Molecular Targets of Aging-Triggered Memory Decline in a Stress-Reactive Rat Strain

In addition to aging, major risk factors for cognitive decline include female gender, stress, and stress-related disorders such as depression. Both major depressive disorder (MDD) and dementia are more common in women than in men. In this proposal, we aim to investigate the sex and strain-specific epigenetic changes that parallel cognitive decline in an animal model of increased stress-reactivity and depression-like traits. Through selective and long-term full-sib breeding, we obtained two inbred strains from the parental Wistar Kyoto (WKY) rat strain: the WKY More Immobile (WMI), and its isogenic control, the WKY Less Immobile (WLI). Compared to WLIs, both male and female WMIs show consistently greater despair-like behavior in the forced swim test. Hippocampus-dependent contextual fear memory did not differ between young WLI and WMI males and females. However, by 12 months of age (middle-age), female WMIs showed a significant decline in fear memory compared to young WMI females, while no decrease in fear memory was observed in female WLIs and male WLIs and WMIs. We propose to test the hypothesis that age-induced strain-specific changes in the hippocampal methylome and transcriptome of female WMIs are associated with the decline in their fear memory. Aim 1 will evaluate the sex-specific strain differences between young (6 months) and middle-aged (12 months) WMI and WLI in the contextual fear conditioning, novel object recognition and Morris water maze paradigms. Additionally, WMI and WLI males and females will be maintained from 6 to 12 months of age in an enriched environment (EE), that is known to enhance memory, and at 12 months of age tested in the same learning/memory tests. Aim 2 will identify differentially expressed genes (DEG) within differentially methylated regions using integrative analysis of Reduced Representation Bisulfite Sequencing and RNA-seq between additional groups of young and middle-aged WMI and WLI males and females after being exposed to the contextual fear conditioning test. Comparisons will be prioritized to those DEGs that differ exclusively between WMI females of 6- and 12-months of age. The WMI and WLI genome have been sequenced and will serve as the reference genomes for these analyses. Expression of DEGs within differentially methylated regions will be measured by quantitative RT-PCR in the hippocampus of young and middle-aged male and female WMIs and WLIs with and without enriched environment housing from 6 to 12 months of age (Aim 1) to identify changes induced by age and EE. Candidate DEGs will also be measured in the hippocampi of all animals from Aim 2, when expression data of DEGs will be associated with contextual fear memory in the same animals. Prediction models for memory will be built using the obtained and confirmed DEGs and methylation data.