Hypothesis\n\nIn aging neurons, oxidative stress promotes S-glutathionylation of APE1 at Cys65, which masks its nuclear localization signal and diverts the protein to cytoplasmic stress granules, thereby reducing nuclear BER capacity independent of APE1 expression levels.\n\n## Rationale\n- OGG1 and APE1 activity decline with age, but total APE1 protein often remains unchanged, suggesting post‑translational regulation {[https://pmc.ncbi.nlm.nih.gov/articles/PMC5576886/] [https://www.aginganddisease.org/EN/10.14336/AD.2022.0331]}.\n- S‑glutathionylation is a reversible redox modification known to affect protein‑protein interactions and subcellular trafficking.\n- Cytoplasmic sequestration of APE1 would limit its ability to process abasic sites in nuclear DNA, exacerbating 8‑oxoG persistence and mtDNA release {[https://pmc.ncbi.nlm.nih.gov/articles/PMC10247526/]}.\n\n## Novel Mechanistic Insight\nWe propose that the redox environment of the aged neuronal cytosol shifts the glutathione disulfide (GSSG)/reduced glutathione (GSH) ratio, favoring mixed‑disulfide formation on APE1. This modification does not degrade the protein but alters its conformational exposure of a basic NLS, leading to CRM1‑dependent export or retention in stress granules. Consequently, neurons exhibit a functional APE1 deficit despite normal mRNA and protein levels.\n\n## Testable Predictions\n1. Detection – Immunoprecipitation of APE1 from young vs. aged mouse brain lysates followed by anti‑glutathione Western blot will show increased S‑glutathionylation in aged samples.\n2. Localization – Proximity ligation assay or subcellular fractionation will reveal increased cytoplasmic APE1 co‑localizing with stress‑granule markers (G3BP1, TIA‑1) in aged neurons.\n3. Functional rescue – Overexpression of glutaredoxin‑2 (GRX2) or treatment with a cell‑permeable de‑glutathionylating agent will reduce APE1‑SSG, restore nuclear APE1, lower nuclear 8‑oxoG levels, and decrease cGAS‑STING activation {[https://pmc.ncbi.nlm.nih.gov/articles/PMC10247526/]}.\n4. Phenotypic outcome – GRX2 overexpression in aged mice will improve performance in hippocampal‑dependent memory tasks (e.g., Morris water maze) and reduce microglial activation compared with controls.\n5. Loss‑of‑function – Neuronal knockdown of GRX2 or expression of a Cys65‑serine APE1 mutant (cannot be glutathionylated) will exacerbate cytoplasmic APE1 accumulation, increase mtDNA release, and accelerate cognitive decline.\n\n## Experimental Approach\n- Biochemical: IP‑Western for APE1‑SSG, mass spec to confirm Cys65 modification.\n- Imaging: Confocal microscopy of primary neurons transfected with APE1‑GFP and stress‑granule markers; quantify nuclear/cytoplasmic ratio.\n- Functional BER assay: Measure incision activity of nuclear extracts using a fluorescent abasic substrate.\n- In vivo: AAV‑mediated GRX2 overexpression or CRISPR‑based knock‑in of APE1 C65S in aged mice; assess 8‑oxoG (immunodot), mtDNA in cytoplasm (qPCR), cGAS‑STING signaling (phospho‑TBK1/IRF3), cytokine profile, and behavior.\n- Intervention: Treat aged mice with a reduced glutathione ester (GSH‑EE) to shift redox balance and monitor APE1 modification.\n\n## Falsifiability\nIf S‑glutathionylation of APE1 does not increase with age, or if manipulation of glutathionylation status fails to alter nuclear APE1 levels, BER activity, or neurodegenerative phenotypes, the hypothesis would be refuted.