Gustducin and transducin: a tale of two G proteins

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.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Sweet taste receptor signaling network: possible implication for cognitive functioning

Sweet taste receptors are transmembrane protein network specialized in the transmission of information from special "sweet" molecules into the intracellular domain. These receptors can sense the taste of a range of molecules and transmit the information downstream to several acceptors, modulate cell specific functions and metabolism, and mediate cell-to-cell coupling through paracrine mechanism. Recent reports indicate that sweet taste receptors are widely distributed in the body and serves specific function relative to their localization. Due to their pleiotropic signaling properties and multisubstrate ligand affinity, sweet taste receptors are able to cooperatively bind multiple substances and mediate signaling by other receptors. Based on increasing evidence about the role of these receptors in the initiation and control of absorption and metabolism, and the pivotal role of metabolic (glucose) regulation in the central nervous system functioning, we propose a possible implication of sweet taste receptor signaling in modulating cognitive functioning.

REEP2 enhances sweet receptor function by recruitment to lipid rafts

Heterologously expressed sensory receptors generally do not achieve the ligand sensitivity observed in vivo, and may require specific accessory proteins to ensure optimal function. We searched for taste cell-expressed receptor transporting protein (RTP) and receptor expression enhancing protein (REEP) family members that might serve as accessory molecules to enhance gustatory receptor function. We determined that REEP2 is an integral membrane protein expressed in taste cells, physically associates with both subunits of the type 1 taste receptor 2 and type 1 taste receptor 3 sweet receptor and specifically enhances responses to tastants of heterologously expressed sweet and bitter taste receptors. Downregulation of endogenously expressed REEP2 in the chemosensory enteroendocrine GLUTag cell line dramatically reduced sensitivity of endogenous sweet receptors. In contrast to the observation that RTP1, RTP2, and REEP1 enhance function of olfactory receptors by promoting their transit to the cell surface, we found that REEP2 does not increase cell surface expression of sweet receptors but instead alters their spatial organization. REEP2 recruits sweet receptors into lipid raft microdomains localized near the taste cell's apical region, thereby improving G-protein-coupled receptor signaling and promoting receptor access to tastants arriving through the apical taste pore.

Immunocytochemical evidence for co-expression of Type III IP3 receptor with signaling components of bitter taste transduction

BACKGROUND

Taste receptor cells are responsible for transducing chemical stimuli into electrical signals that lead to the sense of taste. An important second messenger in taste transduction is IP3, which is involved in both bitter and sweet transduction pathways. Several components of the bitter transduction pathway have been identified, including the T2R/TRB taste receptors, phospholipase C beta2, and the G protein subunits alpha-gustducin, beta3, and gamma13. However, the identity of the IP3 receptor subtype in this pathway is not known. In the present study we used immunocytochemistry on rodent taste tissue to identify the IP3 receptors expressed in taste cells and to examine taste bud expression patterns for IP3R3.

RESULTS

Antibodies against Type I, II, and III IP3 receptors were tested on sections of rat and mouse circumvallate papillae. Robust cytoplasmic labeling for the Type III IP3 receptor (IP3R3) was found in a large subset of taste cells in both species. In contrast, little or no immunoreactivity was seen with antibodies against the Type I or Type II IP3 receptors. To investigate the potential role of IP3R3 in bitter taste transduction, we used double-label immunocytochemistry to determine whether IP3R3 is expressed in the same subset of cells expressing other bitter signaling components. IP3R3 immunoreactive taste cells were also immunoreactive for PLCbeta2 and gamma13. Alpha-gustducin immunoreactivity was present in a subset of IP3R3, PLCbeta2, and gamma13 positive cells.

CONCLUSIONS

IP3R3 is the dominant form of the IP3 receptor expressed in taste cells and our data suggest it plays an important role in bitter taste transduction.

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.

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.

Human taste cells express the G protein alpha-gustducin and neuron-specific enolase

Expression of the alpha-subunit of the taste-specific G protein alpha-gustducin and the glycolytic enzyme neuron-specific enolase (NSE) was investigated immunohistochemically in human circumvallate and foliate taste papillae. Immunofluorescence for alpha-gustducin was observed in taste cells of both types of papillae and exhibited two patterns of immunofluorescence, plasmalemmal and cytosolic. The plasmalemmal pattern showed intense immunofluorescence localized to the apical region, and was exhibited by most immunoreactive taste cells. In contrast, the cytosolic pattern, observed in one or two immunoreactive cells in a taste bud per section, showed immunofluorescence distributed throughout the cytoplasm. A subpopulation of alpha-gustducin-immunoreactive taste receptor cells, most of which exhibited the cytosolic pattern, also expressed NSE. Optical sectioning, using confocal laser scanning microscopy, demonstrated the highest level of expression of alpha-gustducin in the apical microvillar region of the taste cells in close apposition to the taste pore. These studies indicate conservation of epitopes of alpha-gustducin in humans and rats, and suggest that this G protein is associated with taste transduction in both rats and humans. The patterns of expression of alpha-gustducin, and coexpression with NSE, may correlate with specialized subtypes or developmental stages of taste receptor cells.

Some taste substances are direct activators of G-proteins

Amphiphilic substances may stimulate cellular events through direct activation of G-proteins. The present experiments indicate that several amphiphilic sweeteners and the bitter tastant, quinine, activate transducin and Gi/Go-proteins. Concentrations of taste substances required to activate G-proteins in vitro correlated with those used to elicit taste. These data support the hypothesis that amphiphilic taste substances may elicit taste through direct activation of G-proteins.