Summary Dynamic changes in 5-methylcytosine (5mC) have been implicated in the regulation of gene expression critical for consolidation of memory. However, little is known about how these changes in 5mC are regulated in the adult brain. The enzyme Tet methylcytosine dioxygenase 1 (TET1) has been shown to promote active DNA demethylation in the nervous system. Therefore, we took a viral-mediated approach to overexpress the enzyme in the hippocampus and test its potential involvement in memory formation. We found that Tet1 is a neuronal-activity regulated gene and that its overexpression leads to changes in global modified cytosine levels. Furthermore, expression of TET1 or a catalytically inactive mutant (TET1m) resulted in the up-regulation of several neuronal memory-associated genes and impaired contextual fear memory. In summary, we show that neuronal Tet1 regulates DNA methylation levels and that its expression, independent of its catalytic activity, regulates the expression of CNS activity- dependent genes and memory formation.

Epigenetic mechanisms including altered DNA methylation are critical for altered gene transcription subserving synaptic plasticity and the retention of learned behavior. Here we tested the idea that one role for activity-dependent altered DNA methylation is stabilization of cognition-associated hippocampal place cell firing in response to novel place learning. We observed that a behavioral protocol (spatial exploration of a novel environment) known to induce hippocampal place cell remapping resulted in alterations of hippocampal Bdnf DNA methylation. Further studies using neurophysiological in vivo single unit recordings revealed that pharmacological manipulations of DNA methylation decreased long-term but not short-term place field stability. Together our data highlight a role for DNA methylation in regulating neurophysiological spatial representation and memory formation.