Acid tests of N-methyl-D-aspartate receptor gating basics.

In this issue of Molecular Pharmacology, Low et al. (2003) delve into the mechanisms by which pH modulates the glutamate receptor subtype specifically activated by N–methylD-aspartate (NMDA). Based on a combination of scanning mutagenesis, patch clamp physiology, and molecular modeling, the authors conclude that amino acids involved in NMDA receptor modulation by protons are clustered in regions of the receptor that are likely to link agonist binding with channel opening. pH plays an exceptionally broad role in mammalian physiology. Ion channels constitute a major class of proteins that are modulated by protons. Ion channels that exhibit sensitivity to pH changes near the physiological range include voltage-gated K , Na , and Ca channels, inward-rectifier K channels, gap junction channels, ClC chloride channels, Ca -activated K channels, degenerin/epithelial Na ion channels (including acid-sensing ion channels), vanilloid receptors, muscle and neuronal nicotinic acetylcholine receptors, GABAA receptors, and NMDA receptors. Proton effects on ion channels have been proposed to be mediated by one or more of a diversity of mechanisms, including protonation of: membrane surface charges (Hille, 1968), carbohydrate moieties added by glycosylation (Freeman et al., 2000), and amino acids involved in agonist binding (Abdrakhmanova et al., 2002), the ion conduction pathway (Woodhull, 1973), “ball-and-chain” style channel gating (Morley et al., 1996), and channel gating caused by generalized structural rearrangements (Schulte and Fakler, 2000). Because of the variety of potential actions of protons, dissecting the mechanisms by which pH modulates the activity of any channel type is challenging. Some of the effects of pH on channel function have been studied for decades yet remain incompletely understood. Interest in the pH sensitivity of NMDA receptor has been spurred by the diverse and powerful effects of NMDA receptor activation on the mammalian nervous system. NMDA receptors have been implicated in a remarkable range of nervous system physiology (from synapse stabilization during development to synaptic plasticity in adults) and nervous system pathology (from schizophrenia to excitotoxic neuronal death after stroke). Thus, it has been proposed that changes in brain pH may provide, for example, protection of neurons from glutamate excitotoxicity. In addition, determining the location and makeup of the “proton sensor” on NMDA receptors may lead to improved understanding of receptor structure. NMDA receptors are thought to be heterotetramers, composed predominantly of NR1 subunits combined with NR2A, NR2B, NR2C, and/or NR2D subunits. Despite extensive research, such basic questions as the nature of the channel gate and how it functions remain unresolved. The insights into proton modulation of NMDA receptors provided by Low et al. (2003) may have important implications for channel gating. Numerous excellent studies, many involving the authors of Low et al. (2003), underlie our current understanding of NMDA receptor modulation by protons. At physiological extracellular pH, NMDA receptors are about 50% inhibited by protons. Increasing the extracellular pH potentiates NMDA responses, whereas decreasing the extracellular pH leads to full inhibition of NMDA responses. Fitting of [H ]-NMDA response curves reveal in some instances a Hill coefficient near 1, an observation that seems consistent with the idea that protonation of a single proton sensor in NMDA receptors leads to receptor inactivity. In seeming contrast to this straightforward idea, site-directed mutagenesis studies have revealed that remarkably many amino acids, spread over multiple regions of NR1 and NR2 subunits, influence the proton sensitivity of NMDA receptors. In addition, numerous other modulators of NMDA receptor function, including Zn , polyamines, and ifenprodil, act, at least in part, by affecting the proton sensitivity of NMDA receptors. Low et al. (2003) integrate an extensive volume of data on the diverse locations of amino acids in NMDA receptors involved in proton sensitivity. They first expanded the list of amino acids that affect the proton sensitivity of NMDA receptors by making 88 mutations of amino acids in NR1, along with several mutations in NR2. They combined the results of their mutational analysis with data on 53 other NR1 mu-