MOLECULAR CHARACTERIZATION OF GLUTAMATE RECEPTOR EXPRESSION IN BRAIN

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Glutamate receptors (GluRs) have been implicated in simple forms of learning and memory and are the site of action of several psychoactive drugs. GluRs have been broadly classified in the non-NMDA and NMDA subtypes. Molecular cloning of non-NMDA GluR cDNAs has revealed a large family of related receptor subunits that can be classified into distinct groups by their sequence similarity and binding preferences for either AMPA (GluRs 1-4) or kainate (GluRs 5-7 and KAR 1,2). 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. Previously, we reported on the cloning of a novel GluR 4 transcript that results from the differential RNA processing of sequences coding for an alternative C-terminus; we designated this isoform GluR-4c. We observed on Northern blots containing cerebellar RNA, that GluR-4 oligonucleotide probes corresponding to the N-terminal extracellular domain of the receptor hybridized to bands of 6.2, 4.2, and 3.0 kb. However, probes derived from sequences corresponding to the middle or C-terminal end of the protein hybridized exclusively to the larger bands. We now have obtained CDNAS corresponding to the short 3.0 kb transcript; they have been called GluR-4s. DNA sequencing of these cDNAs revealed that the 3.0 kb mRNAs are derived from GluR-4 nascent transcripts by differential splicing, and that they code for a protein that is identical to GluR-4c for most of the N-terminal region, but diverge and terminate to yield a truncated polypeptide. In contrast to the other GluR-4 isoforms, which are expressed in neurons and glia, GluR-4s transcripts are predominantly expressed in cerebellar granule neurons. In collaboration with Dr. Mayer's laboratory, we have begun to identify the non-NMDA GluR isoforms expressed by dorsal root ganglia (DRG) and determine the molecular components underlying the heterogeneity of DRG responses to glutamate analogs. The effects of denervation on GluR expression have also been studied.