Hypothesis

Aging brains exhibit epigenetic rigidity not because of global histone hypoacetylation but because excess nuclear acetyl‑CoA, generated by ACSS2 from acetate, preferentially acetylates promoters of genes that suppress synaptic plasticity (e.g., protein phosphatase 1, REST). This targeted acetylation stabilizes repressive chromatin states that lock neural circuits into over‑consolidated configurations, reducing tolerance for surprise and manifesting as cognitive inflexibility.

Mechanistic Rationale

1. Compartmentalized acetyl‑CoA pools – In young neurons, mitochondrial citrate export supplies nuclear acetyl‑CoA for activity‑dependent histone acetylation at plasticity genes (BDNF, Arc). With age, mitochondrial dysfunction lowers citrate flux, decreasing this pool.
2. Compensatory ACSS2 up‑regulation – To sustain nuclear acetyl‑CoA, aged astrocytes and neurons increase ACSS2 expression and AMPK‑dependent phosphorylation (Ser659), driving acetate conversion to acetyl‑CoA directly at chromatin.
3. Promoter selectivity – ACSS2 is recruited to genomic sites via interaction with repressive transcription factors (REST, NuRD complex). Consequently, acetate‑derived acetyl‑CoA acetylates histones at suppressor loci, enhancing their expression and reinforcing LTD‑like signaling.
4. Feedback loop – Elevated phosphatase activity dephosphorylates CREB and reduces BDNF transcription, further diminishing plasticity‑gene acetyl‑CoA demand and shunting more acetate flux toward suppressor promoters.

This model explains why global HDAC inhibition rescues LTP: it overrides the acetate‑driven acetylation at suppressor sites, while acetate supplementation alone would worsen rigidity if not paired with strategies to redirect ACSS2 activity.

Testable Predictions

• Prediction 1: In hippocampal tissue from aged mice, nuclear acetyl‑CoA levels measured by targeted mass spectrometry will be unchanged or slightly elevated relative to young, but acetate‑derived acetyl‑CoA (tracked with ^13C‑acetate) will be enriched at promoters of Ppp1ca and Rest compared with Bdnf and Arc.
• Prediction 2: Genetic knock‑down of ACSS2 specifically in forebrain excitatory neurons will reduce ^13C‑acetate incorporation into suppressor‑gene promoters, increase histone acetylation at BDNF/Arc, and restore LTP and spatial memory in aged mice without altering global histone acetylation levels.
• Prediction 3: Pharmacological enhancement of mitochondrial citrate export (e.g., with dichloroacetate) will decrease reliance on ACSS2, lower acetate‑derived nuclear acetyl‑CoA at suppressor promoters, and improve cognitive flexibility in aged animals.
• Prediction 4: Exposing aged mice to novelty‑rich environments (controlled uncertainty) will increase neuronal activity‑dependent citrate export, further shifting acetyl‑CoA usage toward plasticity genes and ameliorating over‑consolidation.

Experimental Approach

1. Isotope tracing: Inject ^13C‑acetate into young (3 mo) and aged (20 mo) mice, isolate nuclear fractions, and perform ChIP‑seq for H3K27ac coupled with ^13C‑label detection to map acetate‑dependent acetylation sites.
2. Cell‑specific manipulation: Use AAV‑Cre to delete Acss2 in Camk2a‑positive neurons; assess nuclear acetyl‑CoA, promoter‑specific acetylation (ChIP‑qPCR), LTP (slice electrophysiology), and behavior (novel object recognition, reversal learning).
3. Metabolic rescue: Treat aged mice with dichloroacetate to activate pyruvate dehydrogenase, measure citrate efflux, and repeat the above assays.
4. Environmental intervention: House aged mice in a rotating‑object novelty paradigm for 4 weeks, then evaluate the same molecular and functional readouts.

Falsifiability

If acetate‑derived nuclear acetyl‑CoA does not show preferential enrichment at suppressor‑gene promoters in aged brains, or if ACSS2 deletion fails to reduce suppressor‑gene acetylation and does not rescue LTP/memory, the hypothesis would be refuted. Conversely, confirmation would support the notion that age‑related cognitive inflexibility stems from maladaptive, locus‑specific acetylation driven by acetate metabolism, suggesting that precision metabolic‑epigenetic therapies—not broad HDAC inhibition—are required to re‑introduce controlled uncertainty into overly consolidated neural circuits.