Sweetness and Sweeteners - American Chemical Society

Chapter 15

cAMP: A Role in Sweet Taste Adaptation 1

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Downloaded by NANYANG TECHNOLOGICAL UNIV on June 1, 2016 | http://pubs.acs.org Publication Date: March 4, 2008 | doi: 10.1021/bk-2008-0979.ch015

Nirupa Chaudhari and Sue C. Kinnamon 1

Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, F L 33136 Department of Biomedical Sciences, Colorado State University, Fort Collins, C O 80523 2

Sweet taste transduction is initiated by sugars or synthetic sweeteners binding to the G protein coupled receptor, T1R2+T1R3, and activating G protein(s) and downstream signaling effectors. Recent genetic and functional studies implicated phospholipase C (PLCβ2) and C a release from intracellular stores in sweet transduction. Considerable evidence suggests that c A M P also plays a role in the sweet response. Initial observations pointed to c A M P as the second messenger, because sweet stimuli modulate c A M P levels in taste tissue, and because membrane permeant c A M P strongly influences the physiological response of taste buds to sweet stimuli. Further, enzymes that regulate c A M P levels, adenylyl cyclases (ACs) and phosphodiesterases (PDEs) and gustducin, a G protein that can activate PDEs, are all expressed in many sweet-sensitive taste ceils. And, loss of gustducin impairs sweet responses. Yet, the precise role of c A M P in sweet taste remains unclear. Here, we review evidence that c A M P is produced as a direct consequence of receptor activation, that c A M P directly depolarizes mammalian taste cells, and that cAMP-dependent Protein Kinase ( P K A ) likely underlies adaptation to the sweet response. We suggest a model in which the role of α-gustducin is to keep c A M P levels low to prevent chronic adaptation of sweet-sensitive taste cells. 2+

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Introduction Sweet taste transduction involves binding of sweet ligands (sugars, synthetic sweeteners, D-amino acids, and some sweet proteins) to G protein-coupled taste receptors of the T1R class . T1R3, the third member of this class to be identified, was cloned from the murine sac locus. Sac "taster" strains show increased preferences and neural responses to sucrose and saccharin compared to "nontaster" strains " . When co-transfected in cell culture, TIR3 and the related GPCR, T1R2, form heterodimeric receptors that can be activated by sucrose and many synthetic sweeteners . In contrast, T1R3+T1R1 heterodimers can be activated by L-amino acids, not by sugars or sweeteners ' . In mouse, T1R3 is abundantly expressed all taste fields, while T1R2 is more prevalent in palate and vallate taste buds, and T1R1 is more abundant in fungiform taste buds . Nevertheless, all three TIRs are expressed to some extent in all taste fields. In analogy to the thoroughly investigated steps in bitter transduction, ligand binding to sweet receptors is thought to result in Θβγ activation of the phospholipase, PLCp2, producing the second messengers, IP and diacylglycerol (DAG) and eventually releasing C a from intracellular stores . The cation channel, T R P M 5 * , is an essential component of sweet, bitter and umami transduction, although its exact role in the transduction cascade remains unclear. Knockout of either PLCP2 or T R P M 5 severely impacts sweet transduction , although a recent study shows that mice retain some sweet sensitivity after knockout of T R P M 5 . Despite the recent emphasis on phosphoinositide signaling, a longstanding literature suggests that c A M P also plays a significant role in sweet transduction. Taste cells express several adenylyl cyclases (ACs) , phosphodiesterases (PDEs) , G proteins capable of activating A C s and PDEs " , and direct cyclic nucleotide modulated ion channels ' . In fact, cAMP was originally proposed to be the main second messenger in sweet transduction, based on the observation that sucrose and saccharin stimulated A C activity in rat anterior tongue epithelium . Direct measurements of c A M P in circumvallate or fungiform taste buds showed that both sucrose and/or synthetic sweeteners increase c A M P 1


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Source of the cAMP signal Synthesis of c A M P could be a direct consequence of receptor activation, perhaps by G a subunits stimulating A C . Conversely, c A M P synthesis could be secondary to the PLC-mediated increase in intracellular C a . Several ACs have s

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been identified in taste cells, including A C 8 , which is stimulated by C a . To determine i f the sucrose-evoked c A M P was C a dependent, paired halves of C V epithelium were stimulated with 500 m M sucrose + 0.3 m M I B M X , or with 0.3 m M I B M X alone. Sucrose stimulation resulted in a substantial accumulation of c A M P , compared with the control (Figure 1, left panel; ) , as has been documented previously . We then repeated this stimulation paradigm under conditions where C a was lacking in the extracellular milieu or when release of intracellular C a was blocked with a P L C inhibitor (Figure 1, middle and right panels; ) . Depleting C a did not eliminate the sucrose-stimulated c A M P accumulation, suggesting that c A M P is produced directly as a consequence of receptor activation rather than downstream of C a signaling. 2 +

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Physiological role of cAMP in taste Pharmacological agents that modulate c A M P strongly influence taste cell and neural responses to sweeteners. Using loose patch recording from hamster taste buds in situ, we showed that membrane permeant analogs of c A M P , as well as I B M X and forskolin (each of which increase cAMP), elicited action potentials in sweet responsive taste buds (Figure 2). Further, whole-cell patch clamp recordings showed that both synthetic sweeteners and membrane permeant c A M P analogs depolarized sweet-sensitive taste cells by blocking voltage-gated K currents and a resting K conductance (Figure 3 and ). Although the molecular identity of this K channel has not been determined, we expect it would act in concert with TRPM5 to depolarize taste cells in response to sweet stimuli. In the loose-patch recording configuration, responses to sucrose and the synthetic sweetener, NC01, persist in the presence of a membrane permeant P K A inhibitor, and even increase (Figure 4 and ). We interpret the results (Figures 2, 3, 4) to indicate that sweetener-evoked c A M P elevation has two sequential effects. First, c A M P directly modulates a membrane K conductance to depolarize taste cells. Subsequently, c A M P produces adaptation by activating P K A , and phosphorylating signaling proteins. This phosphorylation could occur at several levels including the taste receptor itself, or various effectors of the P L C signaling pathway. Indeed, PKA-mediated phosphorylation is known to inhibit both PLCP2 and IP R3 * , both integral components of the phosphoinositide signaling pathway in taste cells. Phosphorylation-mediated suppression would develop slowly and with a delay. In contrast, the neural response to sweet compounds typically begins within seconds. 27

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Figure 1. Sucrose stimulates cAMP accumulation in normal Tyrode's (left), Ca /Mg *free (CMF) Tyrode's (middle), or in the presence of a PLC inhibitor, U73122 (right). (Adapted, with permission, from reference 26. Copyright 2006 The American Physiological Society.)



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Figure 2. Loose patch recordings from hamster taste buds in situ showed that membrane permeant cAMP analogs (top) and agents that increase intracellular cAMP (middle, bottom) elicit action potentials in sweet responsive taste buds. (Adapted, with permission, from reference 27. Copyright 1993 The American Physiological Society)


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Figure 3. Whole-cell voltage clamp recordingfroma hamster fungiform taste cell showed that cAMP and saccharin both inhibited a tonically-active voltagegated if current. The effect of cAMP and sweetener was not additive. (Adapted, with permission, from reference 28. Copyright 1996 The American Phsiological Society.)

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Figure 4. The PKA inhibitor H-89 increases thefrequencyof action potentials elicited by sucrose and the synthetic sweetener NC01. Sweetener-elicited frequency in the presence of inhibitor is shown normalized tofrequencyin absence of inhibitor. (Adapted, with permission, from reference 29. Copyright 2000 The American Physiological Society.)



Figure 5. The proposed role of cAMP in sweet taste adaptation, a-gustducin, tonically active through an undefined receptor, keeps cAMP levels low at rest. In the absence of gustducin, increased basal levels of cAMP keep PKA activity elevated. This phosphorylates sweet taste signaling proteins, causing the taste cell to be chronically adapted to sweet stimuli.


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Role of α-gustducin in sweet taste 32

α-Gustducin knockout mice are compromised to sweet stimuli , yet the mechanism for this has never been explained, especially considering the apparent centrality of the P L C signaling pathway . Further, biochemical measurements have generally shown that sweet stimuli elevate, rather than decrease c A M P levels. The knockout effect for sweet appears to be predominantly limited to fungiform and palatal taste fields, where TIRs and agustducin are expressed in the same taste cells ' . Yet, even in these anterior taste fields, biochemical measurements have shown that sweet stimuli increase c A M P levels . We present a model (Figure 5), based on new preliminary data obtained from circumvallate taste buds, that α-gustducin knockout mice have elevated resting levels of c A M P . If this situation holds up for anterior taste fields, it would suggest that the role of α-gustducin is tonically to activate PDE, keeping basal c A M P levels low. If cAMP-dependent phosphorylation normally mediates adaptation, as we propose here, then taste buds in α-gustducin knockout mice would be in a chronically adapted state and unable to respond normally to sweet stimuli. Further studies, measuring c A M P levels in individual taste cells of gustducin knockout mice, will be required to verify i f this model explains the gustducin knockout effect on sweet taste.


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Sweetness and Sweeteners - American Chemical Society

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