The article by Miller and Sweatt (2007) examines the possible role of DNA methylation as an epigenetic mechanism in the regulation of memory in the adult central nervous system. The authors sought to know whether memory regulation, in this case, occurs through the control of transcription in a gene-specific manner in the hippocampus. The specific methylation process under examination was the cytosine-5’ methylation which is a covalent modification aided by the enzyme DNA (cytosine-5’) methyltransferases (DNMTs).
The study utilized adult male Sprague-Dawley rats as animal models and the rats were placed under contextual fear conditioning. Cannula implantation and administration of DNMT inhibitors were done on the animals while real-time RT-PCR and DNA methylation assays were used to quantify RNA as a way of assessing gene transcription. Several statistical tests were used including a one-sample t-test, one-way ANOVA, and Tukey-Kramer test as a post-doc test with all tests being performed at a significance level of.05.
The study revealed that for memory to be built, the activity of the enzyme DNA methyltransferase is necessary. This conclusion was arrived at by infusing some animals with 5-AZA, a DNMT inhibitor while the comparison group animals were vehicle-treated whereby DNMT-inhibited animals had less freezing compared to the vehicle-treated animals. It was evident that memory consolidation is blocked by DNMT inhibition in a plastic manner as evidenced by the ability to form fear in the test animals later in the experimentation.
From this study, Miller and Sweatt (2007) were able to identify that contextual fear conditioning leads to an increase in DNA methylation of a memory suppressor gene, and this process is enhanced as training progresses. It was observed that silencing of a memory suppressor gene was prevented by DNMT inhibition since the increase in gene methylation that would lead to fear conditionings are inhibited.
Another finding from this study was that the process of memory promoting gene transcription is controlled by rapid demethylation. This was determined using the reelin gene since it enhances LTP induction. Further, the study revealed that continued demethylation of reelin is brought about by DNMT inhibition in a fear conditioning situation. The c-fos, as well as DNMT1, confirmed the gene-specificity of DNA methylation. The c-fos was identified as a good example of demonstrating interactions that occur in the hippocampus during DNA methylation thus adding to reelin. Finally, Miller and Sweatt (2007) confirmed that the changes in the process of DNA methylation occurring in the hippocampus are usually highly dynamic.
It was confirmed that the adult CNS experiences changes in DNA methylation but the changes are not permanent and instead they can be both reversible as well as dynamic. This was confirmed by looking at the levels of methylation of reelin and PP1 24 hours post-training followed with euthanizing of the animals after 24 hours where the levels of both genes went back to baseline.
References
Miller, C. A. and Sweatt, J. D. (2007). Covalent modification of DNA regulates memory formation. Neuron, 53:857–869. Web.