Some Taste Substances Are Direct Activators of G-Proteins
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Biochem. Biochem. J. (1994) 297, 451-454 (Printed in Great Britain) 451 RESEARCH COMMUNICATION Some taste substances are direct activators of G-proteins Michael NAIM,*tT Roland SEIFERT,* Bernd NURNBERG,* Lore GRUNBAUM* and Gunter SCHULTZ* *Institut fur Pharmakologie, Freie Universitat Berlin, D-14195 Berlin, Federal Republic of Germany, and tDepartment of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76-100, Israel Amphiphilic substances may stimulate cellular events through taste substances required to activate G-proteins in vitro correlated direct activation of G-proteins. The present experiments indicate with those used to elicit taste. These data support the hypothesis that several amphiphilic sweeteners and the bitter tastant, quin- that amphiphilic taste substances may elicit taste through direct ine, activate transducin and G1/G.-proteins. Concentrations of activation of G-proteins. INTRODUCTION cules in mediation of sweet and bitter taste transduction has been shown [13,14,19,20]. Moreover, gustducin, a novel G-protein G-proteins transfer and amplify signals between specific receptors closely related to transducin (the major G-protein of the retina), and effectors through the exchange of GDP for GTP [1,2]. G- has been identified and cloned from rat gustatory tissue [21]. protein activation is terminated by the hydrolysis of GTP to Interestingly, transducin is also present in gustatory tissue [22]. GDP and P1 through the intrinsic GTPase activity of the a- These findings call for common routes ofG-proteins in the senses subunits [3,4]. Recent studies have shown that a variety of of vision and taste. cationic-amphiphilic neuropeptides and venom peptides Intriguingly, it has been shown that intravenous and intra- (e.g. substance P, bradykinin and mastoparan) and non-peptide lingual administration of some non-sugar sweeteners may elicit substances (e.g. compound 48/80 and alkylamines) activate G- taste [23,24] and taste nerve responses [25,26] independently of proteins directly [5-7]. The amphiphilic properties of such any interaction with putative receptors at the apical surface of compounds allow them to penetrate deeply into the plasma the tongue. Since non-sugar sweeteners and bitter substances are membrane, as predicted by Schwyzer's theory of insertion of amphiphilic, and as their hydrophobic characteristics are im- amphiphilic peptides into membranes [8]. As the interaction of portant for their taste potency [27-29], we undertook this study receptor molecules with G-proteins appears to be mediated by to test the hypothesis that such compounds are direct G-protein the third cytoplasmic loop ofreceptor proteins [5-7], it is assumed activators. With the lack of availability of gustducin in amounts that amphiphilic peptides mimic receptor activities by interaction needed for this study, we studied the effects of some known sweet with G-proteins in a similar manner as receptors do. Hence, by and bitter taste substances on the GTPase activity of transducin their direct activation ofG-proteins, amphiphilic substances may and on a purified fraction containing G,/G.-proteins which mimic certain cellular effects of receptor agonist. The physio- belong to the same G-protein superfamily [1,2]. Mastoparan was logical significance of such a signal-transduction pathway is not used as a positive control for G-protein activation. known at the present time. Taste sensation is initiated by an interaction of taste stimuli MATERIALS AND METHODS with the exposed apical surface of the taste receptor cells, leading to membrane depolarization and synaptic transmission to Materials second-order neurons [9]. In contrast with ionic stimuli (salt and Azolectin, mastoparan and the taste substances, sodium sac- sour), the transduction of sweet and bitter tastes has been charin, sodium cyclamate, aspartame, neohesperidin dihydro- proposed to involve putative specific membrane receptors [10]. chalcone (NHD), naringin, quinine chloride (hydrochloride) and Recent studies using PCR in rat lingual epithelia or bovine taste sucrose, were purchased from Sigma Chemical Co. (St. Louis, tissue [11,12] have identified novel G-protein-coupled receptors. MO, U.S.A.). Monellin was purchased from BioResources However, it is not yet known whether these receptors are involved International (Somerset, NJ, U.S.A.). Single-chain monellin was in taste transduction and, to date, no taste receptor has been kindly provided by Kirin Brewery Co. (Kanagawa, Japan). The isolated. Furthermore, multiple transduction mechanisms may guanidine SC-45647 sweetener was kindly given by The Nutra- be operative for both sweet and bitter tastes [13-16]. Hydrophobic Sweet Company (Mt. Prospect, IL, U.S.A.). Guanosine 5'- interactions of bitter stimuli with lipid bilayers of gustatory [y-[35S]thio]triphosphate ([35S]GTP[S]) (1195/mmol) was pur- tissue has led to the hypothesis that specific receptors are not chased from du Pont New England Nuclear (Bad Homburg, needed for bitter sensation [17]. Indeed, bitter stimuli can Germany). Materials used for the GTPase assay were as described depolarize N- 18 mouse neuroblastoma cells, unrelated to taste elsewhere [30]. [18]. Recently [16] it has been hypothesized that direct activation ofG-proteins by amphiphilic and potentially bitter neuropeptides PurIfication (e.g. bradykinin is bitter) is one of the diverse signal-transduction of G-proteins pathways for bitter sensation. G,/G.-proteins were purified from bovine brain membranes by a The involvement of G-proteins and intracellular signal mole- three-step column-chromatography procedure [31]. In brief, Abbreviations used: NHD, neohesperidin dihydrochalcone; GTP[S], guanosine 5'-[y-thio]triphosphate; EC50, concentration giving half-maximal stimulation; DMSO, dimethyl sulphoxide. $ To whom correspondence should be sent at the Hebrew University of Jerusalem. 452 Research Communication cholate extracts were subjected to chromatography on a DEAE- (y-axis) versus the sweetener concentration (x-axis) needed to Sepharose Fast Flow column (Pharmacia, Freiburg, Germany), produce a sweet intensity level equal to that produced by 0.29 M AcA 34 gel filtration (Serva, Heidelberg, Germany) and on a sucrose (commonly used as a reference) [36-38]. The level of heptylamine-Sepharose column. Fractions were analysed for quinine bitter intensity was based on its mid-range bitter intensity GTP[S] binding and immunoreactivity employing specific anti- [39]. bodies [32]. Heterotrimeric G-proteins were identified by SDS/ PAGE and silver staining. Purity was greater than 900%. The RESULTS AND DISCUSSION purified mixture contained mostly Go1, some G02, Gil, Gi2, and traces of Gi3. The results of the present study indicate that some sweet and Heterotrimeric transducin was prepared essentially as de- bitter amphiphilic taste substances are effective G-protein acti- scribed previously [33]. Heterotrimeric transducin was identified vators in vitro (Figure 1). Taste substances activated the GTPase by SDS/PAGE followed by Coomassie Blue and immuno- of either solubilized transducin or a solubilized mixture of staining, and quantified by GTP[S] binding. Purity was greater purified Gi/G0-proteins in a concentration-dependent manner. than 90 %. Both the transducin and the Gi/G.-protein prepar- The bitter taste substance, quinine chloride, and the non-sugar ations were free of ATPase or low-affinity GTPase activities. sweeteners, NHD and sodium saccharin, were the most effective stimuli, showing stimulations of GTPase activity up to 2-3.5- fold. By comparison, mastoparan (400 ,M) stimulated trans- Reconstitution of G,/G.-proteins into phospholipid vesicles ducin and G1/G.-proteins 1.6- and 2.7-fold respectively (results Azolectin/cholate mixtures, 1 and 10 % (w/v) respectively, in the not shown). These results suggest that some taste substances may buffer A, and G,/G.-proteins (28 pmol) were loaded on to a be similarly effective and even more effective activators of G- 10 ml gel-filtration AcA 34 (25 cm x 8.5 mm) column prepared proteins than mastoparan. with degassed buffer A containing 100 mM NaCl, 2 mM MgCl2, Lower concentrations of taste stimuli were usually required to 1 mM EDTA and 20 mM Hepes/NaOH, pH 8.0,4 °C, according stimulate GTPase of transducin than for the activation of G1/G,- to [34] with slight modifications. Liposomes containing Gi/Go- proteins. Activation of transducin by taste substances showed proteins were eluted (200 ,ul fractions) from the column with the saturation (e.g., sodium saccharin) or biphasic concentration- above buffer and were quantified by GTP[S] binding. Pooled response curves (e.g. quinine chloride) (see Figure 1). In most fractions were then used for the GTPase assays. experiments, analysis of variance resulted in high significance of the stimulatory effects of the taste substances (P < 0.01). In the experiments with transducin, EC50 (concn. giving half-maximal GTPase assay stimulation) values of 5 mM for sodium saccharin, 1.3 mM for GTP hydrolysis was determined essentially as described by NHD, 4 mM for sodium cyclamate and 4 mM for quinine Wenzel-Seifert and Seifert [30]. In agreement with a model chloride were observed. The sweet protein, monellin (used up to suggested by Wieland et al. [35], GDP was included in reaction 250,M), did not stimulate transducin, and the stimulation by mixtures, unless otherwise specified, to enhance the relative the dipeptide, aspartame, was significant (P < 0.05) only when a stimulatory effects of taste substances on GTPase activity. For concentration of 8 mM was used. With respect to the G,/G.- solubilized G1/Go-proteins, reaction mixtures (50 ,tl) contained proteins, a saturated concentration-response curve was observed 44 nM of G-proteins, 1 ,tM [y-32P]GTP (0.1 ,uCi/tube), 1 mM only for aspartame (EC50 = 7 mM). All of the six taste stimuli adenosine 5'-[/3,y-imido]triphosphate, 3 ,uM GDP, 0.1 mM 0.5 1 Sacc NHD EGTA, ,uM MgCl2, mM dithiothreitol and 0.20% (w/v) 400 160 250 250 BSA in 65 mM triethanolamine/HCI, pH 7.0. Reaction mixtures 300 --140 200 200 contained taste substances and mastoparan at various concen- A~~~~~~~~~~~ trations.