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 increasing evidence that dysfunction of GluR activity mediates several central nervous system diseases as well as damage during stroke, the Laboratory of Cellular and Molecular Neurophysiology (LCMN) is directing considerable effort to analysis of GluR function at the molecular level, with the goal of obtaining atomic-resolution structural data. The data will provide a framework for designing experiments to define mechanisms underlying ligand recognition and gating, which, in turn, will allow development of subtype-selective antagonists and allosteric modulators with novel therapeutic applications.

The ionotropic glutamate receptors in humans are encoded by seven gene families named after ligands that were first used to identify the major receptor 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 binding of agonists and antagonists and has provided insight into mechanisms of activation and desensitization.

During the past year, the LCMN undertook 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 high Ångstrom resolution. The structures revealed hyperextension of the ligand-binding core compared with AMPA receptor antagonist structures. They also revealed a dimer assembly with a 20 Å extension between glutamate and antagonist structures. As a result, the linkers connecting the ion channel to the ligand-binding core must be capable of supporting much larger movements than those suggested by earlier work.

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. NR3A's binding profile is strikingly different from that of the related NR1 subunit. Crystals for the glycine and D-serine complexes diffract to 1.7 and 1.5 Å resolution; work is in progress to obtain additional ligand complexes.