Na+-dependent Glutamate Transporters (EAAT1, EAAT2, and EAAT3) of the Blood-Brain Barrier

Abstract

Na؉ -dependent transporters for glutamate exist on astrocytes (EAAT1 and EAAT2) and neurons (EAAT3). These transporters presumably assist in keeping the glutamate concentration low in the extracellular fluid of brain. Recently, Na ؉ -dependent glutamate transport was described on the abluminal membrane of the bloodbrain barrier. To determine whether the above-mentioned transporters participate in glutamate transport of the blood-brain barrier, total RNA was extracted from bovine cerebral capillaries. cDNA for EAAT1, EAAT2, and EAAT3 was observed, indicating that mRNA was present. Western blot analysis demonstrated all three transporters were expressed on abluminal membranes, but none was detectable on luminal membranes of the blood-brain barrier. Measurement of transport kinetics demonstrated voltage dependence, K ؉ -dependence, and an apparent K m of 14 M (aggregate of the three transporters) at a transmembrane potential of ؊61 mV. Inhibition of glutamate transport was observed using inhibitors specific for EAAT2 (kainic acid and dihydrokainic acid) and EAAT3 (cysteine). The relative activity of the three transporters was found to be approximately 1:3:6 for EAAT1, EAAT2, and EAAT3, respectively. These transporters may assist in maintaining low glutamate concentrations in the extracellular fluid.

Consensus meeting: monosodium glutamate – an update

“…Plasma GLU concentrations appear to rise only when pharmacologic doses of MSG are administered. Hence, if the BBB were to become permeable (for review see Ballabh et al, 2004;Neuwelt, 2004), or BBB GLU transporters were to become compromised (they normally function to transport GLU out of the brain (O'Kane et al, 1999), one might imagine that synaptic GLU concentrations could rise, which would be sufficient to stimulate GLU receptors.…”

“…However, several caveats should be noted: (1) The GLU transporters at the BBB appear to be on the abluminal membrane, and function to transport GLU out of the brain (O'Kane et al, 1999). These transporters presumably would still function in situations in which BBB permeability has increased; (2) Glial and neuronal GLU transporters (Goldsmith, 2000;Meldrum, 2000) would also presumably remain functional under conditions of increased BBB permeability (except if the brain is ischemic, and thus oxygen deprived, such as during a stroke/vascular occlusion or under conditions of increased intracranial pressure), and help to keep brain ECF and basal synaptic GLU concentrations low; and (3) Dietary GLU and MSG, even at a very high dose in the daily diet (Tsai and Huang, 1999), do not raise plasma GLU concentrations (MSG intake is self-limiting, since it is not palatable at high concentrations in foods (Yamaguchi, 1987)); hence, dietary GLU or MSG should not influence synaptic GLU concentrations, per se, if BBB permeability were to be increased.…”

High blood glutamate oxaloacetate transaminase levels are associated with good functional outcome in acute ischemic stroke

“…Previous studies have described that glutamate can diffuse through brain capillary endothelial cells from the brain to the blood torrent in a unidirectional manner following a gradient of concentration (Hawkins, 2009;O'Kane et al, 1999;Teichberg et al, 2009). As ischemic stroke is associated with an excessive release of glutamate into the brain parenchyma (Castillo et al, 1996(Castillo et al, , 1997, a decrease in blood glutamate levels provides a mechanism to increase the gradient concentration and to remove this neurotransmitter from the brain at early times, with possible therapeutic implications after an ischemic insult (Gottlieb et al, 2003;Teichberg et al, 2009).…”

Na+-dependent neutral amino acid transporters A, ASC, and N of the blood-brain barrier: mechanisms for neutral amino acid removal

“…The two membranes of the BBB appear to transport NAA in a fashion similar to that previously described for glutamine and acidic amino acids such as glutamate (12,18,22). Na ϩ -dependent carriers are capable of pumping both glutamine (system N) and glutamate (see the EAAT1, -2, and -3 of Ref.…”

“…Na ϩ -dependent carriers are capable of pumping both glutamine (system N) and glutamate (see the EAAT1, -2, and -3 of Ref. 22) from the ECF into endothelial cells. The luminal facilitative carriers for both glutamate (12,22,23) and glutamine (12,18) can then transport them to the plasma.…”