Cloning, expression and pharmacological characterization of a human glutamate receptor: hGluR4

A member of the ionotropic family of glutamate receptors, hGluR4, was isolated from a human cDNA library and characterized following expression in mammalian cell lines. Human GluR4 possessed a 99% amino acid and 92% nucleotide homology to that of its rat counterpart with sequence differences restricted to the carboxy and amino terminal regions of the molecule. Transfection of simian kidney cells (COS-1) with an hGluR4 expression plasmid resulted in the transient formation of a membrane protein that possessed high specific binding for [3H](RS)-alpha-amino- 3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA) but not [3H]kainate. Competition studies yielded a displacement profile of AMPA = quisqualate > glutamate > domoate > kainate >> N-methyl-D-aspartate (NMDA) or dihydrokainate. Whole-cell, voltage-clamp recordings from a human embryonic kidney cell line (HEK 293) stably expressing hGluR4 confirmed the presence of constitutively active, ligand-gated ion channels activated by AMPA, glutamate and kainate but not N-methyl-D-aspartate. Kainate-evoked currents were reversibly attenuated by 6-cyano-7-nitro- quinoxaline-2,3-dione (CNQX) but not DL-2-amino-5- phosphonovalerate (DL-AP5). Agonist-evoked currents exhibited inward rectification and ion substitution experiments indicated that hGluR4 receptor-linked ion channels in their homomeric state are permeable to both CA2+ and Na+ ions. In the same cell line antibody to rat GluR4 immunoprecipitated a major protein band at approximately 108 kDa and a minor one at approximately 340 kDa. The immunoblot analysis of membranes chemically crosslinked with dithiobis(succinimidylpropionate) showed a broad band at 550-600 kDa suggesting that the GluR4 receptor forms a pentamer in situ. This is the first report of the cloning of hGluR4 receptor and its stable expression in a human cell line.