Hypothesis

Transiently senescent mesenchymal stem cells (MSCs) maintain the positional identity of neighboring progenitors by secreting a specific SASP repertoire that stabilizes HOX gene expression; senolytic ablation of these cells erodes this cue, leading to HOX drift, loss of positional memory, and aberrant tissue remodeling.

Mechanistic Model

We propose that senescent MSCs release exosomes enriched in miR‑126 and Wnt5a, together with soluble BMP2, which activate non‑canonical Wnt/BMP signaling in adjacent MSCs and fibroblasts. This signaling sustains a chromatin state that preserves the collinear HOX code (e.g., HOXA5‑HOXA9) through reduced H3K27me3 deposition and increased H3K4me3 at HOX loci. In young tissue, this SASP pulse is transient, supporting repair. With age, senescent cells accumulate, and the same SASP becomes chronic, shifting the balance toward inflammation while still providing positional cues; removing the cells eliminates both the inflammatory and the instructive signals, causing progenitors to lose HOX fidelity and adopt a maladaptive, pro‑fibrotic phenotype.

Testable Predictions

1. Single‑cell RNA‑seq of MSCs isolated from young mice after ABT‑263 treatment will show a significant increase in transcriptional noise and a shift in HOX gene expression variance compared with vehicle controls.
2. ATAC‑seq will reveal decreased accessibility at HOX promoters and enhancers in MSC progeny following senolytic clearance.
3. Co‑culture of senescent MSCs (irradiated or oncogene‑induced) with naïve MSCs will preserve HOXA5‑HOXA9 expression; neutralizing antibodies against Wnt5a or BMP2, or exosome depletion, will abolish this effect.
4. In vivo, local delivery of recombinant Wnt5a/BMP2 or exosome mimetics after senolytic treatment will rescue HOX patterning and reduce fibrosis in a murine model of myocardial infarction.

Falsifiable Outcomes

If senolytic treatment does not alter HOX variance or chromatin accessibility, or if adding back the proposed SASP factors fails to restore HOX expression, the hypothesis is refuted.

Broader Impact

This framework reframes senescent cells not merely as sources of damage but as niche components that encode positional memory, suggesting that senolytic regimens should be timed or combined with SASP replacement to preserve tissue architecture.