, 2010). By demonstrating a regulatory role of DNA demethylation in cognitive function (Rudenko and Tsai, 2013), these studies provide the rationale to further study the role of the Tet proteins in the nervous system. In the current work, we show that the expression of a number of genes is dysregulated in the cortex and hippocampus of Tet1 knockout (Tet1KO) mice. Interestingly, the most prominent Nintedanib cost category of downregulated genes is comprised of multiple neuronal activity-regulated genes that include Npas4, c-Fos, Arc, Egr2, and Egr4 ( Loebrich and Nedivi, 2009 and Ebert

et al., 2013). We also found that while Tet1KO mice display normal memory formation, they showed specific impairments in extinction this website learning. Moreover, we show that while hippocampal long-term potentiation was intact in Tet1KO animals, they had abnormally enhanced long-term depression compared to controls. We performed methylation analysis of a key upstream neuronal activity-regulated gene, Npas4, and found hypermethylation

of the promoter region in Tet1KO animals compared to controls, both in naive mice and after extinction training, which could lead to the reduced expression of Npas4 and its downstream targets. Our study identifies an important role for Tet1 in regulating the neuronal activity-regulated genes, hippocampal synaptic plasticity, and memory all extinction. Reports of high levels of 5hmC in the CNS genome (Kriaucionis and Heintz, 2009 and Szulwach et al., 2011) prompted a search for potential functions for the Tet1 methylcytosine dioxygenase in the mouse brain. We utilized a previously characterized Tet1 knockout (Tet1KO) mouse strain in which exon 4 of Tet1 is deleted, leading to an out-of-frame fusion of exons 3 and 5 and creating a Tet1 null allele ( Dawlaty et al., 2011). Loss of Tet1 mRNA was confirmed by real-time quantitative PCR in cortex and hippocampus ( Figure S1A available online). We also quantified

all three Tet mRNA levels in hippocampal and cortical tissues from wild-type mice and found that all three Tets are expressed in both hippocampus and cortex ( Figure S1B). The presence of all Tet proteins in the CNS may lead to potential compensatory effects caused by the loss of a single Tet family member. Since Tet proteins are responsible for the conversion of 5mC to 5hmC, we wanted to determine how Tet1 ablation affects 5mC and 5hmC levels in the brain. Global genomic 5mC and 5hmC contents in the hippocampi and cortices of 4-month-old Tet1KO and control Tet1+/+ mice were assessed by immunohistochemistry (Figure 1A) and quantified by liquid chromatography combined with tandem mass spectrometry using multiple reaction monitoring (LC/MS/MS-MRM).