Hypothesis

Age‑linked decline in nuclear HDAC1 reduces its cytoplasmic availability, leading to hyperacetylation of the mitochondrial import receptor TOMM20. Acetylated TOMM20 exhibits lower affinity for OGG1, impairing mitochondrial OGG1 import and causing mtDNA 8‑oxoG accumulation, cGAS‑STING activation, and hippocampal neurodegeneration.

Mechanistic Rationale

• HDAC1 is known to promote OGG1 activity on nuclear DNA by deacetylating OGG1 or associated factors [3]. Recent data show that mitochondrial OGG1 import, not just its expression, dictates mtDNA 8‑oxoG levels [2].
• TOMM20, the primary translocase for cytosolic proteins into mitochondria, is regulated by lysine acetylation; acetylation reduces its binding affinity for presequences [9].
• In aged neurons, HDAC1 shuttling to the cytosol diminishes, paralleling reduced mitochondrial OGG1 [2]. Hyperacetylated TOMM20 would therefore fail to recruit OGG1 efficiently, creating a specific import bottleneck.
• This links nuclear epigenetic decline (HDAC1 loss) to a mitochondrial repair defect, explaining why hippocampal neurons—highly dependent on HDAC1 for transcription of synaptic genes—show the greatest 8‑oxoG rise [1].

Testable Predictions

1. Acetylation state: TOMM20 from aged hippocampal neurons will show increased lysine acetylation compared to young controls.
2. Binding assay: Acetylated TOMM20 will pull down less OGG1 in vitro; deacetylation by HDAC1 or SIRT3 will restore binding.
3. Rescue: Neuronal overexpression of HDAC1 (or a mitochondria‑targeted HDAC1 fragment) in aged mice will decrease TOMM20 acetylation, increase mitochondrial OGG1 import, lower mtDNA 8‑oxoG, and reduce cGAS‑STING signaling.
4. Phenocopy: CRISPR‑mediated acetylation‑mimic mutations (K→Q) in TOMM20 will reproduce the import defect even in young neurons, elevating mtDNA 8‑oxoG and inflammatory markers.
5. Specificity: Manipulating TOMM20 acetylation will not affect nuclear OGG1 activity, distinguishing the import effect from previously reported nuclear HDAC1‑OGG1 interactions.

Experimental Approach

• Biochemistry: Isolate mitochondria from young and aged mouse hippocampi; immunoprecipitate TOMM20 and probe acetyl‑lysine levels (Western blot). Perform GST‑TOM20 pull‑downs with recombinant OGG1 ± HDAC1/SIRT3 treatment.
• Cellular: Use primary hippocampal neurons transfected with acetyl‑mimic (K→Q) or acetyl‑null (K→R) TOMM20 constructs; measure mitochondrial OGG1 by immunofluorescence and mtDNA 8‑oxoG by dot‑blot or qPCR after oxidative stress.
• In vivo: AAV‑mediated HDAC1 overexpression (wild‑type or mitochondria‑targeted) in 18‑month‑old mice; assess TOMM20 acetylation, mitochondrial OGG1 levels (subcellular fractionation), mtDNA 8‑oxoG, cGAS‑STING activation (phospho‑TBIRF3, IFN‑β), and hippocampal‑dependent behavior (Morris water maze).
• Controls: Include SIRT3 overexpression to test whether mitochondrial deacetylation can bypass HDAC1 loss; use MUTYH knockdown to confirm that observed toxicity is not due to futile BER cycles.

Potential Outcomes

• Support: Increased TOMM20 acetylation correlates with reduced OGG1 import; HDAC1 rescue normalizes import, lowers 8‑oxoG, and ameliorates inflammation and cognitive decline.
• Refutation: No change in TOMM20 acetylation with age, or HDAC1 manipulation fails to affect mitochondrial OGG1 levels despite altering nuclear OGG1 activity, suggesting alternative import regulators.

This hypothesis directly connects nuclear epigenetic aging to mitochondrial DNA repair via a specific protein‑acetylation mechanism, offering a clear, falsifiable path to explain regional vulnerability in the hippocampus and to identify combinatorial targets (HDAC1 + TOMM20 acetylation) for therapeutic intervention.