Mark L. Mayer, PhD, Chief

Ionotropic glutamate receptors (GluRs) are molecular pores that mediate signal transmission at the majority of excitatory synapses in the mammalian nervous system. Given their essential role in normal brain function and development and in view of accumulating evidence that dysfunction of GluR activity mediates several central nervous system diseases and damage during stroke, the Laboratory of Cellular and Molecular Neurophysiology (LCMN) is directing considerable effort to the analysis of GluR function at the molecular level, with the goal of obtaining atomic-resolution structural data. The data will provide a framework with which to design experiments that define the mechanisms underlying ligand recognition and gating, thereby allowing the development of subtype-selective antagonists and allosteric modulators with novel therapeutic applications.

Human ionotropic glutamate receptors are encoded by seven gene families named after the ligands first used to identify the major subtypes on a functional basis: AMPA, kainate, and NMDA. The recent crystallization of the ligand-binding cores of an AMPA receptor subunit and a related bacterial receptor from the photosynthetic bacterium Synechocystis sp. PCC 6803 has revealed the molecular mechanisms underlying the binding of agonists and antagonists, providing insight into the mechanisms of activation and desensitization.

During the past year, the LCMN has undertaken similar structural studies on members of the kainate receptor gene family as well as on the NMDA receptor NR3a subunit. The Laboratory solved crystal structures of GluR5 ligand-binding cores (S1S2 complexes with novel selective antagonists at 1.76 and 1.86 Å resolution). The structures reveal a hyperextension of the ligand-binding core as compared with the AMPA receptor antagonist structures and a dimer assembly with a 20 Å extension between the glutamate and antagonist structures. As a result, the linkers connecting the ion channel with the ligand-binding core must be capable of supporting much larger movements than earlier work suggested.

Surprisingly, the ease of expression and crystallization of individual GluR subtypes varies considerably such that some species have proven refractory to structural work. The Laboratory established an expression and purification scheme for NR3A and conducted an extensive series of ligand-binding assays. The binding profile is strikingly different from that for the related NR1 subunit. Crystals for the glycine complex diffract to 2 Å resolution; work is in progress to obtain additional ligand complexes.