Hypothesis: The age-related shift from ACLY- to ACSS2-dependent nuclear acetyl-CoA supply leads to localized histone deacetylation at retrotransposon loci, triggering their transcription and driving cellular senescence in the brain.

Mechanistic Rationale: Young tissues rely on ACLY for nuclear acetyl-CoA, which supports broad histone acetylation and retrotransposon silencing Nuclear acetyl-CoA promotes longevity.... Aging induces metabolic stress and neuronal specialization, upregulating ACSS2 Three distinct enzymes supply.... However, ACSS2 might prioritize specific promoters, like TFEB targets for autophagy In neurons, ACSS2 is critical..., leaving repetitive regions underserved. This creates a compartmentalized acetyl-CoA deficit: nuclear pools dip specifically at heterochromatin, allowing retrotransposon derepression—a known senescence driver Progressive loss of histone silencing....

Testable Predictions:

• Correlative Evidence: In aged mouse hippocampi, ACLY mRNA and nuclear protein levels will decline, while ACSS2 increases. ChIP-seq will show reduced H3K9ac at retrotransposon families (e.g., LINE-1), correlating with their increased RNA-seq signal and senescence markers (p16, SA-β-gal).
• Interventional Test: AAV-mediated ACLY overexpression or acetate supplementation in aged mice will restore nuclear acetyl-CoA, elevate H3K9ac at retrotransposons, suppress their transcription, and improve cognitive scores A decline in acetyl-CoA....
• HDAC Inhibitor Synergy: In aged neuronal cultures, HDAC inhibitors (e.g., sodium butyrate) will fail to increase H3K9ac at retrotransposons unless co-administered with acetate to replenish acetyl-CoA substrate HDAC inhibitors increase longevity.... Falsification: if retrotransposon expression remains unchanged after acetyl-CoA boost, the hypothesis is invalid.

Implications: This frames brain aging as a metabolic-epigenetic nexus where enzyme switching precipitates genome instability. Therapeutic strategies should target nuclear acetyl-CoA delivery specifically to senescent cells, avoiding global perturbations that might disrupt autophagy in cytoplasm.