Expression of phospholipase C-beta4 in rat circumvallate taste buds.

The heteromer of T1R1 and T1R3 and taste-mGluR4 function as receptors for glutamate (umami) taste sensation (Chaudhari et al., 2000; Nelson et al., 2002). Metabotropic glutamate receptor type 1 (mGluR1α) is expressed in taste receptor cells in rat gustatory papillae (Toyono et al., 2003). It has been known that mGluR1α couples preferentially to an α-subunit of the Gαq family, leading to activation of phospholipase C-β (PLC-β) and the consequent mobilization of intracellular Ca2+ levels in the central nervous system (Hermans and Challiss, 2001). The inositol tri-phosphate (IP3) pathway is involved in taste transduction for glutamate in mouse fungiform papilla (Ninomiya et al., 2000). Applications of glutamate and the mixture of GMP and glutamate to rat taste cells increase intracellular Ca2+ levels (Lin et al., 2003). Thus, mGluR1α may be a candidate for another type of umami receptors because mGluR1α plays some roles in IP3 pathway and the mobilization of intracellular Ca2+. Recent studies have provided evidence that the members of Gαq family, Gαq, Gα14, Gα15 and PLC-β2, are expressed in rat taste buds (Kusakabe et al., 1998; Rössler et al., 1998). Further, PLC-β2 is known to co-expressed with IP3 type III receptor (IP3R3) in rodent circumvallate taste buds (Clapp et al., 2001). PLC-β2 generates IP3, which then activates IP3R3 of intracellular Ca2+ stores in taste cells. In this context, it is conceivable that there may exist a similar signaling cascade via mGluR1α in umami taste sensation. However, no mention has so far been made of the expression patterns of mGluR1α and these signaling molecules in rat taste bud cells. There are four different PLC-β isoforms (PLC-β1–4) that have been cloned (Rhee and Bae, 1997). They are all regulated by heterotrimeric G proteins and there is evidence suggesting that different isoforms may be involved in a variety of signaling circuits. PLC-β can be activated by both the Gα subunits of the Gαq family and by the βγ subunits generated by a number of different heterotrimeric G proteins. On the other hand, PLC-β4 can be activated by Gαq but not by βγ-subunits of G-proteins (Jiang et al., 1994). The major molecular cascade from mGluR1 to PLC-β is considered to be mGluR1–Gαq–PLC–β4 in Purkinje cells (Hirono et al., 2001). In view of these respects, we deduced that PLC-β4 might contribute the mGluR1-mediated signal transduction in taste sensation. However, a search of the literatures fails to reveal the expression of PLC-β4 in rat taste tissues. In the present study, we examined for the first time the expression patterns of mGluR1α and taste signaling molecules, Gαq and PLCβ2 in rat circumvallate papillae. In addition, we examined the expression PLC-β4 mRNA and its protein in gustatory papillae and taste buds by using reverse transcription–polymerase chain reaction (RT– PCR) and immunohistochemistry.