A Comparison of Two Alternatively Spliced Forms of a Metabotropic Glutamate Receptor Coupled to Phosphoinositide Turnover

Abstract: A comparison of the pharmacological and physiological properties of the metabotropic glutamate 1α and 1β receptors (mGluR1α and mGluR1β) expressed in baby hamster kidney (BHK 570) cells was performed. The mGluR1β receptor is an alternatively spliced form of mGluR1α with a modified carboxy terminus. Immunoblots of membranes from the two cell lines probed with receptor-specific antipeptide antibodies showed that mGluRIa migrated with an M_r= 154, 000, whereas mGluR1β migrated with an M_r= 96, 000. Immunofluorescence imaging of receptors expressed in BHK 570 cells revealed that the mGluR1α receptor was localized to patches along the plasmalemma and on intracellular membranes surrounding the nucleus, whereas mGluR1β was distributed diffusely throughout the cell. Agonist activation of the mGluR1α and the mGluR1β receptors stimulated phosphoinositide hydrolysis. At both receptors, glutamate, quisqualate, and ibotenate were full agonists, whereas trans -(+)-1-aminocyclopentane-1, 3-dicarboxylate appeared to act as a partial agonist. The stimulation of phosphoinositide hydrolysis by mGluR1α showed pertussis toxin-sensitive and insensitive components, whereas the mGluR1β response displayed only the toxin-insensitive component. The mGluR1α and mGluR1β receptors also increased intracellular calcium levels by inducing release from intracellular stores. These results indicate that the different carboxy terminal sequences of the two receptors directly influences G protein coupling and subcellular deposition of the receptor polypeptides and suggest that the two receptors may subserve different roles in the nervous system.