JNK2‑cJun‑H2A.J feed‑forward loop locks the SASP in senescent cells

Hypothesis

Sustained JNK2 activity drives a feed‑forward loop that locks the SASP through coordinated phosphorylation of c‑Jun and stabilization of the H2A.J histone variant.

Mechanistic model

1. Persistent ROS or mitochondrial damage keeps JNK2 active in senescent cells.
2. Active JNK2 phosphorylates c‑Jun on Ser63/Ser73, increasing its affinity for DNA and its ability to recruit the histone chaperone HIRA.
3. Phospho‑cJun–HIRA complexes promote deposition of H2A.J at promoters of IL6, IL8, and other SASP genes.
4. The same phospho‑cJun recruits p300/CBP, which acetylates H2A.J (e.g., H2A.JK5ac). Acetylated H2A.J stabilizes nucleosome positioning and maintains an open chromatin state.
5. Acetyl‑H2A.J further enhances AP‑1 binding, creating a self‑reinforcing circuit that preserves transcriptional activity even when upstream stress wanes.

Testable predictions

• Isoform specificity: JNK2 knock‑down or CRISPR knockout will reduce H2A.J incorporation and H2A.J acetylation at SASP promoters without affecting the initial p53‑dependent growth arrest, whereas JNK1 loss will impair the arrest but leave SASP largely intact.
• Phospho‑cJun requirement: Expressing a phospho‑deficient c‑Jun mutant (S63A/S73A) in JNK2‑proficient senescent cells will phenocopy the JNK2 loss—decreased H2A.J loading, lower SASP mRNA, and reduced chromatin accessibility.
• Epigenetic read‑out: ChIP‑seq for H2A.J and H2A.JK5ac will show coincident enrichment at SASP loci in wild‑type senescent cells; this enrichment will be lost in JNK2‑deficient or phospho‑cJun‑mutant cells.
• Functional rescue: Overexpressing an acetyl‑mimic H2A.J (K5Q) in JNK2‑deficient senescent cells will restore SASP expression despite low c‑Jun phosphorylation, confirming that H2A.J acetylation is a downstream effector.
• Pharmacological test: Selective JNK2 inhibitors (but not JNK1/3 inhibitors) will decrease SASP cytokine secretion in aged mouse tissues and reduce H2A.J acetylation, while leaving p53‑mediated senescence markers unchanged.

Experimental approach

• Use primary human fibroblasts irradiated to induce senescence; generate isoform‑specific shRNA or CRISPR lines.
• Measure SASP cytokines by ELISA, p53/p21 levels by Western blot.
• Perform CUT&Tag for H2A.J, H2A.JK5ac, and phospho‑cJun.
• Assess chromatin accessibility via ATAC‑seq.
• Validate in vivo with aged JNK2‑heterozygous mice and SASP profiling.

It's known that persistent ROS activates JNK2, and we don't expect off‑target effects from isoform‑specific tools.

If these predictions hold, the JNK2‑cJun‑H2A.J axis would constitute a molecular switch that converts transient stress signaling into a stable inflammatory epigenetic state, offering a precise target for delaying inflammaging without blocking beneficial stress responses.