Elucidation of the role of kainate receptor subtypes in hippocampal synaptic function using novel pharmacological tools

The main objective of the proposed research is to design and synthesize new chemical substances that can be used as tools to study some of the fundamental mechanisms by which the brain exercises its multiple functions. Specifically we aim to provide pharmacological tools to elucidate the mechanisms by which one nerve cell (neuron) communicates with others in the brain. This communication is effected at the junctions (synapses) between neurons. Our previous work has helped to establish that one of the main mechanisms by which one cell in a neuronal chain or network communicates with the next cell is by releasing an amino acid, glutamate from its multiple synaptic endings. This amino acid flows across the synaptic gap to the next neuron and there interacts with a protein (glutamate receptor) situated on the surface of the next neuron, to produce electrochemical and/or biochemical changes that control the electrical activity of the second cell. We have helped establish that glutamate can interact with a family of structurally related proteins known as glutamate receptor subtypes each performing different functions in the central nervous system. The present work aims to synthesize chemicals (pharmacological tools) that block the action of glutamate at particular glutamate receptor subtypes that are activated by the natural product kainic acid (kainate receptors). Kainate receptors are tetramers made up of a combination of protein subunits known as GluR5, GluR6, GluR7, KA1 and KA2. Whereas glutamate itself can bind to all of these subunits leading to receptor activation, the aim is to design and chemically synthesise agents known as antagonists that can selectively block the activation of GluR6, GluR7 or KA1/KA2 subunits. We have generated computer models of the ligand binding cores of GluR6, GluR7 and KA1 based on our X-ray crystal structures of the ligand binding core of GluR5 in complex with selective GluR5 subunit antagonists. We plan to use these models to design molecules that interact selectively with subunit specific amino acid residues in the ligand binding core, thereby producing selective GluR6, GluR7 or KA1/KA2 antagonists. By observing what the effect of specific blockade of each of the kainate receptor subunits has on the functioning of the central nervous system, one can deduce the particular roles of that receptor subunit in the integrated pattern of central nervous activity. Kainate receptors are thought to play a role in the fundamental mechanisms by which part of the brain known as the hippocampus stores memories, though the role of each individual subunit in these processes is still controversial due to the lack of specific pharmacological tools. Subunit specific pharmacological tools developed in this project will enable us to understand the role of GluR6, GluR7 and KA1/KA2 in these mechanisms. In addition, we plan to produce radioisotope labelled subunit selective kainate receptor antagonists. This will enable us to visualise the location of the GluR6, GluR7 or KA1/KA2 subunits within particular brain regions such as the hippocampus.

Native kainate receptors in the mammalian central nervous system are tetramers comprised of different combinations of GluR5, GluR6, GluR7, KA1 and KA2 subunits. The lack of subunit selective kainate receptor antagonists is the principal impediment to the elucidation of the physiological roles of native kainate receptor subtypes in the central nervous system. We plan to adopt an integrated multidisciplinary approach involving synthetic organic chemistry, X-ray crystallography, computer-aided drug design, molecular biology and electrophysiology to efficiently identify novel pharmacological tools to characterize the physiological roles of individual kainate receptor subunits. In particular, we will develop potent and selective antagonists for GluR6, GluR7 and KA1/KA2 subunits based on leads we have already identified by compound library screening and those generated by computer-aided drug design using X-ray crystal structures and homology models, so that the role of these receptors in hippocampal synaptic transmission and synaptic plasticity can be established. In addition, we plan to radiolabel the subunit selective kainate receptor antagonists. This will enable us to visualise the location of the GluR6, GluR7 or KA1/KA2 subunits in various brain regions such as the hippocampus.