Cyclic OSK Reprogramming Converts Chronic Senescence into a Transient, Pro-Regenerative State to Enhance Tissue Repair

Hypothesis

Repeated, low‑dose expression of the Yamanaka factors OCT4, SOX2, and KLF4 (OSK) in senescent fibroblasts rewires the epigenetic landscape of the senescence‑associated secretory phenotype (SASP) from a chronic, pro‑inflammatory mode to a transient, wound‑healing‑promoting mode without inducing pluripotency.

Mechanistic Rationale

Chronic senescence is marked by stable heterochromatin at promoters of regenerative SASP factors (e.g., PDGF‑AA, CCN1) and open chromatin at inflammatory loci (IL‑6, IL‑8). OSK factors are known to recruit histone demethylases and acetyltransferases that can remodel H3K9me3 marks at pluripotency genes. We propose that brief OSK pulses partially reset this landscape:

• OSK binding displaces HP1γ from H3K9me3‑rich regions, reducing repression of pro‑regenerative SASP genes.
• Simultaneously, OSK‑mediated recruitment of BRD4 increases acetylation at enhancers of PDGF‑AA and CCN1, boosting their transcription.
• Inflammatory SASP genes, already embedded in a more permissive chromatin state, are not further amplified because OSK does not sustain NF‑κB activation; instead, a feedback loop involving increased miR‑146a dampens NF‑κB signaling. The net effect is a SASP profile that mimics the acute, pro‑regenerative senescence seen in young wound models and salamander limb blastemas, where senescent cells act as chaperones rather than chronic irritants.

Experimental Design

1. Model: Use p16‑3MR mice (which allow fluorescent tracking and inducible ablation of p16+ cells) aged 20‑24 months.
2. Intervention: Generate a doxycycline‑inducible OSK construct knocked into the Rosa26 locus (Rosa26‑rtTA‑TetO‑OSK). Administer doxycycline in 2‑day pulses every 7 days for 4 weeks (mimicking cyclic partial reprogramming). Control groups receive vehicle or doxycycline without OSK.
3. Readouts:
   • SASP profiling: Multiplex cytokine assay of skin interstitial fluid collected 24 h after each pulse; focus on PDGF‑AA, CCN1, IL‑6, IL‑8, MMP‑3.
   • Chromatin state: CUT&RUN for H3K9me3, H3K27ac, and OSK binding at Pdgfa, Ccn1, Il6 promoters in FACS‑sorted p16+ fibroblasts.
   • Wound healing: Full‑thickness excisional wounds on dorsal skin; measure closure rate by planimetry and histologic re‑epithelialization at days 3, 7, 14.
   • Functional senescence: SA‑β‑gal staining and EdU incorporation to ensure cells remain non‑proliferative.
   • Safety: Monitor for teratoma formation and tumorigenesis over 6 months.

Predicted Outcomes

• Cyclic OSK will increase H3K27ac and decrease H3K9me3 at Pdgfa and Ccn1 promoters in p16+ cells, correlating with a 2‑3‑fold rise in PDGF‑AA and CCN1 levels in the SASP.
• IL‑6 and IL‑8 will show no significant increase or a modest decline compared with baseline chronic senescence.
• Wound closure in OSK‑treated aged mice will accelerate to rates comparable to young adult controls (≈ 90 % closure by day 10 vs. ≈ 60 % in vehicle).
• Histology will reveal a thicker granulation layer with increased collagen deposition, mirroring the pro‑regenerative SASP signature.
• No teratomas or ectopic pluripotency markers (OCT4, NANOG) will be detected in treated tissues.

Falsifiability

If OSK pulses fail to alter the chromatin state of regenerative SASP genes, or if the SASP remains dominated by inflammatory cytokines without improvement in wound healing, the hypothesis is refuted. Likewise, evidence of sustained pluripotency induction or tumorigenesis would invalidate the proposed mechanism.

Significance

This work shifts the senolytic paradigm from eradication to phenotypic restoration, leveraging the intrinsic plasticity of senescent cells to recover their chaperone function in aged tissues.