Hypothesis

Aged tissues exhibit spatially confined senescence-associated secretory phenotype (SASP) gradients that directly modulate extracellular matrix (ECM) stiffness through integrin‑mediated mechanotransduction, creating a self‑reinforcing loop that accelerates functional decline.

Rationale

• Single‑cell and bulk transcriptomics have shown increased SASP factor expression in aged tissues, but spatial context remains unclear.
• Emerging expansion‑based methods like Seq‑Scope‑X now enable subcellular resolution mapping of transcripts in complex tissues Recent hypothesis on waste disposal.
• Prior work links senescent cell accumulation to altered tissue mechanics in skeletal muscle regeneration and neurodegenerative settings Frontiers in Aging Neuroscience and PMC9135953.

We propose that SASP components such as IL‑6, TGF‑β1, and MMPs are not uniformly secreted; instead, they form steep concentration gradients emanating from senescent cell clusters. These gradients engage mechanosensitive integrins on neighboring fibroblasts and epithelial cells, triggering focal adhesion kinase (FAK) signaling and downstream actin‑cytoskeletal remodeling. The resulting increase in actomyosin contractility elevates local ECM crosslinking and deposition, which in turn amplifies integrin activation—a positive feedback loop that propagates stiffness outward from the senescence niche.

Testable Predictions

1. Spatial correlation – High‑resolution spatial transcriptomics of aged mouse liver will reveal SASP mRNA hotspots whose intensity decays with distance, inversely correlating with measured ECM stiffness (via atomic force microscopy) at matching coordinates.
2. Mechanotransduction dependence – Pharmacological inhibition of FAK or integrin β1 in aged tissue explants will attenuate the SASP‑stiffness correlation without altering overall senescent cell burden.
3. Gradient disruption – Genetic ablation of a key SASP regulator (e.g., NF‑κB p65) specifically in senescent cells will flatten the SASP gradient and prevent age‑related stiffening, measurable by second‑harmonic generation imaging.
4. Cross‑species validation – Human biopsy samples from young vs. aged donors will show conserved spatial SASP patterns and stiffness gradients, supporting translational relevance.

Experimental Approach

• Apply Seq‑Scope‑X or equivalent expansion sequencing to cryosections of aged (24‑month) and young (3‑month) murine tissues (liver, kidney, skin).
• Simultaneously perform spatial proteomics (e.g., CODEX) for phospho‑FAK and collagen deposition.
• Use laser‑capture microdissection of gradient zones to validate transcript‑protein concordance.
• Perturb integrin‑FAK signaling with small‑molecule inhibitors or conditional knockouts and reassess spatial maps.

Falsifiability

If spatial transcriptomics fails to detect organized SASP gradients, or if manipulating mechanotransduction does not alter the SASP‑stiffness relationship, the hypothesis would be refuted. Conversely, observing the predicted gradient‑dependent mechanochemical coupling would substantiate a novel axis by which cellular senescence reshapes tissue architecture during aging.