Hypothesis

Aging triggers a tissue‑specific shift in the relative expression of nuclear versus cytoplasmic ACSS2 isoforms, altering the balance between locally generated acetyl‑CoA and mitochondrial acetyl‑CoA export. In brain and retinal pigment epithelium, reduced nuclear ACSS2 limits acetate‑derived acetyl‑CoA at histone acetyltransferase sites, promoting H3K9 hypoacetylation. Conversely, in heart and skeletal muscle, stress‑induced nuclear translocation of ACSS2 (via AMPK‑S659 phosphorylation) surplus acetate‑derived acetyl‑CoA outcompetes dwindling mitochondrial export, leading to hyperacetylation despite low NAD+‑dependent sirtuin activity. This isoform switch creates a bidirectional metabolic‑epigenetic feedback that drives opposing acetylation phenotypes across tissues.

Mechanistic Rationale

1. Isoform‑specific chromatin targeting – Nuclear ACSS2 contains a nucleolar localisation signal absent in the cytoplasmic variant, enabling preferential binding to promoters of autophagy and lysosomal genes (see AMPK‑S659 translocation)[2]. Aging‑linked AMPK hyperactivity in muscle favors nuclear ACSS2 accumulation, whereas chronic oxidative stress in brain promotes proteasomal degradation of the nuclear isoform, reducing its chromatin residence.
2. Acetate‑CoA competition – Mitochondrial acetyl‑CoA export via the citrate carrier declines with age[5]. When export falls, nuclear ACSS2‑derived acetyl‑CoA becomes the dominant substrate for HATs like p300/CBP[1]. In tissues where nuclear ACSS2 is upregulated (muscle), this surplus drives hyperacetylation; where it is downregulated (brain), hypoacetylation prevails.
3. NAD+‑SIRT1 interplay – Elevated nuclear acetyl‑CoA consumes acetyl groups that would otherwise be deacetylated by SIRT1, exacerbating hyperacetylation when NAD+ is low[3]. In brain, low acetyl‑CoA limits SIRT1 substrate availability, further depressing deacetylase activity and locking in a hypoacetylated state.

Testable Predictions

• Prediction 1: In aged mouse skeletal muscle, CRISPR‑mediated knock‑in of a nuclear‑export‑deficient ACSS2 mutant will decrease nuclear acetyl‑CoA, reduce H3K27ac at autophagy promoters, and increase NAD+/SIRT1 activity, rescuing the hyperacetylated phenotype.
• Prediction 2: Viral delivery of a brain‑targeted, nuclear‑localized ACSS2 isoform to aged SAMP8 mice will elevate nuclear acetyl‑CoA, restore H3K9ac at synaptic plasticity genes, and improve cognitive performance.
• Prediction 3: Pharmacological inhibition of AMPK (e.g., Compound C) in aged muscle will prevent ACSS2 S659 phosphorylation, block its nuclear translocation, lower histone acetylation, and ameliorate sarcopenia markers.

Experimental Approach

• Generate tissue‑specific ACSS2 isoform‑swap mouse lines (Cre‑loxP driven by Myh6 for heart, Camk2a for brain, Acta3 for skeletal muscle).
• Measure nuclear acetyl‑CoA levels via LC‑MS/MS of isolated nuclei, histone acetylation patterns (H3K9ac, H3K27ac) by ChIP‑seq, and NAD+/SIRT1 activity using fluorometric assays.
• Assess functional outcomes: cognitive tests (Morris water maze), cardiac echocardiography, and grip strength/endurance treadmill.
• Include rescue arms with acetate supplementation (2 g/L drinking water) to test whether exogenous acetate bypasses isoform limitations.

Falsifiability

If nuclear ACSS2 manipulation does not inversely correlate with histone acetylation levels in the predicted direction, or if acetate supplementation fails to modify acetylation despite altered isoform expression, the hypothesis would be refuted. Conversely, consistent isoform‑dependent shifts in acetyl‑CoA pools and reciprocal acetylation changes across brain versus muscle would support the model.

References

• Acetyl-CoA from nuclear ACLY and ACSS2 directly fuels histone acetyltransferases
• AMPK phosphorylation at S659 triggers nuclear translocation during glucose deprivation
• In neurons and glia, chromatin-bound ACSS2 preferentially drives histone acetylation over ACLY-derived pools
• Age-related global histone loss and altered histone acetylation in RPE
• Mitochondrial decline reduces acetyl-CoA output, contributing to global hypoacetylation