Most cells in your adult body are “terminally differentiated” – meaning that they have developed from stem cells into the final liver, or heart, or muscle or endothelial cell that they were meant to be. From that point onward, cells are able to “remember” to stay in this final state – in part – via stable patterns of DNA methylation that reinforce the regulation of “the end state” of gene expression for that cell. As evidence for this role of DNA methylation, it has been observed that levels of DNA methyl transferase (DNMT) decline when cells are fully differentiated and thus, cannot modify or disrupt their patterns of methylation.
NOT the case in the brain! Even though neurons in the adult brain are fully differentiated, levels of methyl transferases – DO NOT decline. Why not? Afterall, we wouldn’t want our neurons to turn into liver cells, or big toe cells, would we?
One hypothesis, suggested by David Sweatt and colleagues is that neurons have more important things to “remember”. They suggest in their fee and open research article, “Evidence That DNA (Cytosine-5) Methyltransferase Regulates Synaptic Plasticity in the Hippocampus” [doi: 10.1074/jbc.M511767200] that:
DNA methylation could have lasting effects on neuronal gene expression and overall functional state. We hypothesize that direct modification of DNA, in the form of DNA (cytosine-5) methylation, is another epigenetic mechanism for long term information storage in the nervous system.
By measuring methylated vs. unmethylated DNA in the promoter of the reelin and BDNF genes and relating this to electrophysiological measures of synaptic plasticity, the research team finds correlations between methylation status and synaptic plasticity. More specifically, they find that zebularine (an inhibitor of DNMT) CAN block long-term potentiation (LTP), but NOT block baseline synaptic transmission nor the ability of synapses to fire in a theta-burst pattern (needed to induce LTP).
This suggests that the epigenetic machinery used for DNA methylation may have a role in the formation of cellular memory – but not in the same sense as in other cells in the body – where cells remember to remain in a terminally differentiated state.
In the brain, this epigenetic machinery may help cells remember stuff that’s more germane to brain function … you know … our memories and stuff.