Hormesis as a Localized Threat Signal That Fails to Propagate in Aged Tissue\n\nCentral hypothesis: Hormetic activation of stress‑response pathways (HSP70, Nrf2 targets, UPR) is not a systemic rejuvenation program but a tightly confined danger signal that radiates from sites of cellular damage. In young tissue this signal can diffuse sufficiently to prep‑air neighboring cells for repair, whereas in aged tissue the extracellular matrix (ECM) sequesters and immobilizes these effectors, aborting the diffusion gradient and turning hormesis into a failure mode that sustains senescence‑bystander effects.\n\n### Mechanistic Basis\n- Stress‑response effectors are secreted or leak from stressed/senescent cells as soluble chaperones, phosphorylated Nrf2, or UPR ligands. Their activity depends on free diffusion through the interstitial space.\n- Aged ECM is enriched in advanced glycation end‑products (AGEs) and cross‑linked collagen, which bind HSP70, Nrf2, and GRP78 with high affinity, creating a molecular sink that limits their range.\n- Senescent cells upregulate TIMP‑1 and LOX, further stabilizing the matrix and reducing protease‑mediated release of sequestered chaperones.\n- Consequently, the concentration profile of stress‑response markers shows a steep, short‑range decay from p16⁺/p21⁺ foci in old muscle, whereas young tissue displays a shallow, longer‑range gradient that can reach regenerative progenitor niches.\n\n### Testable Predictions\n1. Spatial transcriptomics / proteomics of young (3 mo) and aged (24 mo) mouse skeletal muscle will reveal a significantly steeper decay constant (λ) for HSP70, Nrf2 targets (Nqo1, Ho‑1), and UPR markers (Grp78, Chop) around p16⁺ cells in aged tissue.\n2. Enzymatic disruption of AGEs (e.g., with alagebrium) or LOX inhibition (β‑aminopropionitrile) in aged mice will flatten the λ, extending the stress‑signal radius without altering senescent cell burden.\n3. Flattened gradients will correlate with improved satellite‑cell activation and fiber hypertrophy after exercise or ischemia‑reperfusion injury.\n4. Artificial tethering of HSP70 to a soluble, non‑binding carrier (e.g., PEGylated HSP70) injected into aged muscle will rescue the gradient and regeneration even when ECM remains cross‑linked.\n\n### Potential Falsification\nIf high‑resolution spatial profiling shows no age‑dependent difference in the decay length of stress‑response signatures, or if ECM‑modifying interventions fail to alter the gradient or regenerative outcome, the hypothesis that hormesis is a localized threat signal hampered by aged matrix would be falsified. Conversely, confirmation would reframe hormesis as a proximity‑dependent danger response whose efficacy is gated by tissue microenvironment, reconciling the paradox that health‑promoting stresses mimic imminent death signals only when the signal can reach its targets.
