Glutamate receptors (GluRs), broadly classified in the NMDA and non- NMDA subtypes, have been implicated in simple forms of learning and memory and are the site of action of several psychoactive drugs. Molecular cloning of GluR cDNAs has revealed a large family of distantly related receptor subunits that can be classified into groups by their sequence similarity and/or agonist binding preferences. These groups include: NMDA-R1 and NMDA-R2 A-D (agonist is NMDA), GluR 1-4 (AMPA- preferring), and GluR 5-7 and KA1,2 (kainate-preferring). The genes coding for GluR subunits are differentially expressed in regions of the CNS, and during development. We have begun to isolate and characterize the transcription regulatory sequences of NMDA-R genes, in order to identify the elements that impart developmental and regional-specific expression, and confer activity-dependent regulation. Alternative splicing and RNA editing add further complexity to the different receptor isoforms that can be generated from a single GluR gene, resulting in the assembly of receptors that exhibit marked differences in ion-selectivity and desensitization properties. In the case of the GluR 4 gene, we have identified 6.2 and 4.2 transcripts that code for functional GluR receptors. In addition, we have identified a related abundant 3.0 kb cerebellar transcript that codes for a putative truncated protein missing the 4 transmembrane spanning regions present in other GluR subunits. This protein has been called GluR-4s (""""""""s"""""""" for short). Southern and PCR analysis demonstrate that the GluR 4 gene is present in a single copy, and that GluR 4s results from differential RNA processing. The expression of this poly-adenylated transcript is developmentally regulated and predominantly confined to neurons, in contrast to the other GluR mRNAs which are expressed in Bergmann glia and astrocytes. In collaboration with Dr. Mayer's laboratory, we have analyzed the differential expression of non-NMDA GluR subunits in dorsal root ganglia (DRG), forebrain and hippocampus. DRG were found to predominantly express kainate-preferring GluR subunits, whereas both families of receptor mRNAs were found in cerebellum and forebrain. Expression of recombinant receptors in Xenopus oocytes showed that kainate responses at AMPA-and kainate- preferring receptors are differentially potentiated by Con A and cyclothiazide. Oocytes injected with combinations of kainate-preferring receptors showed similar pharmacological responses to those observed in DRG, whereas those injected with AMPA-preferring subunits elicited similar responses to those found in hippocampal neurons. These results suggest that different families of GluR subunits selectively assemble to form functional receptors with unique properties.
