Information from Lay-Language Summaries is Embargoed Until the Conclusion of the Scientific Presentation
635—Addiction: Genetics and Treatment, Preclinical, and Clinical Studies
Tuesday, November 12, 2013, 1:00 pm - 5:00 pm
635.02: Nucleosome repositioning as a novel mechanism for drug-induced epigenetic changes in the brain
Location: Halls B-H
*A. N. BROWN1, J. H. DENNIS2, P. H. BHIDE3; 1Biomed. Sci., 2Dept. of Biol. Sci., Florida State Univ., Tallahassee, FL; 3Biomed. Sciences, Florida State Univ. Col. of Med., Ctr. for Brain Repair, Tallahassee, FL
Abstract Body: Drugs of abuse produce changes in gene expression in the brain that ultimately manifest as behavioral alterations. Recent data suggest that gene expression can be regulated via epigenetic mechanisms. However, the molecular mechanisms facilitating drug-induced epigenetic changes in neurons, specifically the masking or unmasking of the DNA to the action of transcription factors, have remained elusive. We exposed human neuroblastoma cells (SH-SY5Y) to nicotine and cocaine, two of the most highly abused drugs in our society today, to examine changes in nucleosome positioning. Nucleosome occupancy at/near transcriptional start-sites, in cooperation with histone modifications, is believed to regulate DNA accessibility to the transcriptional machinery, resulting in epigenetic alterations of gene expression. We used Comparative Genome Hybridization (CGH) to examine the temporal and genomic distribution of nucleosomes following the drug exposure, and correlated nucleosome position with changes in transcriptional activity. In additional studies, we used neurotransmitter receptor agonists and antagonists to activate or block specific receptors prior to the drug exposure to determine how specific receptor activation contributed to nucleosome positioning and/or transcriptional changes. Based on our data, we propose that the effects of drugs of abuse on nucleosome positioning represent an initial genome-wide mechanism, and neurotransmitter receptor induced alteration of the intracellular transcription factor profile as the next cell-type specific mechanism in the epigenetic cascade of gene expression produced by drugs of abuse. The genome-wide changes are distinctly different between nicotine and cocaine, and cell-type specific changes likely confer cell-, and tissue-specific transcriptional responses. Detailed characterization of these mechanisms for each drug of abuse will further our understanding of drug-induced behavioral responses, pathologies, mechanisms of transgenerational inheritance and potential therapeutic targets for future studies.
Lay Language Summary: Our research shows that drugs of abuse modify gene expression via a combination of genome-wide nucleosome “sliding” and neurotransmitter receptor specific transcriptional activation, and offers insights into why certain drugs may be more addictive than others. Drug abuse and addiction remain major public health concerns worldwide. Some drugs alter mood and behavior within socially and culturally acceptable parameters. Others violate those parameters, and are not only illegal but promote risky behaviors, dependence and ultimately addiction. Why are some drugs more addictive than others? Is it merely the magnitude or frequency of usage, or are molecular mechanisms of drug action to blame? Our research offers novel clues that may help answer these questions. We have shown that "nucleosome sliding" is another layer of epigenetic modification at the heart of differential drug action. Epigenetic modifications, which are distinct from genetic modification of the DNA sequence, tighten or loosen spools of DNA wrapped around nucleosomes, the basic DNA packaging units in the nucleus. A tight wrapping prevents access to the DNA by transcription factors (molecular triggers for gene expression), whereas loosely wrapped DNA is more accessible. Thus, epigenetic modifications can profoundly influence the probability of gene expression. Moreover, nucleosome position along the DNA strands (the exact location of DNA - nucleosome physical contact), with respect to the gene transcription start site, also influences DNA accessibility, and therefore gene transcription. Thus, nucleosome “sliding” along the DNA strands is another epigenetic mechanism of gene expression. Recently we developed a method to identify nucleosome positions genome-wide and assay nucleosome sliding. We used this method to study how nicotine and cocaine, two drugs with different addiction profiles and mechanisms of action in the brain, affected nucleosome position. We used human neuroblastoma cells as our model system. We exposed these cells to nicotine or cocaine, and examined how each drug affected nucleosome position across multiple genes and at various periods following the drug exposures. We found many examples where the two drugs produced different effects. For some genes associated with addiction, cocaine caused nucleosomes to vacate transcription start sites promoting gene transcription, while nicotine promoted nucleosome retention in these genes, preventing gene transcription, or vice versa. Some other genes were unaffected by either drug. What does the differential effect of the drugs on nucleosome positioning tell us about specificity of drug effects on the brain and behavior? Since drugs and environmental signals can cause global or genome-wide nucleosome sliding, at first glance it appeared that nucleosome sliding might have nothing to do with specificity of drug effects. However, a closer examination revealed interesting clues. Although every drug, addictive or innocuous, may produce genome-wide nucleosome sliding and other epigenetic transformations, the specificity of a given drug’s effects on gene expression may be derived from drug action at cell surface receptors. For example, neurotransmitter receptors on neuronal cell surface are targeted differently by different drugs. Cocaine targets monoamine receptors while nicotine targets cholinergic receptors. Receptor activation triggers a cell signaling cascade ultimately activating intracellular transcription factors, which then provoke gene expression. Therefore, although every cell exposed to a given drug may undergo global nucleosome sliding, the gene expression profile induced by a given drug in a given cell is likely a function of the receptors on the cell surface and their sensitivity to the drug. A similar mechanism may also confer drug-specific responses to specific brain regions. Our findings offer novel insights into how certain drugs may become addictive or sensitize the individual to new addictions, how certain people may be more prone to addiction, and open the doors to personalized treatments for drug addiction.
Neuroscience 2013 (43rd annual meeting of the Society for Neuroscience)Exit