Endothelin downregulates the glutamate transporter GLAST in cAMP‐differentiated astrocytes in vitro

Abstract

Endothelin (ET) is a putative pathogenetic mediator associated with brain trauma and ischemia. Because a link between neuronal damage after these injuries and glial Na(+)-dependent L-glutamate transporter activity has been suggested, we investigated the effect of ET on the glutamate clearance ability of astrocytes. Dibutyryl cyclic adenosine monophosphate (dBcAMP), which is widely used to induce differentiation of cultured astrocytes, markedly increased [(3)H]glutamate transport activity in a concentration- and time-dependent manner. In the presence of ET, however, dBcAMP decreased the glutamate uptake. This effect was efficiently prevented by an antagonist of ET(B) receptor, but not of ET(A) receptor. ET per se was virtually ineffective. Eadie-Hofstee analysis demonstrated that dBcAMP increased the V(max) value of glutamate uptake activity by 43.4% in the absence of ET, but decreased it by 41.4% in the presence of ET, without apparent changes in the K(m) value. Accordingly, Western blot analysis indicated that the change in transport activity correlated closely with that in expression level of the glial glutamate transporter GLAST. These results may represent the mechanisms by which ET aggravates trauma- and ischemia-elicited neuronal damage.

β-Amyloid Enhances Glial Glutamate Uptake Activity and Attenuates Synaptic Efficacy

“…Our study indicated, however, that at less than 20 M concentration, A␤ (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) caused a substantial increase in glutamate uptake. In transgenic mice expressing mutant ␤APP, the concentration of A␤ in the brain is not more than the low micromolar range (1 to 4 M), and such low concentrations are sufficient to cause marked impairment in learning and memory (50).…”

“…Although the difference in sequence between A␤ (1-42) and A␤ (1-40) is only two residues of C terminus, A␤ (1-42) aggregates more rapidly than A␤ (1-40) (28). Like A␤ (1-42), A␤ (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35), a biologically active, hydrophobic fragment of A␤ (29), is also highly prone to aggregation (30). This subfragment could also reproduce the effect of A␤ (1-42) (data not shown).…”

“…Previous studies showed that the fragment A␤ (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) induces a decrease in glutamate uptake of rat-cultured astrocytes when applied at a high concentration of 100 M (48, 49). Our study indicated, however, that at less than 20 M concentration, A␤ (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) caused a substantial increase in glutamate uptake.…”

Reactive astrocytes and activated microglial cells express EAAT1, but not EAAT2, reflecting a neuroprotective potential following ischaemia

“…[3][4][5] Expression of EAAT1 on astrocytes can be modulated pharmacologically, and pharmacological up-regulation of EAAT1 is associated with reduced infarct volume and improved neurological outcome following middle cerebral artery occlusion in rats. 10,35 However, no data exist on the expression of EAAT1 or EAAT2 in the human brain following ischaemia.…”

Autoantigen specific T cells inhibit glutamate uptake in astrocytes by decreasing expression of astrocytic glutamate transporter GLAST: a mechanism mediated by tumor necrosis factor‐α

“…Moreover, GLAST expression largely prevails over GLT-1 in primary cultured astrocytes (15). GLAST is extensively regulated (15)(16)(17)(18)(19)(20)(21) and subsequently extracellular glutamate levels can be kept constantly low. It has become clear that high extracellular glutamate induces rapid up-regulation of glutamate uptake and GLAST surface expression in astrocytes suggesting a feed-back loop to control the extracellular glutamate load (22).…”