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Stress and Glucocorticoid Receptor Transcriptional Programming in Time and Space: Implications for the Brain–Gut Axis

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Stress and Glucocorticoid Receptor Transcriptional Programming in Time and Space: Implications for the Brain–Gut Axis Neurogastroenterology & Motility Neurogastroenterol Motil (2016) 28, 12–25 doi: 10.1111/nmo.12706 REVIEW ARTICLE Stress and glucocorticoid receptor transcriptional programming in time and space: Implications for the brain–gut axis J. W. WILEY,* G. A. HIGGINS†,‡ & B. D. ATHEY‡ *Division of Gastroenterology, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA †Department of Pharmacogenomic Science, Assurex Health, Inc., Mason, OH, USA ‡Department of Computational Medicine and Bioinformatics, University of Michigan Medical School, Ann Arbor, MI, USA Key Messages • Chronic stress is associated with the exacerbation of common mental health and gastrointestinal disorders, including depression and irritable bowel syndrome. • Emerging evidence indicates that chronic stress alters physiological cortisol-mediated pulsatile (circadian and ultradian) gene transcription in a tissue-, cell-, and allele-specific manner. • Epigenetic studies indicate that chronic stress selectively alters both the DNA methylome and chromatin compaction state affecting gene transcription in central and peripheral pathways involved in a variety of important functions including mood and pain perception. • Chronic and early life stress-associated disruption in glucocorticoid receptor-regulated gene transcription has a potentially significant impact on the brain–gut axis throughout the lifespan and across generations. Abstract corticotropic hormone from the pituitary, ultimately Background Chronic psychological stress is associated leading to the release of cortisol (human) and corti- with enhanced abdominal pain and altered intestinal costerone (rodent) from the adrenal glands. Cortisol barrier function that may result from a perturbation in binds with the GR in the cytosol, translocates to the the hypothalamic–pituitary–adrenal (HPA) axis. The nucleus, and activates the NR3C1 (nuclear receptor glucocorticoid receptor (GR) exploits diverse mecha- subfamily 3, group C, member 1 [GR]) gene. This nisms to activate or suppress congeneric gene expres- review focuses on the rapidly developing observations sion, with regulatory variation associated with stress- that cortisol is responsible for driving circadian and related disorders in psychiatry and gastroenterology. ultradian bursts of transcriptional activity in the Purpose During acute and chronic stress, corti- CLOCK (clock circadian regulator) and PER (period cotropin-releasing hormone drives secretion of adreno- circadian clock 1) gene families, and this rhythm is disrupted in major depressive disorder, bipolar disor- der, and stress-related gastrointestinal and immune Address for Correspondence John W. Wiley, MD, Professor, Internal Medicine, University disorders. Glucocorticoid receptor regulates different of Michigan Health System, 1150 West Medical Center Drive, sets of transcripts in a tissue-specific manner, through Ann Arbor, MI 48109, USA. pulsatile waves of gene expression that includes Tel: +1 (734) 615-6621; fax: +1 (734) 763-2535; e-mail: occupancy of glucocorticoid response elements located [email protected] within constitutively open spatial domains in chro- Received: 29 July 2015 Accepted for publication: 20 September 2015 matin. Emerging evidence supports a potentially piv- 12 © 2015 John Wiley & Sons Ltd Volume 28, Number 1, January 2016 Chronic stress and GR function otal role for epigenetic regulation of how GR interacts transcription.31 Epigenetics refers to external modifi- with other chromatin regulators to control the expres- cations to DNA and histones affecting gene transcrip- sion of its target genes. Dysregulation of the central tion that are independent of DNA sequence, but are and peripheral GR regulome has potentially signifi- often driven by DNA sequence variation. One example cant consequences for stress-related disorders affect- of an epigenetic change is DNA methylation, which ing the brain–gut axis. involves the addition of a methyl group to specific DNA sites, thereby preventing the expression of Keywords chronic stress, CLOCK genes, epigenomics, specific genes. Another example is histone modifica- intestinal barrier function, irritable bowel syndrome, tion. DNA wraps around histone proteins to form mental health disorders. compact DNA-histone complexes. Modifications that relax the DNA-histone compaction state, such as BACKGROUND acetylation at specific histone sites, allow accessibility to proteins that ‘read’ genes and, thereby, promote gene The human glucocorticoid receptor (GR) a isoform, transcription.32 herein referred to as GR, is responsible for the main- tenance of physiological homeostasis. Glucocorticoid INSIGHTS FROM THE CENTRAL receptor has been implicated in the etiology of a wide NERVOUS SYSTEM AND PSYCHIATRY range of stress-related disorders. Molecular mecha- nisms underlying GR gene expression are widespread Expression quantitative trait loci (eQTLs) are genomic and diverse in the human genome. They include loci that regulate gene expression. The eQTLs may act opportunistic nucleosome occupancy, preprogramming in cis (locally) or trans (at a distance) to a gene. Cis- of chromatin to permit indiscriminate binding of eQTLs, methylation-quantitative trait loci, and QTLs steroid receptors, and distal enhancer–promoter loop- associated with the histone mark H3K27ac, which – ing to transcriptionally poised genes.1 9 The GR con- indicates enhancer and promoter sites of active gene trols target genes through long-distance looping to regulation, are significantly enriched in domains con- promoters within the three-dimensional milieu of taining the GR response element (GRE).33 These – euchromatin.10 12 A range of cooperative interactions domains are targeted by cis-regulatory, transcription with trophic factors, coactivators, corepressors, and factor-enriched sites within the human NR3C1 gene other nuclear receptors, including brain-derived neu- that contain individual SNPs, and elsewhere, depend- rotrophic factor (BDNF), are important in GR regula- ing on cell type.3,34 Chromatin remodeling, which can tion.13 Although greater than 1% of all transcripts in be observed near GR-regulated genes using live cell the human genome are regulated by the GR, circum- imaging immediately after treatment with the syn- scribed cell- and tissue-type expression is a prominent thetic glucocorticoid dexamethasone, occurs with a characteristic of glucocorticoid regulation. well-defined ultradian rhythm.6,9,35 Numerous studies Allele-specific bias is not uncommon among genes have shown that loss of data about the chromatin that are glucocorticoid responsive, including asymmet- environment of the human NR3C1 gene limits under- ric expression of targets resulting from allele-specific standing of the clinical consequences of genetic vari- methylation and parent-of-origin of monoallelic ation.36–38 In the context of genetic association studies, effects.14 In brain, studies in humans and in rodent one confounding problem has been the influence of models show that chronic stress and early-life adver- population structure, as the allele frequency of a given sity disrupts normal ultradian GR pulsatile chromatin variant, but also haplotype structure, varies widely interactions during critical periods of development in depending on ethnicity.39 an allele-specific manner. This stress-induced disrup- Most studies show that abuse, neglect, and/or lack of tion has been associated with psychiatric disorders parenting during early childhood are associated with which persists and may be transmitted through mater- blunted cortisol cycling in adolescence21,40,41; although – nal or paternal alleles across generations.15 26 Simi- the directionality of effect is still controversial as a larly, chronic stress and altered regulation of GR consequence of variability in study designs.42 Blunted expression and function have been implicated in cortisol and adrenocorticotropic hormone responses irritable bowel syndrome, intestinal barrier dysfunc- are associated with major depressive disorder, espe- tion, inflammatory bowel diseases, and alterations in cially in females.43 In humans, chronic stress of the – the gut microbiome.27 30 mother while the infant is in utero, and early life stress Epigenetic modifications are known to play an between birth and 5 years of age has been shown to important role in chronic stress-associated altered gene affect DNA methylation of specific sites within the © 2015 John Wiley & Sons Ltd 13 J. W. Wiley et al. Neurogastroenterology and Motility NR3C1 gene.25,26,44 It has been proposed that there is translocator-like protien (ARNTL; formally known as a critical period during child development when brain and muscle ARNT-like 1 [BMAL1]), which abuse and trauma can exert an irreversible lifelong program transcriptional dynamics of rhythmicity, and transgenerational impact.24 Hyper-suppression of including those of the period (PER1, PER2, and PER3) plasma cortisol is a feature of adult patients with and cryptochrome (CRY1 and CRY2) gene families. major depressive disorder and/or posttraumatic stress Maintenance of the circadian and ultradian rhythmic- disorder who have been abused as children. The ity of cortisol release appears to be regulated through flattening of the normal circadian cortisol curve in acetylation and deacetylation of the GR by the CLOCK women who were abused as children suggests a gene,46,54,55 and through CRY1 and CRY2 rhythmic mechanism that involves genomic variants in critical repression of GR expression in tissues, such as the genes that govern
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