While there’s no doubting that one month of continuous cocaine exposure is going to have an effect on the brain, it seems that some of the molecular culprits behind the response and long-term effects are 5hmC and the TET protein family. An international research team studying addiction via cocaine show that repeat exposure downregulates the TET machinery in the brain’s reward centre, the nucleus accumbens, once the mice get hooked.
5hmC is an intermediate step of active DNA demethylation, but rather than being just a transient by-product it also serves as an epigenetic mark in its own right. 5hmC appears to be particularly relevant to the brain and other neural contexts, where it is dynamically created by the TET enzymes catalyzing the oxidation of 5mC. This creates an indicator for the removal of DNA cytosine methylation and leads to altered transcritpomics, including distinct gene expression and alternative splicing profiles. Furthermore, the time scale for the mark is quite rapid and can occur in response to environmental events.
Previously, the group of Eric Nestler at the Icahn School of Medicine at Mount Sini in New York has teamed up with the genome editing pioneers of the Feng Zhang lab at MIT and used epigenome editing via artificial transcription factors to show that specific epigenetic remodelling events control addiction and depression.
This was accomplished using Zinc Finger Protiens (ZFPs) and transcription activator-like effectors (TALEs) fused to effector domains, which allowed for either epigenetic or transcriptional control. Using this biotechnology they showed the ability to target histone methylation (H3K9me2 via G9a) or acetylation (correlated with their transcriptional activator, p65) to transcription factor binding sites (SRF and CREB) of the Fosb promoter in the nucleus accumbens. Furthermore, the systems replicated alterations to transcriptional control that typically occur in response to stress or to drugs, like cocaine.
In their most recent report, the team characterized the natural methylomic response to the drug. Here’s what they found in the brains of mice after a cocaine binge:
- 5hmC is added to putative enhancers and coding regions involved in drug addiction.
- The 5hmC induction can be replicated in TET1 knockdown mice.
- The molecular profile driven by this change involves increased expression and alternative splicing of the genes with the newly acquired 5hmC, roles previously known for 5hmC.
Interestingly, some of the marks persisted for as long as a month after the cocaine exposure, showing off the true long-term nature behind such a dynamic response to environment. This works adds an in vivo environmental context to the initial discovery of 5hmC as a neuroepigenetic mark for alternative splicing and synaptic function in the brain.
Go and stimulate your reward centre with some in vivo insight over at Nature Neuroscience, March 2015