Nuclear ACSS2-mediated histone acetylation in hypothalamic microglia as a switch to curb inflammaging

Hypothesis

Acetate supplementation drives ACSS2 nuclear translocation in hypothalamic microglia, generating a localized acetyl-CoA pool that selectively acetylates histones at anti‑inflammatory gene promoters (e.g., "Il10", "Tgfb1") and deacetylates pro‑inflammatory loci ("Nlrp3", "Casp1") through a SIRT1‑ACSS2 antagonistic complex, thereby suppressing microglial inflammasome activation and breaking the mitochondrial‑epigenetic vicious cycle of aging.

Mechanistic Model

1. Acetate uptake via monocarboxylate transporters (MCT1/2) raises cytosolic acetate.
2. ACSS2 is acetylated at lysine residues by acetyl‑CoA synthetase activity, exposing a cryptic nuclear localization signal (NLS) that promotes importin‑α/β binding.
3. In the nucleus, ACSS2 forms a transient complex with SIRT1; the acetyl‑CoA produced by ACSS2 allosterically inhibits SIRT1’s deacetylase activity, shifting the local equilibrium toward histone acetylation.
4. This acetyl‑CoA/SIRT1 module preferentially targets enhancers of TFEB‑dependent lysosomal genes and promoters of anti‑inflammatory cytokines, increasing H3K27ac and H3K9ac.
5. Concomitantly, the same complex recruits the HDAC3‑NCoR repressor to NF‑κB‑bound promoters, facilitating deacetylation and transcriptional repression of inflammasome components.
6. The net effect is enhanced lysosomal biogenesis, improved mitophagy, and reduced NLRP3 inflammasome‑driven IL‑1β release.

Predictions

• Acetate treatment will increase nuclear ACSS2 signal (immunofluorescence) and H3K27ac at "Il10" and "Tgfb1" promoters in microglia from aged mice.
• Microglia‑specific ACSS2 knockout or NLS mutation will abolish acetate‑induced histone acetylation and fail to suppress NLRP3 activation, despite normal cytosolic acetate levels.
• Pharmacologic inhibition of SIRT1 (e.g., "EX527") will mimic acetate’s effect, whereas SIRT1 overexpression will blunt it.
• In vivo, chronic acetate supplementation in aged mice will lower hypothalamic IL‑1β, improve systemic markers of inflammaging (serum IL‑6, TNFα), and extend median lifespan by ~10%.

Experimental Approach

• Cellular: Isolate primary microglia from young (3 mo) and aged (24 mo) mice; treat with sodium acetate (5 mM) ± ACSS2 inhibitor (DOPC) or SIRT1 modulator; perform subcellular fractionation, western blot for ACSS2, immunofluorescence for nuclear ACSS2, ChIP‑qPCR for H3K27ac at target loci, and measure NLRP3 inflammasome activation (ASC speck formation, caspase‑1 cleavage).
• Genetic: Generate microglia‑specific ACSS2 floxed mice crossed with Cx3cr1‑CreER; induce deletion in adult mice; assess acetate response.
• In vivo: Feed aged mice 2% sodium acetate in drinking water for 6 months; monitor hypothalamic cytokine levels by ELISA, systemic inflammaging markers, frailty index, and survival.
• Rescue: Overacetylate SIRT1 (constitutively active) in microglia to test whether it blocks acetate benefits.

This framework is falsifiable: if acetate fails to alter nuclear ACSS2 or histone acetylation, or if microglial ACSS2 loss does not impair the anti‑inflammatory effect, the hypothesis would be refuted. Conversely, confirmation would position nuclear ACSS2 as a druggable node linking nutrient sensing to hypothalamic control of organismal aging.