Hypothesis

We propose that chronic JNK/AP‑1 signaling in aging parenchyma is locked in place by a feed‑forward epigenetic circuit in which mitochondrial ROS‑activated JNK phosphorylates 53BP1, promoting its translocation to the cytoplasm where it mediates extrusion of chromatin fragments. These fragments act as decoy substrates that sequester histone deacetylases (HDACs) and DNA methyltransferases (DNMTs) away from AP‑1 target enhancers, thereby maintaining an open chromatin state that sustains AP‑1 binding and transcription of SASP genes even when the initial ROS stimulus wanes.

Mechanistic Model

• Step 1 – ROS‑JNK activation: Persistent mitochondrial dysfunction releases superoxide that activates JNK in parenchymal cells (https://pmc.ncbi.nlm.nih.gov/articles/PMC10352569/).
• Step 2 – 53BP1 phosphorylation: JNK directly phosphorylates 53BP1 on serine residues that enhance its affinity for double‑strand breaks and promote its cytoplasmic shuttling (https://doi.org/10.1101/gad.331272.119).
• Step 3 – Chromatin extrusion: Cytoplasmic 53BP1 nucleates the formation of micron‑sized chromatin fragments that bud off from the nucleus, a process observed in senescent cells (https://pmc.ncbi.nlm.nih.gov/articles/PMC9106822/).
• Step 4 – Epitope sequestration: These fragments bind HDAC1/2 and DNMT3A via exposed histone tails, reducing their nuclear availability (https://pmc.ncbi.nlm.nih.gov/articles/PMC10228795/).
• Step 5 – Enhancer hypo‑repression: Loss of HDAC/DNMT activity at AP‑1‑bound enhancers prevents deacetylation and methylation, preserving a transcriptionally permissive chromatin landscape that sustains c‑Jun/c‑Fos binding and SASP output (https://doi.org/10.1038/s41598-021-95344-5/).
• Step 6 – Feedback to mitochondria: SASP cytokines (TNF‑α, IL‑6) reinforce mitochondrial ROS production via NF‑κB signaling, completing the loop.

Testable Predictions

1. Phospho‑53BP1 levels in cytoplasmic extracts will correlate with AP‑1 target gene expression across aging mouse kidney and liver, and will decline when JNK is inhibited pharmacologically.
2. HDAC1/2 nuclear occupancy at AP‑1‑dependent enhancers (e.g., Il6, Tnf promoters) will be reduced in aged parenchyma; rescuing nuclear HDAC levels (via HDAC overexpression or fragment‑binding peptides) will diminish SASP without affecting acute JNK activation after injury.
3. Chromatin fragment burden measured by extracellular DNA staining will predict SASP intensity better than p16 or p21 levels in single‑cell RNA‑seq datasets.
4. Genetic disruption of 53BP1’s JNK phosphorylation site (Ser→Ala knock‑in) will break the feed‑forward loop, leading to transient AP‑1 activation after stress but failure to sustain SASP in aged mice, while preserving wound‑healing responses.
5. Therapeutic blockade of fragment‑HDAC interaction (using cell‑permeable peptides mimicking the HDAC‑binding domain of chromatin) will attenuate inflammaging in aged mice without increasing tumorigenesis or impairing stress‑induced JNK signaling.

Experimental Approach

• In vivo: Generate kidney‑specific JNK‑conditional knockout and 53BP1 phospho‑mutant mice; assess mitochondrial ROS (MitoSOX), cytoplasmic chromatin fragments (Cyto‑DNA ELISA), AP‑1 ChIP‑seq, and SASP cytokines across lifespan.
• In vitro: Treat primary mouse renal tubular epithelial cells with rotenone to induce mitochondrial ROS; use live‑cell imaging to track 53BP1 translocation and fragment formation; apply HDAC rescue constructs and measure Il6 transcription via qPCR.
• Human relevance: Isolate primary proximal tubule cells from young and older donors; quantify cytoplasmic 53BP1 fragments, nuclear HDAC levels, and AP‑1 activity; test peptide inhibitor efficacy.

If these predictions hold, the model redefines the transition from acute to chronic JNK/AP‑1 signaling as an epigenetically stabilized state driven by mitochondrial ROS‑dependent chromatin extrusion, offering a precise intervention point that preserves protective stress responses while dismantling the inflammaging loop.