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

Lead Research Organisation: University of Sheffield
Department Name: Biomedical Science

Abstract

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.

Publications

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Title BisPCR2 
Description Next-generation sequencing (NGS) techniques offer single base resolution of CpG methylation levels with high statistical significance, but are also high cost if performed genome-wide. The BisPCR2 method is a simplified targeted bisulfite sequencing approach in which DNA sequencing libraries are prepared following sodium bisulfite conversion and two rounds of PCR for target enrichment and sample barcoding. 
Type Of Material Physiological assessment or outcome measure 
Year Produced 2015 
Provided To Others? Yes  
Impact The BisPCR2 method has been used to identify specific CpG sites in genes that are regulated by Glucocortoid Receptor function. 
URL https://www.ncbi.nlm.nih.gov/pubmed/26236400
 
Description PubhD talks in Sheffield 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact These organised talks are given in the pub to a mixed audience, the majority having no prior knowledge of the science or the field. The talk was given in layman's terms, which led to a lively discussion on how stress can become embedded in our epigenomes.
Year(s) Of Engagement Activity 2018
 
Description Sheffield Epigenetics Symposium 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact This interdisciplinary symposium brought together students, researchers and academics in different epigenetics-related subjects to share their research and to debate the frontiers of the field. This was an opportunity to engage and strengthen cross-collaboration and knowledge exchange in the area of epigenetics.

The symposium was anchored around three different themes, spanning from (1) insights from emerging technologies, passing through (2) epigenetics in health, disease and development, and leading to (3) epigenetics and the bigger picture.

Prof Vincent Colot from IBENS-CNRS Paris, France, gave the invited keynote seminar, entitled: "Mobile DNA, epigenetics and rapid adaptation".

My personal role, along with another PhD researcher, was to obtain funding for the symposium, to invite and engage with speakers from Sheffield and across the UK, and to organise the symposium.

Furthermore, I gave a 15-minute presentation on my own research as part of the symposium.
Year(s) Of Engagement Activity 2019