Aging Mouse Brain Reveals Epigenetic Marks of Cellular Senescence

Scientists have long suspected that aging leaves chemical fingerprints on the DNA packaging inside our cells. Now a team at the University of Turin has captured those fingerprints in unprecedented detail across the lifespan of the mouse brain, and what they found may reshape how we think about cognitive decline. In a study published in Frontiers in Cell Death, Marco Sbriz, Maftuna Shamshiddinova, Laura Lossi, and Adalberto Merighi mapped two epigenetic modifications of histone H4 and the senescence marker lamin B1 in the brains of postnatal day 5 pups, mature adults aged 9 to 10 months, and old mice at 24 months. Their results converge on a striking conclusion: the aging hippocampus and cerebral cortex acquire a cellular phenotype consistent with senescence, blurring the line between normal brain aging and the cellular state that precedes neurodegeneration.

Histones are the spool-like proteins around which DNA winds, and chemical tags added to specific amino acid residues on these proteins act like molecular switches that turn genes on or off. The Turin researchers focused on two such tags: trimethylation of histone H4 at lysine 20, known as H4K20me3, which is associated with compacted, transcriptionally silent heterochromatin, DNA repair, and replication timing; and acetylation of histone H4 at lysine 12, or H4K12ac, which loosens the DNA-histone interaction and is linked to active gene transcription and histone deposition. Previous work had shown that H4K12ac declines at memory-related genes in cognitively impaired old mice, while H4K20me3 has been implicated in maintaining genome stability and controlling cell fate decisions in neural progenitors. What remained unknown was how these two opposing marks are distributed across the brain, in which cell types they appear, and how their expression changes from birth to old age.

Using immunofluorescence on paraffin-embedded brain sections, the team first confirmed that both epigenetic tags are present in the nuclei of neurons and astrocytes throughout all major brain regions, from the olfactory bulb to the cerebellum. They verified these anatomical observations with Western blot analysis of forebrain, cerebellum, and brainstem extracts. Crucially, they also measured the expression of naive histone H4 itself across the three age groups and found no statistically significant differences in the percentage of H4-positive nuclei or in staining intensity. This control experiment was essential: it ruled out the possibility that any age-related changes in the epigenetic marks were simply due to a global loss of the histone protein, rather than a genuine reprogramming of the chemical tags.

The quantitative analysis centered on the hippocampal dentate gyrus, a region critical for memory consolidation and one of the few areas where new neurons continue to be generated throughout life. Using automated cell counting with the Hough Circle Transform plugin in Fiji, the researchers calculated the percentage of immunoreactive nuclei relative to the total number of DAPI-stained cells, measured nuclear shape through Hough scores, and quantified staining intensity via mean gray density and integrated density. For H4K20me3, the percentage of positive cells rose from about 68 percent in P5 pups to 87 percent in adults, then fell back to roughly 63 percent in old mice. Nuclear shape became more regularly circular in adults but turned irregular again in old animals. Staining intensity followed the same trajectory, peaking in adulthood and dropping significantly in aged mice.

H4K12ac told a parallel story. The percentage of immunoreactive cells in the dentate gyrus increased from about 50 percent at P5 to 76 percent in adults, then declined to 46 percent in old mice. Nuclear circularity and staining intensity both decreased in aged animals compared to adults. These findings align closely with earlier reports that H4K12ac is reduced in the CA1 region of cognitively impaired 20-month-old mice, where its loss downregulates genes associated with neuronal and synaptic function. The Turin team’s data extend that observation to the dentate gyrus and demonstrate that the reduction carries a biologically meaningful effect size, not merely a statistically detectable one.

Perhaps the most dramatic age-related change was in lamin B1, a structural protein of the nuclear lamina that maintains nuclear envelope integrity. LB1 immunostaining appeared as a fluorescent ring surrounding the nucleoplasm, and its expression declined progressively across the three age groups. The percentage of LB1-positive cells in the dentate gyrus dropped from about 50 percent at P5 and 62 percent in adults to just 34 percent in old mice. Staining intensity fell even more steeply, from a mean gray density of 123 units at P5 to 57 units in old animals. Nuclear profiles became fragmented and irregular in aged mice, consistent with the nuclear deformation that characterizes senescent cells. Because LB1 loss is now widely regarded as a hallmark of cellular senescence, and because LB1 decline has been shown to underlie the age-related loss of hippocampal neurogenesis, these quantitative data suggest that a large population of dentate gyrus cells acquires a senescent phenotype by 24 months of age.

The researchers then explored whether the two histone marks coexist within the same nuclei, a question that touches on the concept of bivalent chromatin states. Using double immunolabeling with a paraformaldehyde vapor blocking method to allow two rabbit primary antibodies in the same section, they quantified coexistence through Pearson’s correlation coefficients and fluorescence overlapping areas in the hippocampus, and through object-based colocalization analysis in the cerebral cortex. In the hippocampus, coexistence increased significantly from P5 to adulthood, with a slight but non-significant reduction in old mice. In the cerebral cortex, the number of double-labeled nuclei per microscopic field fell from about 65 at P5 to roughly 25 in old animals, a statistically significant decline with a large effect size. The coexistence of an activating acetylation mark with a repressive methylation mark is unusual under normal conditions, but such combinatorial patterns have been described in embryonic stem cells, where they maintain undifferentiated, poised chromatin states. The authors caution that their light-microscopy approach demonstrates spatial overlap at the cellular level, not necessarily co-enrichment on the same nucleosome; resolving that question would require ChIP-seq or CUT&RUN at high resolution.

Exploratory experiments also examined the relationship between H4K20me3 and activated caspase 3, an executioner caspase central to apoptotic cell death. Double-stained nuclei were observed in adult hippocampus but only very occasionally in old animals, consistent with the well-established resistance of senescent cells to apoptosis. Intriguingly, the team detected strongly caspase 3-immunoreactive granular material in the cytoplasm of many dentate gyrus cells at both adult and old ages, which displayed orange autofluorescence under UV light, identifying it as lipofuscin, the wear-and-tear pigment that accumulates with aging. Lipofuscin aggregates have been shown to induce apoptosis through proteasome inhibition and caspase activation, suggesting that the senescent phenotype in the aged brain may coexist with ongoing cellular stress driven by this persistent debris.

The convergence of declining H4K20me3, declining H4K12ac, and declining LB1 in the old hippocampus, each with biologically meaningful effect sizes, paints a coherent molecular portrait of brain aging. The authors propose that the border between physiological cell aging and true senescence is quite blurry in old animals, and that the senescent state may represent a transitional phase between normal aging and neurodegeneration. Because histone methylation marks such as H4K20me3 have been reported as dysregulated in Alzheimer’s disease, and because LB1 loss correlates with nuclear invaginations seen in both physiological aging and tauopathies, the molecular pathways identified here may offer clues to understanding age-related cognitive disorders. The study also raises the possibility that interventions targeting these epigenetic marks, such as the one-carbon metabolite cocktail that recently reduced H4K20me3 levels in aged mouse hippocampus, could potentially rejuvenate the aging brain. As the global population ages and neurodegenerative diseases place increasing pressure on healthcare systems, understanding the epigenetic and structural changes that drive cellular senescence in the brain becomes not just an academic pursuit but a potential roadmap for therapeutic intervention.

Subject of Research: aging, mouse, brain, histone H4, lamin B1, cell death

Article Title: Expression of two histone H4 epigenetic modifications (lysine 12 acetylation and lysine 20 trimethylation), and the senescence marker lamin B1 in the postnatal, adult, and old mouse brain

Article References: Expression of two histone H4 epigenetic modifications (lysine 12 acetylation and lysine 20 trimethylation), and the senescence marker lamin B1 in the postnatal, adult, and old mouse brain. (n.d.). https://doi.org/10.3389/fceld.2025.1632653

Image Credits: AI Generated

DOI: 10.3389/fceld.2025.1632653

Keywords: Expression, histone, epigenetic, modifications, lysine, acetylation, trimethylation, senescence, marker, lamin, postnatal, adult