Glucocorticoids regulate the expression of NGF, bFGF, and S100-beta in cultured hippocampal astrocytes: Impact on hippocampal neuron survival

Glucocorticoids play complex roles in the development, survival, and aging of hippocampal neurons; however, the mechanisms underlying these phenomena are unclear. Although several hypotheses have been proposed and tested, few studies have focused on how glucocorticoids influence neuron-astrocyte interactions. Astrocytes synthesize and secrete a wide variety of neurotrophic factors, through which they may regulate neuronal activity and survival. Therefore, I hypothesize that glucocorticoids regulate the expression of astrocyte-derived neurotrophic factors in the hippocampus. This affects the ability of astrocytes to support neuron survival, which may, in turn, contribute to the complex roles that glucocorticoids play in development, maintenance and aging of the hippocampus. To test the hypothesis, I monitored the effects of glucocorticoids on gene expression of three astrocyte-derived neurotrophic factors which are known to be important for hippocampal neuron development and survival, NGF{dollar}beta{dollar}, bFGF and S100{dollar}beta{dollar}, in cultured hippocampal astrocytes. Enriched type I astrocyte cultures from hippocampus were treated with dexamethasone, a synthetic glucocorticoid. Neurotrophic factor mRNAs were measured by solution hybridization-RNase protection assays. Dexamethasone suppressed NGF{dollar}beta{dollar} mRNA. In contrast, it stimulated bFGF mRNA levels and induced a biphasic response in S100{dollar}beta{dollar} gene expression. Furthermore, I monitored the protein levels of these three neurotrophic factors using Sandwich ELISA. Some of the dexamethasone-induced changes in NGF{dollar}beta{dollar}, bFGF, and S100{dollar}beta{dollar} mRNA levels were translated into parallel changes in their intracellular protein levels or in their levels in the conditioned medium. These results demonstrate that glucocorticoids differentially regulate the expression of important neurotrophic factors in hippocampal astrocytes in vitro. In addition, I observed the impact of glucocorticoid-induced changes in astrocytes on hippocampal neuron survival and on the vulnerability of the neurons to excitatory stimuli. Hippocampal neuronal cell cultures were treated with the conditioned medium which was collected from purified type I hippocampal astrocyte cultures that had been treated with dexamethasone. Neuron survival rates were calculated by counting viable neurons in cell culture. Conditioned medium from dexamethasone-treated astrocytes increased neuron survival under basal conditions; whereas, it decreased the survival after exposure to glutamate. These results indicate dexamethasone affects the ability of astrocyte-conditioned medium to support neuron survival and the ability of astrocyte-conditioned medium to protect neurons against the excitatory neurotoxicity. At the present time, it is unclear which astrocyte-derived factors are responsible for the dexamethasone effects; however, it is possible that the changes in the expression of NGF, bFGF, and S100{dollar}beta{dollar} which I observed previously are involved. Together, these studies suggest that one of the possible mechanisms underlying the complex roles of glucocorticoids played in the hippocampus is through the regulation of astrocyte-derived neurotrophic factors.