Adenylyl cyclase expression and modulation of cAMP in rat taste cells

Abstract

Abaffy, Tatjana, Kristina R. Trubey, and Nirupa Chaudhari. Adenylyl cyclase expression and modulation of cAMP in rat taste cells. Am J Physiol Cell Physiol 284: C1420-C1428, 2003. First published February 26, 2003 10.1152/ajpcell.00556.2002cAMP is a second messenger implicated in sensory transduction for taste. The identity of adenylyl cyclase (AC) in taste cells has not been explored. We have employed RT-PCR to identify the AC isoforms present in taste cells and found that AC 4, 6, and 8 are expressed as mRNAs in taste tissue. These proteins are also expressed in a subset of taste cells as revealed by immunohistochemistry. Alterations of cAMP concentrations are associated with transduction of taste stimuli of several classes. The involvement of particular ACs in this modulation has not been investigated. We demonstrate that glutamate, which is a potent stimulus eliciting a taste quality termed umami, causes a decrease in cAMP in forskolin-treated taste cells. The potentiation of this response by inosine monophosphate, the lack of response to D-glutamate, and the lack of response to umami stimuli in nonsensory lingual epithelium all suggest that the cAMP modulation represents umami taste transduction. Because cAMP downregulation via ACs can be mediated through G␣ i proteins, we examined the colocalization of the detected ACs with G␣i proteins and found that 66% of AC8 immunopositive taste cells are also positive for gustducin, a taste-specific G␣ i protein. Whether AC8 is directly involved in signal transduction of umami taste remains to be established. immunohistochemistry; glutamate; umami; taste transduction; gustducin MEMBRANE-ASSOCIATED ADENYLYL CYCLASES (AC) are enzymes that are central in the regulation of intracellular cAMP levels. ACs, comprising a family of nine isoforms (AC1-9), are integral membrane proteins, composed of two cytosolic catalytic domains, C1 and C2, each preceded by six transmembrane domains (16, 34). The catalytic domains are highly conserved across all ACs and even across many guanylate cyclases and are responsible for the synthesis of cAMP using ATP as a substrate. Physiologically, G protein-coupled receptors (GPCRs) regulate ACs through heterotrimeric G protein subunits, specifically the stimulatory G␣ s , the inhibitory G␣ i , and/or certain G␤␥-subunits (9, 45). All AC isoforms can be stimulated by G␣ s and by forskolin (FSK), a diterpene commonly used in cAMP analysis.Inhibition by G␣ i is limited to a subset of AC isoforms. Inhibitory G␣ i subunits are able to overcome the FSKmediated activation of AC1, 3, 5, 6, 8, and 9, presumably because FSK binds near the catalytic site, distant from the G␣ i binding site (16,55). ACs also differ in their response to modulators such as Ca 2ϩ and calmodulin. Indeed, a unified classification of ACs incorporates both such regulatory properties and their sequence similarities.Taste buds are heterogeneous clusters of taste receptor cells, each of which responds to subsets of taste stimuli and contains distinct populations of cell signaling mole...

UmamiTaste Responses Are Mediated by α-Transducin and α-Gustducin

“…Ex vivo stimulation with MSG and/or IMP of fungiform papillae from C57BL mice increased the concentrations of both cAMP and IP 3 , suggesting that downstream effectors may be adenylyl cyclase and phospholipase C, as has been inferred for sweet compounds (Bernhardt et al, 1996). In contrast, rat circumvallate papillae taste tissues responded to MSG and L-AP-4 by decreasing cAMP (Abaffy et al, 2003), as is the case with bitter compounds (Lindemann, 1996;Yan et al, 2001).…”

Angiotensin II Modulates Salty and Sweet Taste Sensitivities

“…Taste cells express several isoforms of adenylyl cyclase (Abaffy et al, 2003) and phosphodiesterase (McLaughlin et al, 1994), which synthesize and degrade cAMP, respectively. The intracellular machinery for cAMP signaling is present in taste buds and cAMP changes occur following gustatory stimulation (Nakashima and Ninomiya, 1999;Roberts et al, 2009).…”

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.…”