Functional Significance of the Preconditioning-induced Down-regulation of Glutamate Transporter GLT-1 in Neuron/astrocyte Co-cultures

Abstract

In the brain, prior sublethal ischemia (preconditioning, PC) is known to produce tolerance of neurons to subsequent lethal ischemia. This study aims at elucidating what alterations were induced in neurons and/or astrocytes by PC treatment. The rise in the extracellular concentration of glutamate during ischemia was markedly suppressed by the prior PC treatment. Immunocytochemical and Western blot analyses demonstrated that the expression of the astrocytic glutamate transporter GLT-1 was transiently down-regulated after the PC insult. The PC insult possibly suppressed the neuron-derived factors up-regulating GLT-1. Here we show that PC-induced down-regulation of GLT-1 is crucial for the increased neuronal resistance to subsequent severe ischemic insult.

Molecular Physiology of Preconditioning-Induced Brain Tolerance to Ischemia

“…However, with neuron/astrocyte cocultures, another group of investigators reported that a downregulation of the EAAT2 glutamate was associated with ischemic tolerance induced by ischemic preconditioning (210,442).…”

Nitric Oxide Produced During Sublethal Ischemia Is Crucial for the Preconditioning-Induced Down-Regulation of Glutamate Transporter GLT-1 in Neuron/Astrocyte Co-Cultures

“…Culture methods were described previously in detail (3,4,10,11). In brief, both neurons and astrocytes were prepared from 17-day old embryonic rat cortices and grown in Dulbecco's modified Eagle's medium (DMEM, Gibco, Grand Island, NY) which was supplemented with 10% heat-inactivated fetal bovine serum (FBS), 10%…”

“…In the brain, Kitagawa et al (2) first reported that gerbils subjected to sublethal transient global ischemia exhibited reduced hippocampal neuronal death after a severe ischemic insult 24-28 h later. We have recently demonstrated in mixed neuron/astrocyte cultures that the ischemia-induced rise in the extracellular concentration of L-glutamate (Glu) is significantly suppressed by prior PC treatment, and this suppression during ischemia primarily contributes to the PC-induced ischemic tolerance of neurons (3).…”

“…In addition, we have recently revealed that in neuron/astrocyte co-cultures the expression of the astrocytic glutamate transporter GLT-1 is transiently down-regulated after PC treatment, and the ischemia-induced rise in the extracellular concentration of glutamate via a reversed uptake of glutamate by GLT-1 is significantly suppressed (3). The question then arises as to whether NO produced during PC is involved in the PC-induced down-regulation of GLT-1, and in the resultant ischemic tolerance of neurons.…”

Glutamate transporters in brain ischemia: to modulate or not?

“…In cortical cultures exposed to low-oxygen concentrations, ischemic tolerance was associated with the up-regulation of GLT-1 and EAAC1 [204] . The co-culture of cortical neurons with astrocytes demonstrated the down-regulation of GLT-1 after OGD preconditioning, associated with the increased resistance of these co-cultures to more severe ischemic episodes through OGD [205] . Mild ischemia might reverse neuronal Glu transport through EAAC1, while a severe ischemia episode, causing brain damage, might also affect glial transporters, including GLT-1.…”