Kathryn Partin - US grants

Glutamate receptors are proteins that are essential for communication between brain cells (neurons), and function in brain processes including learning and memory. Mammalian glutamate receptors have evolved into a multi-gene family containing subtypes with genetic, functional and pharmacological similarities. Glutamate receptors are activated when they bind extracellular glutamate (ligand). The binding to the receptor allows the protein to open a channel (pore) through which ions flow across the neuron's membrane; converting a chemical signal (glutamate) into an electrical signal (membrane depolarization). The proposed study will test the hypothesis that one region common to all glutamate receptors (the M3-M4 extracellular domain) participates in a common mechanism to relay the signal between the ligand binding site and the pore (allosteric signal transduction). Elucidation of a conserved functional role of the M3-M4 extracellular domain would provide important insights into how ligand-gated ion channels function.

DESCRIPTION (provided by applicant): The goal of this research is to understand how allosteric modulators of glutamate receptors exert their actions. Glutamate is the primary excitatory neurotransmitter in the brain, and the actions of glutamate receptors underlie normal and pathophysiological brain function. Drugs that specifically enhance or diminish glutamate receptor activity have potential for treating cognitive impairment following stroke, brain injury or neurodegenerative disease. We have identified two classes of positive allosteric modulators: one that slows channel closing (deactivation) and another that slows entry into the desensitized state. We will use a prototypical modulator of deactivation (l-BCP) and of desensitization (cyclothiazide, CTZ) to dissect the molecular mechanism of allosteric modulation. Two potential drug-binding sites have been identified for ANPA receptors, but it is not known whether these sites represent a true drug-binding site, or domains that regulate receptor conformations which secondarily affect drug modulation. We will use site-directed mutagenesis and patch-clamp electrophysiology to determine the extent to which perturbation of each site alters the efficacy of CTZ or l-BcP, or alters gating kinetics in the absence of any drug. Our hypotheses state that one of the identified sites (Site I) is not a drug-binding site, but rather regulates allosteric transitions that affect drug sensitivity secondarily; and, that the second site (Site II) represents the CTZ binding site. Mutations of each site will be tested for control kinetics of deactivation and desensitization, and then for slowing of either deactivation or desensitization in the presence of CTZ or l-ECP. The results from these experiments will allow us to determine if either of these sites is a good candidate for a drug-binding site, and if so, what the important chemical aspects of the site are with respect to activity. The results from these experiments will permit the rational design of modulatory drugs with improved selectivity and potency, as well as contributing to our understanding of the fundamental processes underlying glutamate receptor gating.