Expression of Galpha14 in sweet-transducing taste cells of the posterior tongue

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 PLC beta 2-mediated release of stored Ca2+. Whereas G alpha gus (gustducin) couples to the T2R (bitter) receptors, which G alpha-subunit couples to the sweet (T1R2 + T1R3) receptor is presently not known. We utilized RTPCR, immunocytochemistry and single-cell gene expression profiling to examine the expression of the Gaq family (q, 11, 14) in mouse taste buds. Results: By RT-PCR, G alpha 14 is expressed strongly and in a taste selective manner in posterior ( vallate and foliate), but not anterior ( fungiform and palate) taste fields. G alpha q and G alpha 11, although detectable, are not expressed in a taste-selective fashion. Further, expression of G alpha 14 mRNA is limited to Type II/Receptor cells in taste buds. Immunocytochemistry on vallate papillae using a broad Gaq family antiserum reveals specific staining only in Type II taste cells (i.e. those expressing TrpM5 and PLC beta 2). This staining persists in Gaq knockout mice and immunostaining with a G alpha 11-specific antiserum shows no immunoreactivity in taste buds. Taken together, these data show that G alpha 14 is the dominant Gaq family member detected. Immunoreactivity for G alpha 14 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 G alpha 14 and both T1R2 and T1R3, the receptor combination that forms sweet taste receptors. Conclusion: G alpha 14 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.