Abstract
Background"Type II"/Receptor cells express G protein-coupled receptors (GPCRs) for sweet, umami (T1Rs and mGluRs) or bitter (T2Rs), as well as the proteins for downstream signalling cascades. Transduction downstream of T1Rs and T2Rs relies on G-protein and PLCβ2-mediated release of stored Ca2+. Whereas Gαgus (gustducin) couples to the T2R (bitter) receptors, which Gα-subunit couples to the sweet (T1R2 + T1R3) receptor is presently not known. We utilized RT-PCR, immunocytochemistry and single-cell gene expression profiling to examine the expression of the Gαq family (q, 11, 14) in mouse taste buds.ResultsBy RT-PCR, Gα14 is expressed strongly and in a taste selective manner in posterior (vallate and foliate), but not anterior (fungiform and palate) taste fields. Gαq and Gα11, although detectable, are not expressed in a taste-selective fashion. Further, expression of Gα14 mRNA is limited to Type II/Receptor cells in taste buds. Immunocytochemistry on vallate papillae using a broad Gαq family antiserum reveals specific staining only in Type II taste cells (i.e. those expressing TrpM5 and PLCβ2). This staining persists in Gαq knockout mice and immunostaining with a Gα11-specific antiserum shows no immunoreactivity in taste buds. Taken together, these data show that Gα14 is the dominant Gαq family member detected. Immunoreactivity for Gα14 strongly correlates with expression of T1R3, the taste receptor subunit present in taste cells responsive to either umami or sweet. Single cell gene expression profiling confirms a tight correlation between the expression of Gα14 and both T1R2 and T1R3, the receptor combination that forms sweet taste receptors.ConclusionGα14 is co-expressed with the sweet taste receptor in posterior tongue, although not in anterior tongue. Thus, sweet taste transduction may rely on different downstream transduction elements in posterior and anterior taste fields.
Highlights
Taste buds, the end-organs for gustation, detect and respond to a variety of macronutrient and aversive compounds to generate taste perception
We have used RT-PCR, immunocytochemistry and single-cell gene expression profiling to examine the expression of the Gαq family[32] in mouse taste buds and establish which of these are co-expressed with T1R2 and T1R3, subunits that constitute the sweet taste G protein coupled receptors (GPCRs)
Gαq family members are differentially expressed in different taste fields First, we used end-point RT-PCR to evaluate the expression of Gαq family subunits in taste buds to assess which members, if any, of the Gαq family (Gαq, Gα11 and Gα14) are expressed in taste buds
Summary
The end-organs for gustation, detect and respond to a variety of macronutrient and aversive compounds to generate taste perception. Compounds that evoke bitter taste bind to one or more G protein coupled receptors (GPCRs) of the T2R family of taste receptors [13]. Sugars and a variety of other sweeteners bind to the heterodimeric receptor, T1R2+T1R3[5,8] Most of these various taste GPCRs appear to all couple to a common transduction pathway that includes the heterotrimeric G protein subunits, Gβ3 and Gγ13, a phospholipase C, PLCβ2, and a transient receptor potential ion channel, TrpM5 [9,10,11,12,13,14]. The primary cellular response triggered by tastants appears to be a Ca2+ signal Much of this signal is produced via the action of Gβγ subunits[9], the contribution of the Gαq family has not been examined in taste buds
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