conditions of the assay. We conclude that the model of essential coactivation best describes the activation of PDE by T(*) in a reconstituted vertebrate visual cascade using recombinant heterotrimeric transducin.

T2Rs function as bitter taste receptors

Bitter taste perception provides animals with critical protection against ingestion of poisonous compounds. In the accompanying paper, we report the characterization of a large family of putative mammalian taste receptors (T2Rs). Here we use a heterologous expression system to show that specific T2Rs function as bitter taste receptors. A mouse T2R (mT2R-5) responds to the bitter tastant cycloheximide, and a human and a mouse receptor (hT2R-4 and mT2R-8) responded to denatonium and 6-n-propyl-2-thiouracil. Mice strains deficient in their ability to detect cycloheximide have amino acid substitutions in the mT2R-5 gene; these changes render the receptor significantly less responsive to cycloheximide. We also expressed mT2R-5 in insect cells and demonstrate specific tastant-dependent activation of gustducin, a G protein implicated in bitter signaling. Since a single taste receptor cell expresses a large repertoire of T2Rs, these findings provide a plausible explanation for the uniform bitter taste that is evoked by many structurally unrelated toxic compounds.

Effect of the umami peptides on the ligand binding and function of rat mGlu4a receptor might implicate this receptor in the monosodium glutamate taste transduction

1. The effect of several metabotropic ligands and di- or tripeptides were tested on the binding of [3H]-L(+)-2-amino-4-phosphonobutyric acid ([3H]-L-AP4) on rat mGlu4 receptor. For selected compounds, the functional activity was determined on this receptor using the guanosine-5'[gamma-35S]-thiotriphosphate [gamma-35S]-GTP binding assay. 2. Using the scintillation proximity assay, [3H]-L-AP4 saturation analysis gave binding parameters K(D) and Bmax values of 150 nM and 9.3 pmoles mg-1 protein, respectively. The specific binding was inhibited concentration-dependently by several mGlu receptor ligands, and their rank order of affinity was established. 3. Several peptides inhibited the [3H]-L-AP4 binding with the following rank order of potency: glutamate-glutamate>glutamate-glutamate-leucine=aspartate - glutamate>>glutamate - glutamate-aspartate>lactoyl-glutamate>>aspartate-aspartate. Aspartate-phenylalanine-methyl ester (aspartame) was inactive up to 1 mM and guanosine-5'-monophosphate and inosine-5'-monophosphate were inactive up to 100 micronM. 4. The [gamma-35S]-GTP binding functional assay was used to determine the agonist activities of the different compounds. For the rat mGlu4 agonists, L-AP4 and L-glutamate, the correlation between their occupancy and activation of the receptor was close to one. The peptides, Glu-Glu, Asp-Glu and Glu-Glu-Asp failed to stimulate the [gamma-35S]-GTP binding at receptor occupancy greater than 80% and Glu-Glu-Leu appeared to be a weak partial agonist. These peptides did not elicit a clear dose-dependent umami perception. However, Glu-lac showed a good correlation between its potency to stimulate the [gamma-35S]-GTP binding and its affinity for displacement of [3H]-L-AP4 binding. These data are in agreement with the peptide taste assessment in human subjects, which showed that the acid derivatives of glutamate had characteristics similar to umami.

Mechanism of allosteric regulation of the rod cGMP phosphodiesterase activity by the helical domain of transducin alpha subunit

The G protein alpha subunit (Galpha) is composed of two distinct folding domains: a GTP-binding Ras-like domain and an alpha helical domain (HD). We have recently reported that the helical domain (HDt) of the vertebrate visual transducin alpha subunit (Galphat) synergizes activation of retinal cyclic GMP phosphodiesterase (PDE) by activated Galphat (Liu, W., and Northup, J. K., (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12878-12883). Here, we examine the molecular basis for this HD-based signaling regulation, and we provide a new model for the activation of the target effector. The HD proteins derived from visual transducin or taste gustducin alpha subunits, but no other Galpha HD proteins, each attenuate the PDE catalytic core (Palphabeta) and synergize Galphat stimulation of the holoPDE (Palphabetagamma2) with similar apparent affinities. The data from studies of both HDt-mediated attenuation and stimulation indicate that the HDt and the PDE inhibitory subunit (Pgamma) interact with PDE at independent sites and that Palphabeta contains the binding sites for HD. The saturation of both processes by HDt displays positive cooperativity with Hill coefficients of 1.5 for the attenuation of Palphabeta activity and 2.1 for synergism of holoPDE activation. Our data suggest the that Galphat-HDt regulates PDE by allosterically decreasing the affinity of Palphabeta for Pgamma and thus simultaneously facilitating the interaction of the activated Galphat-Ras-like domain with Pgamma. Thus, we propose a new model for the high efficiency of PDE activation as well as deactivation, and, overall, a novel mechanism for controlling fidelity, sensitivity, and efficacy of G protein signaling.

The helical domain of a G protein alpha subunit is a regulator of its effector

The alpha subunit (Galpha) of heterotrimeric G proteins is a major determinant of signaling selectivity. The Galpha structure essentially comprises a GTPase "Ras-like" domain (RasD) and a unique alpha-helical domain (HD). We used the vertebrate phototransduction model to test for potential functions of HD and found that the HD of the retinal transducin Galpha (Galphat) and the closely related gustducin (Galphag), but not Galphai1, Galphas, or Galphaq synergistically enhance guanosine 5'-gamma[-thio]triphosphate bound Galphat (GalphatGTPgammaS) activation of bovine rod cGMP phosphodiesterase (PDE). In addition, both HDt and HDg, but not HDi1, HDs, or HDq attenuate the trypsin-activated PDE. GalphatGDP and HDt attenuation of trypsin-activated PDE saturate with similar affinities and to an identical 38% of initial activity. These data suggest that interaction of intact Galphat with the PDE catalytic core may be caused by the HD moiety, and they indicate an independent site(s) for the HD moiety of Galphat within the PDE catalytic core in addition to the sites for the inhibitory Pgamma subunits. The HD moiety of GalphatGDP is an attenuator of the activated catalytic core, whereas in the presence of activated GalphatGTPgammaS the independently expressed HDt is a potent synergist. Rhodopsin catalysis of Galphat activation enhances the PDE activation produced by subsaturating levels of Galphat, suggesting a HD-moiety synergism from a transient conformation of Galphat. These results establish HD-selective regulations of vertebrate retinal PDE, and they provide evidence demonstrating that the HD is a modulatory domain. We suggest that the HD works in concert with the RasD, enhancing the efficiency of G protein signaling.

Characterization and solubilization of bitter-responsive receptors that couple to gustducin

The tastes of many bitter and sweet compounds are thought to be transduced via guanine nucleotide binding protein (G-protein)-coupled receptors, although the biochemical nature of these receptors is poorly understood at present. Gustducin, a taste-specific G-protein closely related to the transducins, is a key component in transducing the responses to compounds that humans equate with bitter and sweet. Rod transducin, which is also expressed in taste receptor cells, can be activated by the bitter compound denatonium in the presence of bovine taste membranes. In this paper, we show that gustducin is expressed in bovine taste tissue and that both gustducin and transducin, in the presence of bovine taste membranes, can be activated specifically by several bitter compounds, including denatonium, quinine, and strychnine. We also demonstrate that the activation in response to denatonium of gustducin by presumptive bitter-responsive receptors present in taste membranes depends on an interaction with the C terminus of gustducin and requires G-protein betagamma subunits to provide the receptor-interacting heterotrimer. The taste receptor-gustducin interaction can be competitively inhibited by peptides derived from the sites of interaction of rhodopsin and transducin. Finally, as the initial step toward purifying taste receptors, we have solubilized this bitter-responsive taste receptor and maintained its biological activity.

Gustducin and its role in taste

The mechanisms responsible for taste signal transductions are very complex. A key molecule, alpha-gustducin, a primarily taste-specific G protein alpha-subunit, was discovered in 1992 and was later found to be involved in both bitter and sweet taste transduction. A proposed mechanism for alpha-gustducin involves coupling specific cell-surface receptors with a cyclic nucleotide phosphodiesterase which would open a cyclic nucleotide-suppressible cation channel leading to influx of calcium, and ultimately leading to release of neurotransmitter. Although "knock-out" animals deficient in the alpha-gustducin gene clearly demonstrate that gustducin is an essential molecule for tasting certain bitter and sweet compounds, the precise role of alpha-gustducin in bitter and sweet taste is presently unclear. Indeed, there are several other signaling mechanisms in sweet and bitter taste, apparently unrelated to alpha-gustducin, that increase cyclic AMP or inositol 1,4,5 trisphosphate. Thus, proposed models for alpha-gustducin and those found by other laboratories may be parallel and interdependent.

Sodium saccharin inhibits adenylyl cyclase activity in non-taste cells

We have studied the in vitro effect of sodium saccharin (NaSacch) on the rat adipocyte adenylyl cyclase complex. NaSacch (2.5-50 mM) inhibited significantly in a dose-dependent manner basal and isoproterenol-stimulated cAMP accumulation on isolated rat adipocytes. Similarly, NaSacch (2.5-50 mM) inhibited forskolin-stimulated adenylyl cyclase activity measured in the presence of Mg(2+)-ATP on adipocyte, astrocyte and thyrocyte membrane fractions. In contrast, NaSacch did not inhibit but slightly increased the forskolin-stimulated adenylyl cyclase activity measured in the presence of Mn(2+)-ATP and GDP beta S, a stable GDP analogue. The effect of NaSacch was not mediated through either the A1-adenosine receptor (A1R) or the alpha 2-adrenergic receptor (alpha 2AR). The inhibitory effect of NaSacch was additive to that of A1R agonist and was not blocked by the addition of the alpha 2AR antagonist RX 821002. Pretreatment of adipocytes with pertussis toxin slightly attenuated but did not abolish the inhibitory effect of NaSacch on forskolin-stimulated adenylyl cyclase activity on membrane fractions. These data suggest that the inhibitory effect of NaSacch on forskolin stimulated-adenylyl cyclase in adipocytes does not imply only Gi protein but also other direct or indirect inhibitory pathway(s) which remain to be determined.

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.

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.