The Role of Glucocorticoid Receptors on the Expression of Transposable Elements in the Rat Hippocampus

Stress disorders are widespread and difficult to treat due to individual variations in biological symptoms. The major stress hormone in rats is corticosterone (CORT), which works primarily by activating glucocorticoid receptors (GR), a transcription factor than can regulate gene expression in a tissue- and context-specific manner. Variation in gene expression explains some individual differences in stress responses. However, few genetic variations have been found in protein-coding regions to explain the genesis of these differences. Transposable elements (TEs) are mobile genetic elements that have copied themselves or moved to different locations in the genome. With divergent, non-functional remnants constituting a large portion of mammalian DNA, TEs contribute to overall genetic uniqueness, even among direct ancestors. TEs are a potential source of regulatory sequences with which GR may interact. Previous work has shown that stress leads to a tissue-specific transcription of TEs, but a direct GR-TE relationship has not been tested, and the mechanism by which stress influences TE transcription is unknown. This dissertation tests the possibility that GR activation can result in binding to, and transcription of, TEs in the rat hippocampus. To examine the interaction of GR and TEs, Experiment 1 utilized an adrenalectomized rat model and ChIP-Seq to determine GR was bound to TEs in the hippocampus after CORT administration, but this binding was not associated with any changes in gene expression. Experiment 2 utilized RNA-Seq of two different animal models—adrenalectomy and acute restraint stress (ARS)—to identify several TE transcripts that were differentially regulated and downstream of intergenic GR binding sites. Experiment 3 analyzed the TE transcripts in silico and determined that they were unlikely to be translated proteins, had few conserved domains or recognizable motifs, but had high similarity to non-coding RNAs. Experiment 4 used ChIP-Seq, RNA-Seq and RT-qPCR to show that GR also bound to mitochondrial DNA, but had mixed results on CORT’s effects on mitochondrial gene transcription. Overall, GR is shown to interact with TEs, and possibly drive the transcription of ncRNAs comprised of domesticated transposable elements. This interaction may be partly responsible for the variation seen in stress response