GROUP-III HUMAN METABOTROPIC GLUTAMATE RECEPTOR-4, RECEPTOR-7 AND RECEPTOR-8 - MOLECULAR-CLONING, FUNCTIONAL EXPRESSION, AND COMPARISON OF PHARMACOLOGICAL PROPERTIES IN RGT CELLS

Cloning and expression in a stable mammalian cell line co-transfected with a glutamate transporter (RGT cells) were used as tools for studyi ng the functions and pharmacological properties of group III metabotro pic glutamate receptors (mGluRs). Complementary DNAs (cDNAs) encoding the human mGluR4, human mGluR7, and human mGluR8 were isolated from hu man cerebellum, fetal brain or retinal cDNA libraries. The human mGluR 4, mGluR7 and mGluR8 receptors were 912, 915 and 908 amino acid residu es long and share 67-70% amino acid similarity with each other and 42- 45% similarity with the members of mGluR subgroups I and II. The human mGluR4, and mGluR7 had amino acid identity of 96% and 99.5% with rat mGluR4 and 7, respectively, whereas the human mGluR8 has 98.8% amino a cid identity with the mouse mGluR8. The nucleotide and amino acid sequ ences in the coding region of human mGluR4 and mGluR7 were found to be identical to the previously published sequences by Flor et al. and Ma koff et al. Following stable expression in RGT cells, highly significa nt inhibitions of forskolin stimulation of cAMP production by group II I agonists were found for each receptor. The relative potencies of the group III agonist L-AP4 varied greatly between the group III clones, being mGluR8 > mGluR4 much greater than mGluR7. The reported group II mGluR agonist L-CCG-I was a highly potent mGluR8 agonist (EC50 = 0.35 mu M), with significant agonist activities at both mGluR4 (EC50 = 3.7 mu M) and mGluR7 (EC50 = 47 mu M). The antagonist potency of the purpo rted group III mGluR antagonist MPPG also varied among the receptors b eing human mGluR8 much greater than mGluR4 = mGluR7. The expression an d second messenger coupling of human group III mGluRs expressed in the RGT cell line are useful to clearly define the subtype selectivities of mGluR ligands. (C) 1998 Elsevier Science B.V.