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In the central nervous system, glycine is a coagonist with glutamate at the N-methyl-D-aspartate subtype of ionotropic glutamate receptors. The GLYT1b subtype of glycine transporters is expressed in similar regions of the brain as the excitatory N-methyl-D-aspartate receptors and has been postulated to regulate glycine concentrations within excitatory synapses. We have expressed GLYT1b in Xenopus laevis oocytes and used electrophysiological techniques to investigate the pH regulation of glycine transporter function. We found that H(+) inhibits glycine transport by a noncompetitive mechanism, with half-maximal inhibition occurring at concentrations found in both physiological and pathological conditions. Charge-to-flux experiments revealed that the decreased current measured corresponds to a decreased influx of [(3)H]glycine and that the proton inhibition of GLYT1b does not alter the coupling ratio of transport. The membrane potential does not affect proton inhibition of transport, suggesting that the site of action on GLYT1b is not within the electric field of the membrane. Mutation of histidine 421 to an alanine residue, in the fourth extracellular loop of GLYT1b, renders the transporter insensitive to regulation by pH, but does not seem to alter the kinetics of glycine transport. These results suggests that histidine 421 is responsible for mediating the inhibitory actions of protons. Proton modulation of GLYT1b may be an important factor in determining the dynamics of excitatory neurotransmission.

Zinc ions (Zn2+) are stored in synaptic vesicles with glutamate in a number of regions of the brain. When released into the synapse, Zn2+ modulates the activity of various receptors and ion channels. Excitatory amino acid transporters (EAATs) maintain extracellular glutamate concentrations below toxic levels and regulate the kinetics of glutamate receptor activation. We have investigated the actions of Zn2+ on two of the most abundant human excitatory amino acid transporters, EAAT1 and EAAT2. Zn2+ is a noncompetitive, partial inhibitor of glutamate transport by EAAT1 with an IC50 value of 9.9 +/- 2.3 microM and has no effect on glutamate transport by EAAT2 at concentrations up to 300 microM. Glutamate and aspartate transport by EAAT1 are associated with an uncoupled chloride conductance, but Zn2+ selectively inhibits transport and increases the relative chloride flux through the transporter. We have investigated the molecular basis for differential inhibition of EAAT1 and EAAT2 by Zn2+ using site-directed mutagenesis and demonstrate that histidine residues of EAAT1 at positions 146 and 156 form part of the Zn2+ binding site. EAAT2 contains a histidine residue at the position corresponding to histidine 146 of EAAT1, but at the position corresponding to histidine 156 of EAAT1, EAAT2 has a glycine residue. Mutation of this glycine residue in EAAT2 to histidine generates a Zn2+ sensitive transporter, further confirming the role of this residue in conferring differential Zn2+ sensitivity.

Glutamate transporters serve the important function of mediating removal of glutamate released at excitatory synapses and maintaining extracellular concentrations below excitotoxic levels. Excitatory amino acid transporter subtypes EAAT1 and EAAT2 have a high degree of sequence homology and similar predicted topology and yet display a number of functional differences. Several recombinant chimeric transporters were generated to identify domains that contribute to functional differences between EAAT1 and EAAT2. Wild-type transporters and chimeric transporters were expressed in Xenopus laevis oocytes, and electrogenic transport was studied under voltage clamp conditions. The differential sensitivity of EAAT1 and EAAT2 to transport blockers, kainate, threo-3-methylglutamate, and (2S, 4R)-4-methylglutamate as well as L-serine-O-sulfate transport and chloride permeability were employed to characterize chimeric transporters. One particular region, transmembrane domains 9 and 10, plays an important role in defining these functional differences. The intracellular carboxyl-terminal region may also play a minor role in conferring an effect on chloride permeability. This study provides important insight into the identification of functional domains that determine differences among glutamate transporter subtypes.

We have investigated the mechanism of action of a series of glutamate derivatives on the cloned excitatory amino acid transporters 1 and 2 (EAAT1 and EAAT2), expressed in Xenopus laevis oocytes. The compounds were tested as substrates and competitive blockers of the glutamate transporters. A number of compounds showed contrasting mechanisms of action on EAAT1 compared with EAAT2. In particular, (2S,4R)-4-methylglutamate and 4-methylene-glutamate were transported by EAAT1, with Km values of 54 microM and 391 microM, respectively, but potently blocked glutamate transport by EAAT2, with Kb values of 3.4 microM and 39 microM, respectively. Indeed, (2S,4R)-4-methylglutamate is the most potent blocker of EAAT2 yet described. (+/-)-Threo-3-methylglutamate also potently blocked glutamate transport by EAAT2 (Kb = 18 microM), but was inactive on EAAT1 as either a substrate or a blocker at concentrations up to 300 microM. In contrast to (2S,4R)-4-methylglutamate, L-threo-4-hydroxyglutamate was a substrate for both EAAT1 and EAAT2, with Km values of 61 microM and 48 microM, respectively. It seems that the chemical nature and also the orientation of the group at the 4-position of the carbon backbone of glutamate is crucial in determining the pharmacological activity. The conformations of these molecules have been modeled to understand the structural differences between, firstly, compounds that are blockers versus substrates of EAAT2 and, secondly, the pharmacological differences between EAAT1 and EAAT2.