Background: Microglia, the innate immune cells of the central nervous system (CNS), are the first cellular responders to stroke. Microglia populate the brain during development and age throughout the life of the individual. With aging, microglia transition to an activated, pro-inflammatory phenotype, making them primed to respond in a malicious manner following an ischemic event. These primed microglia are the same cells that populated the brain during development but now have an altered gene expression profile due to the inflammaging environment (e.g. cell damage, inflammatory cytokine milieu). Cells, including microglia, may respond to inflammaging by altering their epigenetic landscape to ultimately result in changes in gene expression.
Hypothesis: We hypothesized that epigenetic mechanisms are responsible for the changes observed in microglia gene expression with aging, and that by definition these changes are reversible.
Methods: We utilized a model of heterochronic parabiosis in which young (2-3-month) and aged (18-20-month) male C57Bl/6 mice (Charles River, NIA) were surgically attached and share a common blood supply. Pairs were attached for 8 weeks. Young and aged isochronic pairs served as surgical controls. Brains were collected for immunohistochemical (IHC), Western blot, and multiplex cytokine analysis. P0.5-P3 neonatal pups were used for in vitro culture of primary microglia and were stimulated with lipopolysaccharide (LPS) and interferon-γ for 24hr.
Results: We screened 10 histone modifications and found that aging causes an imbalance in the epigenetic status of the CNS, with 1.26±0.08-fold higher histone H3 lysine 27 trimethylation (H3K27me3) and 0.649±0.063-fold reduced H3K27me1 (n=3, p<0.05). Pro-inflammatory stimulation of primary microglia resulted in elevated H3K27me3 (n=3, p<0.01), suggesting this mark is central to the pro-inflammatory status of aged microglia. Heterochronic parabiosis reversed these age-related changes, with reduced H3K27me3 in the brains of aged heterochronic mice (rejuvenation) and reduced H3K27me1 in young heterochronic mice (accelerated aging) (n=3-4, p<0.05).
Conclusion: We conclude that the aged CNS has an imbalanced epigenetic landscape, and that this is reversible.