Characterization of adversity-induced phenotypic outcomes and prevention of adversity-induced Bdnf methylation via pharmacological manipulation of the epigenome
Date
2019
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Publisher
University of Delaware
Abstract
The early postnatal environment, particularly in the context of the caregiving relationship, plays a major role in the establishment of typical and atypical behavioral and biological trajectories. Biological embedding of early adverse experiences disrupts function of the hypothalamic-pituitary-adrenal (HPA) axis, alters plasticity of the central nervous system (CNS), and increases risk of cardiac and enteric disease. Behaviorally, this often manifests as mood dysregulation, cognitive dysfunction, and physical health issues. Though the mechanisms underlying these outcomes remain unclear, shifting of the epigenetic landscape is a promising candidate. Previous work from our lab has uncovered adversity-induced methylation that rises at specific loci of the Brain-derived Neurotrophic factor (Bdnf) gene, which is a major player in both development and lifelong CNS plasticity. We hypothesize that these epigenetic changes are significant contributors to behavioral outcomes following early adversity. To test this hypothesis, our lab employs a rodent model of disrupted caregiving called the “Scarcity-Adversity Model of Low Nesting Resources”. This model capitalizes on the principle of resource deprivation to induce poor caregiving behavior toward pups, allowing us to examine behavior and brain differences in nurtured versus maltreated subjects. ☐ Chapter 1 of this dissertation introduces the reader to epigenetic modifications and their potential role in behavioral plasticity in response to environmental conditions. After a brief introduction to some specific epigenetic modifications and enzymes, Chapter 1 focuses on literature on the developing epigenome in the context of postnatal adversity. ☐ Chapter 2 details the characterization of behavioral outcomes in subjects exposed to our model of early caregiving adversity. Literature establishes a clear link between early adversity and atypical behavioral outcomes, but the form of early adversity varies widely across publications. To move forward in our investigation of the epigenetics of early stress, we first had to characterize the behavioral outcomes of our specific model of early adversity. We examined behavior in adolescence and adulthood following exposure to our model in infancy. We chose tasks to characterize depressive- and anxiety-like behavior as well as learning and memory performance, and discovered that maltreated animals exhibited age and sex-specific differences on various tasks. While adult maltreated subjects exhibited atypical behavioral patterns in the forced-swim task (arguably a measure of depressive-like behavior) and novelobject recognition (a learning and memory task), adult maltreated males exhibited significant deficits on a fear extinction task (a learning and memory task often used to study human disorders such as post-traumatic stress disorder, or PTSD). Characterizing the behavioral outcomes of our model allows us to move forward in our attempt to understand the molecular mechanisms that underlie them. ☐ Chapter 3 of this dissertation is meant to establish our ability to manipulate the epigenome, something that will be required in order for us to understand how it contributes to behavioral phenotypes. A handful of drugs are known to affect the epigenome via actions on epigenetic enzymes. One of these drugs, sodium butyrate, is a histone deacetylase inhibitor. Histone deacetylases remove acetyl groups from the tails of histones, the proteins around which DNA wrap, making the charge of the histone-DNA complex such that transcription is reduced. Thus, giving a histone deacetylase inhibitor such as sodium butyrate should create a more permissive transcriptional state. In addition, DNA methylation is affected via extensive crosstalk between histone modifications and methylation levels. We chose to use this drug to investigate whether we could prevent a previously reported, stable, long-term increase in methylation at Bdnf exon IX in the prefrontal cortex (PFC) following maltreatment in infancy. Initial results revealed that a 300 mg/kg dose was not sufficient to prevent this mark in the maltreated PFC. However, the mark was successfully prevented in the maltreated male PFC when the dose was increased to 400 mg/kg, a finding that did not hold in females. Examination of global levels of methylation and of another Bdnf exon not affected by stress at this time point revealed no differences between groups. Though this suggests a promising level of specificity, the mechanisms of that potential specificity are unclear. Taken together, these results allow us to move forward and investigate the behavioral impact of preventing a stable epigenetic mark of early adversity. ☐ Chapter 4 summarizes the findings, limitations, and potential implications of the data contained in this dissertation. ☐ The data presented in this document have helped characterize the behavioral implications of our model of early-adversity, have reinforced previously reported data on the epigenetic implications of our model, and have established our ability to manipulate adversity-induced methylation of Bdnf to better understand how it contributes to behavioral pathology. Further research is needed to move this work forward in females and to better understand the off-target effects of this drug treatment, objectives which are being addressed in ongoing projects in our lab. Overall, the data contained here provide insights that will facilitate our attempts to understand how early adversity affects health across the lifespan, knowledge we hope will one day translate to both prevention and intervention efforts.