Epigenetics of cocaine addiction

Cocaine addiction is the compulsive use of cocaine despite adverse consequences. It arises through epigenetic modification (e.g., through HDAC, sirtuin, and G9a) and transcriptional regulation (primarily through ΔFosB's AP-1 complex) of genes in the nucleus accumbens. Histone deacetylase inhibitors (HDAC inhibitors) have been implicated as a potential treatment for cocaine addicts. HDACs are enzymes that can deacetylate the histones associated with genes. This can activate genes for transcription. Several experiments have shown that inhibiting HDACs involved in histone H3K9 deacetylation reduces drug seeking behavior. It has been known that epigenetic regulations, such as the methylation of H3K9, have a key role in the mechanism of addiction. Recent studies have shown that administering HDAC inhibitors can help reduce the craving for cocaine in rats. Trichostatin A (TsA) is an HDAC inhibitor associated with reduced cocaine-seeking behaviors; it inhibits HDAC classes 1, 3, 4, 6, and 10. Since this HDAC inhibitor has such a significant effect on cocaine-seeking behaviors, scientists have speculated about their ability to reduce a cocaine addict's risk of relapse in the rat model system during rehab. After several tests in which rats were exposed to cocaine followed by either an HDAC inhibitor or a placebo, it was found that HDAC inhibitors had a significant effect on lowering cocaine-seeking behavior. This also suggests an epigenetic mechanism involved in HDAC chromatin regulation. The data is crucial to proving the hypothesis that trichostatin A can remodel chromatin structure and prevent behavioral changes following cocaine exposure. Tests also revealed that HDAC inhibitor administration can not only prevent addiction, but also helps reduce the risk of a relapse in cocaine addicts in the rat model system. As the previous findings suggest, chronic cocaine use caused both alterations in the chromatin remodeling activity of HDACs and drug seeking behavior. Renthal et al. focused specifically on the class II histone deacetylase, HDAC5, since it was known to have activity-dependent regulation in neurons. In fact, they found that HDAC5 was a central regulator of the actions of chronic cocaine use and contributed to the behavioral adaptations with its deacetylase activity. Chronic cocaine injections increased HDAC5 phosphorylation at Ser259 in the nucleus accumbens (NAc) within 30 minutes. This provides docking sites for 14-3-3 proteins, which mediate the export of HDAC5 out of the nucleus. They also found that CaMKII was necessary for depolarization-induced HDAC5 phosphorylation in NAc tissue, highlighting its role as a kinase for HDAC5. Experiments with mutant proteins and HDAC inhibitors suggested that HDAC5’s action is mediated through its catalytic histone deacetylase domain. Rapid phosphorylation and the export of HDAC5 from the nucleus following cocaine use most likely leads to increased “pulses” of acetylation, targeted gene activation, and behavioral adaptations to long-term cocaine exposure. The second set of experiments that Renthal et al. performed showed that chronic cocaine use induced upregulation of the NK1 receptor protein in HDAC5 knockout mice, which is associated with hyperacetylation of H3 at the NK1R gene promoter. The NK1R gene promoter has been associated with enhanced response to cocaine reward, meaning HDAC5 in normal genomes may decrease cocaine reward with chronic cocaine exposure. They also found key pathways that were implicated in neural plasticity and reward behavior, which included DA receptor signaling, ATF2/CREB signaling, NF-κB, NFAT, cytoskeletal remodeling proteins, and ion channels. Their data implicated chromatin remodeling as a mechanism that drives altered gene activation and behavioral responses to cocaine. Using this they were able to conclude that within normal (wild type) genomes, the response to chronic cocaine includes phosphorylation of HDAC5 and export of the deacetylase out of the nucleus to activate downstream target genes. Between exposure and 24 hours after, HDAC5 returns to the cell nucleus to limit expression of these cocaine regulated genes by histone deacetylation. Their experiments with HDAC5 knockout mice lent additional support for this hypothesis. Since HDAC5 isn’t there to limit the gene’s expression, it begins to accumulate with repeated cocaine exposure, with the end result being increased sensitivity to cocaine reward. Modifications to histones such as methylations and acetylations can change gene expression patterns by activating or deactivating a region of DNA for transcription. The H3K9 position has been shown by several studies to be altered by chronic cocaine use. Addictive behavior observed from long-term cocaine users can be due to changes of the gene expression profiles in the brain’s reward circuitry. Most research has been focused on the active regions of the reward-related genes, but Maze et al. focuses at what happens to the heterochromatic regions. Maze et al. showed that heterochromatic regions in the nucleus accumbens (NAc), a major reward circuit in the brain, are significantly altered in the H3K9me3 position. Acute cocaine exposure leads to a rapid increase in H3K9me3 within half an hour and decreases back to normal levels within 24 hours. Chronic cocaine exposure leads to a slower increase in H3K9me3 within an hour (although it reaches the same level as acute by this time) and a 50% decrease from normal baseline levels within 24 hours. This chronic exposure was proposed to decrease heterochromatization (destabilization) within this brain region in patients given repeated cocaine exposure, which implies that the long-term addictive behaviors are affected by this epigenetic mark. They used ChIP-seq to provide supporting evidence that the H3K9me3 modification is mainly localized to intergenic regions. In these areas of the genome, 17 regions of repeat elements (SINEs, LINEs, LTRs, etc.) had significant H3K9me3 state changes in chronic cocaine exposure mouse models. They used quantitative PCR to determine that of these significant elements, the LINE-1 region showed a significant increase in expression levels. LINE-1 is a retrotransposon, so expressing it inappropriately can activate the transposon to insert itself within important genes and destabilizing the DNA. They conclude their findings by suggesting that LINE-1 retrotransposon insertions cause inappropriate or disrupted expression of genes leading to the addictive behavior.