Bernhard Bettler

Cloning of a novel glutamate receptor subunit, GluR5: Expression in the nervous system during development

We have isolated cDNAs encoding a glutamate receptor subunit, designated GluR5, displaying 40%-41% amino acid identity with the kainate/AMPA receptor subunits GluR1, GluR2, GluR3, and GluR4. This level of sequence similarity is significantly below the approximately 70% intersubunit identity characteristic of kainate/AMPA receptors. The GluR5 protein forms homomeric ion channels in Xenopus oocytes that are weakly responsive to L-glutamate. The GluR5 gene is expressed in subsets of neurons throughout the developing and adult central and peripheral nervous systems. During embryogenesis, GluR5 transcripts are detected in areas of neuronal differentiation and synapse formation.

Agonist selectivity of glutamate receptors is specified by two domains structurally related to bacterial amino acid-binding proteins.

By exchanging portions of the AMPA receptor subunit GluR3 and the kainate receptor subunit GluR6, we have identified two discontinuous segments of approximately 150 amino acid residues each that control the agonist pharmacology of these glutamate receptors. The first segment (S1) is adjacent and N-terminal to the putative transmembrane domain 1 (TM1), whereas the second segment (S2) is located between the putative TM3 and TM4. Only the simultaneous exchange of S1 and S2 converts the pharmacological profile of the recipient to that of the donor subunit. The two segments identified in this study share sequence similarities with the ligand-binding site of several bacterial periplasmic amino acid-binding proteins. Based on the X-ray structure of these proteins, we propose a model for the glutamate-binding site of ionotropic glutamate receptors.

Cloning of a putative glutamate receptor: A low affinity kainate-binding subunit

Kainate, a glutamate receptor agonist, is a potent neuroexcitatory agent that produces epileptiform activity and selective neuronal degeneration. Binding studies using neuronal membrane homogenates or brain sections have identified sites having either high or low affinity for [3H]kainate. Here we report the cloning of a gene, GluR7, with approximately 75% sequence identity with the previously cloned GluR5 and GluR6 subunit genes. Transcripts of the GluR7 gene are evident in brain areas that bind [3H]kainate and are susceptible to kainate-induced neurotoxicity. We have performed ligand binding studies with membranes of transfected HeLa cells expressing GluR6 or GluR7 subunits. Our data show that the GluR6 and GluR7 subunits have a rank order of agonist affinity (domoate greater than kainate much greater than L-glutamate, quisqualate much greater than AMPA, NMDA) and a dissociation constant for kainate (95 and 77 nM, respectively) characteristic of the low affinity kainate-binding sites described in the brain.

Subunit composition of kainate receptors in hippocampal interneurons

Kainate receptor activation affects GABAergic inhibition in the hippocampus by mechanisms that are thought to involve the GluR5 subunit. We report that disruption of the GluR5 subunit gene does not cause the loss of functional KARs in CA1 interneurons, nor does it prevent kainate-induced inhibition of evoked GABAergic synaptic transmission onto CA1 pyramidal cells. However, KAR function is abolished in mice lacking both GluR5 and GluR6 subunits, indicating that KARs in CA1 stratum radiatum interneurons are heteromeric receptors composed of both subunits. In addition, we show the presence of presynaptic KARs comprising the GluR6 but not the GluR5 subunit that modulate synaptic transmission between inhibitory interneurons. The existence of two separate populations of KARs in hippocampal interneurons adds to the complexity of KAR localization and function.

The Glutamate Receptors: Genes, Structure and Expression

Many plausible theories of learning, pattern recognition and memory depend upon changes in the efficiency of chemical synapses (Cajal 1911; Hebb 1949; Hopfield 1982; Kandel and Schwartz 1982). It seems unlikely that these theories will be testable until the structure, function and regulation of receptor and ion channel molecules are understood in detail.