Glucocorticoid Receptor-regulated gene transcription in the epigenetic embedding of neural stress responses: towards a mechanistic understanding of ch

The Hypothalamo-Pituitary-Adrenal (HPA) axis is the body's main regulator of physiological responses to external stressors and is highly conserved within vertebrates. In response to a biological stressor, the hypothalamus produces Corticotrophin Releasing Hormone (CRH), stimulating the pituitary to release Adrenocorticotrophic Hormone (ACTH), which causes the adrenal/interrenal glands to release cortisol. In the brain, cortisol is a ligand for the Glucocorticoid Receptor (GR) transcription factor, NR3C1, which regulates expression of neuroendocrine and neural activity-regulated genes. In order to ensure that stress-induced HPA axis activity depends on the persistence of an external stressor, cortisol-bound NR3C1 exerts negative feedback on the HPA axis by repressing transcription of CRH and ACTH. Failure to provide this feedback, by transcriptional inhibition or mutation of NR3C1, causes sustained HPA axis activation, hypercortisolaemia and glucocorticoid resistance - all of which are characteristics of depression. In the brains of suicide completers with histories of childhood abuse and depression, the NR3C1 gene exhibits both hypermethylation and reduced transcription [1]. Similar changes are found in the brains of rodents with depressive behaviours caused by early life deprivation of maternal care [2]. Moreover, genetic inactivation of NR3C1 causes Chrousos Syndrome, a genetic disorder characterised by chronic fatigue, hypercortisolaemia, profound anxiety and depression. While these findings imply that loss of NR3C1 activity is a causal step in the pathogenesis of depression, the downstream epigenomic and transcriptional consequences of reduced NR3C1 function remain unknown.
In order to investigate rigorously the role of the epigenome in the regulation of depressive behaviours [3], we are currently identifying the targets of NR3C1 signalling within the neural methylome of the zebrafish, a highly tractable model organism for genetic and epigenetic analysis of development and disorders of the nervous system. Mutation of zebrafish nr3c1 causes hyperactivation of the HPA axis, hypercortsolaemia and depressive behaviours in homozygous adults [4], providing a valid model for Chrousos Syndrome and other depressive disorders. The proposed PhD project will take forward our ongoing research by comparing the neural transcriptomes of adult wild-type and nr3c1 mutant fish to identify Nr3c1-responsive mRNAs, and then integrating this transcriptomic data with our emerging epigenomic data using cutting-edge bioinformatics techniques. This integrative approach, which we previously adopted to elucidate the epigenetic regulation of neuronal specification in zebrafish [5], will allow specific Nr3c1-associated epigenomic signals to be linked to the functional impacts of Nr3c1 on target gene transcription within the brain. Using in situ hybridization, Nr3c1-regulated genes will be classified into synexpression groups to understand how loss of nr3c1 function promotes depression through changes to patterns of neural gene transcription. The project will thus identify transcriptional regulatory modules that facilitate behavioural responses to stressors, through use of a combination of wet lab techniques and novel bioinformatics tools.

References:
[1] McGowan PO et al. (2009). Nature Neurosci. 12: 342-348.
[2] Weaver IC et al. (2004). Nature Neurosci. 7: 847-854.
[3] Cunliffe VT (2015). WIRES Syst. Biol. Med. 7: 52-71.
[4] Ziv L et al. (2012). Mol. Psychiatry 18: 681-691.
[5] Harrison MR et al. (2011). BMC Genomics 12: 24.