Hypothesis

Transient expression of OSK (Oct4, Sox2, Klf4) in senescent fibroblasts reprograms the secretory phenotype toward a developmental, regenerative SASP while maintaining the p16^INK4a‑mediated cell‑cycle arrest that suppresses tumorigenesis.

Mechanistic Rationale

Young senescent fibroblasts are enriched for developmental and morphogenetic programs that promote regeneration [1]. Inflammation‑driven SASP in aged senescence loses this coordination [1]. OSK specifically downregulates inflammatory SASP genes without altering the core arrest [4]. We propose that a short OSK pulse remodels the epigenetic landscape of senescent cells, shifting SASP composition from IL‑6/IL‑8‑dominant to osteopontin‑rich, fibroblast‑growth‑factor‑rich patterns that activate stem‑cell niches via CD44 signaling [2] while keeping p16^INK4a expression intact [3] and thus preserving tumor‑suppressive arrest.

Predictions

1. In murine excisional wound models, localized, transient OSK delivery (2‑day doxycycline pulse in inducible OSK fibroblasts) will increase wound closure rate and collagen organization compared with controls, without increasing hyperplastic lesions or tumor incidence over 6 months.
2. Senescent cells isolated from OSK‑treated wounds will show reduced expression of inflammatory SASP cytokines (IL‑1β, IL‑6, TNF‑α) and elevated osteopontin and FGF‑7 levels, while p16^INK4a and SA‑β‑gal activity remain unchanged.
3. Blocking CD44 with neutralizing antibodies or genetic knockdown will abolish the OSK‑induced improvement in regeneration, indicating that the regenerative effect depends on the osteopontin‑CD44 axis.
4. Chronic or repeated OSK exposure (e.g., weekly pulses) will eventually erode p16^INK4a‑mediated arrest and lead to hyperplastic lesions, establishing a dose‑dependent safety window.

Experimental Design

• Use Col1a2‑CreERT2;Rosa26‑LSL‑OSK mice to induce OSK in fibroblasts upon tamoxifen; administer a single 2‑day doxycycline pulse to synchronize expression.
• Create 4 mm dorsal skin wounds; treat groups: (a) vehicle, (b) senolytic (dasatinib + quercetin) as positive control for clearance, (c) transient OSK, (d) OSK + CD44 blockade.
• Measure wound area daily, histology at days 4 and 14, and long‑term skin tumor formation at 6 months.
• Perform single‑cell RNA‑seq on FACS‑sorted PDGFRβ^+ fibroblasts to quantify SASP shifts and p16^INK4a expression.
• Validate findings in human senescent fibroblast cultures exposed to 4‑hour OSK mRNA electroporation, assessing secreted osteopontin and proliferation of co‑cultured hair follicle stem cells.

Falsifiability

If transient OSK fails to improve wound healing despite confirmed reduction of inflammatory SASP, or if healing improves but p16^INK4a expression is significantly lowered correlating with tumor formation, the hypothesis that OSK reprograms senescence without compromising tumor‑suppressive arrest would be falsified. Conversely, a successful outcome preserving arrest while enhancing regeneration would support the notion that senescent cells can be ‘rejuvenated’ negotiators rather than removed.