ASK1 cysteine oxidation as a molecular timer that locks AP-1 into a constitutive senescent state

Hypothesis

Persistent oxidation of specific cysteine residues on ASK1 acts as a redox‑based switch that converts transient JNK signaling into a self‑sustaining AP‑1 program during the 48‑96 hour window after senescence onset.

Mechanistic Basis

Mitochondrial minor MOMP (miMOMP) releases bursts of superoxide that diffuse to the cytosol and oxidize the thioredoxin‑binding pocket of ASK1 (Cys32 and Cys35). Oxidation prevents binding of reduced thioredoxin, allowing ASK1 autophosphorylation and continuous activation of the MKK4/7→JNK cascade mitochondrial ROS activates JNK. Sustained JNK drives c‑JUN phosphorylation, which in turn recruits histone acetyltransferases to AP‑1 sites on SASP promoters, establishing an epigenetic imprint that maintains transcription even if mitochondrial ROS fluctuate. This model extends the feed‑forward loop by proposing that the redox state of ASK1, not merely ROS levels, determines whether the signal is reversible or locked in ASK1 maintains SASP.

Testable Predictions

1. Cells expressing ASK1 Cys32/35→Ser mutants will show normal early JNK activation after stress but fail to maintain phospho‑c‑JUN and SASP expression beyond 72 h, despite persistent miMOMP‑derived ROS.
2. Pharmacological reduction of ASK1 cysteines with cell‑permeable thioredoxin mimetics administered at 60 h post‑induction will reverse AP‑1‑dependent chromatin acetylation at SASP enhancers and reduce IL‑6/IL‑1β secretion.
3. Proximity labeling will reveal that oxidized ASK1 preferentially partitions into stress‑granule‑like condensates that contain JNK scaffolds, whereas reduced ASK1 remains diffusely cytosolic.

Experimental Approach

• Generate isogenic human fibroblast lines with CRISPR‑knockin of ASK1 C32S/C35S and appropriate controls.
• Induce senescence with doxorubicin; monitor mitochondrial ROS (MitoSOX), phospho‑ASK1, phospho‑JNK, and phospho‑c‑JUN at 0, 24, 48, 72, 96 h RUNX1 feed‑forward loop.
• Assess SASP mRNA (IL6, IL1B) and protein secretion; perform ATAC‑seq and H3K27ac ChIP‑seq to quantify enhancer acetylation.
• Apply a thioredoxin mimetic (e.g., TXN‑peptide) at 60 h and repeat measurements.
• Use BioID‑ASK1 (wild‑type vs Cys mutants) to map interacting proteins; evaluate condensate formation by fluorescence recovery after photobleaching (FRAP).

If the cysteine‑mutant cells display transient JNK activity and reversible SASP, while wild‑type cells show persistent signaling, the hypothesis is supported. Conversely, if mutating the cysteines does not affect the duration of AP‑1 activity, the redox‑switch model would be falsified.