Cloning and characterization of a novel mGluR1 variant from vallate papillae that functions as a receptor for L-glutamate stimuli.

Monosodium L-glutamate (MSG) functions as a signal for dietary protein. In the tongue, glutamate binds presumably to taste cell chemoreceptors which, upon being activated, alter the firing rate of innervating sensory nerves. It has just recently been hypothesized that several G protein coupled receptors (GPCR) acted as umami receptors (Chaudhari et al., 2000; Li et al., 2002; Nelson et al., 2002; Toyono et al., 2003). However, the receptor mechanism for umami taste perception is still in doubt (Damak et al., 2003; Zhao et al., 2003). The data from gustatory nerve recordings, receptor distribution and taste cell electrophysiological function cannot all be reconciled solely by reference to already known receptors (Hoon et al., 1999; Ninomiya et al., 2000; Kim et al., 2003). The glossopharyngeal nerve that innervates folliate and vallate papillae at the posterior tongue contains fibers that are highly sensitive and selective to umami substances. Such umami-sensitive fibers were not observed in the chorda tympani nerve innervating fungiform papillae of anterior tongue (Ninomiya et al., 2000). However, the proposed amino-acid receptor T1R1 + T1R3 seems to be mostly abundant in fungiform papillae (Hoon et al., 1999; Kim et al., 2003). Furthermore, since knocking out T1r3 does not affect umami responses originating from the back of the tongue, other receptors must be considered (Damak et al., 2003). In this study, we describe a novel metabotropic glutamate receptor 1 (mGluR1) variant. This variant was cloned from rat vallate tissue and has a unique 5′ end sequence. In-frame with the long open reading frame there is a stop codon suggesting the presence of an un-translated 5′ region producing a short extracellular domain. Truncated mGluR1 generated intracellular Ca2+ mobilization in Xenopus oocytes when L-glutamate was applied at concentrations that elicit the umami taste.