The mechanism of GLT-1 mediating cerebral ischemic injury depends on the activation of p38 MAPK.

The previous studies have shown that glial glutamate transporter-1 (GLT-1) participates in cerebral ischemic injury in rats. However, the mechanism involved remains to be elucidated. This study was undertaken to investigate whether p38 MAPK was involved in regulating GLT-1 in the process. At first, it was observed that global brain ischemia for 8 min led to obvious delayed neuronal death, GLT-1 down-regulation and p-p38 MAPK up-regulation in CA1 hippocampus in rats. Then, whether p-p38 MAPK was involved in regulating GLT-1 during cerebral ischemic injury was studied in vitro. Astrocyte-neuron co-cultures exposed to oxygen and glucose deprivation (OGD) were used to mimic brain ischemia. It was observed that lethal OGD (4-h OGD) decreased GLT-1 expression and increased p-p38 MAPK expression in astrocytes. The p-p38 MAPK protein rised from 0 min to 48 h that is the end time of the observation, and the peak value was at 12 h, which was 12.45 times of the control group. Moreover, pre-administration of p38 MAPK inhibitor SB203580 or its siRNA dose-dependently increased GLT-1 expression, and meanwhile alleviated the neuronal death induced by lethal OGD. The above results indicated that p38 MAPK signaling pathway participated in regulating GLT-1 during OGD injury in vitro. Finally, back to in vivo experiment, it was found that pre-administration of SB203580 by intracerebroventricular injection dose-dependently reversed the down-regulation of GLT-1 expression and attenuated the delayed neuronal death normally induced by global brain ischemia in CA1 hippocampus in rats. Taken together, it can be concluded that the mechanism of GLT-1 mediating cerebral ischemic injury depends on the activation of p38 MAPK.

p38 MAPK Participates in the Mediation of GLT-1 Up-regulation During the Induction of Brain Ischemic Tolerance by Cerebral Ischemic Preconditioning.

Our previous study has proved that the up-regulation of glial glutamate transporter 1 (GLT-1) played an important role in the acquisition of brain ischemic tolerance after cerebral ischemic preconditioning (CIP) in rats. However, little is known about the mechanism involved in the up-regulation of GLT-1 in the process. The present study investigates whether p38 MAPK, ERK1/2, and/or JNK participates in the up-regulation of GLT-1 during the induction of brain ischemic tolerance by CIP. It was found that CIP significantly enhanced the expression of p-p38 MAPK without altering p-ERK1/2 and p-JNK expression in the CA1 hippocampus. Inhibition of p38 MAPK function by its selective inhibitor SB203580 or knockdown p38 MAPK expression by its antisense oligodeoxynucleotides (AS-ODNs) suppressed the induction of brain ischemic tolerance. Furthermore, p38 MAPK was activated earlier than the up-regulation of GLT-1 in the CA1 hippocampus after CIP. Meanwhile, the expression of p-p38 MAPK by astrocytes was increased, and p38 MAPK AS-ODNs dose-dependently inhibited the up-regulation of GLT-1 after CIP. Taken together, it could be concluded that p38 MAPK participates in the mediation of GLT-1 up-regulation during the induction of brain ischemic tolerance after CIP.

Up-regulation of neuronal and astrocytic GLT-1 mRNA in the hippocampal CA1 subfield during the induction of brain ischemic tolerance in rats.

Our previous study showed that 3-min cerebral ischemic preconditioning (CIP) up-regulated the expression of glial glutamate transporter-1 (GLT-1) protein, which protects pyramidal neurons, allowing them to survive an 8-min ischemic insult that usually induces severe delayed neuronal death in the hippocampal CA1 subfield. In the present study, in situ hybridization and immunohistochemistry were used to observe whether GLT-1 mRNA is modulated and whether actrocytes and/or neurons express GLT-1 mRNA during the induction of brain ischemic tolerance. We observed that GLT-1 mRNA is expressed in neurons and astrocytes in the hippocampal CA1 subfield. The expression of GLT-1 mRNA was significantly down-regulated in both neurons and astrocytes after the 8-min lethal ischemic insult. CIP for 3 min increased the expression of GLT-1 mRNA in neurons and astrocytes, and induced the elongation of the astrocytic processes around pyramidal neurons. It may be concluded that CIP performed 2 days before lethal ischemic insult activated astrocytes, which resulted in an increased number of lengthened processes expressing high levels of GLT-1, which protected the neurons and allowed them to survive 8-min ischemic insult that is usually lethal to neurons in the hippocampal CA1 subfield.

Intermittent hypobaric hypoxia preconditioning induced brain ischemic tolerance by up-regulating glial glutamate transporter-1 in rats.

Several studies showed that the up-regulation of glial glutamate transporter-1 (GLT-1) participates in the acquisition of brain ischemic tolerance induced by cerebral ischemic preconditioning or ceftriaxone pretreatment in rats. To explore whether GLT-1 plays a role in the acquisition of brain ischemic tolerance induced by intermittent hypobaric hypoxia (IH) preconditioning (mimicking 5,000 m high-altitude, 6 h per day, once daily for 28 days), immunohistochemistry and western blot were used to observe the changes in the expression of GLT-1 protein in hippocampal CA1 subfield during the induction of brain ischemic tolerance by IH preconditioning, and the effect of dihydrokainate (DHK), an inhibitor of GLT-1, on the acquisition of brain ischemic tolerance in rats. The basal expression of GLT-1 protein in hippocampal CA1 subfield was significantly up-regulated by IH preconditioning, and at the same time astrocytes were activated by IH preconditioning, which appeared normal soma and aplenty slender processes. The GLT-1 expression was decreased at 7 days after 8-min global brain ischemia. When the rats were pretreated with the IH preconditioning before the global brain ischemia, the down-regulation of GLT-1 protein was prevented clearly. Neuropathological evaluation by thionin staining showed that 200 nmol DHK blocked the protective role of IH preconditioning against delayed neuronal death induced normally by 8-min global brain ischemia. Taken together, the up-regulation of GLT-1 protein participates in the acquisition of brain ischemic tolerance induced by IH preconditioning in rats.

High expression of GLT-1 in hippocampal CA3 and dentate gyrus subfields contributes to their inherent resistance to ischemia in rats.

It is well known that neurons in the CA3 and dentate gyrus (DG) subfields of the hippocampus are resistant to short period of ischemia which is usually lethal to pyramidal neurons in hippocampal CA1 subfield. The present study was undertaken to clarify whether the inherent higher resistance of neurons in CA3 and DG to ischemia is associated with glial glutamate transporter-1 (GLT-1) in rats. Western blot analysis and immunohistochemistry assay showed that the basal expressions of GLT-1 in both CA3 and DG were much higher than that in CA1 subfield. Mild global brain ischemia for 8 min induced delayed death of almost all CA1 pyramidal neurons and marked GLT-1 down-regulation in the CA1 subfield, but it was not lethal to the neurons in either CA3 or DG and induced GLT-1 up-regulation and astrocyte activation showed normal soma and aplenty slender processes in the both areas. When the global brain ischemia was prolonged to 25 min, neuronal death was clearly observed in CA3 and DG accompanied with down-regulation of GLT-1 expression and abnormal astrocytes represented with hypertrophic somas, but shortened processes. After down-regulating of GLT-1 expression and function by its antisense oligodeoxynucleotides or inhibiting GLT-1 function by dihydrokainate, an inhibitor of GLT-1, the mild global brain ischemia for 8 min, which usually was not lethal to CA3 and DG neurons, induced the neuronal death in CA3 and DG subfields. Taken together, the higher expression of GLT-1 in the CA3 and DG contributes to their inherent resistance to ischemia.