Water and urea permeation pathways of the human excitatory amino acid transporter EAAT1.

Glutamate transport is coupled to the co-transport of 3 Na(+) and 1 H(+) followed by the counter-transport of 1 K(+). In addition, glutamate and Na(+) binding to glutamate transporters generates an uncoupled anion conductance. The human glial glutamate transporter EAAT1 (excitatory amino acid transporter 1) also allows significant passive and active water transport, which suggests that water permeation through glutamate transporters may play an important role in glial cell homoeostasis. Urea also permeates EAAT1 and has been used to characterize the permeation properties of the transporter. We have previously identified a series of mutations that differentially affect either the glutamate transport process or the substrate-activated channel function of EAAT1. The water and urea permeation properties of wild-type EAAT1 and two mutant transporters were measured to identify which permeation pathway facilitates the movement of these molecules. We demonstrate that there is a significant rate of L-glutamate-stimulated passive and active water transport. Both the passive and active L-glutamate-stimulated water transport is most closely associated with the glutamate transport process. In contrast, L-glutamate-stimulated [(14)C]urea permeation is associated with the anion channel of the transporter. However, there is also likely to be a transporter-specific, but glutamate independent, flux of water via the anion channel.

Lipid inhibitors of high affinity glycine transporters: identification of a novel class of analgesics.

Glycine plays a key role in regulating inhibitory neurotransmission in the spinal cord and concentrations of glycine in the CNS are regulated by two subtypes of high affinity glycine transporters, GlyT1 and GlyT2. In this mini review we will discuss a series of lipid inhibitors of GlyT2 that show promise as analgesics in the treatment of neuropathic and inflammatory pain. N-arachidonyl-glycine inhibits the rate of transport by GlyT2, but has very little or no activity on GlyT1. We will discuss structure-activity studies of the actions of related lipids on GlyT2 and also the characterization of a more potent lipid inhibitor of GlyT2, oleoyl-l-carnitine. Both N-arachidonyl-glycine and oleoyl-l-carnitine show specificity for GlyT2 over GlyT1, which has allowed the use of chimeric GlyT1/GlyT2 transporters to begin characterizing the molecular basis for specificity and mechanism of action of these lipid inhibitors. Although our understanding of the molecular basis for lipid inhibition is still in its infancy, it appears that extracellular loop 4 of GlyT2 plays an important role in the inhibitory mechanism.