Tastants evoke cAMP signal in taste buds that is independent of calcium signaling

Abstract

We previously showed that rat taste buds express several adenylyl cyclases (ACs) of which only AC8 is known to be stimulated by Ca 2ϩ . Here we demonstrate by direct measurements of cAMP levels that AC activity in taste buds is stimulated by treatments that elevate intracellular Ca 2ϩ . Specifically, 5 M thapsigargin or 3 M A-23187 (calcium ionophore), both of which increase intracellular Ca 2ϩ concentration ([Ca 2ϩ ]i), lead to a significant elevation of cAMP levels. This calcium stimulation of AC activity requires extracellular Ca 2ϩ , suggesting that it is dependent on Ca 2ϩ entry rather than release from stores. With immunofluorescence microscopy, we show that the calcium-stimulated AC8 is principally expressed in taste cells that also express phospholipase C␤2 (i.e., cells that elevate [Ca 2ϩ ]i in response to sweet, bitter, or umami stimuli). Taste transduction for sucrose is known to result in an elevation of both cAMP and calcium in taste buds. Thus we tested whether the cAMP increase in response to sucrose is a downstream consequence of calcium elevation. Even under conditions of depletion of stored and extracellular calcium, the cAMP response to sucrose stimulation persists in taste cells. The cAMP signal in response to monosodium glutamate stimulation is similarly unperturbed by calcium depletion. Our results suggest that tastant-evoked cAMP signals are not simply a secondary consequence of calcium modulation. Instead, cAMP and released Ca 2ϩ may represent independent second messenger signals downstream of taste receptors.calcium-sensitive adenylyl cyclase; capacitative entry; cross talk; taste transduction DURING THE PAST DECADE, numerous advances have been made in our understanding of taste transduction mechanisms. Most tastants of the sweet, bitter, and umami classes are thought to activate G protein-coupled taste receptors and their heterotrimeric G proteins. The G␥ 13 subunit appears to be taste specific and associated with G␤ 1 (14). This G␤ 1 ␥ 13 dimer has been shown directly to activate a phospholipase C (PLC␤ 2 ), stimulating the production of inositol 1,4,5-trisphosphate (IP 3 ) and eventually triggering an increase in cytoplasmic Ca 2ϩ (14,23,35,38). Strong support for this sequence of signaling events is derived from the pronounced taste deficit that results from genetic ablation of PLC␤ 2 (45). Although release of stored Ca 2ϩ (triggered by PLC␤ 2 activity) is essential, taste-evoked calcium transients in taste cells may also include a component of capacitative entry from the extracellular medium (28). Despite the recent emphasis on this IP 3 -mediated calcium release pathway, there is evidence that changes in cAMP and cGMP concentration occur after tastant stimulation. Cyclic nucleotide modulation has been demonstrated after stimulation with some sweet, bitter, and umami stimuli (1,39,44), and exogenously applied cAMP appears to mimic tastant-evoked activity (8). Nevertheless, the significance and source of cAMP and cGMP in taste transduction are unclear, given the essential role of...

Signal transduction and information processing in mammalian taste buds

“…Decades ago, Striem et al [101] reported that treating membranes derived from the anterior surface of rat tongues with sucrose stimulated adenylyl cyclase, implying the presence of G as proteins and Focussed search with primers based on synaptic glutamate receptors cAMP pathways in taste. These studies have been replicated and extended, using intact taste tissue [102]. Further, cAMP depolarizes frog and mouse taste cells, presumably due to cAMP-dependent protein kinase-phosphorylating and closing potassium channels [103,104].…”

“…Indeed, direct measurements of cAMP after bitter taste verified that this second messenger declines with bitter stimulation [107]. Trubey et al [102] also showed that umami-taste stimulation also causes a decrease in cAMP in rat taste buds, emphasizing the presence of transducin-like G proteins in taste cells. That G a gustducin is intimately involved in bitter, umami, and sweet stimulation has been verified by showing that these taste qualities are significantly compromised in G a gustducin knockout mice [108][109][110][111].…”

Caffeine induces gastric acid secretion via bitter taste signaling in gastric parietal cells

“…That leaves the question of how caffeine can stimulate GAS via TAS2Rs. So far, only for sweet and glutamate taste receptors (TAS1R1/3) has an increase of cAMP levels via activation of adenylyl cyclase been demonstrated (39). The signaling cascade of bitter taste receptors has been proposed to reduce cAMP levels by activation of PDE via α-gustducin or transducin (23,26,40).…”

Functional expression of the extracellular-Ca2+-sensing receptor in mouse taste cells

“…With such a diversity of CASR-mediated intracellular signaling, it is not unlikely that instead of Ca 2+ mobilization in type I cells, CASR is coupled to some other signaling pathways, perhaps, with cAMP as a second messenger (see Hofer and Brown, 2003;Hofer et al, 2004;Breitwieser et al, 2004). Consistent, at least in the case with Glu, is that this amino acid has been documented to modulate cAMP production in the taste tissue (Abaffy et al, 2003;Trubey et al, 2006), although a cellular source of Glu-dependent cAMP signals and the underlying mechanisms remain to be identified.…”