Glucocorticoid regulation of the astrocyte transcriptome in vitro and in vivo.

Abstract

The broad goal of this thesis has been to gain understanding about the influence of cell type on transcriptional regulation in the brain by characterizing glucocorticoid-mediated regulation in astrocytes. Glucocorticoids are the primary mediator of the stress response facilitated through the hypothalamic-pituitary-adrenal axis. At the cellular level, glucocorticoids act as transcriptional regulators by binding their receptors and upregulating or downregulating target genes. All brain cell types (i.e. neurons and glia) express glucocorticoid receptors, yet most studies have interpreted data exclusively in terms of impact on neurons. We hypothesized that glucocorticoid mRNA regulation in the brain varies by cell type. We characterized glucocorticoid receptor-mediated mRNA regulation in mouse primary astrocyte cultures using microarrays, qPCR, and bioinformatics analyses. We identified numerous mRNAs regulated more than two-fold by the glucocorticoids dexamethasone and corticosterone, including 36 mRNAs demonstrating novel glucocorticoid regulation. Interestingly, glucocorticoids appear to selectively target astrocyte-enriched mRNAs in astrocyte in vitro in a time-dependent manner.

To test the physiological relevance of the observed glucocorticoid regulation in astrocytes in vitro, we measured a subset of regulated mRNAs by qPCR in two paradigms of in vivo corticosterone exposure in mice (acute/chronic). The majority of measured mRNAs regulated by glucocorticoids in astrocytes in vitro were also regulated by corticosterone exposure in vivo. Acute corticosterone exposure regulated distinct mRNAs compared with chronic corticosterone exposure. Chronic corticosterone exposure in vivo also regulated additional mRNAs not regulated by acute corticosterone exposure in vivo or glucocorticoid treatment of astrocytes in vitro. In situ hybridization experiments revealed gene expression patterns for a subset of mRNAs regulated by chronic corticosterone exposure that were consistent with astrocyte localization. Together, these data indicate that astrocyte mRNAs can indeed be regulated by glucocorticoids and suggest that glucocorticoids selectively regulate mRNAs by cell type in the brain. Our findings contribute further knowledge to understanding the stress response in the brain and may have clinical relevance for further understanding psychiatric disorders, particularly depression because of the established associations between depression and hypercortisolemia as well as astrocyte pathology.