A Novel Splice Variant of a Metabotropic Glutamate Receptor, Human mGluR7b

Two splice variants of the human metabotropic glutamate receptor 7, named hmGluR7a and hmGluR7b, were isolated from a human brain cDNA library. The isoforms differ by an out-of-frame insertion of 92 nucleotides close to the C-terminus of the hmGluR7 coding region. hmGluR7a has a length of 915 amino acids and represents the human homolog of the recently cloned rat mGluR7. hmGluR7b is seven amino acids longer and exhibits a novel C-terminus of 23 amino acids in length. RT-PCR analysis demonstrated the existence of mGluR7b transcripts in wild-type mouse brain and its absence in mGluR7 knockout mice. Northern blot analyses indicate that mGluR7 expression is developmentally regulated. It is expressed at high levels in human fetal brain and at a lower level in many regions of adult human brain. Stimulation of hmGluR7b with L-2-amino-4-phosphonobutyrate (L-AP4), L-serine-O-phosphate (L-SOP) or L-glutamate in stably transfected Chinese hamster ovary (CHO) cells depressed forskolin-induced cAMP accumulation, whereas (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) and quisqualate (both at 1 mM) had no significant effects. As described for rat mGluR7, the rank order of agonist potencies is: L-SOP, L-AP4 > L-glutamate > (1S,3R)-ACPD, quisqualate.