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.
Transcriptional and epigenetic mechanisms Role of HDAC inhibitors in cocaine addiction 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 person who is addicted to cocaine'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 relapse in cocaine addicts in the rat model system.
Role of HDAC5 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. With this information, they concluded 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 the 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 result being increased sensitivity to cocaine reward.
Changes in critical H3K9me3 modifications 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 in 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 heterochromatinization (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.
Role of G9a in changes to H3K9me2 modification Like Maze et al., studies by Covington et al. focused on histone modification in the nucleus accumbens. They suggest that the H3K9me2 modification in this area of the brain plays a role in the stress and depression pathway. Their idea was that cocaine modifies this epigenetic mark and that this increases an addicted person's vulnerability to stress and depression, which leads to the addictive effects of these reagents. A methyltransferase, G9a, was found to have reduced expression in the nucleus accumbens in cocaine addicts, therefore causing reduced H3K9me2 levels. The genes that are unsilenced via acetylation of the heterochromatin abnormally express the genes involved in BDNF-TrkB-CREB signaling pathway. This causes enhanced phosphorylation of CREB downstream in the pathway. CREB causes enhanced acetylation and dis-regulation of the stress and depression signaling pathways.
Cocaine induces epigenetic changes that lead to neuronal morphology and behavioral changes. Most changes have to do with the disruption of heterochromatin caused by reduced levels of methylation on histones, namely H3K9. This decrease is mediated by the repression of G9a, a histone-lysine N-methyltransferase which is regulated by ΔFosB. ΔFosB is a cocaine-induced transcription factor that accumulates in the nucleus accumbens (NAc) and acts to repress G9a. When ΔFosB is overexpressed, G9a levels are reduced, and H3K9 dimethylation levels are reduced in the NAc. Maze et al. was interested in determining how the reduced levels affects the behavior of cocaine users. Several studies were done in rats and it was concluded that G9a overexpression, and thus the presence of H3K9 dimethylation, caused a decreased preference for cocaine in rats. Researchers then looked at the nuclear volume of rats exposed to cocaine and found that down-regulation of G9a increased the amount of dendritic spines in the nucleus accumbens leading to increased cocaine-seeking behavior.
Only in the nucleus accumbens do G9a levels matter for cocaine addiction. Studies have been done in which G9a levels and H3K9me2 levels were changed in other areas of the brain, but these other locations had no effect on the cocaine-seeking behavior of rats.
Gene expression studies in the mesolimbic pathway The role of sirtuin deacetylases have also been shown to play a role in mediating cocaine addiction. To determine their role, the two transcription factors previously mentioned, ΔFosB and CREB, first had to be analyzed. It has been previously stated that ΔFosB increases the cocaine-seeking behavior of users. This is because ΔFosB has a uniquely stable structure that allows it to persist and accumulate in the body. Unlike ΔFosB, CREB is responsible for reducing sensitivity to cocaine which causes negative symptoms during the withdrawal period. Each of these transcription factors increases the cocaine-seeking behavior of addicts. Once this connection between transcription factors had been established, the researchers were curious if other genes caused addictive behaviors too and explored the ones that were significant markers for cocaine addiction, one of which, was the sirtuin gene family. Sirtuins are Class III NAD-dependent histone deacetylases. Not only do sirtuins deacetylate histones, but are also responsible for the deacetylation of tubulin, p53, and NFKB. Several studies have explored the role sirtuins played in cocaine-seeking behavior. In one set of experiments, it was found that a significant increase in ΔFosB in the nucleus accumbens led to ΔFosB binding to the SIRT2 promoter. This increased binding caused acetylation of H3 which is associated with increased Sir2 mRNA. It was also found that the H3 acetylation induced by cocaine increased Sirt1 in the nucleus accumbens. Thus, repeated cocaine use causes an increase in both Sirt1 and Sirt2. Scientists were then interested in determining how increased Sirt1 and Sirt2 affected nuclear volume because it has been previously shown that repeated cocaine use does so. Scientists studying Sirt1 and Sirt2 have also found that these transcription factors led to increased nuclear volume. Therefore, it has been concluded that ΔFosB, CREB, Sirt1, and Sirt2 all play an integral role in cocaine-seeking behaviors.