Two newly identified exons in human GRM1 express a novel splice variant of metabotropic glutamate 1 receptor

To date, five human metabotropic glutamate (mGlu) 1 receptor splice variants (1a, 1b, 1d, 1f, and 1g) have been described, all of which involve alternative C-terminal splicing. mGlu1a receptor contains a long C-terminal domain (341 amino acids), which has been shown to scaffold with several proteins and contribute to the structure of the post-synaptic density. However, several shorter mGlu1 receptor splice variants lack the sequence required for these interactions, and no major functional differences between these short splice variants have been described. By using RT-PCR we have shown that two human melanoma cell lines express both mGlu1a and mGlu1b receptors. In addition, using 3′RACE, we identified three previously unknown mGlu1 receptor mRNAs. Two differ in the length of their 3′ untranslated region (UTR), and encode the same predicted protein as mGlu1g receptor—the shortest of all mGlu1 receptor splice variants. The third mRNA, named mGlu1h, encodes a predicted C-terminal splice variant of 10 additional amino acids. mGlu1h mRNA was observed in two different melanoma cell lines and is overexpressed, compared with melanoma precursor cells, melanocytes. Most importantly, this new splice variant, mGlu1h receptor, is encoded by two previously unidentified exons located within the human GRM1 gene. Additionally, these new exons are found exclusively within the GRM1 genes of higher primates and are highly conserved. Therefore, we hypothesize that mGlu1h receptors play a distinct role in primate glutamatergic signaling.     

Activation of group II mGlu receptors inhibits voltage-gated Ca2+ currents in myenteric neurons

The enteric nervous system (ENS) contains functional ionotropic and group I metabotropic glutamate (mGlu) receptors. In this study, we determined whether enteric neurons express group II mGlu receptors and the effects of mGlu receptor activation on voltage-gated Ca(2+) currents in these cells. (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC), a group II mGlu receptor agonist, reversibly suppressed the Ba(2+) current in myenteric neurons isolated from the guinea pig ileum. Significant inhibition was also produced by L-glutamate and the group II mGlu receptor agonists, (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I), with a rank order potency of 2R,4R-APDC > DCG-IV > L-glutamate > L-CCG-I, and was reduced by the group II mGlu receptor antagonist LY-341495. Pretreatment of neurons with pertussis toxin (PTX) reduced the action of mGlu receptor agonists, suggesting participation of G(i)/G(o) proteins. Finally, omega-conotoxin GVIA blocked current suppression by DCG-IV, suggesting modulation of N-type calcium channels. mGlu2/3 receptor immunoreactivity was displayed by neurons in culture and in the submucosal and myenteric plexus of the ileum. A subset of these cells displayed a glutamatergic phenotype as shown by the expression of vesicular glutamate transporter 2. These results provide the first evidence for functional group II mGlu receptors in the ENS and show that these receptors are PTX sensitive and negatively coupled to N-type calcium channels. Inhibition of N-type calcium channels produced by activation of group II mGlu receptors may modulate enteric neurotransmission.     

Localization and function of group III metabotropic glutamate receptors in rat pancreatic islets

Pancreatic islets contain ionotropic glutamate receptors that can modulate hormone secretion. The purpose of this study was to determine whether islets express functional group III metabotropic glutamate (mGlu) receptors. RT-PCR analysis showed that rat islets express the mGlu8 receptor subtype. mGlu8 receptor immunoreactivity was primarily displayed by glucagon-secreting alpha-cells and intrapancreatic neurons. By demonstrating the immunoreactivities of both glutamate and the vesicular glutamate transporter 2 (VGLUT2) in these cells, we established that alpha-cells express a glutamatergic phenotype. VGLUT2 was concentrated in the secretory granules of islet cells, suggesting that glutamate might play a role in the regulation of glucagon processing. The expression of mGlu8 by glutamatergic cells also suggests that mGlu8 may function as an autoreceptor to regulate glutamate release. Pancreatic group III mGlu receptors are functional because mGlu8 receptor agonists inhibited glucagon release and forskolin-induced accumulation of cAMP in isolated islets, and (R,S)-cyclopropyl-4-phosphonophenylglycine, a group III mGlu receptor antagonist, reduced these effects. Because excess glucagon secretion causes postprandial hyperglycemia in patients with type 2 diabetes, group III mGlu receptor agonists could be of value in the treatment of these patients.