Most people cannot remember anything before the age of three. However, the brain seems to use mechanisms important in fetal development for lifelong learning and memory. During mitosis, epigenetic modifications, including DNA methylation, enable parent cells to pass phenotypes on to daughter cells. Now Miller and Sweatt report that methylation of DNA is important for memory formation in a recent article in Neuron.
The addition of methyl groups to DNA suppresses gene expression by physically blocking transcription factor binding to DNA. DNA methyltransferase adds methyl groups to cytosine-guanine nucleotide pairs.
The authors found increased expression of DNA methyltransferases during memory formation in the adult hippocampus. In contextual fear conditioning, animals learn to associate a shock with an environment. When they are returned to the shock environment, rats freeze with fear. Thirty minutes after contextual fear training, rats showed increased hippocampal expression of mRNA encoding the DNA methyltransferases (3a and 3b) that are important during development.
Inhibition of DNA methyltransferases blocked memory formation. The authors infused DNA methyltransferase inhibitors or vehicle into the hippocampus following training and tested freezing in the shock environment 24 hours later. Rats treated with inhibitors showed reduced freezing relative to rats treated with vehicle. DNA methyltransferase inhibitors did not permanently impair memory formation. After testing, the authors retrained the rats. Twenty-four hours later, rats that had previously been treated with DNA methyltransferase inhibitors showed freezing comparable to vehicle-treated rats tested 24 hours after training.
Protein phosphatase 1 (PP1) suppresses memory formation, and PP1 inhibition increases long-term potentiation and associative learning. The authors used methylation-specific real-time PCR to quantify methylated and unmethylated PP1 after contextual fear conditioning. One hour after training, rats trained to associate a shock with an environment showed increased methylation of hippocampal PP1 relative to rats exposed only to the shock or the shock environment and reduced expression of PP1 mRNA relative to rats exposed only to the shock environment. Relative to vehicle-treated controls, rats treated with DNA methyltransferase inhibitors immediately after training showed less methylated PP1, more unmethylated PP1 and greater expression of PP1 mRNA. Together, these data suggest that DNA methylation suppresses PP1 expression during associative learning.
Associative learning decreased methylation of reelin, another gene that is important in memory formation. One hour after training, rats trained to associate shock with an environment showed less methylated and more unmethylated reelin DNA relative to rats exposed only to shock or the shock environment and increased reelin mRNA expression relative to rats exposed only to the shock environment.
DNA methyltransferase inhibition did not alter PP1 mRNA expression in untrained rats, suggesting that methylation of PP1 in the adult hippocampus is specific to memory formation. Expression of both PP1 and reelin returned to baseline levels within 24 hours of contextual fear conditioning training, suggesting that regulation of DNA methylation is both rapid and dynamic.
Rett syndrome is associated with mutations in methyl-CpG-binding protein 2 (MeCP2), which binds to methylated DNA, and schizophrenia is associated with hypermethylation of reelin, suggesting that disruption of DNA methylation can result in neurological disorders that impair memory and cognition.