Chronic cocaine exposure increases FosB-mediated transcription of sirtuins 1 and 2, altering MAPK signaling in the nucleus accumbens.
Addictive drugs alter gene expression in the brain's reward system, notably in the nucleus accumbens (NAc). To investigate the underlying mechanisms, Renthal et al. carried out a genome-wide analysis of chronic cocaine-induced chromatin modifications and transcription-factor binding, revealing a role for sirtuins 1 and 2 (Sirt1 and Sirt2) in the rewarding effects of cocaine.
The authors used chromatin immunoprecipitation (ChIP) to analyse NAc lysates of mice injected with cocaine or saline for a week and detect changes in histone acetylation and methylation (which respectively permit and repress gene expression). ChIP–chip assays revealed cocaine-induced changes in the acetylation of histones H3 and H4 around the transcription start site of hundreds of genes, including those whose expression is known to be upregulated by cocaine. For both histones, cocaine exposure induced hyperacetylation at far more genes than hypoacetylation, indicating that it predominantly results in gene activation. Cocaine exposure also affected histone methylation, with methylation greatly exceeding demethylation. There was little overlap between the genes that showed changes in H3 methylation, H3 acetylation and H4 acetylation, indicating that these chromatin modifications are generally independent mechanisms by which cocaine affects gene expression.
Cocaine exposure is known to induce expression of the transcription factors FOSB and cAMP-responsive element-binding protein (CREB). Using ChIP to assess the DNA binding of FosB and CREB, the authors showed that chronic cocaine exposure significantly increased the number of FosB-bound gene promoters. Approximately one-fifth of these genes also showed increased histone acetylation or methylation. Binding of activated CREB was also increased after chronic cocaine exposure and, again, around one-fifth of the affected genes also showed changes in histone acetylation or methylation. These results indicate that FosB and CREB can act as both transcriptional activators and repressors.
Although many of the genes highlighted in this study had previously been implicated in cocaine-induced behaviour, genes that had not previously been implicated were also affected, including Sirt1 and Sirt2. Repeated cocaine exposure not only induced FosB binding at Sirt2 and increased acetylation of Sirt1 and Sirt2, it also increased Sirt1 and Sirt2 expression and activity levels. A sirtuin inhibitor decreased NAc medium spiny neuron excitability in vitro and decreased the behavioural response to cocaine in vivo, whereas a sirtuin activator had the opposite effect, suggesting that sirtuins are key regulators of the neuronal and behavioural response to cocaine. Sirtuins can deacetylate cellular proteins and so affect intracellular signalling pathways. Indeed, in NAc slices a sirtuin inhibitor reduced phosphorylation of mitogen-activated protein kinase 3 (MAPK3) and MAPK1, indicating that MAPK activation may mediate the sirtuin regulation of cocaine actions in the NAc.
These findings show that chronic cocaine exposure affects the chromatin status and consequently the transcription of hundreds of genes. This type of analysis could identify new potential treatment targets for addiction, of which Sirt1 and Sirt2 are examples.
Leonie Welberg
References
Renthal, W. et al. Genome-wide analysis of chromatin regulation by cocaine reveals a role for sirtuins. Neuron62, 335–348 (2009) | Article | PubMed |
Tsankova, N. et al. Epigenetic regulation in psychiatric disorders. Nature Rev. Neurosci.8, 355–367 (2007) | Article | PubMed |
FosB-mediated transcription of sirtuins 1 and 2, altering MAPK signaling in the nucleus accumbens.
