Hypothesis

Transient senescent fibroblasts in healing wounds limit excessive myofibroblast conversion by capturing and inactivating active TGF‑β through SASP‑secreted latent‑TGF‑β binding proteins (LTBPs). Removal of these senescent cells before the resolution phase unleashes uncontrolled TGF‑β signaling, driving pathological fibrosis.

Rationale

• Acute senescence promotes tissue repair via SASP‑mediated immune recruitment and matrix remodeling [PMC9620855].
• During wound healing, waves of p16+/p21+ senescent fibroblasts appear early and disappear as granulation tissue matures [10.1101/2025.06.08.658533].
• SASP can reinforce senescence autocrinely and spread paracrine senescence to limit proliferation of minimally damaged cells [JCI 95148].
• TGF‑β is a master driver of myofibroblast differentiation; its activity is tightly controlled by extracellular latent complexes and LTBPs.

We propose that senescent fibroblasts secrete specific LTBPs (e.g., LTBP1, LTBP3) as part of their SASP. These proteins bind latent TGF‑β, keeping it sequestered in the extracellular matrix. When senescent cells are cleared prematurely, LTBP levels drop, latent TGF‑β is rapidly activated by integrins or proteases, and neighboring fibroblasts differentiate into myofibroblasts unchecked.

Testable Predictions

1. LTBP enrichment in senescent SASP – Conditioned medium from irradiated or oncogene‑induced senescent fibroblasts will contain higher levels of LTBP1/3 compared with proliferating fibroblasts, detectable by ELISA or mass spectrometry.
2. Loss of LTBP correlates with TGF‑β activation – In murine excisional wounds, genetic ablation of p16+ cells using p16‑3MR at day 3 post‑injury will reduce LTBP deposition in the wound bed and increase phospho‑SMAD2/3 signaling in adjacent fibroblasts by day 5.
3. Fibrosis exacerbation after early senolysis – Mice receiving a senolytic (e.g., navitoclax) at day 3 will show increased α‑SMA+ myofibroblast density and collagen deposition at day 14 compared with vehicle controls, despite similar early wound closure rates.
4. Rescue by LTBP supplementation – Exogenous recombinant LTBP1 co‑administered with early senolytic will normalize TGF‑β activity and prevent excessive fibrosis, confirming that LTBP loss mediates the phenotype.
5. Human relevance – Human keloid fibroblasts, which exhibit persistent senescence markers, will display elevated LTBP secretion; siRNA knock‑down of LTBP1 in these cells will increase TGF‑β signaling and collagen production.

Experimental Approach

• In vitro: Isolate primary mouse dermal fibroblasts, induce senescence via etoposide or IR, collect SASP, quantify LTBPs by western blot and ELISA. Treat naïve fibroblasts with senescent SASP ± LTBP‑neutralizing antibodies and measure p‑SMAD2/3 and α‑SMA expression.
• In vivo: Use p16‑3MR mice; administer ganciclovir or navitoclax at defined post‑wounding intervals (day 0, 3, 5). Harvest wounds for immunofluorescence (LTBP1, p‑SMAD2/3, α‑SMA), hydroxyproline assay, and RNA‑seq to assess fibrosis pathways.
• Intervention: Deliver recombinant LTBP1 via hydrogels or osmotic pumps alongside senolytics.
• Human tissue: Analyze LTBP1 expression in normal scar versus keloid biopsies via immunohistochemistry; correlate with senescence markers (p16, p21, SASP).

Potential Outcomes and Implications

If validated, this hypothesis reframes senescent fibroblasts not merely as transient immunomodulators but as active gatekeepers of growth‑factor bioavailability. It suggests that the timing of senolytic interventions is critical: early clearance may disrupt a protective LTBP‑TGF‑β buffer, converting a regenerative response into a fibrotic one. Consequently, therapeutic strategies might combine senolysis with LTBP supplementation or TGF‑β neutralizing antibodies to preserve the beneficial SASP while eliminating detrimental persistent senescence.

By linking senescence‑derived extracellular matrix modulators to TGF‑β homeostasis, the hypothesis provides a mechanistic bridge between the "hostage negotiator" concept and the dual nature of senescence in tissue repair versus pathology.