mTOR/S6 kinase pathway contributes to astrocyte survival during ischemia

Neurons are highly dependent on astrocyte survival during brain damage. To identify genes involved in astrocyte function during ischemia, we performed mRNA differential display in astrocytes after oxygen and glucose deprivation (OGD). We detected a robust down-regulation of S6 kinase 1 (S6K1) mRNA that was accompanied by a sharp decrease in protein levels and activity. OGD-induced apoptosis was increased by the combined deletion of S6K1 and S6K2 genes, as well as by treatment with rapamycin that inhibits S6K1 activity by acting on the upstream regulator mTOR (mammalian target of rapamycin). Astrocytes lacking S6K1 and S6K2 (S6K1;S6K2-/-) displayed a defect in BAD phosphorylation and in the expression of the anti-apoptotic factors Bcl-2 and Bcl-xL. Furthermore reactive oxygen species were increased while translation recovery was impaired in S6K-deficient astrocytes following OGD. Rescue of either S6K1 or S6K2 expression by adenoviral infection revealed that protective functions were specifically mediated by S6K1, because this isoform selectively promoted resistance to OGD and reduction of ROS levels. Finally, "in vivo" effects of S6K suppression were analyzed in the permanent middle cerebral artery occlusion model of ischemia, in which absence of S6K expression increased mortality and infarct volume. In summary, this article uncovers a protective role for astrocyte S6K1 against brain ischemia, indicating a functional pathway that senses nutrient and oxygen levels and may be beneficial for neuronal survival.