P.3.008 S-Adenosyl-methionine impairs forced swimming-induced behavioural immobility by inhibiting gene expression in dentate gyrus neurons

Abstract

The consolidation of stress-induced adaptive behaviours, such as the learned immobility response in the forced swim (FS) test, depends on specific epigenetic modifications underlying gene transcriptional responses in dentate gyrus (DG) granule neurons of the hippocampus. In these neurons FS evokes the activation of two interacting signalling pathways, i.e. the glucocorticoid receptor (GR) and the NMDAR/ERK1/2/MSK1-Elk-1 pathways, resulting in phosphorylation of serine10 and acetylation of lysine14 at histone H3 (H3S10p-K14ac) which leads to induction of immediate early genes (IEGs) c-Fos and Egr-1 [1,2]. These molecular responses are critical for the consolidation of the behavioural immobility response [1,2]. The drug and endogenous methyl-donor S-adenosylmethionine (SAM) impairs the consolidation of the behavioural immobility response [3] suggesting the involvement of histone methylation and/or DNA methylation in gene transcriptional control underlying the behavioural response but this is unknown. Therefore, to understand the mechanism of action of SAM and to gain insight into the involvement of histone/DNA methylation, rats were injected with SAM (100 mg/kg s.c.) 30 minutes before FS (15 min, 25oC). Twenty-four hours later they were subjected to FS again and immobility behaviour were scored in 10 s bins. SAM had no effect on immobility in the initial FS test (mean±SEM; vehicle: 15.8±1.6 bins, n = 8; SAM: 15.0±1.9 bins, n = 9; P>0.05 post-hoc Bonferroni test). However, in the retest, immobility in the SAM group was significantly lower (11.4±3.3 bins, n = 9) than that in the vehicle group (20.6±2.3 bins, n = 8; P<0.05), confirming that SAM indeed impairs the consolidation of the behavioural immobility response [3]. To study whether SAM's effects on behaviour could be explained through effects on FS-induced epigenetic and transcriptional responses in DG neurons, rats were pre-treated with SAM or vehicle and submitted to FS or not (baseline control) and killed 1 h later. Compared to vehicle, SAM evoked a significant decrease in FSinduced c-Fos and Egr-1in DG neurons (Vehicle: c-Fos: 127.1±4.6 neurons n = 5, Egr-1: 18.4±2.5 n = 5; SAM: c-Fos 82.6±5.0 neurons n = 6, Egr-1 8.5±1.2 n = 5; both c-Fos and Egr-1: P<0.01). However, SAM had no effect on stress-induced H3S10p-K14ac in DG neurons suggesting that the drug effect was independent of this dual epigenetic mark. Next, we investigated the effect of SAM and FS on the methylation status of histone H3 lysine residues. Chromatin immuno-precipitation (ChIP) analysis revealed that SAM and FS had no significant effects on H3K4me3, H3K9me3 and H3K27me3 at the c-Fos and Egr-1 promoters. Recently, we have started investigating DNA methylation using bisulfite pyrosequencing and methyl-DNA IP (MeDIP) to assess the cytosine methylation status of CpG islands within the c-fos and egr-1 gene promoters. We found that cytosine methylation of both promoters was very low (<5%) in the DG, the rest of hippocampus, and the neocortex and did not change after FS. Presently we are S64 Epigenetics: towards new drug targets studying the effects of SAM on de novo DNA methylation of the gene promoters. We conclude that SAM impairs the FS-induced behavioural immobility response through inhibition of gene transcription in DG neurons. This drug effect appears to be independent of histone modifications.