Caloric-restriction induced alterations in CG and non-CG methylation attenuate age-associated changes in the old brain.
Brain aging is marked by cognitive decline and susceptibility to neurodegeneration. Caloric-restriction (CR) increases neurogenesis, improves memory function, and protects from age-associated neurological disorders. Epigenetic mechanisms, including DNA methylation, are vital to normal CNS cellular and memory functions, and are dysregulated with aging. The beneficial effects of CR have been proposed to work through epigenetic processes, but this is largely unexplored. We therefore tested whether life-long CR prevents age-related DNA methylation changes in the brain. Hippocampal DNA from young (3 months) and old (24 months) male mice fed ad libitum and 24 month old mice fed a 40% calorierestricted diet from 3 months of age were examined by genome-wide bisulfite sequencing to measure methylation with base-specificity. Over 27 million CG and CH (non-CG) sites were examined. Of the ~40,000 differentially methylated CGs (dmCGs) and ~80,000 CHs (dmCHs) with aging, >1/3 were prevented by CR and were found across genomic regulatory regions and gene pathways. CR also caused alterations to CG and CH methylation at sites not differentially methylated with aging, and these CR-specific changes demonstrated a different pattern of regulatory element and gene pathway enrichment than those affected by aging. CR-specific DNMT1 and TET3 promoter hypermethylation corresponded to reduced gene expression. These findings demonstrate that CR attenuates age-related CG and CH hippocampal methylation changes, in combination with CR-specific methylation that may also contribute to the neuroprotective effects of CR. The prevention of age-related methylation alterations is also consistent with the pro-longevity effects of CR working through an epigenetic mechanism.